Showing papers in "Tellus A in 2010"

Large-scale atmospheric circulation changes are associated with the recent loss of Arctic sea ice

Abstract: Recent loss of summer sea ice in the Arctic is directly connected to shifts in northern wind patterns in the following autumn, which has the potential of altering the heat budget at the cold end of the global heat engine.With continuing loss of summer sea ice to less than 20% of its climatological mean over the next decades,we anticipate increased modification of atmospheric circulation patterns. While a shift to a more meridional atmospheric climate pattern, the Arctic Dipole (AD), over the last decade contributed to recent reductions in summer Arctic sea ice extent, the increase in late summer open water area is, in turn, directly contributing to a modification of large scale atmospheric circulation patterns through the additional heat stored in the Arctic Ocean and released to the atmosphere during the autumn season. Extensive regions in the Arctic during late autumn beginning in 2002 have surface air temperature anomalies of greater than 3 °C and temperature anomalies above 850 hPa of 1 °C. These temperatures contribute to an increase in the 1000–500 hPa thickness field in every recent year with reduced sea ice cover. While gradients in this thickness field can be considered a baroclinic contribution to the flow field from loss of sea ice, atmospheric circulation also has a more variable barotropic contribution. Thus, reduction in sea ice has a direct connection to increased thickness fields in every year, but not necessarily to the sea level pressure (SLP) fields. Compositing wind fields for late autumn 2002–2008 helps to highlight the baroclinic contribution; for the years with diminished sea ice cover there were composite anomalous tropospheric easterly winds of∼1.4 m s –1 , relative to climatological easterly winds near the surface and upper troposphericwesterlies of∼3 m s –1 . Loss of summer sea ice is supported by decadal shifts in atmospheric climate patterns. A persistent positive Arctic Oscillation pattern in late autumn (OND) during 1988–1994 and in winter (JFM) during 1989–1997 shifted to more interannual variability in the following years. An anomalous meridional wind pattern with high SLP on the North American side of the Arctic—the AD pattern, shifted from primarily small interannual variability to a persistent phase during spring (AMJ) beginning in 1997 (except for 2006) and extending to summer (JAS) beginning in 2005.

Asynchronous data assimilation with the EnKF

Abstract: This study revisits the problem of assimilation of asynchronous observations, or four-dimensional data assimilation, with the ensemble Kalman filter (EnKF). We show that for a system with perfect model and linear dynamics the ensemble Kalman smoother (EnKS) provides a simple and efficient solution for the problem: one just needs to use the ensemble observations (that is, the forecast observations for each ensemble member) from the time of observation during the update, for each assimilated observation. This recipe can be used for assimilating both past and future data; in the context of assimilating generic asynchronous observations we refer to it as the asynchronous EnKF. The asynchronous EnKF is essentially equivalent to the four-dimensional variational data assimilation (4D-Var). It requires only one forward integration of the system to obtain and store the data necessary for the analysis, and therefore is feasible for large-scale applications. Unlike 4D-Var, the asynchronous EnKF requires no tangent linear or adjoint model.

Sensitivity study of heavy precipitation in Limited Area Model climate simulations: influence of the size of the domain and the use of the spectral nudging technique

Abstract: We assess the impact of two sources of uncertainties in a limited area model (LAM) on the representation of intense precipitation: the size of the domain of integration and the use of the spectral nudging technique (driving of the large-scale within the domain of integration) We work in a perfect-model approach where the LAM is driven by a general circulation model (GCM) run at the same resolution and sharing the same physics and dynamics as the LAM A set of three 50 km resolution simulations run over Western Europe with the LAM ALADIN-Climate and the GCM ARPEGE-Climate are performed to address this issue Results are consistent with previous studies regarding the seasonal-mean fields Furthermore, they show that neither the use of the spectral nudging nor the choice of a small domain are detrimental to the modelling of heavy precipitation in the present experiment

Data assimilation with the weighted ensemble Kalman filter

Abstract: In this paper, two data assimilation methods based on sequential Monte Carlo sampling are studied and compared: the ensemble Kalman filter and the particle filter. Each of these techniques has its own advantages and drawbacks. In this work, we try to get the best of each method by combining them. The proposed algorithm, called the weighted ensemble Kalman filter, consists to rely on the Ensemble Kalman Filter updates of samples in order to define a proposal distribution for the particle filter that depends on the history of measurement. The corresponding particle filter reveals to be efficient with a small number of samples and does not rely anymore on the Gaussian approximations of the ensemble Kalman filter. The efficiency of the new algorithm is demonstrated both in terms of accuracy and computational load. This latter aspect is of the utmost importance in meteorology or in oceanography since in these domains, data assimilation processes involve a huge number of state variables driven by highly non-linear dynamical models. Numerical experiments have been performed on different dynamical scenarios. The performances of the proposed technique have been compared to the ensemble Kalman filter procedure, which has demonstrated to provide meaningful results in geophysical sciences.

1D+3DVar assimilation of radar reflectivity data: a proof of concept.

Abstract: An original one-dimensional (1-D) retrieval followed by a three-dimensional variational (1D+3DVar) assimilation technique is being developed to assimilate volumes of radar reflectivity data in the high-resolution numerical weather prediction Arome model. The good performance of the 1-D retrieval is shown for an isolated storm over southwestern France through an observing system simulation experiment. The full method is applied with real data to a flash-flood event, which occurred in a mountainous area. For this complex case, the assimilation of reflectivity data improves short-term precipitation forecasts. The assimilation of reflectivity data has a positive impact on the convective system’s dynamics by feeding the cold pool under the storm, which controls the intensity and location of the updrafts. A one-hourly update cycle of 3 h further improves these results. A sensitivity study is also presented to evaluate the assimilation method for this flash-flood event in different conditions. The smoothing coefficient involved in the 1-D retrieval is shown to have a very small impact on analyses and quantitative precipitation forecasts. The assimilation of reflectivity data is found to be able to cause the creation of a cold pool, which modifies favourably the precipitation quantitative forecast. Finally, results from an 8-d-long assimilation cycle are presented.

The distribution of eddy kinetic and potential energies in the global ocean

Abstract: Understanding of the major sources, sinks, and reservoirs of energy in the ocean is briefly updated in a diagram. The nature of the dominant kinetic energy reservoir, that of the balanced variablity, is then found to be indistinguishable in the observations from a sum of barotropic and first baroclinic ordinary quasi-geostrophic modes. Little supporting evidence is available to partition the spectra among forced motions and turbulent cascades, along with significant energy more consistent with weakly non-linear wave dynamics. Linear-response wind-forced motions appear to dominate the high frequency (but subinertial) mooring frequency spectra. Turbulent cascades appear to fill the high wavenumber spectra in altimetric data and numerical simulations. Progress on these issues is hindered by the difficulty in connecting the comparatively easily available frequency spectra with the variety of theoretically predicted wavenumber spectra.

Extreme rainfall events in southern Sweden: where does the moisture come from?

Abstract: The atmospheric transport of moisture leading to extreme summer precipitation events in southern Sweden was investigated using a Lagrangian trajectory model. Surprisingly, we found that the trajectories crossed continental Europe and the Baltic Sea before arriving over Sweden; they did not arrive directly from the North Sea. Such transport pathways were not seen for a control sample of non-extreme rainfall events. We then used a new source region identification technique to investigate the hypothesis that Europe and the Baltic are important sources of the moisture that is rained out in the extreme events. Although the results varied between events, we found that this is indeed the case. Our results establish the atmospheric transport patterns that are apparently a pre-requisite for extreme rainfall events to occur in southern Sweden, and further suggest regional moisture availability may also play a key role.

Estimation of wind storm impacts over Western Germany under future climate conditions using a statistical–dynamical downscaling approach

Abstract: A statistical–dynamical regionalization approach is developed to assess possible changes in wind storm impacts. The method is applied to North Rhine-Westphalia (Western Germany) using the FOOT3DK mesoscale model for dynamical downscaling and ECHAM5/OM1 global circulation model climate projections. The method first classifies typical weather developments within the reanalysis period using K-means cluster algorithm. Most historical wind storms are associated with four weather developments (primary storm-clusters). Mesoscale simulations are performed for representative elements for all clusters to derive regional wind climatology. Additionally, 28 historical storms affecting Western Germany are simulated. Empirical functions are estimated to relate wind gust fields and insured losses. Transient ECHAM5/OM1 simulations show an enhanced frequency of primary storm-clusters and storms for 2060–2100 compared to 1960–2000. Accordingly, wind gusts increase over Western Germany, reaching locally +5% for 98th wind gust percentiles (A2-scenario). Consequently, storm losses are expected to increase substantially (+8% for A1B-scenario, +19% for A2-scenario). Regional patterns show larger changes over north-eastern parts of North Rhine-Westphalia than for western parts. For storms with return periods above 20 yr, loss expectations for Germany may increase by a factor of 2. These results document the method’s functionality to assess future changes in loss potentials in regional terms.

The mid-depth circulation of the Nordic Seas derived from profiling float observations

Abstract: The trajectories of 61 profiling Argo floats deployed at mid-depth in the Nordic Seas—the Greenland, Lofoten and Norwegian Basins and the Iceland Plateau—between 2001 and 2009 are analysed to determine the pattern, strength and variability of the regional circulation. The mid-depth circulation is strongly coupled with the structure of the bottom topography of the four major basins and of the Nordic Seas as a whole. It is cyclonic, both on the large-scale and on the basin scale, with weak flow (<1 cm s −1 ) in the interior of the basins and somewhat stronger flow (up to 5 cm s −1 ) at their rims. Only few floats moved from one basin to another, indicating that the internal recirculation within the basins is by far dominating the larger-scale exchanges. The seasonal variability of the mid-depth flow ranges from less than 1 cm s −1 over the Iceland Plateau to more than 4 cm s −1 in the Greenland Basin. These velocities translate into internal gyre transports of up to 15 ± 10 × 10 6 m 3 s −1 , several times the overall exchange between the Nordic Seas and the subpolar North Atlantic. The seasonal variability of the Greenland Basin and the Norwegian Basin can be adequately modelled using the barotropic vorticity equation, with the wind and bottom friction as the only forcing mechanisms. For the Lofoten Basin and the Iceland Plateau less than 50% of the variance can be explained by the wind.

Assimilation of Image Sequences in Numerical Models

Abstract: Understanding and forecasting the evolution of geophysical fluids is a major scientific and societal challenge. Forecasting algorithms should take into account all the available information on the considered dynamic system. The variational data assimilation (VDA) technique combines all these informations in an optimality system (O.S.) in a consistent way to reconstruct the model inputs. VDA is currently used by the major meteorological centres. During the last two decades about 30 satellites were launched to improve the knowledge of the atmosphere and of the oceans. They continuously provide a huge amount of data that are still underused by numerical forecast systems. In particular, the dynamic evolution of certain meteorological or oceanic features (such as eddies, fronts, etc.) that the human vision may easily detect is not optimally taken into account in realistic applications of VDA. Image Assimilation in VDA framework can be performed using ‘pseudo-observation’ techniques: they provide apparent velocity fields, which are assimilated as classical observations. These measurements are obtained by certain external procedures, which are decoupled with the considered dynamic system. In this paper, we suggest a more consistent approach, which directly incorporates image sequences into the O.S.

Probabilistic hindcasts and projections of the coupled climate, carbon cycle and Atlantic meridional overturning circulation system: a Bayesian fusion of century-scale observations with a simple model

Abstract: How has the Atlantic Meridional Overturning Circulation (AMOC) varied over the past centuries and what is the risk of an anthropogenic AMOC collapse? We report probabilistic projections of the future climate which improve on previous AMOC projection studies by (i) greatly expanding the considered observational constraints and (ii) carefully sampling the tail areas of the parameter probability distribution function (pdf). We use a Bayesian inversion to constrain a simple model of the coupled climate, carbon cycle and AMOC systems using observations to derive multicentury hindcasts and projections. Our hindcasts show considerable skill in representing the observational constraints. We show that robust AMOC risk estimates can require carefully sampling the parameter pdfs. We find a low probability of experiencing an AMOC collapse within the 21st century for a business-as-usual emissions scenario. The probability of experiencing an AMOC collapse within two centuries is 1/10. The probability of crossing a forcing threshold and triggering a future AMOC collapse (by 2300) is approximately 1/30 in the 21st century and over 1/3 in the 22nd. Given the simplicity of the model structure and uncertainty in the forcing assumptions, our analysis should be considered a proof of concept and the quantitative conclusions subject to severe caveats.

On the low‐frequency phase relation between the Denmark Strait and the Faroe‐Bank Channel overflows

Abstract: Numerical simulations of the Atlantic and Arctic Oceans using an ocean-sea ice model are analysed with respect to interannual-to-decadal variability (1948–2008) of the dense water overflows east and west of Iceland. Besides the good agreement between observed and simulated Denmark Strait and Faroe-Bank Channel overflows, the model shows an antiphase relation between both transports at time scales of 3–10 yr. The analysis of wind stress curl over the Nordic Seas suggests that the phase relation is locally forced. The main mode of atmospheric variability in the Atlantic reinforces the dense overflow west of Iceland during positive phases of the North Atlantic Oscillation (NAO) and decreases it during negative phases. East of Iceland the overflow is decreased during positive phases of the NAO and increased during negative phases. After the mid-1990s, the antiphase relation is less clear due to a reduction of wind stress curl variance and a change in the production of dense water in the Nordic Seas. In that period, the Norwegian Sea experienced a reduction of its cyclonicity, leading to the export of a surplus of dense water across the Faroe-Shetland Channel. The forcing of the gyre variability is attributed to large-scale wind stress curl changes and to shifts of convection regimes linked to Arctic-Nordic Seas dense water exchange modifications at Fram Strait. Simulated sea surface height variability is highly correlated with the dense water volume in the Greenland and Norwegian Seas gyres, implying that altimetric measurements are good indicators of dense water content in the Nordic Seas and therefore provide a potential for monitoring the reservoirs and explain/anticipate changes in the overflows.

Role of convective parameterization in simulations of a convection band at grey-zone resolutions

Abstract: In this study, we investigate the role of convective parameterization (CP) in simulations of a convection band over the mid-Korean Peninsula at grey-zone resolutions, at which convection is partially resolved, partially subgrid. An approach similar to that used in ‘observing system simulation experiment’ is adopted. Simulations with a 500-m grid size serve as benchmark simulations. The impacts of resolution and convective parameterization at grey-zone resolutions (i.e. 3, 6 and 9 km) are then investigated. Results indicate that a grid size of 3 km is sufficient to resolve the convection band and CP for this size grid is not necessary. With 6 and 9 km grids, explicit simulations or those based on a Kain–Fritsch CP scheme do not simulate the atmospheric structure surrounding the band accurately. A major problem with CP is excessive triggering of parameterized convection. False triggers of CP in the band adjacent area suppress evolution of the resolved convection band through excessive stabilization of inflow air. We obtain significant improvements by using a modified trigger function, resulting in reduction of the area of parameterized convection, which in turn leads to stronger development of a resolved convection band. Furthermore, our approach reduces bias in the domain-averaged vertical thermodynamic structure.

Constraints on exchanges in the Arctic Mediterranean—do they exist and can they be of use?

Abstract: Extensive changes have been reported from the Arctic Mediterranean. The ice cover is retreating, the temperature in the Atlantic layer has been increasing, the salinity in the upper layers shows large variations and deep waters in the Greenland Sea have become warmer and more saline. These changes all appear externally forced; by the radiation balance, by the atmosphere, and by ocean advection. The question arises—are there processes inherent to the Arctic Ocean, which can constrain changes induced by external forcing? Three features are examined; the storage and export of liquid freshwater in the upper layers, the heat loss of the Atlantic water encountering sea ice and the possibility to define a salinity separating the two roles of the Arctic Mediterranean, as estuary and as concentration basin. If the freshwater outflow in the upper layer is rotationally controlled, the liquid freshwater storage and export only depend upon the freshwater input. The melting rate of sea ice is affected both by the heat transport and by the temperature of the inflowing Atlantic water. A salinity separating the estuarine and the deep-water circulation is proposed depending upon the salinity and the temperature of the Atlantic water as it encounters sea ice.

Sensitivity of arctic summer sea ice coverage to global warming forcing: towards reducing uncertainty in arctic climate change projections

Abstract: Substantial uncertainties have emerged in Arctic climate change projections by the fourth Intergovernmental Panel on Climate Change assessment report climate models. In particular, the models as a group considerably underestimate the recent accelerating sea ice reduction. To better understand the uncertainties, we evaluated sensitivities of summer sea ice coverage to global warming forcing in models and observations. The result suggests that the uncertainties result from the large range of sensitivities involved in the computation of sea ice mass balance by the climate models, specifically with the changes in sea ice area (SIA) ranging from 0.09 × 10 6 to −1.23 × 10 6 km 2 in response to 1.0 K increase of air temperature. The sensitivities also vary largely across ensemble members in the same model, indicating impacts of initial condition on evolution of feedback strength with model integrations. Through observationally constraining, the selected model runs by the sensitivity analysis well captured the observed changes in SIA and surface air temperatures and greatly reduced their future projection uncertainties to a certain range from the currently announced one. The projected ice-free summer Arctic Ocean may occur as early as in the late 2030s using a criterion of 80% SIA loss and the Arctic regional mean surface air temperature will be likely increased by 8.5 ± 2.5 °C in winter and 3.7 ± 0.9 °C in summer by the end of this century.

A hybrid approach to estimating error covariances in variational data assimilation

Abstract: Data assimilation (DA) involves the combination of observational data with the underlying dynamical principles governing the system under observation. In this work we combine the advantages of the two prominent DA systems: the 4D-Var and the ensemble methods. The hybrid method described in this paper consists of identifying the subspace spanned by the major 4D-Var error reduction directions. These directions are then removed from the background covariance through a Galerkin-type projection, and are replaced by estimates of the analysis error obtained through a low-rank Hessian inverse approximation. The updated error covariance in one window can be used as the background covariance for the next window thus better capturing the ‘error of the day’. The numerical results for a non-linear model demonstrate how the hybrid method leads to a good estimate of the true error covariance, and improves the 4D-Var analysis results.

Fine-scale model simulation of gravity waves generated by Basen nunatak in Antarctica

Abstract: Gravity waves over a small nunatak in Dronning Maud Land, Antarctica, were simulated applying the mesoscale model Weather Research and Forecasting (WRF). Most model experiments were made using three nested domains with 0.9 km horizontal resolution in the finest grid. The model results were validated against observations from a vertically pointing 54.5 MHz VHF radar, which provided profiles of the vertical and horizontal wind components over the nunatak. The WRF model generated gravity waves in the periods when these were observed. The modelled wave characteristics were qualitatively similar to the observed ones, although quantitative differences existed. The gravity wave had a typical vertical wave length of 3–4 km, and a maximum amplitude of 1 m s −1 in the vertical velocity field. The amplitude was largest at altitude of 2.0–3.5 km. The wave was strongest during nighttime, when the surface sensible heat flux was downwards. The evolution of vertical profiles of the observed rms fluctuation of vertical wind velocity and the modelled bulk Richardson number resembled each other. Gravity waves may affect the safety of aircraft landing and take-off on the lee side of the nunatak.

Non-stationary drainage flows and motions in the cold pool

Abstract: The initial formation of drainage flows and subsequent interaction with the cold pool are examined by contrasting winds measured with 2-sonic anemometers at three stations along a gentle slope for a 45-d observational period in late summer and early fall. On clear nights with weak winds, the station at the bottom of the slope experiences downslope flow only at the beginning of the evening, which quickly yields to light winds of variable direction after formation of a cold pool on the valley floor. A second station, several hundred metres up the slope, experiences drainage flow in the first part of the evening, which diminishes and yields to light and variable winds in the middle of the night as the influence of the cold pool deepens and engulfs the station. Drainage flow continues throughout the night at the third station located still farther up the slope and above the cold pool throughout the night. The interplay between the drainage flow, large-scale flow and submeso motions leads to frequent large shifts of wind direction. The character of the wind-direction variability differs substantially between the three stations. Large remaining uncertainties are noted.

Mixed Gaussian‐lognormal four‐dimensional data assimilation

Abstract: In this paper, the current methods that are used for deriving the Gaussian cost function in four-dimensional variational (4D VAR) data assimilation, are extended to lognormal and mixed, lognormal and Gaussian, random variables (rv). It is also shown that transforming a lognormal rv into a Gaussian rv to use in a Gaussian based 4D VAR results in the analysis state being similar to a median in lognormal space. An alternative version of the functional approach is derived so that the minimum of this alternative cost function is the mode in lognormal space. However, the current approaches do not allow for a generalized probability density function (pdf) approach, to overcome this a general probability model is derived so that the mode is found for any pdf from Bayesian networks and conditional independence properties. It is shown that the current Gaussian cost function and the lognormal can be found by the probability model method. The paper is finished by comparing the median approach with the modal approach in the Lorenz 1963 model for both weak and strong constraint 4D VAR and show that the mode is a more reliable analysis statistic than the median under certain circumstances.

Climatology of the Nordic (Irminger, Greenland, Barents, Kara and White/Pechora) Seas ice cover based on 85 GHz satellite microwave radiometry: 1992–2008

Abstract: The ice cover of the Arctic peripheral seas bordering the Northern North Atlantic is examined for 1992–2008 using the ARTIST Sea Ice (ASI) algorithm applied to derive the sea ice concentration from 85 GHz SSM/I measurements. Our analysis reveals a 2 months longer ice-free season in the Irminger Sea (IS), and reductions in ice area and extent between 1992–1999 and 2000–2008 by 10–20% during winter and 30–55% in summer. Barents Sea (BS) ice-cover anomalies (ICA) persist twice as long as ICA in the other regions. Early winter ICA in region IS are correlated to late summer/fall Greenland Sea (GS) ICA. Summertime GS and wintertime IS ICA are correlated to winter Fram Strait ice-area flux anomalies. The wintertime GS ice-cover decrease is associated with less Is Odden events. Our analysis suggests a large-scale, interregional ocean–ice–atmosphere feedback mechanism involving regions BS, Kara (KS) and White/Pechora Sea (WPS). To understand this mechanism the current and preceding general atmospheric circulation, associated variations in Arctic Ocean ice export and oceanic heat advection are needed. However, our results suggest (1) BS ICA could play a key role to predict subsequent KS ICA and (2) anomalous Arctic Ocean ice export into BS could trigger long-lasting BS ICA.

Improving passive microwave sea ice concentration algorithms for coastal areas: applications to the Baltic Sea

Abstract: Sea ice concentration can be retrieved from passive microwave data using the NASA Team algorithm or the Artist Sea Ice (ASI) algorithm, for example. The brightness temperature measurements obtained from the Special Sensor Microwave Imager (SSM/I) instrument or the Advanced Microwave Scanning Radiometer-EOS (AMSR-E) are commonly used for this purpose. Due to the coarse resolution of these instruments considerable systematic ice concentration errors in coastal regions occur. In the vicinity of the coast the instrument footprints usually contain both land and sea surfaces. Compared to sea surfaces, land surfaces are characterized by higher emissivities and lower polarization differences at the involved microwave channels. Thus, a systematic overestimation of coastal ice concentration is caused. In this paper, a method is developed to remove the land impact on the observed radiation. Combining a high-resolution data set for the shoreline and the antenna gain function the brightness temperature contribution originating from land surfaces can be identified. The brightness temperature related to the ocean fraction within the considered footprint can then be extracted. This separation technique is applied to SSM/I measurements in the Baltic Sea and the resulting ice concentration fields are compared to high-resolution satellite images. The highly complex shoreline of the Baltic Sea region provides an ideal area for testing the method. However, the presented approach can as well be applied to Arctic coastal regions. It is shown that the method considerably improves ice concentration retrieval in regions influenced by land surfaces without removing actually existing sea ice.

Spatio-temporal evolution of perturbations in ensembles initialized by bred, Lyapunov and singular vectors

Abstract: We study the evolution of finite perturbations in the Lorenz ‘96 model, a meteorological toy model of the atmosphere. The initial perturbations are chosen to be aligned along different dynamic vectors: bred, Lyapunov, and singular vectors. Using a particular vector determines not only the amplification rate of the perturbation but also the spatial structure of the perturbation and its stability under the evolution of the flow. The evolution of perturbations is systematically studied by means of the so-called mean-variance of logarithms diagram that provides in a very compact way the basic information to analyse the spatial structure. We discuss the corresponding advantages of using those different vectors for preparing initial perturbations to be used in ensemble prediction systems, focusing on key properties: dynamic adaptation to the flow, robustness, equivalence between members of the ensemble, etc. Among all the vectors considered here, the so-called characteristic Lyapunov vectors are possibly optimal, in the sense that they are both perfectly adapted to the flow and extremely robust.

The influence of synoptic scale flow on sea breeze induced surface winds and calm zones

Abstract: High-latitude sea breezes and the related daytime calm zones were studied through fine-scale two-dimensional idealized simulations by varying the direction and speed of the ambient large-scale geostrophic flow in small steps. Strongest coastal afternoon breezes were obtained for moderate large-scale flows 45°–90° left from the pure offshore direction (as seen from the sea), while calm zones appeared near the coast for weak to moderate large-scale flows about 30°–90° right from offshore, and at the seaward edge of the breeze cell for weak ambient flows 45°–90° right from offshore. The complex daytime evolution of the wind field for ambient winds roughly opposing the sea breeze is illustrated via a few key cases.

Variable source regions of Denmark Strait and Faroe Bank Channel overflow waters

Abstract: The pathways of the water that overflows the Greenland–Scotland Ridge and their source regions are studied with an ocean circulation model. Depending on the size of wind stress curl around Iceland, different water mass properties are found in the less dense fraction of the overflows on both sides of Iceland. Although in both cases, this fraction contributes to only a small part of the overflow, their pathways and associated source regions characterize also changes in the pathways of the denser fractions which is confirmed by backward float experiments. In detail, large Denmark Strait overflow during high wind stress curl conditions is associated with a primary pathway along the East Greenland Current with one branch encircling the Greenland Sea that is supplied by regions further southward while the other branch enters through Fram Strait. During low forcing conditions, the source region shifts into the Iceland Sea associated with a reversal of a current north of Iceland. The Faroe Bank Channel overflow is primarily fed by sources in the Norwegian Sea with water flowing southward along the Norwegian shelf. A smaller contribution that is enhanced during high forcing follows along the Jan Mayen Ridge and provides water from the Iceland Sea and the Greenland Sea.

Seasonal to inter-annual variability of temperature and salinity in the Greenland Sea Gyre: heat and freshwater budgets

Abstract: Six years of autonomous profiling float data from the Greenland Sea Gyre are used to detect changes in temperature and salinity of the water column on time scales from seasonal to inter-annual. The effect of ocean–atmosphere and internal ocean fluxes on heat and freshwater is largely (about 90%) confined to the upper 700 m. Throughout the water column a warming at a mean rate of 0.05 K year −1 is observed, whereas the freshwater content is dominated by inter-annual changes not containing trends. In the annual mean the Gyre exports freshwater across its boundary throughout the water column. Import of freshwater takes place only in the upper 50 m during summer. Heat is exported in the upper 50 m, while below the gyre cools the surrounding. The net effect of the gyre on the water mass conversion in the Arctic Mediterranean is small and the gyre does not re-enforce the Nordic Seas overturning circulation.

Cessation and partial reversal of deep water freshening in the northern North Atlantic: observation-based estimates and attribution

Abstract: Recent decadal salinity changes in the Greenland-Scotland overflow-derived deep waters are quantified using CTD data from repeated hydrographic sections in the Irminger Sea. The Denmark Strait Overflow Water salinity record shows the absence of any net change over the 1980s–2000s; changes in the Iceland–Scotland Overflow Water (ISOW) and in the deep water column (σ 0 > 27.82), enclosing both overflows, show a distinct freshening reversal in the early 2000s. The observed freshening reversal is a lagged consequence of the persistent ISOW salinification that occurred upstream, in the Iceland Basin, after 1996 in response to salinification of the northeast Atlantic waters entrained into the overflow. The entrainment salinity increase is explained by the earlier documented North Atlantic Oscillation (NAO)-induced contraction of the subpolar gyre and corresponding northwestward advance of subtropical waters that followed the NAO decline in the mid-1990s and continued through the mid-2000s. Remarkably, the ISOW freshening reversal is not associated with changes in the overflow water salinity. This suggests that changes in the NAO-dependent relative contributions of subpolar and subtropical waters to the entrainment south of the Iceland–Scotland Ridge may dominate over changes in the Nordic Seas freshwater balance with respect to their effect on the ISOW salinity.

Extreme value statistics for North Atlantic cyclones

Abstract: Extremes of the cyclone intensity measures geopotential height (z 1000 ), mean horizontal gradient (∇z), cyclone depth ( D ), and relative vorticity (ζ 850 ), are analysed in re-analysis data (ERA40) and model simulations (ECHAM5/MPIOM) in the North Atlantic region for extended winter seasons. Generalized Pareto distributions (GPD) are estimated for model validation and climate change assessment. Covariates, linear trend and North Atlantic Oscillation (NAO) are included to analyse the dependancies of the extremes. In ERA40 no significant linear trend can be detected, while evidence for a NAO impact on z 1000 , ∇z and ζ 850 extremes is found. Model validation yields good agreement with consistent scale and shape, but a shift to lower values is notable. Like in ERA40 no trend is found in the simulation. The evidence for an NAO impact on cyclone extremes is less corroborated in the simulation, pointing to sample size effects. In the warmer climate scenario (A1BS) extreme value statistics shows an intensification for all variables. Significant differences in GPD are obtained through testing for lower (higher) parameters. In contrast, considering all cyclones an increase is only present for z 1000 , while a decrease is found for ∇z and ζ 850 and no change for D .

The effect of mechanical stirring on horizontal convection

Abstract: An important experimental result, as yet poorly understood, is that mechanical stirring can significantly enhance the strength of horizontal convection. A contentious issue is whether this necessarily implies that the mechanical stirring replaces the buoyancy forcing as the main source of energy driving the observed overturning circulation, as has been suggested for the Atlantic meridional overturning circulation (AMOC). In this paper, rigorous energetics considerations and idealized numerical experiments reveal that the rate at which the surface buoyancy forcing supplies energy to the fluid, as measured by the production rate of available potential energy G(APE), does not solely depend upon the buoyancy forcing, as is often implicitly assumed, but also upon the vertical stratification, such that the deeper the thermocline depth, the larger G(APE). This suggests that mechanical stirring enhances horizontal convection because it causes more energy to be extracted from the buoyancy forcing. It does so by enhancing turbulent mixing, which allows surface heating to reach greater depths, which increases the thermocline depth and hence G(APE). This paper therefore proposes a new hypothesis, namely that mechanically stirred horizontal convection and the AMOC are best described as mechanically controlled heat engines.

Climatology and variability of Southern Hemisphere marine cold-air outbreaks.

Abstract: Marine cold air outbreaks (MCAOs) are events where cold air flows over a relatively warm sea surface. Such outbreaks are associated with severe mesoscale weather systems that are not generally resolved in global climate models, such as polar lows and boundary-layer fronts. Here, an analysis of winter climatology and variability of MCAOs in the Southern Hemisphere (SH) is presented. Near the sea ice edge, north–south fluctuations of the Southern Annular Mode (SAM) index are key, while further north, large-scale wave disturbances are needed to move air masses far enough away from the Antarctic continent to instigate MCAOs. Unlike in the Northern Hemisphere (NH), the spatial patterns of mean and extreme values of the MCAO index differ considerably. Near 60°S, both mean and extreme values of the index are similar to those found in the main MCAO regions in the NH. Further north, the mean MCAO index is quite high, but the extreme values are much lower than in the NH. We conclude that MCAOs in the SH are as widespread and can be as strong as in the NH, but severe MCAOs near densely populated regions such as the Tasman Sea are less common than in the Nordic Seas and near Japan.

The simulated sea ice thermal microwave emission at window and sounding frequencies

Abstract: Although the sea ice microwave emissivity is well defined in terms of brightness temperature and the effective temperature it is difficult to derive in practice and its link to physical processes in the snow and sea ice cover is not understood in detail. Future applications of assessing the sea ice microwave emission potentially include atmospheric sounding over sea ice for channels peaking near the surface. Here the microwave emission processes from sea ice are simulated using the combination of a one-dimensional thermodynamic snow and sea ice model and a microwave emission model. The emission model is a sea ice version of the Microwave Emission Model for Layered Snow-packs (MEMLS). It is demonstrated that the simulated seasonal variability of the emissivity, the spectral gradient and the polarization are comparable to satellite measurements. The spectral gradient ratio defined as the difference over the sum of the 18 and 36 GHz at vertical polarization is related to the emissivity at the atmospheric temperature sounding channels at around 50 GHz. Further the brightness temperature at neighbouring channels is highly correlated. However, the effective temperature at lower frequencies (18, 23, 36 and 50 GHz) is poorly correlated with the surface or air temperature.