Showing papers in "Ocean Science in 2021"
••
TL;DR: In this paper, the authors describe the underlying hydrodynamic and data-assimilation design and accuracy assessment for the latest FES2014 atlas, especially for the altimetry de-aliasing purposes.
Abstract: . Since the mid-1990s, a series of FES (finite element solution) global ocean
tidal atlases has been produced and released with the primary objective to
provide altimetry missions with tidal de-aliasing correction at the best
possible accuracy. We describe the underlying hydrodynamic and data
assimilation design and accuracy assessments for the latest FES2014 release
(finalized in early 2016), especially for the altimetry de-aliasing
purposes. The FES2014 atlas shows extremely significant improvements
compared to the standard FES2004 and (intermediary) FES2012 atlases, in all
ocean compartments, especially in shelf and coastal seas, thanks to the
unstructured grid flexible resolution, recent progress in the (prior to
assimilation) hydrodynamic tidal solutions, and use of ensemble data
assimilation technique. Compared to earlier releases, the available tidal
constituent's spectrum has been significantly extended, the overall
resolution has been augmented, and additional scientific byproducts such as loading
and self-attraction, energy diagnostics, or lowest astronomical tides have
been derived from the atlas and are available. Compared to the other
available global ocean tidal atlases, FES2014 clearly shows improved
de-aliasing performance in most of the global ocean areas and has
consequently been integrated in satellite altimetry geophysical data records
(GDRs) and gravimetric data processing and adopted in recently renewed ITRF
standards (International Terrestrial Reference System, 2020). It also
provides very accurate open-boundary tidal conditions for regional and
coastal modelling.
134 citations
••
TL;DR: In this article, the authors present the newly updated CNES-CLS18 MDT, which uses updated input datasets: the GOCO05S geoid model is used based on the complete GOCE mission (November 2009-October 2013) and 10.5 years of GRACE data, together with all drifting buoy velocities (SVP-type and Argo floats) and hydrological profiles (CORA database) available from 1993 to 2017 (instead of the 1993-2012).
Abstract: . The mean dynamic topography (MDT) is a key reference surface for altimetry. It is needed for the calculation of the ocean absolute dynamic topography, and under the geostrophic approximation, the estimation of surface currents. CNES-CLS mean dynamic topography (MDT) solutions are calculated by merging information from altimeter data, GRACE, and GOCE gravity field and oceanographic in situ measurements (drifting buoy velocities, hydrological profiles). The objective of this paper is to present the newly updated CNES-CLS18 MDT. The main improvement compared to the previous CNES-CLS13 solution is the use of updated input datasets: the GOCO05S geoid model is used based on the complete GOCE mission (November 2009–October 2013) and 10.5 years of GRACE data, together with all drifting buoy velocities (SVP-type and Argo floats) and hydrological profiles (CORA database) available from 1993 to 2017 (instead of 1993–2012). The new solution also benefits from improved data processing (in particular a new wind-driven current model has been developed to extract the geostrophic component from the buoy velocities) and methodology (in particular the computation of the medium-scale GOCE-based MDT first guess has been revised). An evaluation of the new solution compared to the previous version and to other existing MDT solutions show significant improvements in both strong currents and coastal areas.
62 citations
••
TL;DR: In this article, the authors quantify biases in Antarctic Bottom Water (AABW) and North Atlantic Deep Water (NADW) formation, properties, transport, and global extent in 35 climate models that participated in the latest Climate Model Intercomparison Project (CMIP6).
Abstract: . Deep and bottom water formation are crucial components of the global ocean circulation, yet they were poorly represented in the previous generation of climate models. We here quantify biases in Antarctic Bottom Water (AABW) and North Atlantic Deep Water (NADW) formation, properties, transport, and global extent in 35 climate models that participated in the latest Climate Model Intercomparison Project (CMIP6). Several CMIP6 models are correctly forming AABW via shelf processes, but 28 models in the Southern Ocean and all 35 models in the North Atlantic form deep and bottom water via open-ocean deep convection too deeply, too often, and/or over too large an area. Models that convect the least form the most accurate AABW but the least accurate NADW. The four CESM2 models with their overflow parameterisation are among the most accurate models. In the Atlantic, the colder the AABW, the stronger the abyssal overturning at 30 ∘ S, and the further north the AABW layer extends. The saltier the NADW, the stronger the Atlantic Meridional Overturning Circulation (AMOC), and the further south the NADW layer extends. In the Indian and Pacific oceans in contrast, the fresher models are the ones which extend the furthest regardless of the strength of their abyssal overturning, most likely because they are also the models with the weakest fronts in the Antarctic Circumpolar Current. There are clear improvements since CMIP5: several CMIP6 models correctly represent or parameterise Antarctic shelf processes, fewer models exhibit Southern Ocean deep convection, more models convect at the right location in the Labrador Sea, bottom density biases are reduced, and abyssal overturning is more realistic. However, more improvements are required, e.g. by generalising the use of overflow parameterisations or by coupling to interactive ice sheet models, before deep and bottom water formation, and hence heat and carbon storage, are represented accurately.
59 citations
••
TL;DR: In this article, the characteristics of the major water masses in the Atlantic Ocean are described by six properties taken from the biased-adjusted Global Ocean Data Analysis Project version 2 (GLODAPv2) data product.
Abstract: . A large number of water masses are presented in the Atlantic Ocean,
and knowledge of their distributions and properties is important for
understanding and monitoring of a range of oceanographic phenomena. The
characteristics and distributions of water masses in biogeochemical space
are useful for, in particular, chemical and biological oceanography to
understand the origin and mixing history of water samples. Here, we define
the characteristics of the major water masses in the Atlantic Ocean as
source water types (SWTs) from their formation areas, and map out their
distributions. The SWTs are described by six properties taken from the
biased-adjusted Global Ocean Data
Analysis Project version 2 (GLODAPv2) data product, including both conservative
(conservative temperature and absolute salinity) and non-conservative
(oxygen, silicate, phosphate and nitrate) properties. The distributions of
these water masses are investigated with the use of the optimum
multi-parameter (OMP) method and mapped out. The Atlantic Ocean is divided
into four vertical layers by distinct neutral densities and four zonal
layers to guide the identification and characterization. The water masses in
the upper layer originate from wintertime subduction and are defined as
central waters. Below the upper layer, the intermediate layer consists of
three main water masses: Antarctic Intermediate Water (AAIW), Subarctic
Intermediate Water (SAIW) and Mediterranean Water (MW). The North Atlantic
Deep Water (NADW, divided into its upper and lower components) is the
dominating water mass in the deep and overflow layer. The origin of both the
upper and lower NADW is the Labrador Sea Water (LSW), the Iceland–Scotland
Overflow Water (ISOW) and the Denmark Strait Overflow Water (DSOW). The
Antarctic Bottom Water (AABW) is the only natural water mass in the bottom
layer, and this water mass is redefined as Northeast Atlantic Bottom Water
(NEABW) in the north of the Equator due to the change of key properties,
especially silicate. Similar with NADW, two additional water masses,
Circumpolar Deep Water (CDW) and Weddell Sea Bottom Water (WSBW), are
defined in the Weddell Sea region in order to understand the origin of AABW.
39 citations
••
TL;DR: In this article, a hierarchy of global 1/4 ∘ ǫ (ORCA025) and Atlantic Ocean 1/20 ∘ nested (VIKING20X) ocean-sea-ice models is described.
Abstract: . A hierarchy of global 1 / 4 ∘ (ORCA025) and Atlantic Ocean 1 / 20 ∘ nested (VIKING20X) ocean–sea-ice models is described. It is shown that the eddy-rich configurations performed in hindcasts of the past 50–60 years under CORE and JRA55-do atmospheric forcings realistically simulate the large-scale horizontal circulation, the distribution of the mesoscale, overflow and convective processes, and the representation of regional current systems in the North and South Atlantic. The representation of the Atlantic Meridional Overturning Circulation (AMOC), and in particular the long-term temporal evolution, strongly depends on numerical choices for the application of freshwater fluxes. The interannual variability of the AMOC instead is highly correlated among the model experiments and also with observations, including the 2010 minimum observed by RAPID at 26.5 ∘ N. This points to a dominant role of the wind forcing.
The ability of the model to represent
regional observations in western boundary current (WBC) systems at 53 ∘ N, 26.5 ∘ N and 11 ∘ S is explored. The question is investigated of whether WBC systems are able to represent the AMOC, and in particular whether these WBC systems exhibit similar temporal evolution to that of the zonally integrated AMOC. Apart from the basin-scale measurements at 26.5 ∘ N, it is shown that in particular the outflow of North Atlantic Deepwater at 53 ∘ N is a good indicator of the subpolar AMOC trend during the recent decades, once provided in density coordinates. The good reproduction of observed AMOC and WBC trends in the most reasonable simulations indicate that the eddy-rich VIKING20X is capable of representing realistic forcing-related and ocean-intrinsic trends.
30 citations
••
TL;DR: In this paper, the authors describe the new developments of the Ship-Traffic Emission Assessment Model (STEAM) which enable the modelling of pollution discharges to water from ships, including nutrients from black/grey water discharges as well as from food waste.
Abstract: . This paper describes the new developments of the Ship
Traffic Emission Assessment Model (STEAM) which enable the modelling of
pollutant discharges to water from ships. These include nutrients from
black/grey water discharges as well as from food waste. Further, the
modelling of contaminants in ballast, black, grey and scrubber water, bilge
discharges, and stern tube oil leaks are also described as well as releases of
contaminants from antifouling paints. Each of the discharges is regulated
by different sections of the IMO MARPOL convention, and emission patterns of
different pollution releases vary significantly. The discharge patterns and
total amounts for the year 2012 in the Baltic Sea area are reported and open-loop SO x scrubbing effluent was found to be the second-largest pollutant
stream by volume. The scrubber discharges have increased significantly in
recent years, and their environmental impacts need to be investigated in
detail.
27 citations
••
TL;DR: In this article, a detailed comparison and validation of these internal-tide models is proposed using existing satellite altimeter databases. And the results show a significantly significant altimeter variance reduction when using internal tide correction.
Abstract: . Altimeter measurements are corrected for several
geophysical parameters in order to access ocean signals of interest, like
mesoscale or sub-mesoscale variability. The ocean tide is one of the most
critical corrections due to the amplitude of the tidal elevations and to the
aliasing phenomena of high-frequency signals into the lower-frequency band, but
the internal-tide signatures at the ocean surface are not yet corrected
globally. Internal tides can have a signature of several centimeters at the surface with
wavelengths of about 50–250 km for the first mode and even smaller scales for
higher-order modes. The goals of the upcoming Surface Water Ocean Topography
(SWOT) mission and other high-resolution ocean measurements make the
correction of these small-scale signals a challenge, as the correction of
all tidal variability becomes mandatory to access accurate measurements of
other oceanic signals. In this context, several scientific teams are working on the development of
new internal-tide models, taking advantage of the very long altimeter time
series now available, which represent an unprecedented and valuable global
ocean database. The internal-tide models presented here focus on the
coherent internal-tide signal and they are of three types: empirical models
based upon analysis of existing altimeter missions, an assimilative model and a three-dimensional hydrodynamic model. A detailed comparison and validation of these internal-tide models is
proposed using existing satellite altimeter databases. The analysis focuses
on the four main tidal constituents: M 2 , K 1 , O 1 and S 2 . The validation
process is based on a statistical analysis of multi-mission altimetry
including Jason-2 and Cryosphere Satellite-2 data. The results show a
significant altimeter variance reduction when using internal-tide
corrections in all ocean regions where internal tides are
generating or propagating. A complementary spectral analysis also gives some
estimation of the performance of each model as a function of wavelength and
some insight into the residual non-stationary part of internal tides in the
different regions of interest. This work led to the implementation of a new
internal-tide correction (ZARON'one) in the next geophysical data records version-F (GDR-F) standards.
27 citations
••
TL;DR: In this paper, the authors introduce a similar metric for sea-level response, which is defined as the increase in the sea level rate associated with a given warming in units of meters per century per kelvin.
Abstract: . Recent assessments from the Intergovernmental Panel on
Climate Change (IPCC) imply that global mean sea level is unlikely to rise
more than about 1.1 m within this century but will increase further beyond
2100. Even within the most intensive future anthropogenic greenhouse gas
emission scenarios, higher levels are assessed to be unlikely. However, some
studies conclude that considerably greater sea level rise could be realized,
and a number of experts assign a substantially higher likelihood of such a
future. To understand this discrepancy, it would be useful to have scenario-independent metrics that can be compared between different approaches. The
concept of a transient climate sensitivity has proven to be useful to
compare the global mean temperature response of climate models to specific
radiative forcing scenarios. Here, we introduce a similar metric for sea
level response. By analyzing the mean rate of change in sea level (not sea level
itself), we identify a nearly linear relationship with global mean surface
temperature (and therefore accumulated carbon dioxide emissions) both in model projections and in observations on a century scale. This
motivates us to define the “transient sea level sensitivity” as the increase
in the sea level rate associated with a given warming in units of
meters per century per kelvin. We find that future projections estimated on climate model
responses fall below extrapolation based on recent observational records.
This comparison suggests that the likely upper level of sea level
projections in recent IPCC reports would be too low.
21 citations
••
TL;DR: This paper developed a higher-fidelity empirical model of AMOC variability based on RAPID data and associated physically with changes in thickness of the persistent upper, intermediate, and deep water masses at 26 ∘ ǫ n and associated transports.
Abstract: . A decline in Atlantic meridional overturning circulation (AMOC) strength has been observed between 2004 and 2012 by the RAPID-MOCHA-WBTS (RAPID – Meridional Overturning Circulation and Heatflux Array – Western Boundary Time Series, hereafter RAPID array) with this weakened state of the AMOC persisting until 2017. Climate model and paleo-oceanographic research suggests that the AMOC may have been declining for decades or even centuries before this; however direct observations are sparse prior to 2004, giving only “snapshots” of the overturning circulation. Previous studies have used linear models based on upper-layer temperature anomalies to extend AMOC estimates back in time; however these ignore changes in the deep circulation that are beginning to emerge in the observations of AMOC decline. Here we develop a higher-fidelity empirical model of AMOC variability based on RAPID data and associated physically with changes in thickness of the persistent upper, intermediate, and deep water masses at 26 ∘ N and associated transports. We applied historical hydrographic data to the empirical model to create an AMOC time series extending from 1981 to 2016. Increasing the resolution of the observed AMOC to approximately annual shows multi-annual variability in agreement with RAPID observations and shows that the downturn between 2008 and 2012 was the weakest AMOC since the mid-1980s. However, the time series shows no overall AMOC decline as indicated by other proxies and high-resolution climate models. Our results reinforce that adequately capturing changes to the deep circulation is key to detecting any anthropogenic climate-change-related AMOC decline.
21 citations
••
TL;DR: In this article, two long-lived anthropogenic radionuclides 129I and 236U together with two age models were used to constrain the pathways and circulation times of Atlantic Water in the surface (10-35 m depth) and in the mid-depth Atlantic layer (250-800 m depth).
Abstract: . The inflow of Atlantic Water to the Arctic Ocean is a crucial determinant for the future trajectory of this ocean basin with regard to warming, loss
of sea ice, and ocean acidification. Yet many details of the fate and circulation of these waters within the Arctic remain unclear. Here, we use the
two long-lived anthropogenic radionuclides 129I and 236U together with two age models to constrain the pathways and circulation
times of Atlantic Water in the surface (10–35 m depth) and in the mid-depth Atlantic layer (250–800 m depth). We thereby benefit
from the unique time-dependent tagging of Atlantic Water by these two isotopes. In the surface layer, a binary mixing model yields tracer ages of
Atlantic Water between 9–16 years in the Amundsen Basin, 12–17 years in the Fram Strait (East Greenland Current), and up to 20 years in the Canada
Basin, reflecting the pathways of Atlantic Water through the Arctic and their exiting through the Fram Strait. In the mid-depth Atlantic layer
(250–800 m ), the transit time distribution (TTD) model yields mean ages in the central Arctic ranging between 15 and 55 years, while the
mode ages representing the most probable ages of the TTD range between 3 and 30 years. The estimated mean ages are overall in good agreement with
previous studies using artificial radionuclides or ventilation tracers. Although we find the overall flow to be dominated by advection, the shift in
the mode age towards a younger age compared to the mean age also reflects the presence of a substantial amount of lateral mixing. For applications
interested in how fast signals are transported into the Arctic's interior, the mode age appears to be a suitable measure. The short mode ages
obtained in this study suggest that changes in the properties of Atlantic Water will quickly spread through the Arctic Ocean and can lead to
relatively rapid changes throughout the upper water column in future years.
18 citations
••
Cooperative Institute for Research in Environmental Sciences1, Earth System Research Laboratory2, University of Gothenburg3, Alfred Wegener Institute for Polar and Marine Research4, National Oceanography Centre, Southampton5, Remote Sensing Center6, University of Texas at Austin7, National Institute of Polar Research8, Danish Meteorological Institute9, University of Alaska Fairbanks10
TL;DR: In this article, the authors assess the variability of Arctic freshwater, which plays a fundamental role in the Arctic climate system by impacting ocean stratification and sea ice formation or melt.
Abstract: . The Arctic climate system is rapidly transitioning into a new
regime with a reduction in the extent of sea ice, enhanced mixing in the
ocean and atmosphere, and thus enhanced coupling within the
ocean–ice–atmosphere system; these physical changes are leading to ecosystem
changes in the Arctic Ocean. In this review paper, we assess one of the
critically important aspects of this new regime, the variability of Arctic
freshwater, which plays a fundamental role in the Arctic climate system by
impacting ocean stratification and sea ice formation or melt. Liquid and solid
freshwater exports also affect the global climate system, notably by
impacting the global ocean overturning circulation. We assess how freshwater
budgets have changed relative to the 2000–2010 period. We include
discussions of processes such as poleward atmospheric moisture transport,
runoff from the Greenland Ice Sheet and Arctic glaciers, the role of snow on
sea ice, and vertical redistribution. Notably, sea ice cover has become
more seasonal and more mobile; the mass loss of the Greenland Ice Sheet
increased in the 2010s (particularly in the western, northern, and southern regions)
and imported warm, salty Atlantic waters have shoaled. During 2000–2010, the
Arctic Oscillation and moisture transport into the Arctic are in-phase and
have a positive trend. This cyclonic atmospheric circulation pattern forces
reduced freshwater content on the Atlantic–Eurasian side of the Arctic Ocean
and freshwater gains in the Beaufort Gyre. We show that the trend in Arctic
freshwater content in the 2010s has stabilized relative to the 2000s,
potentially due to an increased compensation between a freshening of the
Beaufort Gyre and a reduction in freshwater in the rest of the Arctic Ocean.
However, large inter-model spread across the ocean reanalyses and
uncertainty in the observations used in this study prevent a definitive
conclusion about the degree of this compensation.
••
TL;DR: In this article, the authors presented a refined method for its determination, which is based on the combination of absolute satellite altimetry (SAT) sea level measurements and relative sea level changes recorded by tide gauges (TGs).
Abstract: . Vertical land motion (VLM) at the coast is a substantial contributor to relative sea level change. In this work, we present a refined method for its
determination, which is based on the combination of absolute satellite altimetry (SAT) sea level measurements and relative sea level changes
recorded by tide gauges (TGs). These measurements complement VLM estimates from the GNSS (Global Navigation Satellite System) by increasing their spatial
coverage. Trend estimates from the SAT and TG combination are particularly sensitive to the quality and resolution of applied altimetry data as well as
to the coupling procedure of altimetry and TGs. Hence, a multi-mission, dedicated coastal along-track altimetry dataset is coupled with
high-frequency TG measurements at 58 stations. To improve the coupling procedure, a so-called “zone of influence” (ZOI) is defined, which confines
coherent zones of sea level variability on the basis of relative levels of comparability between TG and altimetry observations. Selecting 20 %
of the most representative absolute sea level observations in a 300 km radius around the TGs results in the best VLM estimates in terms of
accuracy and uncertainty. At this threshold, VLM SAT-TG estimates have median formal uncertainties of
0.58 mm yr−1 . Validation against GNSS VLM estimates yields a root mean square (rms ΔVLM ) of VLM SAT-TG and
VLM GNSS differences of 1.28 mm yr−1 , demonstrating the level of accuracy of our approach. Compared to a reference
250 km radius selection, the 300 km zone of influence improves trend accuracies by 15 % and uncertainties by 35 %. With
increasing record lengths, the spatial scales of the coherency in coastal sea level trends increase. Therefore, the relevance of the ZOI for
improving VLM SAT-TG accuracy decreases. Further individual zone of influence adaptations offer the prospect of bringing the accuracy of
the estimates below 1 mm yr−1 .
••
TL;DR: In this paper, the vertical dispersion and distribution of negatively buoyant rigid microplastics within a realistic circulation model of the Mediterranean sea were studied and an equation describing their idealized dynamics was proposed.
Abstract: . We study the vertical dispersion and distribution of negatively
buoyant rigid microplastics within a realistic circulation
model of the Mediterranean sea. We first propose an equation
describing their idealized dynamics. In that framework, we
evaluate the importance of some relevant physical effects (inertia, Coriolis force, small-scale turbulence and
variable seawater density), and we bound the relative error of
simplifying the dynamics to a constant sinking velocity added
to a large-scale velocity field. We then calculate the amount
and vertical distribution of microplastic particles on the
water column of the open ocean if their release from the sea
surface is continuous at rates compatible with observations in
the Mediterranean. The vertical distribution is found to be
almost uniform with depth for the majority of our
parameter range. Transient distributions from flash releases
reveal a non-Gaussian character of the dispersion and various
diffusion laws, both normal and anomalous. The origin of these
behaviors is explored in terms of horizontal and vertical flow
organization.
••
TL;DR: In this paper, the relationship between coastal sea level and the variability of the western boundary currents has been previously studied in each basin separately, but comparison between the two basins is missing.
Abstract: . The northwest basins of the Atlantic and Pacific oceans are regions of intense western boundary currents (WBCs): the Gulf Stream and the Kuroshio. The variability of these poleward currents and their extensions in the open ocean is of major importance to the climate system. It is largely dominated by in-phase meridional shifts downstream of the points at which they separate from the coast. Tide gauges on the adjacent coastlines have measured the inshore sea level for many decades and provide a unique window on the past of the oceanic circulation. The relationship between coastal sea level and the variability of the western boundary currents has been previously studied in each basin separately, but comparison between the two basins is missing. Here we show for each basin that the inshore sea level upstream of the separation points is in sustained agreement with the meridional shifts of the western boundary current extension over the period studied, i.e. the past 7 (5) decades in the Atlantic (Pacific). Decomposition of the coastal sea level into principal components allows us to discriminate this variability in the upstream sea level from other sources of variability such as the influence of large meanders in the Pacific. Our result extends previous findings limited to the altimetry era and suggests that prediction of inshore sea-level changes could be improved by the inclusion of meridional shifts of the western boundary current extensions as predictors. Long-duration tide gauges, such as Key West, Fernandina Beach or Hosojima, could be used as proxies for the past meridional shifts of the western boundary current extensions.
••
TL;DR: In this paper, the impact of the assimilation of HFR (high-frequency radar) observations in a high-resolution regional model is evaluated, focusing on the improvement of the mesoscale dynamics.
Abstract: . The impact of the assimilation of HFR (high-frequency radar) observations in a high-resolution regional model is evaluated, focusing on the improvement of the mesoscale dynamics. The study area is the Ibiza Channel, located in the western Mediterranean Sea. The resulting fields are tested against trajectories from 13 drifters. Six different assimilation experiments are compared to a control run (no assimilation). The experiments consist of assimilating (i) sea surface temperature, sea level anomaly, and Argo profiles (generic observation dataset); the generic observation dataset plus (ii) HFR total velocities and (iii) HFR radial velocities. Moreover, for each dataset, two different initialization methods are assessed: (a) restarting directly from the analysis after the assimilation or (b) using an intermediate initialization step applying a strong nudging towards the analysis fields. The experiments assimilating generic observations plus HFR total velocities with the direct restart provide the best results, reducing by 53 % the average separation distance between drifters and virtual particles after the first 48 h of simulation in comparison to the control run. When using the nudging initialization step, the best results are found when assimilating HFR radial velocities with a reduction of the mean separation distance by around 48 %. Results show that the integration of HFR observations in the data assimilation system enhances the prediction of surface currents inside the area covered by both antennas, while not degrading the correction achieved thanks to the assimilation of generic data sources beyond it.
The assimilation of radial observations benefits from the smoothing effect associated with the application of the intermediate nudging step.
••
TL;DR: In this paper, the authors analyzed the robustness of Agulhas leakage estimates as well as the thermohaline property modifications of the leakage south of Africa using Lagrangian experiments with both the newly developed tool Parcels and the well established tool Ariane.
Abstract: . The inflow of relatively warm and salty water from the Indian Ocean into the South Atlantic via Agulhas leakage is important for the global overturning circulation and the global climate. In this study, we analyse the robustness of Agulhas leakage estimates as well as the thermohaline property modifications of Agulhas leakage south of Africa. Lagrangian experiments with both the newly developed tool Parcels and the well established tool Ariane were performed to simulate Agulhas leakage in the eddy-rich ocean–sea-ice model INALT20 ( 1 / 20 ∘ horizontal resolution) forced by the JRA55-do atmospheric boundary conditions.
The average transport, its variability, trend and the transit time from the Agulhas Current to the Cape Basin of Agulhas leakage is simulated comparably with both Lagrangian tools, emphasizing the robustness of our method. Different designs of the Lagrangian experiment alter in particular the total transport of Agulhas leakage by up to 2 Sv, but the variability and trend of the transport are similar across these estimates. During the transit from the Agulhas Current at 32 ∘ S to the Cape Basin, a cooling and freshening of Agulhas leakage waters occurs especially at the location of the Agulhas Retroflection, resulting in a density increase as the thermal effect dominates. Beyond the strong air–sea exchange around South Africa, Agulhas leakage warms and salinifies the water masses below the thermocline in the South Atlantic.
••
TL;DR: In this article, the impact of air-sea fluxes and the ocean surface density field on the transformation of subpolar mode water (SPMW) in the Iceland Basin, a water mass that “pre-conditions” dense water formation downstream, is analyzed.
Abstract: . Wintertime convection in the North Atlantic Ocean is a key component of the global climate as it produces dense waters at high latitudes that flow
equatorward as part of the Atlantic Meridional Overturning Circulation (AMOC). Recent work has highlighted the dominant role of the Irminger and
Iceland basins in the production of North Atlantic Deep Water. Dense water formation in these basins is mainly explained by buoyancy forcing that
transforms surface waters to the deep waters of the AMOC lower limb. Air–sea fluxes and the ocean surface density field are both key determinants of the buoyancy-driven transformation. We analyze these contributions to the transformation in order to better understand the connection between atmospheric forcing and the densification of surface water. More precisely, we study the impact of air–sea fluxes and the ocean surface density field on the transformation of subpolar mode water (SPMW) in the Iceland Basin, a water mass that “pre-conditions” dense water formation downstream. Analyses using 40 years of observations (1980–2019) reveal that the variance in SPMW transformation is mainly influenced by the
variance in density at the ocean surface. This surface density is set by a combination of advection, wind-driven upwelling and surface fluxes. Our
study shows that the latter explains ∼ 30 % of the variance in outcrop area as expressed by the surface area between the outcropped SPMW
isopycnals. The key role of the surface density in SPMW transformation partly explains the unusually large SPMW transformation in winter 2014–2015 over the Iceland Basin.
••
TL;DR: In this paper, an unsupervised classification method called Gaussian mixture modelling (GMM) and a novel inter-class parameter called the I-metric were used to estimate the relative positions of the fronts in the Southern Ocean.
Abstract: . Oceanographic fronts are transitions between thermohaline structures with different characteristics. Such transitions are ubiquitous, and their locations and properties affect how the ocean operates as part of the global climate system. In the Southern Ocean, fronts have classically been defined using a small number of continuous, circumpolar features in sea surface height or dynamic height. Modern observational and theoretical developments are challenging and expanding this traditional framework to accommodate a more complex view of fronts. Here, we present a complementary new approach for calculating fronts using an unsupervised classification method called Gaussian mixture modelling (GMM) and a novel inter-class parameter called the I -metric. The I -metric approach produces a probabilistic view of front location, emphasising the fact that the boundaries between water masses are not uniformly sharp across the entire Southern Ocean. The I -metric approach uses thermohaline information from a range of depth levels, making it more general than approaches that only use near-surface properties. We train the GMM using an observationally constrained state estimate in order to have more uniform spatial and temporal data coverage. The probabilistic boundaries defined by the I -metric roughly coincide with several classically defined fronts, offering a novel view of this structure. The I -metric fronts appear to be relatively sharp in the open ocean and somewhat diffuse near large topographic features, possibly highlighting the importance of topographically induced mixing. For comparison with a more localised method, we also use an edge detection approach for identifying fronts. We find a strong correlation between the edge field of the leading principal component and the zonal velocity; the edge detection method highlights the presence of jets, which are supported by thermal wind balance. This more localised method highlights the complex, multiscale structure of Southern Ocean fronts, complementing and contrasting with the more domain-wide view offered by the I -metric. The Sobel edge detection method may be useful for defining and tracking smaller-scale fronts and jets in model or reanalysis data. The I -metric approach may prove to be a useful method for inter-model comparison, as it uses the thermohaline structure of those models instead of tracking somewhat ad hoc values of sea surface height and/or dynamic height, which can vary considerably between models. In addition, the general I -metric approach allows front definitions to shift with changing temperature and salinity structures, which may be useful for characterising fronts in a changing climate.
••
TL;DR: In this paper, the authors used two multi-century Community Earth System Model simulations at coarse (1 ∘ ) and fine (0.1 ) ocean model horizontal grid spacing to study the effects of the representation of mesoscale ocean flows on major patterns of multidecadal variability.
Abstract: . Climate variability on multidecadal timescales appears to be organized in pronounced patterns with clear expressions in sea surface temperature, such as the Atlantic Multidecadal Variability and the Pacific Decadal Oscillation.
These patterns are now well studied both in observations and global climate models and are important in the attribution of climate change.
Results from CMIP5 models have indicated large biases in these patterns with consequences for ocean heat storage variability and the global mean surface temperature.
In this paper, we use two multi-century Community Earth System Model simulations at coarse (1 ∘ ) and fine (0.1 ∘ ) ocean model horizontal grid spacing to study the effects of the representation of mesoscale ocean flows on major patterns of multidecadal variability.
We find that resolving mesoscale ocean flows both improves the characteristics of the modes of variability with respect to observations and increases the amplitude of the heat content variability in the individual ocean basins.
In the strongly eddying model, multidecadal variability increases compared to sub-decadal variability.
This shift of spectral power is seen in sea surface temperature indices, basin-scale surface heat fluxes, and the global mean surface temperature.
This implies that the current CMIP6 model generation, which predominantly does not resolve the ocean mesoscale, may systematically underestimate multidecadal variability.
••
TL;DR: In this article, in situ and satellite data were collected during the ARKTIKA-2018 expedition and then complemented with satellite-derived sea surface temperature (SST), salinity (SSS), sea surface height, wind speed, and sea ice concentration.
Abstract: Variability of surface water masses of the Laptev and the East Siberian seas in August–September 2018 is studied using in situ and satellite data In situ data were collected during the ARKTIKA-2018 expedition and then complemented with satellite-derived sea surface temperature (SST), salinity (SSS), sea surface height, wind speed, and sea ice concentration The estimation of SSS fields is challenging in high-latitude regions, and the precision of soil moisture and ocean salinity (SMOS) SSS retrieval is improved by applying a threshold on SSS weekly error For the first time in this region, the validity of DMI (Danish Meteorological Institute) SST and SMOS SSS products is thoroughly studied using ARKTIKA-2018 expedition continuous thermosalinograph measurements and conductivity–temperature–depth (CTD) casts They are found to be adequate to describe large surface gradients in this region Surface gradients and mixing of the river and the sea water in the ice-free and ice-covered areas are described with a special attention to the marginal ice zone at a synoptic scale We suggest that the freshwater is pushed northward, close to the marginal ice zone (MIZ) and under the sea ice, which is confirmed by the oxygen isotope analysis The SST-SSS diagram based on satellite estimates shows the possibility of investigating the surface water mass transformation at a synoptic scale and reveals the presence of river water on the shelf of the East Siberian Sea The Ekman transport is calculated to better understand the pathway of surface water displacement on the shelf and beyond
••
TL;DR: In this paper, bottom pressure observations on both sides of theAtlantic basin, combined with satellite measurements of sea level anomalies and wind stress data, are utilized to estimate variations of the AtlanticMeridional Overturning Circulation (AMOC) at 11 ∘ S.
Abstract: . Bottom pressure observations on both sides of the
Atlantic basin, combined with satellite measurements of sea level anomalies
and wind stress data, are utilized to estimate variations of the Atlantic
Meridional Overturning Circulation (AMOC) at 11 ∘ S. Over the
period 2013–2018, the AMOC and its components are dominated by seasonal
variability, with peak-to-peak amplitudes of 12 Sv for the upper-ocean
geostrophic transport, 7 Sv for the Ekman and 14 Sv for the AMOC transport.
The characteristics of the observed seasonal cycles of the AMOC and its
components are compared to results from an ocean general circulation model,
which is known to reproduce the variability of the Western Boundary Current
on longer timescales. The observed seasonal variability of zonally
integrated geostrophic velocity in the upper 300 m is controlled by pressure
variations at the eastern boundary, while at 500 m depth contributions from
the western and eastern boundaries are similar. The model tends to
underestimate the seasonal pressure variability at 300 and 500 m depth,
especially at the western boundary, which translates into the estimate of
the upper-ocean geostrophic transport. In the model, seasonal AMOC
variability at 11 ∘ S is governed, besides the Ekman transport, by
the geostrophic transport variability in the eastern basin. The geostrophic
contribution of the western basin to the seasonal cycle of the AMOC is
instead comparably weak, as transport variability in the western basin
interior related to local wind curl forcing is mainly compensated by the
Western Boundary Current. Our analyses indicate that while some of the
uncertainties of our estimates result from the technical aspects of the
observational strategy or processes not being properly represented in the
model, uncertainties in the wind forcing are particularly relevant for the
resulting uncertainties of AMOC estimates at 11 ∘ S.
••
TL;DR: In this article, the authors investigated the impact of atmospheric forcing uncertainties on the prediction of the dispersion of pollutants in the marine environment and proposed an oil spill prediction system based on model uncertainty of the atmospheric forcing.
Abstract: . We investigate the impact of atmospheric forcing uncertainties on
the prediction of the dispersion of pollutants in the marine environment.
Ensemble simulations consisting of 50 members were carried out using the
ECMWF ensemble prediction system and the oil spill model MEDSLIK-II in the
Aegean Sea. A deterministic control run using the unperturbed wind of the
ECMWF high-resolution system served as reference for the oil spill
prediction. We considered the oil spill rates and duration to be similar to major
accidents of the past (e.g., the Prestige case) and we performed simulations
for different seasons and oil spill types. Oil spill performance metrics and
indices were introduced in the context of probabilistic hazard assessment.
Results suggest that oil spill model uncertainties were sensitive to the
atmospheric forcing uncertainties, especially to phase differences in the
intensity and direction of the wind among members. An oil spill ensemble
prediction system based on model uncertainty of the atmospheric forcing,
shows great potential for predicting pathways of oil spill transport
alongside a deterministic simulation, increasing the reliability of the
model prediction and providing important information for the control and
mitigation strategies in the event of an oil spill accident.
••
TL;DR: In this article, the authors investigated the long-term changes of the principal tidal component M2 along North Atlantic coasts, from 1846 to 2018, and found that M2 variations are consistent at all the stations in the North-East Atlantic (Cuxhaven, Delfzijl, Hoek van Holland, Newlyn, Brest).
Abstract: . We investigated the long-term changes of the principal tidal component M2 along North Atlantic coasts, from 1846 to 2018. We analysed 18 tide gauges with time series starting no later than 1940. The longest is Brest with 165 years of observations. We carefully processed the data, particularly to remove the 18.6-year nodal modulation. We found that M2 variations are consistent at all the stations in the North-East Atlantic (Cuxhaven, Delfzijl, Hoek van Holland, Newlyn, Brest), whereas some discrepancies appear in the North-West Atlantic. The changes started long before the 20th century and are not linear. The secular trends in M2 amplitude vary from one station to another; most of them are positive, up to 2.5 mm/yr at Wilmington since 1910. Since 1990, the trends switch from positive to negative values in the North-East Atlantic. Concerning the possible causes of the observed changes, the similarity between the North Atlantic Oscillation and M2 variations in the North-East Atlantic suggests a possible influence of the large-scale atmospheric circulation on the tide. Our statistical analysis confirms large correlations at all the stations in the North-East Atlantic. We discuss a possible underlying mechanism. A different spatial distribution of mean sea level (corresponding to water depth) from one year to another, depending on the low-frequency sea-level pressure patterns, could impact the propagation of the tide in the North Atlantic basin. However, the hypothesis is at present unproven.
••
TL;DR: In this paper, a 3D hydrodynamic model was used to reproduce the flow patterns of the Liao River estuary and to explore the variation in salinity under different scenario.
Abstract: . The wetland of Liao River estuary (LRE) in northeastern China is one of the
best-preserved wetlands across the globe. However, it is extremely vulnerable
to hydrological changes as well as other disturbances, particularly upstream
river discharges and the reclamation from anthropogenic activities. In this
study, a 3D hydrodynamic model was used to reproduce the flow patterns of the
LRE and to explore the variation in salinity under different
scenarios. Furthermore, the impact of river discharge and shoreline changes on
the salinity distribution in the LRE was quantitatively analyzed and
discussed through several simulation experiments. The model reasonably
reconstructed the spatiotemporal variability and distribution of salinity in
the Liao River estuary and the wetlands across intertidal areas. Increases in
runoff were demonstrated to significantly decrease the mean salinity values of
the estuary, with changes in salinity negatively correlated to the
longitudinal distance from the estuary mouth. Moreover, the shoreline change
caused by the construction of Panjin Port was observed to have an obvious
influence on the salinity distribution of the LRE, particularly in the lower
reaches of the Pink Beach wetland. Comparisons of the Liao River estuary
residual flow fields under different shorelines revealed that the
establishment of the port resulted in the diffusion of the runoff from the
Daliao River due to the obstruction of the port body, which enhanced the tidal
mixing effect and consequently weakened the dilution effect of freshwater
entering the Pink Beach wetland. Overall, the current study should be helpful
for offering a greater understanding of Suaeda heteroptera vegetation
degradation in the LRE, China, and also provides a new perspective in
investigating the degradation mechanism in other estuarine wetlands.
••
TL;DR: In this article, the authors investigate the freshwater budget of the Atlantic and Arctic oceans in coupled climate change simulations with the Community Earth System Model and compare a strongly eddying setup with 0.1 ∘ ocean grid spacing to a non-eddying 1 ∘ configuration typical of Coupled model Intercomparison Project phase 6 (CMIP6) models.
Abstract: . We investigate the freshwater budget of the Atlantic and Arctic oceans in coupled climate change simulations with the Community Earth System Model and compare a strongly eddying setup with 0.1 ∘ ocean grid spacing to a non-eddying 1 ∘ configuration typical of Coupled Model Intercomparison Project phase 6 (CMIP6) models.
Details of this budget are important to understand the evolution of the Atlantic Meridional Overturning Circulation (AMOC) under climate change.
We find that the slowdown of the AMOC in the year 2100 under the increasing CO 2 concentrations of the Representative Concentration Pathway 8.5 (RCP8.5) scenario is almost identical between both simulations.
Also, the surface freshwater fluxes are similar in their mean and trend under climate change in both simulations.
While the basin-scale total freshwater transport is similar between the simulations, significant local differences exist.
The high-ocean-resolution simulation exhibits significantly reduced ocean state biases, notably in the salt distribution, due to an improved circulation.
Mesoscale eddies contribute considerably to the freshwater and salt transport, in particular at the boundaries of the subtropical and subpolar gyres.
Both simulations start in the single equilibrium AMOC regime according to a commonly used AMOC stability indicator and evolve towards the multiple equilibrium regime under climate change, but only the high-resolution simulation enters it due to the reduced biases in the freshwater budget.
••
TL;DR: In this article, a suspended particulate matter distribution against a hydrographical background was studied at the oceanographic transect across the equatorial Atlantic in the year 2000.
Abstract: . A suspended particulate matter distribution against a hydrographical
background was studied at the oceanographic transect across the equatorial
Atlantic in the year 2000. An area of abnormally high suspended matter volume
concentrations was found above the Sierra Leone Rise in the entire water
column (eastern part of the transect). The suggested explanation for the
anomaly is based on the ballast hypothesis whereby solid particles are
incorporated as ballast into suspended biogenic aggregates, leading to
increased velocities of sinking. This occurs within the Northwest African
upwelling area, where the plankton exposed to the Saharan dust abundance form
a significant number of aggregates, which are later transported equatorward
via the Canary Current. An intermediate nepheloid layer associated with the Deep
Western Boundary Current was recorded from the South American Slope at depths
of 3200–3700 to 4300 m above the Para Abyssal Plain. Antarctic Bottom
Water enriched in suspended matter was found mostly in the troughs at
40–41 ∘ W. It was detached from the bottom, coinciding with the core
of the flow due to the bottom rise “dam” located up-stream. The grain size
of particles along the entire transect has a polymodal distribution with 2–4
and 8–13 µm modes. The registered rise in percentage in some parts of the transect of the 7–21 µm sized particles suggests the
presence of the well-known coarse mode (20–60 µm ) formed by
aggregation of transparent exopolymer particles (mucus).
••
TL;DR: In this paper, a two-band exponential solar absorption scheme was fitted to vertical profiles of photosynthetically active radiation to determine the effective scale depth of blue light in the southern Bay of Bengal during the 2016 summer monsoon.
Abstract: . Chlorophyll has long been known to influence air–sea gas exchange
and CO 2 drawdown. But chlorophyll also influences regional climate
through its effect on solar radiation absorption and thus sea surface
temperature (SST). In the Bay of Bengal, the effect of chlorophyll on SST has
been demonstrated to have a significant impact on the Indian summer
(southwest) monsoon. However, little is known about the drivers and impacts
of chlorophyll variability in the Bay of Bengal during the southwest
monsoon. Here we use observations of downwelling irradiance measured by an
ocean glider and three profiling floats to determine the spatial and
temporal variability of solar absorption across the southern Bay of Bengal
during the 2016 summer monsoon. A two-band exponential solar absorption
scheme is fitted to vertical profiles of photosynthetically active radiation
to determine the effective scale depth of blue light. Scale depths of blue
light are found to vary from 12 m during the highest (0.3–0.5 mg m −3 )
mixed-layer chlorophyll concentrations to over 25 m when the mixed-layer
chlorophyll concentrations are below 0.1 mg m −3 . The Southwest Monsoon
Current and coastal regions of the Bay of Bengal are observed to have higher
mixed-layer chlorophyll concentrations and shallower solar penetration
depths than other regions of the southern Bay of Bengal. Substantial
sub-daily variability in solar radiation absorption is observed, which
highlights the importance of near-surface ocean processes in modulating
mixed-layer chlorophyll. Simulations using a one-dimensional K-profile
parameterization ocean mixed-layer model with observed surface forcing from
July 2016 show that a 0.3 mg m −3 increase in chlorophyll concentration
increases sea surface temperature by 0.35 ∘ C in 1 month, with SST
differences growing rapidly during calm and sunny conditions. This has the
potential to influence monsoon rainfall around the Bay of Bengal and its
intraseasonal variability.
••
TL;DR: In this article, a large ensemble of 50 simulations performed with the same ocean general circulation model (OGCM) driven by the same realistic atmospheric forcing and only differing by a small initial perturbation is analyzed over 1980-2015.
Abstract: . The southwestern Pacific Ocean sits at a bifurcation where southern subtropical
waters are redistributed equatorward and poleward by different ocean
currents. The processes governing the interannual variability of these
currents are not completely understood. This issue is investigated using a
probabilistic modeling strategy that allows disentangling the
atmospherically forced deterministic ocean variability and the chaotic
intrinsic ocean variability. A large ensemble of 50 simulations performed with
the same ocean general circulation model (OGCM) driven by the same realistic
atmospheric forcing and only differing by a small initial perturbation is
analyzed over 1980–2015. Our results show that, in the southwestern Pacific, the
interannual variability of the transports is strongly dominated by chaotic
ocean variability south of 20 ∘ S. In the tropics, while the
interannual variability of transports and eddy kinetic energy modulation are
largely deterministic and explained by the El Nino–Southern Oscillation (ENSO),
ocean nonlinear processes still explain 10 % to 20 % of
their interannual variance at large scale. Regions of strong chaotic variance
generally coincide with regions of high mesoscale activity, suggesting that a
spontaneous inverse cascade is at work from the mesoscale toward lower frequencies
and larger scales. The spatiotemporal features of the low-frequency oceanic
chaotic variability are complex but spatially coherent within certain
regions. In the Subtropical Countercurrent area, they appear as
interannually varying, zonally elongated alternating current structures, while
in the EAC (East Australian Current) region, they are eddy-shaped. Given this
strong imprint of large-scale chaotic oceanic fluctuations, our results
question the attribution of interannual variability to the atmospheric forcing
in the region from pointwise observations and one-member simulations.
••
TL;DR: In this paper, the role of internal forcing induced by changes in the horizontal pressure gradient due to the varying density of the Adriatic Deep Water (AdDW), which spreads into the deep layers of the northern Ionian Sea, was investigated.
Abstract: . The North Ionian Gyre (NIG) displays prominent inversions on
decadal scales. We investigate the role of internal forcing induced by
changes in the horizontal pressure gradient due to the varying density of
Adriatic Deep Water (AdDW), which spreads into the deep layers of the
northern Ionian Sea. In turn, the AdDW density fluctuates according to the
circulation of the NIG through a feedback mechanism known as the bimodal
oscillating system. We set up laboratory experiments with a two-layer
ambient fluid in a circular rotating tank, where densities of 1000 and 1015 kg m −3 characterize the upper and lower layers, respectively. From the
potential vorticity evolution during the dense-water outflow from a marginal
sea, we analyze the response of the open-sea circulation to the along-slope
dense-water flow. In addition, we show some features of the
cyclonic and anticyclonic eddies that form in the upper layer over the slope
area. We illustrate the outcome of the experiments of varying density and
varying discharge rates associated with dense-water injection. When the
density is high (1020 kg m −3 ) and the discharge is large, the kinetic
energy of the mean flow is stronger than the eddy kinetic energy. Conversely, when the density is lower (1010 kg m −3 ) and the discharge
is reduced, vortices are more energetic than the mean flow – that is, the
eddy kinetic energy is larger than the kinetic energy of the mean flow. In
general, over the slope, following the onset of dense-water injection, the
cyclonic vorticity associated with current shear develops in the upper
layer. The vorticity behaves in a two-layer fashion, thereby becoming
anticyclonic in the lower layer of the slope area. Concurrently, over the
deep flat-bottom portion of the basin, a large-scale anticyclonic gyre forms
in the upper layer extending partly toward a sloping rim. The density record
shows the rise of the pycnocline due to the dense-water sinking toward the
flat-bottom portion of the tank. We show that the rate of increase in the
anticyclonic potential vorticity is proportional to the rate of the rise of
the interface, namely to the rate of decrease in the upper-layer thickness
(i.e., the upper-layer squeezing). The comparison of laboratory experiments
with the Ionian Sea is made for a situation when the sudden switch from
cyclonic to anticyclonic basin-wide circulation took place following
extremely dense Adriatic water overflow after the harsh winter in 2012. We
show how similar the temporal evolution and the vertical structure are in
both laboratory and oceanic conditions. The demonstrated similarity further
supports the assertion that the wind-stress curl over the Ionian Sea is not
of paramount importance in generating basin-wide circulation inversions
compared with the internal forcing.
••
University of Tasmania1, University of Massachusetts Dartmouth2, Japan Agency for Marine-Earth Science and Technology3, University of Maryland Center for Environmental Science4, Woods Hole Oceanographic Institution5, University of New South Wales6, Australian Institute of Marine Science7, Chinese Academy of Sciences8, University of East Anglia9, National Oceanography Centre, Southampton10, National Center for Atmospheric Research11, Commonwealth Scientific and Industrial Research Organisation12, Hobart Corporation13, United States Naval Research Laboratory14, Indian National Centre for Ocean Information Services15, University of Tokyo16, University of Paris17, Physical Research Laboratory18, Scripps Institution of Oceanography19, University of Southern Mississippi20
TL;DR: In this article, a review brings together new understanding of the ocean-atmosphere system in the Indian Ocean since the last comprehensive review, describing Indian Ocean circulation patterns, air-sea interactions and climate variability.
Abstract: Over the past decade, our understanding of the Indian Ocean has advanced through concerted efforts toward measuring the ocean circulation and its water properties, detecting changes in water masses, and linking physical processes to ecologically important variables. New circulation pathways and mechanisms have been discovered, which control atmospheric and oceanic mean state and variability. This review brings together new understanding of the ocean-atmosphere system in the Indian Ocean since the last comprehensive review, describing the Indian Ocean circulation patterns, air-sea interactions and climate variability. The second International Indian Ocean Expedition (IIOE-2) and related efforts have motivated the application of new technologies to deliver higher-resolution observations and models of Indian Ocean processes. As a result we are discovering the importance of small scale processes in setting the large-scale gradients and circulation, interactions between physical and biogeochemical processes, interactions between boundary currents and the interior, and between the surface and the deep ocean. In the last decade we have seen rapid warming of the Indian Ocean overlaid with extremes in the form of marine heatwaves. These events have motivated studies that have delivered new insight into the variability in ocean heat content and exchanges in the Indian Ocean, and climate variability on interannual to decadal timescales. This synthesis paper reviews the advances in these areas in the last decade.