Temperature Change on the Antarctic Peninsula Linked to the Tropical Pacific

AbstractSignificant summer warming over the eastern Antarctic Peninsula in the last 50 years has been attributed to a strengthening of the circumpolar westerlies, widely believed to be anthropogenic in origin. On the western side of the peninsula, significant warming has occurred mainly in austral winter and has been attributed to the reduction of sea ice. The authors show that austral fall is the only season in which spatially extensive warming has occurred on the Antarctic Peninsula. This is accompanied by a significant reduction of sea ice cover off the west coast. In winter and spring, warming is mainly observed on the west side of the peninsula. The most important large-scale forcing of the significant widespread warming trend in fall is the extratropical Rossby wave train associated with tropical Pacific sea surface temperature anomalies. Winter and spring warming on the western peninsula reflects the persistence of sea ice anomalies arising from the tropically forced atmospheric circulation changes...

Related papers:

title: Variability and Trends in Antarctic Surface Temperatures from In Situ and Satellite Infrared Measurements
abstract: The surface air temperatures observed from stations in Antarctica have been shown to have predominantly positive trends that are as high as 0.5°C decade−1 along the Antarctic Peninsula. To evaluate whether the trends are caused by a local or large-scale phenomenon in the Antarctic region, surface temperatures inferred from infrared satellite data from 1979 to 1998 have been analyzed in combination with data from 21 stations that have long record lengths. The surface temperatures derived from infrared data are coherent spatially and temporally and are shown to agree well with Antarctic station data with a correlation coefficient of 0.98 and a standard deviation of about 3°C. The trend analysis on station data yielded on the average 0.012 ± 0.008°C yr−1 and −0.008 ± 0.025°C yr−1 for the 45- and 20-yr record, respectively. The latter reasonably agrees with the trend of −0.042 ± 0.067°C yr−1 inferred from the satellite 20-yr record. The 20-yr record length is shown to be about the minimum length requ...

title: Changes in Antarctic Peninsula tropospheric temperatures from 1956 to 1999: a synthesis of observations and reanalysis data
abstract: The surface warming at Faraday station in the western Antarctic Peninsula is one of the largest observed anywhere over the last 50 years, yet the physical mechanisms driving this climate change are poorly understood. In this paper we synthesize radiosonde temperature observations from three Peninsula stations and NCEP-NCAR reanalysis data in order to examine contemporaneous regional tropospheric temperature trends (1956-99), which may in turn help us to understand better the causes of the surface warming. The reanalysis data are utilized in two ways: (i) to provide long-term mean monthly offsets between Faraday, which ceased radiosonde observations in 1982, and two other stations in the region having more recent data, Bellingshausen and Marambio, in order to create post-1982 simulated Faraday data; (ii) after having any spurious trends and bias removed, to provide directly a monthly value for Faraday when no equivalent value from regional observations is available. Using available months of overlap, a comparison between temperature observations and simulated data suggests that the latter are a reasonable facsimile of the former. 
 
The synthesized time-series of tropospheric temperatures reveal a statistically significant mean annual tropospheric (850-300 hPa) warming above Faraday between 1956 and 1999 of similar to0.027 +/- 0.022degreesC year(-1). Winter and summer both show a warming trend, with significance varying with height and season. Annually, the mean tropospheric warming is half that at the surface, Unlike the surface warming, the calculated tropospheric warming trend is no greater than observed at other Antarctic stations, and indeed is not significantly greater than the background global warming trend for most of the period examined. Thus, we cannot dismiss the possibility that the Peninsula surface warming may simply be a response to a global warming magnified by the observed strong regional feedback between sea-ice extent and surface temperature during winter.

title: Non-annular atmospheric circulation change induced by stratospheric ozone depletion and its role in the recent increase of Antarctic sea ice extent
abstract: Based on a new analysis of passive microwave satellite data, we demonstrate that the annual mean extent of Antarctic sea ice has increased at a statistically significant rate of 0.97% dec−1 since the late 1970s. The largest increase has been in autumn when there has been a dipole of significant positive and negative trends in the Ross and Amundsen‐Bellingshausen Seas respectively. The autumn increase in the Ross Sea sector is primarily a result of stronger cyclonic atmospheric flow over the Amundsen Sea. Model experiments suggest that the trend towards stronger cyclonic circulation is mainly a result of stratospheric ozone depletion, which has strengthened autumn wind speeds around the continent, deepening the Amundsen Sea Low through flow separation around the high coastal orography. However, statistics derived from a climate model control run suggest that the observed sea ice increase might still be within the range of natural climate variability.

title: A Synthesis of Antarctic Temperatures
abstract: Monthly surface air temperatures from land surface stations, automatic weather stations, and ship/buoy observations from the high-latitude Southern Hemisphere are synthesized into gridded analyses at a resolution appropriate for applications ranging from spatial trend analyses to climate change impact assessments. Correlation length scales are used to enhance information content while limiting the spatial extent of influence of the sparse data in the Antarctic region. The correlation length scales are generally largest in summer and over the Antarctic continent, while they are shortest over the winter sea ice. Gridded analyses of temperature anomalies, limited to regions within a correlation length scale of at least one observation, are constructed and validated against observed temperature anomalies in single-station-out experiments. Trends calculated for the 1958–2002 period suggest modest warming over much of the 60°–90°S domain. All seasons show warming, with winter trends being the largest a...

title: ADVANCES IN DESCRIBING RECENT ANTARCTIC CLIMATE VARIABILITY
abstract: Antarctica is a challenging region for conducting meteorological research because of its geographic isolation, climate extremes, vastness, and lack of permanent human inhabitants. About 15 observing stations have been in continuous operation since the onset of the modern scientific era in Antarctica during the International Geophysical Year in 1957/58. Identifying and attributing natural- and human-caused climate change signals from the comparatively short Antarctic dataset is confounded by large year-to-year fluctuations of temperature, atmospheric pressure, and snowfall. Yet there is increasing urgency to understand Antarctica's role in the global climate system for a number of reasons, most importantly the potential consequences of ice-mass loss on global sea level rise. Here, we describe recently-created records that allow Antarctic near-surface temperature and snowfall changes to be assessed in all of Antarctica's 24 glacial drainage systems during the past five decades. The new near-surface temperat...

title: An assessment and interpretation of the observed warming of West Antarctica in the austral spring
abstract: We synthesize variability and trends in multiple analyses of Antarctic near-surface temperature representing several independent source datasets and spatially complete reconstructions, and place these into the broader context of the behavior of other components of the climate system during the past 30–50 years. Along with an annual-mean trend during the past 50 years of about 0.1°C/decade averaged over Antarctica, there is a distinct seasonality to the trends, with insignificant change (and even some cooling) in austral summer and autumn in East Antarctica, contrasting with warming in austral winter and spring. Apart from the Peninsula, the seasonal warming is largest and most significant in West Antarctica in the austral spring since the late 1970s. Concurrent trends in sea ice are independent evidence of the observed warming over West Antarctic, with the decrease in sea ice area in the Amundsen and Bellingshausen Seas congruent with at least 50% of the inland warming of West Antarctica. Trends in near surface winds and geopotential heights over the high-latitude South Pacific are consistent with a role for atmospheric forcing of the sea ice and air temperature anomalies. Most of the circulation trend projects onto the two Pacific South American (PSA) modes of atmospheric circulation variability, while the Southern Annular Mode lacks a positive trend in spring that would otherwise cause a cooling tendency. The largest circulation trend is associated with the PSA-1 mode, a wave-train extending from the tropics to the high Southern latitudes. The PSA-1 mode is significantly correlated with SSTs in the southwestern tropical and subtropical Pacific. The increased SSTs in this region, together with the observed increase in rainfall, suggest that anomalous deep convection has strengthened or increased the occurrence of the Rossby wave-train associated with PSA-1. This hypothesis is supported by results from two ensembles of SST-forced atmospheric general circulation model simulations. Finally, the implications of the seasonality, timing, and spatial patterns of Antarctic temperature trends with respect to interpreting the relative roles of stratospheric ozone depletion, SSTs and increased atmospheric concentrations of greenhouse gasses are discussed.

title: Historical SAM Variability. Part II: Twentieth-Century Variability and Trends from Reconstructions, Observations, and the IPCC AR4 Models*
abstract: This second paper examines the Southern Hemisphere annular mode (SAM) variability from reconstructions, observed indices, and simulations from 17 Intergovernmental Panel on Climate Change (IPCC) Fourth Assessment Report (AR4) models from 1865 to 2005. Comparisons reveal the models do not fully simulate the duration of strong natural variability within the reconstructions during the 1930s and 1960s. Seasonal indices are examined to understand the relative roles of forced and natural fluctuations. The models capture the recent (1957‐2005) positive SAM trends in austral summer, which reconstructions indicate is the strongest trend during the last 150 yr; ozone depletion is the dominant mechanism driving these trends. In autumn, negative trends after 1930 in the reconstructions are stronger than the recent positive trend. Furthermore, model trends in autumn during 1957‐2005 are the most different from observations. Both of these conditions suggest the recent autumn trend is most likely natural climate variability, with external forcing playing a secondary role. Many models also produce significant spring trends during this period not seen in observations. Although insignificant, these differences arise because of vastly different spatial structures in the Southern Hemisphere pressure trends. As the trend differences between models and observations in austral spring have been increasing over the last 30 yr, care must be exercised when examining the future SAM projections and their impacts in this season.

title: A tropospheric assessment of the ERA‐40, NCEP, and JRA‐25 global reanalyses in the polar regions
abstract: The reliability of the global reanalyses in the polar regions is investigated. The overview stems from an April 2006 Scientific Committee on Antarctic Research (SCAR) workshop on the performance of global reanalyses in high latitudes held at the British Antarctic Survey. Overall, the skill is much higher in the Arctic than the Antarctic, where the reanalyses are only reliable in the summer months prior to the modern satellite era. In the Antarctic, large circulation differences between the reanalyses are found primarily before 1979, when vast quantities of satellite sounding data started to be assimilated. Specifically for ERA-40, this data discontinuity creates a marked jump in Antarctic snow accumulation, especially at high elevations. In the Arctic, the largest differences are related to the reanalyses' depiction of clouds and their associated radiation impacts; ERA-40 captures the cloud variability much better than NCEP1 and JRA-25, but the ERA-40 and JRA-25 clouds are too optically thin for shortwave radiation. To further contrast the reanalyses skill, cyclone tracking results are presented. In the Southern Hemisphere, cyclonic activity is markedly different between the reanalyses, where there are few matched cyclones prior to 1979. In comparison, only some of the weaker cyclones are not matched in the Northern Hemisphere from 1958–2001, again indicating the superior skill in this hemisphere. Although this manuscript focuses on deficiencies in the reanalyses, it is important to note that they are a powerful tool for climate studies in both polar regions when used with a recognition of their limitations.

title: Recent Rapid Regional Climate Warming on the Antarctic Peninsula
abstract: The Intergovernmental Panel on Climate Change (IPCC) confirmed that mean global warming was 0.6 ± 0.2 °C during the 20th century and cited anthropogenic increases in greenhouse gases as the likely cause of temperature rise in the last 50 years. But this mean value conceals the substantial complexity of observed climate change, which is seasonally- and diurnally-biased, decadally-variable and geographically patchy. In particular, over the last 50 years three high-latitude areas have undergone recent rapid regional (RRR) warming, which was substantially more rapid than the global mean. However, each RRR warming occupies a different climatic regime and may have an entirely different underlying cause. We discuss the significance of RRR warming in one area, the Antarctic Peninsula. Here warming was much more rapid than in the rest of Antarctica where it was not significantly different to the global mean. We highlight climate proxies that appear to show that RRR warming on the Antarctic Peninsula is unprecedented over the last two millennia, and so unlikely to be a natural mode of variability. So while the station records do not indicate a ubiquitous polar amplification of global warming, the RRR warming on the Antarctic Peninsula might be a regional amplification of such warming. This, however, remains unproven since we cannot yet be sure what mechanism leads to such an amplification. We discuss several possible candidate mechanisms: changing oceanographic or changing atmospheric circulation, or a regional air-sea-ice feedback amplifying greenhouse warming. We can show that atmospheric warming and reduction in sea-ice duration coincide in a small area on the west of the Antarctic Peninsula, but here we cannot yet distinguish cause and effect. Thus for the present we cannot determine which process is the probable cause of RRR warming on the Antarctic Peninsula and until the mechanism initiating and sustaining the RRR warming is understood, and is convincingly reproduced in climate models, we lack a sound basis for predicting climate change in this region over the coming century.

title: Variability in krill biomass links harvesting and climate warming to penguin population changes in Antarctica
abstract: The West Antarctic Peninsula (WAP) and adjacent Scotia Sea support abundant wildlife populations, many of which were nearly extirpated by humans. This region is also among the fastest-warming areas on the planet, with 5–6 °C increases in mean winter air temperatures and associated decreases in winter sea-ice cover. These biological and physical perturbations have affected the ecosystem profoundly. One hypothesis guiding ecological interpretations of changes in top predator populations in this region, the “sea-ice hypothesis,” proposes that reductions in winter sea ice have led directly to declines in “ice-loving” species by decreasing their winter habitat, while populations of “ice-avoiding” species have increased. However, 30 y of field studies and recent surveys of penguins throughout the WAP and Scotia Sea demonstrate this mechanism is not controlling penguin populations; populations of both ice-loving Adélie and ice-avoiding chinstrap penguins have declined significantly. We argue in favor of an alternative, more robust hypothesis that attributes both increases and decreases in penguin populations to changes in the abundance of their main prey, Antarctic krill. Unlike many other predators in this region, Adélie and chinstrap penguins were never directly harvested by man; thus, their population trajectories track the impacts of biological and environmental changes in this ecosystem. Linking trends in penguin abundance with trends in krill biomass explains why populations of Adélie and chinstrap penguins increased after competitors (fur seals, baleen whales, and some fishes) were nearly extirpated in the 19th to mid-20th centuries and currently are decreasing in response to climate change.

title: Simulation of Recent Southern Hemisphere Climate Change
abstract: Recent observations indicate that climate change over the high latitudes of the Southern Hemisphere is dominated by a strengthening of the circumpolar westerly flow that extends from the surface to the stratosphere. Here we demonstrate that the seasonality, structure, and amplitude of the observed climate trends are simulated in a state-of-the-art atmospheric model run with high vertical resolution that is forced solely by prescribed stratospheric ozone depletion. The results provide evidence that anthropogenic emissions of ozonedepleting gases have had a distinct impact on climate not only at stratospheric levels but at Earth's surface as well.

title: Southern Hemisphere climate response to ozone changes and greenhouse gas increases
abstract: While most of the Earth warmed rapidly during recent decades, surface temperatures decreased significantly over most of Antarctica. This cooling is consistent with circulation changes associated with a shift in the Southern Annular Mode (SAM). It has been suggested that both Antarctic ozone depletion and increasing greenhouses gases have contributed to these trends. We show that a climate model including the stratosphere and both composition changes reproduces the vertical structure and seasonality of observed trends. We find that the two factors have had comparable surface impacts over recent decades, though ozone dominates above the middle troposphere. Projected impacts of the two factors on circulation over the next fifty years oppose one another, resulting in minimal trends. In contrast, their effects on surface climate reinforce one another, causing a departure from the SAM pattern and a turnabout in Antarctic temperatures, which rise more rapidly than elsewhere in the Southern Hemisphere.

title: Forced annular variations in the 20th century Intergovernmental Panel on Climate Change Fourth Assessment Report models
abstract: We examine the annular mode within each hemisphere (defined here as the leading empirical orthogonal function and principal component of hemispheric sea level pressure) as simulated by the Intergovernmental Panel on Climate Change Fourth Assessment Report ensembles of coupled ocean-atmosphere models. The simulated annular patterns exhibit a high spatial correlation with the observed patterns during the late 20th century, though the mode represents too large a percentage of total temporal variability within each hemisphere. In response to increasing concentrations of greenhouse gases and tropospheric sulfate aerosols, the multimodel average exhibits a positive annular trend in both hemispheres, with decreasing sea level pressure (SLP) over the pole and a compensating increase in midlatitudes. In the Northern Hemisphere, the trend agrees in sign but is of smaller amplitude than that observed during recent decades. In the Southern Hemisphere, decreasing stratospheric ozone causes an additional reduction in Antarctic surface pressure during the latter half of the 20th century. While annular trends in the multimodel average are positive, individual model trends vary widely. Not all models predict a decrease in high-latitude SLP, although no model exhibits an increase. As a test of the models' annular sensitivity, the response to volcanic aerosols in the stratosphere is calculated during the winter following five major tropical eruptions. The observed response exhibits coupling between stratospheric anomalies and annular variations at the surface, similar to the coupling between these levels simulated elsewhere by models in response to increasing GHG concentration. The multimodel average is of the correct sign but significantly smaller in magnitude than the observed annular anomaly. This suggests that the models underestimate the coupling of stratospheric changes to annular variations at the surface and may not simulate the full response to increasing GHGs.

title: Characteristics of Summer Airflow over the Antarctic Peninsula in Response to Recent Strengthening of Westerly Circumpolar Winds
abstract: Summer near-surface temperatures over the northeast coast of the Antarctic Peninsula have increased by more than 2°C over the past 40 years, a temperature increase 3 times greater than that on the northwest coast. Recent analysis has shown a strong correlation between this striking warming trend and significant change in the summer Southern Hemisphere annular mode (SAM), which has resulted in greatly increased summer westerlies across the northern peninsula. It has been proposed that the strengthening westerlies have resulted in increased vertical deflection of relatively warm maritime air over the northern peninsula, contributing significantly to the observed warming and the recent collapse of northern sections of the Larsen Ice Shelf. In this study, laboratory and numerical modeling of airflow incident to the peninsula are employed to further understand this mechanism. It is shown that the effect of the strengthening westerlies has led to a distinct transition from a “blocked” regime to a “flow...

title: Climate Variability in the Amundsen and Bellingshausen Seas
abstract: Satellite data reveal a 20% decline in sea ice extent in the Amundsen and Bellingshausen Seas in the two decades following 1973. This change is negatively correlated with surface air temperatures on the west side of the Antarctic Peninsula, which have increased ;0.58C decade21 since the mid-1940s. The recession was strongest during summer, when monthly average minima in 1991‐92 removed much of the incipient multiyear ice over the continental shelf. This would have lowered the regional-mean ice thickness, impacting snow ice formation, brine production, and vertical heat flux. The northern ice edge contracted by ;18 of latitude in all seasons from 1973‐79 to 1987‐93, returning toward mean conditions in 1993‐95. The decline included multiyear cycles of several years in length, superimposed on high interannual variability. A review of atmospheric forcing shows winds consistent with mean and extreme ice extents, and suggests links to larger-scale circulation changes in the South Pacific. Historical ocean measurements are sparse in this sector, but mixed-layer depths and upper pycnoclines beneath the sea ice resemble those in the Weddell Sea. Weaker surface currents or changes in the upwelling of Circumpolar Deep Water on the continental shelf could have contributed to the anomaly persistence.

title: Mechanism study of the ENSO and southern high latitude climate teleconnections
abstract: Evidence of El Nino-Southern Oscillation (ENSO) teleconnections in the southern high latitude climate has been identified, although the mechanisms that might lead to such far-reaching teleconnections remain unresolved. Here we propose one such mechanism-the regional mean meridional atmospheric circulation (the regional Ferrel Cell)-responsible for the covariability of the ENSO and Antarctic Dipole (ADP; a predominant interannually-varying signal in the southern high latitudes). It is found that the altered storm tracks associated with the ENSO variability influence the regional Ferrel Cell indirectly by changing the meridional eddy heat flux divergence and convergence, and shifting the latent heat release zone. The changes of the regional Ferrel Cell then influence the southern high latitude climate by modulating the mean meridional heat flux.

title: Trends in Antarctic annual sea ice retreat and advance and their relation to El Niño–Southern Oscillation and Southern Annular Mode variability
abstract: Previous studies have shown strong contrasting trends in annual sea ice duration and in monthly sea ice concentration in two regions of the Southern Ocean: decreases in the western Antarctic Peninsula/southern Bellingshausen Sea (wAP/sBS) region and increases in the western Ross Sea (wRS) region. To better understand the evolution of these regional sea ice trends, we utilize the full temporal (quasi-daily) resolution of satellite-derived sea ice data to track spatially the annual ice edge advance and retreat from 1979 to 2004. These newly analyzed data reveal that sea ice is retreating 31 ± 10 days earlier and advancing 54 ± 9 days later in the wAP/sBS region (i.e., total change over 1979-2004), whereas in the wRS region, sea ice is retreating 29 ± 6 days later and advancing 31 ± 6 days earlier. Changes in the wAP/sBS and wRS regions, particularly as observed during sea ice advance, occurred in association with decadal changes in the mean state of the Southern Annular Mode (SAM; negative in the 1980s and positive in the 1990s) and the high-latitude response to El Nino-Southem Oscillation (ENSO). In general, the high-latitude ice-atmosphere response to ENSO was strongest when -SAM was coincident with El Nino and when +SAM was coincident with La Nina, particularly in the wAP/sBS region. In total, there were 7 of 11 -SAMs between 1980 and 1990 and the 7 of 10 +SAMs between 1991 and 2000 that were associated with consistent decadal sea ice changes in the wAP/sBS and wRS regions, respectively. Elsewhere, ENSO/SAM-related sea ice changes were not as consistent over time (e.g., western Weddell, Amundsen, and eastern Ross Sea region), or variability in general was high (e.g., central/ eastern Weddell and along East Antarctica).

title: Has the ozone hole contributed to increased Antarctic sea ice extent
abstract: Since the 1970s sea ice extent has decreased dramatically in the Northern Hemisphere and increased slightly in the Southern Hemisphere, a difference that is potentially explained by ozone depletion in the Southern Hemisphere stratosphere. In this study we consider the impact of stratospheric ozone depletion on Antarctic sea ice extent using a climate model forced with observed stratospheric ozone depletion from 1979 to 2005. Contrary to expectations, our model simulates a year-round decrease in Antarctic sea ice due to stratospheric ozone depletion. The largest percentage sea ice decrease in our model occurs in the austral summer near the coast of Antarctica, due to a mechanism involving offshore Ekman sea ice transport. The largest absolute decrease is simulated in the austral winter away from the coast of Antarctica, in response to an ocean warming that is consistent with a poleward shift of thc large-scale pattern of sea surface temperature. Our model results strongly suggest that processes not linked to stratospheric ozone depletion must be invoked to explain the observed increase in Antarctic sea ice extent.

title: Wind-driven trends in Antarctic sea-ice drift
abstract: The sea-ice cover around Antarctica has experienced a slight expansion in area over the past decades1, 2. This small overall increase is the sum of much larger opposing trends in different sectors that have been proposed to result from changes in atmospheric temperature or wind stress3, 4, 5, precipitation6, 7, ocean temperature8, and atmosphere or ocean feedbacks9, 10. However, climate models have failed to reproduce the overall increase in sea ice11. Here we present a data set of satellite-tracked sea-ice motion for the period of 1992–2010 that reveals large and statistically significant trends in Antarctic ice drift, which, in most sectors, can be linked to local winds. We quantify dynamic and thermodynamic processes in the internal ice pack and show that wind-driven changes in ice advection are the dominant driver of ice-concentration trends around much of West Antarctica, whereas wind-driven thermodynamic changes dominate elsewhere. The ice-drift trends also imply large changes in the surface stress that drives the Antarctic ocean gyres, and in the fluxes of heat and salt responsible for the production of Antarctic bottom and intermediate waters.

title: Southern Hemisphere Circulation Features Associated with El Niño-Southern Oscillation Events
abstract: Composite seasonal mean and anomaly fields prepared from operational numerical analyses have been used to describe the Southern Hemisphere (SH) circulation features associated with El Nino-Southern Oscillation (ENSO) events. The period of analyses available (1972–83) has limited the composites to include only three ENSO events. The reliability and stability of the composites has been tested using multiple permutation methods and by comparison with the results obtained using a longer period (1950–79) of SH rawinsonde station data. In the SH winter, a weak equivalent-barotropic wavetrain pattern of anomalies extends over Australia and the South Pacific Ocean to South America. This wavetrain pattern is quite variable in amplitude and location between ENSO events, although it is more stable over the subtropical Pacific. In the SH summer, the circulation anomalies are more zonally symmetric, with increased height at low and high latitudes and decreased height in middle latitudes. The circulation anoma...

title: Simulation of Polar Antarctic trends: Influence of tropical SST
abstract: The polar Antarctic atmosphere has been recently characterized by a progressive intensification of the circumpolar westerly winds, both in the stratospheric and tropospheric component. We simulate the response of the polar southern hemisphere atmosphere to a prescribed trend of oceanic equatorial temperature to demonstrate a possible link between the progressive warming of tropical sea surface temperature (SST) and Antarctic climate changes in the last decades. Model simulations produce an atmospheric response suggesting an influence of equatorial SST on polar dynamics, particularly during summer months.

title: ENSO-related impacts on Antarctic sea ice: a synthesis of phenomenon and mechanisms
abstract: Many remote and local climate variabilities influence Antarctic sea ice at different time scales. The strongest sea ice teleconnection at the interannual time scale was found between El Niño–Southern Oscillation (ENSO) events and a high latitude climate mode named the Antarctic Dipole. The Antarctic Dipole is characterized by an out-of-phase relationship between sea ice and surface temperature anomalies in the South Pacific and South Atlantic, manifesting itself and persisting 3–4 seasons after being triggered by the ENSO forcing. This study examines the life cycles of ENSO warm and cold events in the tropics and associated evolution of the ADP in high latitudes of the Southern Hemisphere. In evaluating the mechanisms that form the ADP, the study suggests a synthesized scheme that links these high latitude processes with ENSO teleconnection in both the Pacific and Atlantic basins. The synthesized scheme suggests that the two main mechanisms responsible for the formation/maintenance of the Antarctic Dipole are the heat flux due to the mean meridional circulation of the regional Ferrel Cell and regional anomalous circulation generated by stationary eddies. The changes in the Hadley Cell, the jet stream in the subtropics, and the Rossby Wave train associated with ENSO link the tropical forcing to these high latitude processes. Moreover, these two mechanisms operate in phase and are comparable in magnitude. The positive feedback between the jet stream and stationary eddies in the atmosphere, the positive feedback within the air-sea-ice system, and the seasonality all reinforce the anomalies, resulting in persistent Antarctic Dipole anomalies.

title: Southern Hemisphere Circulation and Relations with Sea Ice and Sea Surface Temperature
abstract: Relationships on the seasonal timescale between Southern Hemisphere 500-hPa height, sea surface temperature, and Antarctic sea ice variability have been investigated using NCEP–NCAR reanalyses, NCEP sea surface temperatures, and Met Office sea surface temperature and sea ice data. The dominant region of interannual variability in the Southern Hemisphere circulation, over the southeast Pacific Ocean, is found to be related to ENSO variability in tropical Pacific sea temperatures, as shown in a number of earlier papers. It is also related to Antarctic sea ice variability, where an out-of-phase relationship is found between sea ice extent in the central Pacific and in the southwest Atlantic Ocean. Sea ice extent is enhanced in one region when the atmospheric flow anomaly is equatorward, presumably through a combination of anomalous heat flux and direct advection. At the same time, the atmospheric flow anomaly in the other region tends to be poleward, resulting in a poleward retreat in the sea ice ed...

title: Teleconnections between the tropical Pacific and the Amundsen‐Bellinghausens Sea: Role of the El Niño/Southern Oscillation
abstract: Tropical Pacific–high southern latitude teleconnections are shown to be caused by Rossby wave dynamics and are sensitive to the exact pattern of sea surface temperature (SST) anomalies forcing anomalous ascent. Further, the signal becomes obscured by local natural variability at higher southern latitudes. Here it is shown that a Rossby wave with a source in the tropical Pacific can explain the general pattern of teleconnection found during the southern winter. Upper level divergence associated with deep convection in the tropical Pacific plays a major role in the generation of these Rossby waves. SST anomalies associated with El Nino/Southern Oscillation force the tropical deep convection. However, the relationship between deep convection and SST anomalies is complex and modeling experiments show that in areas of large-scale subsidence positive SST anomalies would have to be larger than any observed anomalies (>4°C) before they have any effect on the vertical motion. Large variations are observed between the high-latitude response to El Nino with apparently similar tropical forcings. This makes accurate prediction of the high-latitude response to tropical change difficult. Ensemble GCM runs show that variations due to the natural variation of the zonal flow in the Southern Hemisphere can swamp the signal resulting from changes in the Rossby wave source region.

title: Winter warming in West Antarctica caused by central tropical Pacific warming
abstract: The Pacific sector of Antarctica has experienced substantial warming in the past 30 years. Observations of global surface temperatures and atmospheric circulation data show that the warming in continental West Antarctica is linked to sea surface temperature changes in the tropical Pacific Ocean.

title: The reliability of Antarctic tropospheric pressure and temperature in the latest global reanalyses
abstract: AbstractIn this study, surface and radiosonde data from staffed Antarctic observation stations are compared to output from five reanalyses [Climate Forecast System Reanalysis (CFSR), 40-yr ECMWF Re-Analysis (ERA-40), ECMWF Interim Re-Analysis (ERA-Interim), Japanese 25-year Reanalysis (JRA-25), and Modern Era Retrospective-Analysis for Research and Applications (MERRA)] over three decades spanning 1979–2008. Bias and year-to-year correlation between the reanalyses and observations are assessed for four variables: mean sea level pressure (MSLP), near-surface air temperature (Ts), 500-hPa geopotential height (H500), and 500-hPa temperature (T500).It was found that CFSR and MERRA are of a sufficiently high resolution for the height of the orography to be accurately reproduced at coastal observation stations. Progressively larger negative Ts biases at these coastal stations are apparent for reanalyses in order of decreasing resolution. However, orography height bias cannot explain large winter warm biases in ...

title: The ERA-Interim reanalysis: configuration and performance of the data assimilation system
abstract: ERA-Interim is the latest global atmospheric reanalysis produced by the European Centre for Medium-Range Weather Forecasts (ECMWF). The ERA-Interim project was conducted in part to prepare for a new atmospheric reanalysis to replace ERA-40, which will extend back to the early part of the twentieth century. This article describes the forecast model, data assimilation method, and input datasets used to produce ERA-Interim, and discusses the performance of the system. Special emphasis is placed on various difficulties encountered in the production of ERA-40, including the representation of the hydrological cycle, the quality of the stratospheric circulation, and the consistency in time of the reanalysed fields. We provide evidence for substantial improvements in each of these aspects. We also identify areas where further work is needed and describe opportunities and objectives for future reanalysis projects at ECMWF.

title: MERRA: NASA’s Modern-Era Retrospective Analysis for Research and Applications
abstract: Modern-Era Retrospective Analysis for Research and Applications (MERRA) was undertaken by NASA’s Global Modeling and Assimilation Office with two primary objectives: to place observations from NASA’s Earth Observing System satellites into a climate context and to improve upon the hydrologic cycle represented in earlier generations of reanalyses. Focusing on the satellite era, from 1979 to the present, MERRA has achieved its goals with significant improvements in precipitation and water vapor climatology. Here, a brief overview of the system and some aspects of its performance, including quality assessment diagnostics from innovation and residual statistics, is given.By comparing MERRA with other updated reanalyses [the interim version of the next ECMWF Re-Analysis (ERA-Interim) and the Climate Forecast System Reanalysis (CFSR)], advances made in this new generation of reanalyses, as well as remaining deficiencies, are identified. Although there is little difference between the new reanalyses i...

title: An Intercomparison of Methods for Finding Coupled Patterns in Climate Data
abstract: This paper introduces a conceptual framework for comparing methods that isolate important coupled modes of variability between time series of two fields. Four specific methods are compared: principal component analysis with the fields combined (CPCA), canonical correlation analysis (CCA) and a variant of CCA proposed by Barnett and Preisendorfer (BP), principal component analysis of one single field followed by correlation of its component amplitudes with the second field (SFPCA), and singular value decomposition of the covariance matrix between the two fields (SVD). SVD and CPCA are easier to implement than BP, and do not involve user-chosen parameters. All methods are applied to a simple but geophysically relevant model system and their ability to detect a coupled signal is compared as parameters such as the number of points in each field, the number of samples in the time domain, and the signal-to-noise ratio are varied. In datasets involving geophysical fields, the number of sampling times ma...

title: Sensitivity of a global climate model to an increase of CO2 concentration in the atmosphere
abstract: This study investigates the response of a global model of the climate to the quadrupling of the CO2 concentration in the atmosphere. The model consists of (1) a general circulation model of the atmosphere, (2) a heat and water balance model of the continents, and (3) a simple mixed layer model of the oceans. It has a global computational domain and realistic geography. For the computation of radiative transfer, the seasonal variation of insolation is imposed at the top of the model atmosphere, and the fixed distribution of cloud cover is prescribed as a function of latitude and of height. It is found that with some exceptions, the model succeeds in reproducing the large-scale characteristics of seasonal and geographical variation of the observed atmospheric temperature. The climatic effect of a CO2 increase is determined by comparing statistical equilibrium states of the model atmosphere with a normal concentration and with a 4 times the normal concentration of CO2 in the air. It is found that the warming of the model atmosphere resulting from the CO2 increase has significant seasonal and latitudinal variation. Because of the absence of an albedo feedback mechanism, the warming over the Antarctic continent is somewhat less than the warming in high latitudes of the northern hemisphere. Over the Arctic Ocean and its surroundings, the warming is much larger in winter than summer, thereby reducing the amplitude of seasonal temperature variation. It is concluded that this seasonal asymmetry in the warming results from the reduction in the coverage and thickness of the sea ice. The warming of the model atmosphere results in an enrichment of the moisture content in the air and an increase in the poleward moisture transport. The additional moisture is picked up from the tropical ocean and is brought to high latitudes where both precipitation and runoff increase throughout the year. Further, the time of rapid snowmelt and maximum runoff becomes earlier.

title: Some simple solutions for heat‐induced tropical circulation
abstract: A simple analytic model is constructed to elucidate some basic features of the response of the tropical atmosphere to diabatic heating. In particular, there is considerable east-west asymmetry which can be illustrated by solutions for heating concentrated in an area of finite extent. This is of more than academic interest because heating in practice tends to be concentrated in specific areas. For instance, a model with heating symmetric about the equator at Indonesian longitudes produces low-level easterly flow over the Pacific through propagation of Kelvin waves into the region. It also produces low-level westerly inflow over the Indian Ocean (but in a smaller region) because planetary waves propagate there. In the heating region itself the low-level flow is away from the equator as required by the vorticity equation. The return flow toward the equator is farther west because of planetary wave propagation, and so cyclonic flow is obtained around lows which form on the western margins of the heating zone. Another model solution with the heating displaced north of the equator provides a flow similar to the monsoon circulation of July and a simple model solution can also be found for heating concentrated along an inter-tropical convergence line.

title: Variability in annual mean circulation in southern high latitudes
abstract: Using a hierarchy of climate models together with observations from gridded analyses, I examine the atmosphere-only and coupled ocean-atmosphere variability in the general circulation for the region south of 40 °S. The variability in mean sea level pressure (MSLP) is well simulated by the coupled models. A complication is that the difference between the two analyses used for verification is comparable to the analysis-model differences. An increase in variability is seen within the hierarchy of model runs although even a model without interannual variations in sea surface temperatures (SSTs) captures most of the observed variability. The temporal variation in MSLP in southern high latitudes has a white spectrum consistent with “random” forcing by weather events and a decoupling from oceanic “integration”. In contrast, the spatial pattern of MSLP variability shows large-scale structure that is consistent between observations and various models, even without interannual variation in SSTs. This shows that the models are sufficiently skillful to reproduce the pattern of observed variability and suggests that the pattern of variability is a characteristic of the land-sea distribution and topography.

title: The ERA‐40 re‐analysis
abstract: ERA-40 is a re-analysis of meteorological observations from September 1957 to August 2002 produced by the European Centre for Medium-Range Weather Forecasts (ECMWF) in collaboration with many institutions. The observing system changed considerably over this re-analysis period, with assimilable data provided by a succession of satellite-borne instruments from the 1970s onwards, supplemented by increasing numbers of observations from aircraft, ocean-buoys and other surface platforms, but with a declining number of radiosonde ascents since the late 1980s. The observations used in ERA-40 were accumulated from many sources. The first part of this paper describes the data acquisition and the principal changes in data type and coverage over the period. It also describes the data assimilation system used for ERA-40. This benefited from many of the changes introduced into operational forecasting since the mid-1990s, when the systems used for the 15-year ECMWF re-analysis (ERA-15) and the National Centers for Environmental Prediction/National Center for Atmospheric Research (NCEP/NCAR) re-analysis were implemented. Several of the improvements are discussed. General aspects of the production of the analyses are also summarized.

A number of results indicative of the overall performance of the data assimilation system, and implicitly of the observing system, are presented and discussed. The comparison of background (short-range) forecasts and analyses with observations, the consistency of the global mass budget, the magnitude of differences between analysis and background fields and the accuracy of medium-range forecasts run from the ERA-40 analyses are illustrated. Several results demonstrate the marked improvement that was made to the observing system for the southern hemisphere in the 1970s, particularly towards the end of the decade. In contrast, the synoptic quality of the analysis for the northern hemisphere is sufficient to provide forecasts that remain skilful well into the medium range for all years. Two particular problems are also examined: excessive precipitation over tropical oceans and a too strong Brewer-Dobson circulation, both of which are pronounced in later years. Several other aspects of the quality of the re-analyses revealed by monitoring and validation studies are summarized. Expectations that the ‘second-generation’ ERA-40 re-analysis would provide products that are better than those from the firstgeneration ERA-15 and NCEP/NCAR re-analyses are found to have been met in most cases.  The contributions of N. A. Rayner and R. W. Saunders are Crown copyright.

title: Examining the Tropical Pacific's Response to Global Warming
abstract: The response of the tropical Pacific to increasing greenhouse gases represents an exciting intersection of theory, modeling, and observations. In this article, we contrast competing theories for the response of the tropical Pacific to global warming, illustrate the utility of models for understanding and reconciling these theories, and highlight the need for improved instrumental and paleoclimatic reconstructions to better evaluate the fidelity of current model projections. 
 
There is a long-standing debate in the climate community as to how the tropical Pacific will respond to increased greenhouse gases: Will the structure of changes in the ocean surface temperature more closely resemble an El Nino or a La Nina [e.g., Knutson and Manabe, 1995; Clement et al., 1996; Meehl and Washington, 1996; Cane et al., 1997; Cobb et al., 2003; Collins et al., 2005; Vecchi et al., 2006; Zhang and Song, 2006]? This distinction is of profound significance because conditions in the tropical Pacific affect a range of weather phenomena including tropical cyclone activity, global patterns of drought and flood, agricultural productivity, and oceanic biological activity. The debate extends beyond global warming, with El Nino- and La Nina-like responses being invoked as frameworks for interpreting past climate changes on timescales of centuries to millions of years [e.g., Koutavas et al., 2002; Stott et al., 2002; Mann et al., 2005; Wara et al., 2005].

title: Reconstruction of Monthly SST in the Tropical Pacific Ocean during 1868–1993Using Adaptive Climate Basis Functions
abstract: An EOF-based reconstruction of monthly SST anomaly fields is calculated for the years 1868–1993 in the tropical Pacific Ocean. The EOFs are computed from optimally interpolated SST anomaly fields from November 1981 to October 1995 from the same region. These are used as a functional basis set for projecting SST anomalies from the Comprehensive Ocean–Atmosphere Data Set (COADS) to produce a smooth, spatially complete reconstruction. The optimal number of EOF modes for each month’s reconstruction is chosen using large-scale error analysis: a modern subset of the COADS SST is subsampled according to the distribution of observations in the historical month being reconstructed, forming a testbed on which to evaluate the reconstruction based on a variable number of modes. The test bed is least squares fit to the basis EOFs. The difference fields between the reconstruction and a smoothed version of the test bed is found. The total variance of difference fields (residual) is minimized with respect to the...

title: Central Pacific El Niño and decadal climate change in the North Pacific Ocean
abstract: Decadal fluctuations of the ocean and atmosphere over the North Pacific Ocean significantly affect the weather and climate of North America and Eurasia. They also cause transitions between different states of marine ecosystems across the Pacific Ocean 1‐3 . An important fraction of North Pacific low-frequency variability is linked to the North Pacific Gyre Oscillation 4 , a climate pattern associated with decadal fluctuations of the ocean circulation. Decadal variations in the North Pacific Gyre Oscillation are characterized by a pattern of sea surface temperature anomalies that resemble the central Pacific El Nino, a dominant mode of interannual variability with far-reaching effects on global climate patterns 5‐7 . Here we use an ensemble of simulations with a coupled ocean‐ atmosphere model to show that the sea surface temperature anomalies associated with central Pacific El Nino force changes in the extra-tropical atmospheric circulation. These changes in turn drive the decadal fluctuations of the North Pacific Gyre Oscillation. Given that central Pacific El Nino events could become more frequent with increasing levels of greenhouse gases in the atmosphere 8 , we infer that the North Pacific