Showing papers in "Journal of Climate in 2017"

The Modern-Era Retrospective Analysis for Research and Applications, Version 2 (MERRA-2)

Abstract: The Modern-Era Retrospective Analysis for Research and Applications, version 2 (MERRA-2), is the latest atmospheric reanalysis of the modern satellite era produced by NASA’s Global Modeling and Assimilation Office (GMAO). MERRA-2 assimilates observation types not available to its predecessor, MERRA, and includes updates to the Goddard Earth Observing System (GEOS) model and analysis scheme so as to provide a viable ongoing climate analysis beyond MERRA’s terminus. While addressing known limitations of MERRA, MERRA-2 is also intended to be a development milestone for a future integrated Earth system analysis (IESA) currently under development at GMAO. This paper provides an overview of the MERRA-2 system and various performance metrics. Among the advances in MERRA-2 relevant to IESA are the assimilation of aerosol observations, several improvements to the representation of the stratosphere including ozone, and improved representations of cryospheric processes. Other improvements in the quality of M...

Extended Reconstructed Sea Surface Temperature, Version 5 (ERSSTv5): Upgrades, Validations, and Intercomparisons

Abstract: The monthly global 2° × 2° Extended Reconstructed Sea Surface Temperature (ERSST) has been revised and updated from version 4 to version 5. This update incorporates a new release of ICOADS release 3.0 (R3.0), a decade of near-surface data from Argo floats, and a new estimate of centennial sea ice from HadISST2. A number of choices in aspects of quality control, bias adjustment, and interpolation have been substantively revised. The resulting ERSST estimates have more realistic spatiotemporal variations, better representation of high-latitude SSTs, and ship SST biases are now calculated relative to more accurate buoy measurements, while the global long-term trend remains about the same. Progressive experiments have been undertaken to highlight the effects of each change in data source and analysis technique upon the final product. The reconstructed SST is systematically decreased by 0.077°C, as the reference data source is switched from ship SST in ERSSTv4 to modern buoy SST in ERSSTv5. Furthermore...

The MERRA-2 Aerosol Reanalysis, 1980 Onward. Part I: System Description and Data Assimilation Evaluation

Abstract: The Modern-Era Retrospective Analysis for Research and Applications, Version 2 (MERRA-2) updates NASA's previous satellite era (1980 - onward) reanalysis system to include additional observations and improvements to the Goddard Earth Observing System, Version 5 (GEOS-5) Earth system model. As a major step towards a full Integrated Earth Systems Analysis (IESA), in addition to meteorological observations, MERRA-2 now includes assimilation of aerosol optical depth (AOD) from various ground- and space-based remote sensing platforms. Here, in the first of a pair of studies, we document the MERRA-2 aerosol assimilation, including a description of the prognostic model (GEOS-5 coupled to the GOCART aerosol module), aerosol emissions, and the quality control of ingested observations. We provide initial validation and evaluation of the analyzed AOD fields using independent observations from ground, aircraft, and shipborne instruments. We demonstrate the positive impact of the AOD assimilation on simulated aerosols by comparing MERRA-2 aerosol fields to an identical control simulation that does not include AOD assimilation. Having shown the AOD evaluation, we take a first look at aerosol-climate interactions by examining the shortwave, clear-sky aerosol direct radiative effect. In our companion paper, we evaluate and validate available MERRA-2 aerosol properties not directly impacted by the AOD assimilation (e.g. aerosol vertical distribution and absorption). Importantly, while highlighting the skill of the MERRA-2 aerosol assimilation products, both studies point out caveats that must be considered when using this new reanalysis product for future studies of aerosols and their interactions with weather and climate.

The MERRA-2 Aerosol Reanalysis, 1980 Onward. Part II: Evaluation and Case Studies

Abstract: The Modern-Era Retrospective Analysis for Research and Applications, version 2 (MERRA-2), is NASA’s latest reanalysis for the satellite era (1980 onward) using the Goddard Earth Observing System, version 5 (GEOS-5), Earth system model. MERRA-2 provides several improvements over its predecessor (MERRA-1), including aerosol assimilation for the entire period. MERRA-2 assimilates bias-corrected aerosol optical depth (AOD) from the Moderate Resolution Imaging Spectroradiometer and the Advanced Very High Resolution Radiometer instruments. Additionally, MERRA-2 assimilates (non bias corrected) AOD from the Multiangle Imaging SpectroRadiometer over bright surfaces and AOD from Aerosol Robotic Network sunphotometer stations. This paper, the second of a pair, summarizes the efforts to assess the quality of the MERRA-2 aerosol products. First, MERRA-2 aerosols are evaluated using independent observations. It is shown that the MERRA-2 absorption aerosol optical depth (AAOD) and ultraviolet aerosol index (AI)...

Land Surface Precipitation in MERRA-2

Abstract: The Modern-Era Retrospective Analysis for Research and Applications, version 2 (MERRA-2), features several major advances from the original MERRA reanalysis, including the use, outside of high latitudes, of observations-based precipitation data products to correct the precipitation falling on the land surface in the MERRA-2 system. The method for merging the observed precipitation into MERRA-2 has been refined from that of the (land-only) MERRA-Land reanalysis. This paper describes the method and evaluates the MERRA-2 land surface precipitation. Compared to monthly GPCPv2.2 observations, the corrected MERRA-2 precipitation (M2CORR) is better than the precipitation generated by the atmospheric models within the cycling MERRA-2 and MERRA systems. M2CORR is also better than MERRA-Land precipitation over Africa because in MERRA-2 a merged satellite–gauge precipitation product is used instead of the gauge-only data used for MERRA-Land. Compared to 3-hourly TRMM observations, the M2CORR diurnal cycle ha...

Assessment of MERRA-2 Land Surface Hydrology Estimates

Abstract: The MERRA-2 atmospheric reanalysis product provides global, 1-hourly estimates of land surface conditions for 1980–present at ~50-km resolution. MERRA-2 uses observations-based precipitation to force the land (unlike its predecessor, MERRA). This paper evaluates MERRA-2 and MERRA land hydrology estimates, along with those of the land-only MERRA-Land and ERA-Interim/Land products, which also use observations-based precipitation. Overall, MERRA-2 land hydrology estimates are better than those of MERRA-Land and MERRA. A comparison against GRACE satellite observations of terrestrial water storage demonstrates clear improvements in MERRA-2 over MERRA in South America and Africa but also reflects known errors in the observations used to correct the MERRA-2 precipitation. Validation against in situ measurements from 220–320 stations in North America, Europe, and Australia shows that MERRA-2 and MERRA-Land have the highest surface and root zone soil moisture skill, slightly higher than that of ERA-Interim...

Increased Quasi Stationarity and Persistence of Winter Ural Blocking and Eurasian Extreme Cold Events in Response to Arctic Warming. Part I: Insights from Observational Analyses

Abstract: Part I of this study examines the relationship among winter cold anomalies over Eurasia, Ural blocking (UB), and the background conditions associated with Arctic warming over the Barents and Kara Seas (BKS) using reanalysis data. It is found that the intensity, persistence, and occurrence region of UB-related Eurasian cold anomalies depend strongly on the strength and vertical shear (VS) of the mean westerly wind (MWW) over mid–high-latitude Eurasia related to BKS warming.Observational analysis reveals that during 1951–2015 UB days are 64% (54%) more frequent during weak MWW (VS) winters, with 26.9 (28.4) days per winter, than during strong MWW (VS) winters. During weak MWW or VS winters, as frequently observed during 2000–15, persistent and large UB-related warming is seen over the BKS together with large and widespread midlatitude Eurasian cold anomalies resulting from increased quasi stationarity and persistence of the UB. By contrast, when the MWW or VS is strong as frequently observed during ...

Heat Waves in Southern China: Synoptic Behavior, Long-Term Change, and Urbanization Effects

Abstract: The characteristics of heat waves (HWs) in southern China in 1979–2010 are studied by using both reanalysis and station datasets. Guangdong Province of China (GDPC) is taken as an example. It is found that the westward movement of the western North Pacific subtropical high (WNPSH) is the primary factor for large-scale HWs occurring in GDPC. When an HW occurs, a hot and dry atmospheric column prevails over southern China. The region is overlaid by anomalous subsidence, which leads to warming, and clear sky, which causes greater solar heating. HWs are accompanied by an anomalous high pressure center and anticyclone near the surface, with anomalous land–sea northwesterly flow, thus reducing sea–land moisture transport and drying the atmosphere over land. The evolution of the high pressure anomaly and high temperature is associated with the westward displacement of WNPSH, with a prominent positive anomaly in 500-hPa height migrating westward. All these features associated with HWs in GDPC coincide wit...

The Northern Hemisphere Extratropical Atmospheric Circulation Response to ENSO: How Well Do We Know It and How Do We Evaluate Models Accordingly?

Abstract: Application of random sampling techniques to composite differences between 18 El Nino and 14 La Nina events observed since 1920 reveals considerable uncertainty in both the pattern and amplitude of the Northern Hemisphere extratropical winter sea level pressure (SLP) response to ENSO. While the SLP responses over the North Pacific and North America are robust to sampling variability, their magnitudes can vary by a factor of 2; other regions, such as the Arctic, North Atlantic, and Europe are less robust in their SLP patterns, amplitudes, and statistical significance. The uncertainties on the observed ENSO composite are shown to arise mainly from atmospheric internal variability as opposed to ENSO diversity. These observational findings pose considerable challenges for the evaluation of ENSO teleconnections in models. An approach is proposed that incorporates both pattern and amplitude uncertainty in the observational target, allowing for discrimination between true model biases in the forced ENSO ...

How Well Are Tropical Cyclones Represented in Reanalysis Datasets

Abstract: Tropical cyclones (TCs) are identified and tracked in six recent reanalysis datasets and compared with those from the IBTrACS best-track archive. Results indicate that nearly every cyclone present in IBTrACS over the period 1979–2012 can be found in all six reanalyses using a tracking and matching approach. However, TC intensities are significantly underrepresented in the reanalyses compared to the observations. Applying a typical objective TC identification scheme, it is found that the largest uncertainties in TC identification occur for the weaker storms; this is exacerbated by uncertainties in the observations for weak storms and lack of consistency in operational procedures. For example, certain types of storms, such as tropical depressions, subtropical cyclones, and monsoon depressions, are not included in the best-track data for all reporting agencies. There are definite improvements in how well TCs are represented in more recent, higher-resolution reanalyses; in particular MERRA-2 is compar...

Attribution of Extreme Events in Arctic Sea Ice Extent

Abstract: Arctic sea ice extent (SIE) has decreased over recent decades, with record-setting minimum events in 2007 and again in 2012. A question of interest across many disciplines concerns the extent to which such extreme events can be attributed to anthropogenic influences. First, a detection and attribution analysis is performed for trends in SIE anomalies over the observed period. The main objective of this study is an event attribution analysis for extreme minimum events in Arctic SIE. Although focus is placed on the 2012 event, the results are generalized to extreme events of other magnitudes, including both past and potential future extremes. Several ensembles of model responses are used, including two single-model large ensembles. Using several different metrics to define the events in question, it is shown that an extreme SIE minimum of the magnitude seen in 2012 is consistent with a scenario including anthropogenic influence and is extremely unlikely in a scenario excluding anthropogenic influenc...

Super-clausius-clapeyron scaling of extreme hourly convective precipitation and its relation to large-scale atmospheric conditions

Abstract: Present-day precipitation–temperature scaling relations indicate that hourly precipitation extremes may have a response to warming exceeding the Clausius–Clapeyron (CC) relation; for the Netherlands the dependency on surface dewpoint temperature follows 2 times the CC relation (2CC). The authors’ hypothesis—as supported by a simple physical argument presented here—is that this 2CC behavior arises from the physics of convective clouds. To further investigate this, the large-scale atmospheric conditions accompanying summertime afternoon precipitation events are analyzed using surface observations combined with a regional reanalysis. Events are precipitation measurements clustered in time and space. The hourly peak intensities of these events again reveal a 2CC scaling with the surface dewpoint temperature. The temperature excess of moist updrafts initialized at the surface and the maximum cloud depth are clear functions of surface dewpoint, confirming the key role of surface humidity on convective a...

Stratospheric control of the Madden-Julian oscillation.

Abstract: Interannual variation of seasonal-mean tropical convection over the Indo-Pacific region is primarily controlled by El Nino–Southern Oscillation (ENSO). For example, during El Nino winters, seasonal-mean convection around the Maritime Continent becomes weaker than normal, while that over the central to eastern Pacific is strengthened. Similarly, subseasonal convective activity, which is associated with the Madden–Julian oscillation (MJO), is influenced by ENSO. The MJO activity tends to extend farther eastward to the date line during El Nino winters and contract toward the western Pacific during La Nina winters. However, the overall level of MJO activity across the Maritime Continent does not change much in response to the ENSO. It is shown that the boreal winter MJO amplitude is closely linked with the stratospheric quasi-biennial oscillation (QBO) rather than with ENSO. The MJO activity around the Maritime Continent becomes stronger and more organized during the easterly QBO winters. The QBO-rela...

Assessing the Climate Impacts of the Observed Atlantic Multidecadal Variability Using the GFDL CM2.1 and NCAR CESM1 Global Coupled Models

Abstract: The climate impacts of the observed Atlantic multidecadal variability (AMV) are investigated using the GFDL CM2.1 and the NCAR CESM1 coupled climate models. The model North Atlantic sea surface temperatures are restored to fixed anomalies corresponding to an estimate of the internally driven component of the observed AMV. Both models show that during boreal summer the AMV alters the Walker circulation and generates precipitation anomalies over the whole tropical belt. A warm phase of the AMV yields reduced precipitation over the western United States, drier conditions over the Mediterranean basin, and wetter conditions over northern Europe. During boreal winter, the AMV modulates by a factor of about 2 the frequency of occurrence of El Nino and La Nina events. This response is associated with anomalies over the Pacific that project onto the interdecadal Pacific oscillation pattern (i.e., Pacific decadal oscillation–like anomalies in the Northern Hemisphere and a symmetrical pattern in the Southern...

Recent Changes in the Moisture Source of Precipitation over the Tibetan Plateau

Abstract: Evidence has suggested a wetting trend over part of the Tibetan Plateau (TP) in recent decades, although there are large uncertainties in this trend due to sparse observations. Examining the change in the moisture source for precipitation over a region in the TP with the most obvious increasing precipitation trend may help understand the precipitation change. This study applied the modified Water Accounting Model with two atmospheric reanalyses, ground-observed precipitation, and evaporation from a land surface model to investigate the change in moisture source of the precipitation over the targeted region. The study estimated that on average more than 69% and more than 21% of the moisture supply to precipitation over the targeted region came from land and ocean, respectively. The moisture transports from the west of the TP by the westerlies and from the southwest by the Indian summer monsoon likely contributed the most to precipitation over the targeted region. The moisture from inside the region...

Positive Trend in the Antarctic Sea Ice Cover and Associated Changes in Surface Temperature

Abstract: The Antarctic sea ice extent has been slowly increasing contrary to expected trends due to global warming and results from coupled climate models. After a record high extent in 2012 the extent was even higher in 2014 when the magnitude exceeded 20 × 106 km2 for the first time during the satellite era. The positive trend is confirmed with newly reprocessed sea ice data that addressed inconsistency issues in the time series. The variability in sea ice extent and ice area was studied alongside surface ice temperature for the 34-yr period starting in 1981, and the results of the analysis show a strong correlation of −0.94 during the growth season and −0.86 during the melt season. The correlation coefficients are even stronger with a one-month lag in surface temperature at −0.96 during the growth season and −0.98 during the melt season, suggesting that the trend in sea ice cover is strongly influenced by the trend in surface temperature. The correlation with atmospheric circulation as represented by th...

Barrier Effect of the Indo-Pacific Maritime Continent on the MJO: Perspectives from Tracking MJO Precipitation

Abstract: Explanations for the barrier effect of the Indo-Pacific Maritime Continent (MC) on the MJO should satisfy two criteria. First, they should include specific features of the MC, namely, its intricate land–sea distributions and elevated terrains. Second, they should include mechanisms for both the barrier effect and its overcoming by some MJO events. Guided by these two criteria, a precipitation-tracking method is applied to identify MJO events that propagate across the MC (MJO-C) and those that are blocked by the MC (MJO-B). About a half of MJO events that form over the Indian Ocean propagate through the MC. Most of them (>75%) become weakened over the MC. The barrier effect cannot be explained in terms of the strength, horizontal scale, or spatial distribution of MJO convection when it approaches the MC from the west. A distinction between MJO-B and MJO-C is their precipitation over the sea versus land in the MC region. MJO-C events rain much more over the sea than over land, whereas rainfall over ...

Storylines of Atmospheric Circulation Change for European Regional Climate Impact Assessment

Abstract: There is increasing interest in understanding the regional impacts of different global warming targets. However, several regional climate impacts depend on the atmospheric circulation, whose response to climate change remains substantially uncertain and not interpretable in a probabilistic sense in multimodel ensemble projections. To account for these uncertainties, a novel approach where regional climate change is analyzed as a function of carbon emissions conditional on plausible storylines of atmospheric circulation change is here presented and applied to the CMIP5 models’ future projections. The different storylines are determined based on the response in three remote drivers of regional circulation: the tropical and polar amplification of global warming and changes in stratospheric vortex strength. As an illustration of this approach, it is shown that the severity of the projected wintertime Mediterranean precipitation decline and central European windiness increase strongly depends on the st...

Contribution of Tropical Cyclones to Rainfall at the Global Scale

Abstract: This study quantifies the relative contribution of tropical cyclones (TCs) to annual, seasonal, and extreme rainfall and examines the connection between El Nino–Southern Oscillation (ENSO) and the occurrence of extreme TC-induced rainfall across the globe. The authors use historical 6-h best-track TC datasets and daily precipitation data from 18 607 global rain gauges with at least 25 complete years of data between 1970 and 2014. The highest TC-induced rainfall totals occur in East Asia (>400 mm yr−1) and northeastern Australia (>200 mm yr−1), followed by the southeastern United States and along the coast of the Gulf of Mexico (100–150 mm yr−1). Fractionally, TCs account for 35%–50% of the mean annual rainfall in northwestern Australia, southeastern China, the northern Philippines, and Baja California, Mexico. Seasonally, between 40% and 50% of TC-induced rain is recorded along the western coast of Australia and in islands of the south Indian Ocean in the austral summer and in East Asia and Mexico...

Simulated Atmospheric Response to Regional and Pan-Arctic Sea Ice Loss

Abstract: The loss of Arctic sea ice is already having profound environmental, societal, and ecological impacts locally. A highly uncertain area of scientific research, however, is whether such Arctic change has a tangible effect on weather and climate at lower latitudes. There is emerging evidence that the geographical location of sea ice loss is critically important in determining the large-scale atmospheric circulation response and associated midlatitude impacts. However, such regional dependencies have not been explored in a thorough and systematic manner. To make progress on this issue, this study analyzes ensemble simulations with an atmospheric general circulation model prescribed with sea ice loss separately in nine regions of the Arctic, to elucidate the distinct responses to regional sea ice loss. The results suggest that in some regions, sea ice loss triggers large-scale dynamical responses, whereas in other regions sea ice loss induces only local thermodynamical changes. Sea ice loss in the Bare...

The Role of Downward Infrared Radiation in the Recent Arctic Winter Warming Trend

Abstract: During the past three decades, the most rapid warming at the surface has occurred during the Arctic winter. By analyzing daily ERA-Interim data, it is found that the majority of the winter warming trend north of 70°N can be explained by the trend in the downward infrared radiation (IR). This downward IR trend can be attributed to an enhanced poleward flux of moisture and sensible heat into the Arctic by poleward-propagating Rossby waves, which increases the total column water and temperature within this region. This enhanced moisture flux is mostly due to changes in the planetary-scale atmospheric circulation rather than an increase in moisture in lower latitudes. The results of this study lead to the question of whether Arctic amplification has mostly arisen through changes in the Rossby wave response to greenhouse gas forcing and its impact on moisture transport into the Arctic.

Projected Future Changes of Tropical Cyclone Activity over the Western North and South Pacific in a 20-km-Mesh Regional Climate Model

Abstract: A high-resolution regional atmospheric model is employed to project the late twenty-first-century changes of tropical cyclone (TC) activity over the western North Pacific (WP) and southwest Pacific (SP). The model realistically reproduces the basic features of the TC climatology in the present-day simulation. Future projections under the representative concentration pathway 4.5 (RCP45) and 8.5 (RCP85) scenarios are investigated. The results show no significant change of TC genesis frequency (TCGF) in the WP by RCP45 due to the cancellation of the reduction over the western part and the increase over the eastern part together with a considerable decrease of TCGF by RCP85 due to the excessive TCGF reduction in the western part. The TCGF over the SP consistently decreases from RCP45 to RCP85. Despite the fact that the simulated maximum surface wind speeds are below 52 m s−1, the change with more strong TCs and fewer weak TCs is robust. The future changes in the TC genesis locations and translational ...

Quantifying Spatiotemporal Variations of Soil Moisture Control on Surface Energy Balance and Near-Surface Air Temperature

Abstract: Soil moisture plays a crucial role for the energy partitioning at Earth’s surface Changing fractions of latent and sensible heat fluxes caused by soil moisture variations can affect both near-surface air temperature and precipitation In this study, a simple framework for the dependence of evaporative fraction (the ratio of latent heat flux over net radiation) on soil moisture is used to analyze spatial and temporal variations of land–atmosphere coupling and its effect on near-surface air temperature Using three different data sources (two reanalysis datasets and one combination of different datasets), three key parameters for the relation between soil moisture and evaporative fraction are estimated: 1) the frequency of occurrence of different soil moisture regimes, 2) the sensitivity of evaporative fraction to soil moisture in the transitional soil moisture regime, and 3) the critical soil moisture value that separates soil moisture- and energy-limited evapotranspiration regimes The results sh

How Suitable is Quantile Mapping For Postprocessing GCM Precipitation Forecasts

Abstract: GCMs are used by many national weather services to produce seasonal outlooks of atmospheric and oceanic conditions and fluxes. Postprocessing is often a necessary step before GCM forecasts can be applied in practice. Quantile mapping (QM) is rapidly becoming the method of choice by operational agencies to postprocess raw GCM outputs. The authors investigate whether QM is appropriate for this task. Ensemble forecast postprocessing methods should aim to 1) correct bias, 2) ensure forecasts are reliable in ensemble spread, and 3) guarantee forecasts are at least as skillful as climatology, a property called “coherence.” This study evaluates the effectiveness of QM in achieving these aims by applying it to precipitation forecasts from the POAMA model. It is shown that while QM is highly effective in correcting bias, it cannot ensure reliability in forecast ensemble spread or guarantee coherence. This is because QM ignores the correlation between raw ensemble forecasts and observations. When raw foreca...

Atmospheric Water Balance and Variability in the MERRA-2 Reanalysis

Abstract: Closing and balancing Earth’s global water cycle remains a challenge for the climate community Observations are limited in duration, global coverage, and frequency, and not all water cycle terms are adequately observed Reanalyses aim to fill the gaps through the assimilation of as many atmospheric water vapor observations as possible Former generations of reanalyses have demonstrated a number of systematic problems that have limited their use in climate studies, especially regarding low-frequency trends This study characterizes the NASA Modern-Era Retrospective Analysis for Research and Applications version 2 (MERRA-2) water cycle relative to contemporary reanalyses and observations MERRA-2 includes measures intended to minimize the spurious global variations related to inhomogeneity in the observational record The global balance and cycling of water from ocean to land is presented, with special attention given to the water vapor analysis increment and the effects of the changing observing s

Atmospheric Dynamic and Thermodynamic Processes Driving the Western North Pacific Anomalous Anticyclone during El Niño. Part I: Maintenance Mechanisms

Abstract: The western North Pacific anomalous anticyclone (WNPAC) is an important low-level circulation system that connects El Nino and the East Asian monsoon. In this study, the mechanisms responsible for the formation and maintenance of the WNPAC are explored. Part I of this study focuses on the WNPAC maintenance mechanisms during El Nino mature winter and the following spring. Moisture and moist static energy analyses indicated that the WNPAC is maintained by both the remote forcing from the equatorial central-eastern Pacific via the atmospheric bridge and the local air–sea interactions. Three pacemaker experiments by a coupled global climate model FGOALS-s2, with upper-700-m ocean temperature in the equatorial central-eastern Pacific restored to the observational anomalies plus model climatology, suggest that about 60% (70%) intensity of the WNPAC during the winter (spring) is contributed by the remote forcing from the equatorial central-eastern Pacific. The key remote forcing mechanism responsible for...

Evaluation of the Ozone Fields in NASA's MERRA-2 Reanalysis.

Abstract: We describe and assess the quality of the assimilated ozone product from the Modern-Era Retrospective Analysis for Research and Applications, Version 2 (MERRA-2) produced at NASA's Global Modeling and Assimilation Office (GMAO) spanning the time period from 1980 to present. MERRA-2 assimilates partial column ozone retrievals from a series of Solar Backscatter Ultraviolet (SBUV) radiometers on NASA and NOAA spacecraft between January 1980 and September 2004; starting in October 2004 retrieved ozone profiles from the Microwave Limb Sounder (MLS) and total column ozone from the Ozone Monitoring Instrument on NASA's EOS Aura satellite are assimilated. We compare the MERRA-2 ozone with independent satellite and ozonesonde data focusing on the representation of the spatial and temporal variability of stratospheric and upper tropospheric ozone and on implications of the change in the observing system from SBUV to EOS Aura. The comparisons show agreement within 10 % (standard deviation of the difference) between MERRA-2 profiles and independent satellite data in most of the stratosphere. The agreement improves after 2004 when EOS Aura data are assimilated. The standard deviation of the differences between the lower stratospheric and upper tropospheric MERRA-2 ozone and ozonesondes is 11.2 % and 24.5 %, respectively, with correlations of 0.8 and above, indicative of a realistic representation of the near-tropopause ozone variability in MERRA-2. The agreement improves significantly in the EOS Aura period, however MERRA-2 is biased low in the upper troposphere with respect to the ozonesondes. Caution is recommended when using MERRA-2 ozone for decadal changes and trend studies.

The Central Role of Ocean Dynamics in Connecting the North Atlantic Oscillation to the Extratropical Component of the Atlantic Multidecadal Oscillation

Abstract: The relationship between the North Atlantic Oscillation (NAO) and Atlantic sea surface temperature (SST) variability is investigated using models and observations. Coupled climate models are used in which the ocean component is either a fully dynamic ocean or a slab ocean with no resolved ocean heat transport. On time scales less than 10 yr, NAO variations drive a tripole pattern of SST anomalies in both observations and models. This SST pattern is a direct response of the ocean mixed layer to turbulent surface heat flux anomalies associated with the NAO. On time scales longer than 10 yr, a similar relationship exists between the NAO and the tripole pattern of SST anomalies in models with a slab ocean. A different relationship exists both for the observations and for models with a dynamic ocean. In these models, a positive (negative) NAO anomaly leads, after a decadal-scale lag, to a monopole pattern of warming (cooling) that resembles the Atlantic multidecadal oscillation (AMO), although with sma...

Interdecadal Variations of the Silk Road Pattern

Abstract: Based on several reanalysis and observational datasets, this study suggests that the Silk Road pattern (SRP), a major teleconnection pattern stretching across Eurasia in the boreal summer, shows clear interdecadal variations that explain approximately 50% of its total variance. The interdecadal SRP features a strong barotropic wave train along the Asian subtropical jet, resembling its interannual counterpart. Additionally, it features a second weak wave train over the northern part of Eurasia, leading to larger meridional scale than its interannual counterpart. The interdecadal SRP contributes approximately 40% of the summer surface air temperature’s variance with little uncertainty and 10%–20% of the summer precipitation’s variance with greater uncertainty over large domains of Eurasia. The interdecadal SRP shows two regime shifts in 1972 and 1997. The latter shift explains over 40% of the observed rainfall reduction over northeastern Asia and over 40% of the observed warming over eastern Europe,...

Atmospheric Response to Arctic and Antarctic Sea Ice: The Importance of Ocean–Atmosphere Coupling and the Background State

Abstract: The atmospheric response to Arctic and Antarctic sea ice changes typical of the present day and coming decades is investigated using the Hadley Centre global climate model (HadGEM3). The response is diagnosed from ensemble simulations of the period 1979 to 2009 with observed and perturbed sea ice concentrations. The experimental design allows the impacts of ocean–atmosphere coupling and the background atmospheric state to be assessed. The modeled response can be very different to that inferred from statistical relationships, showing that the response cannot be easily diagnosed from observations. Reduced Arctic sea ice drives a local low pressure response in boreal summer and autumn. Increased Antarctic sea ice drives a poleward shift of the Southern Hemisphere midlatitude jet, especially in the cold season. Coupling enables surface temperature responses to spread to the ocean, amplifying the atmospheric response and revealing additional impacts including warming of the North Atlantic in response t...