Southern Hemisphere Atmospheric Circulation Research Articles

Enhanced upwelling in the eastern equatorial Pacific at the last five glacial terminations

TEX86H- and UK37′-derived paleotemperatures, and isoprenoid glycerol dialkyl glycerol tetraether (GDGT), and alkenone concentrations were examined for ODP Site 1239 in the eastern equatorial Pacific (EEP) for the last 430kyr. We propose that the difference between TEX86H- and UK37′-derived temperatures (∆T) and the abundance ratio of GDGTs to alkenones (GDGT/alkenone ratio) are potential upwelling indices which show consistent results with other upwelling indices. The ∆T and GDGT/alkenone ratio were maximal during the last five deglaciations, suggesting intensified upwelling. The intensification of upwelling in the EEP coincided with those at the Peru margin and in the Southern Ocean. This coincidence suggests that the reorganization of the Southern Hemisphere atmospheric circulation induced the intensification of the subtropical high-pressure cell, causing stronger southeast trade winds along the west coast of South America and the southern westerlies over the Southern Ocean, enhancing upwelling in both regions.

Sensitivity of Southern Hemisphere circulation to LGM and 4 × CO2 climates

This paper investigates the effect of Last Glacial Maximum (LGM) versus high CO2 world boundary condition on the Southern Hemisphere atmospheric circulation, in particular on the strength and latitudinal position of the near surface Southern Westerly Winds (SWW). PMIP2 and PMIP3 experiments, as well as the “abrupt 4 × CO2” simulations from CMIP5, were analyzed. Robust findings include poleward expansion of the Mean Meridional Circulation (MMC) and intensified and poleward‐shifted SWW in the 4 × CO2 simulations (consistent with recent observations and 21st century climate change projections); and for the LGM, stronger and southward shifted northern hemisphere MMC, and weakened southern Hadley cell. However, six of the eight LGM simulations show a decrease in the SWW, the other two models simulate the opposite. A critical difference between the models is strong coupling between sea‐ice extent, surface temperature gradients, SWW, and Ferrel cell in the two models with stronger and poleward‐shifted SWW.

Modes of interannual variability of Southern Hemisphere atmospheric circulation in CMIP3 models: assessment and projections

An assessment is made of the modes of interannual variability in the seasonal mean summer and winter Southern Hemisphere 500 hPa geopotential height in the twentieth century in models from the Coupled Model Intercomparison Project phase 3 (CMIP3) dataset. The analysis is done for both the intraseasonal and slow components of the geopotential height. When the CMIP3 models are assessed against reanalysis data, the spatial structure and variance of the leading modes in the intraseasonal component are generally well reproduced. There are systematic differences between the models in their reproduction of the leading modes in the slow component. An overall score using the leading modes in the slow component allows a categorisation of CMIP3 model performance. Using an ensemble from four models that suitably reproduce the twentieth century modes, modes of variability in the slow-internal and slow-external components are estimated. The leading mode of the slow-external component is shown to be related to observed changes in greenhouse gas concentrations. In this ensemble, there is little change in the leading modes in the intraseasonal component in the twenty-first century. Larger changes in variance, and subtle changes in regional-scale structure, are found for the leading modes in the slow-internal component. These are related to changes in the slowly varying dynamics of the Southern Annular Mode and the El Nino-Southern Oscillation. By far the biggest change is in the leading mode of the slow-external component. The spatial structure becomes uniform in the twenty-first century, and the variance increases with increasing greenhouse gas concentrations.

Influences of two types of ENSO on South American precipitation

AbstractSea surface temperature (SST) anomalies in the Pacific Ocean during El Niño‐Southern Oscillation (ENSO) episodes exhibit different spatial patterns from year to year. In most ENSO studies, SST anomalies have been analysed in the 3.4 or 3.0 El Niño regions. Recent analyses have considered different SST anomalies areas in the Pacific, such as ENSO Modoki, which takes into account anomalies in the central Pacific which are bounded by opposite anomalies in the eastern and western Pacific. In order to analyse the influence of different Pacific spatial patterns on South American precipitation and on the Southern Hemisphere atmospheric circulation, composites obtained from Canonical ENSO were compared with those from ENSO Modoki in cases when strong anomalies were present in the Central Pacific. During the Canonical El Niño (La Niña), there tends to be a precipitation increase (decrease) in the La Plata Basin (LPB, 45°W–65°W, 15°S–35°S) and a decrease (increase) over northern South America during all seasons. In ENSO Modoki exhibiting strong anomalies in the Central Pacific, these typical patterns are not observed, and in some regions the anomalies even show opposite signs. Precipitation anomaly differences or similarities over South America between the two cases occur in different areas and different seasons. In both cases, differences in tropical South American precipitation during both ENSO types are related to differences in the Walker circulation. In extra‐tropical South America, the precipitation differences are due to differences in the Pacific wavetrains and differences in moisture flux intensity over the continent. Copyright © 2012 Royal Meteorological Society

Seesaw does not explain Antarctic climate change

The Arctic has warmed rapidly in the past 3 decades, but temperature records show that the Antarctic on average has warmed less. This observation hints at a bipolar seesaw mechanism, in which changes in ocean circulation result in changes in heat transport between the hemispheres, linking the warming Arctic to a lack of warming in the Antarctic. However, a new analysis by Schneider and Noone shows that this mechanism cannot explain Antarctic temperature variability or trends. The authors reviewed the data set used as the basis of the bipolar seesaw hypotheses and note that for the Antarctic region the data come from only one station prior to the mid‐1940s and include only a few stations until the late 1950s. They estimated Antarctic climate variability using a variety of independent data, including the long station time series, a reconstruction of the Southern Annular Mode index (a measure of Southern Hemisphere atmospheric circulation variability), and an ice core–based temperature reconstruction.

The Southern Annular Mode Seen through Weather Regimes

This article investigates the prominent features of the Southern Hemisphere (south of 20°S) atmospheric circulation when extracted using EOF analysis and a k-means clustering algorithm. The focus is on the southern annular mode (SAM), the nature of its recent trend, and the zonal symmetry of associated spatial patterns. The study uses the NCEP–Department of Energy Atmospheric Model Intercomparison Project II Reanalysis (NCEP-2) (period 1979–2009) to obtain robust patterns over the recent years and the Twentieth Century Reanalysis Project (period 1871–2008) to document decadal changes. Also presented is a comparison of these signals against a station-based reconstruction of the SAM index and a gridded interpolated dataset [Hadley Centre Sea Level Pressure dataset version 2 (HadSLP2)]. Over their common period, both reanalyses are in fair agreement, both in terms of spatial patterns and temporal variability. In particular, both datasets show weather regimes that can be interpreted as the opposite phases of the SAM. At the decadal time scale, the study shows that the trend toward the positive SAM phase (as inferred from the usual EOF-based index) is related more to an increase in the frequency of clusters corresponding to the positive phase, with little changes in the frequency of the negative SAM events. Similarly, the long-term tropospheric warming trend already discussed in the literature is shown to be related more to a decrease in the number of abnormally cold days, with little changes in the number of abnormally warm days. The cluster analysis therefore allows for complement descriptions based on simple indexes or EOF decompositions, highlighting the nonlinear nature of the decadal changes in the Southern Hemisphere atmospheric circulation and temperature.

Tropical Pacific interactions in the WCRP-CMIP3 multi-model dataset

The ability of numerical models in reproducing the observed statistical links between the sea surface temperature of central and eastern tropical Pacific Ocean is analysed in this paper. Significant links in the whole spectrum of frequency were studied for models of the World Climate Research Program (WCRP-CMIP3/IPCC-AR4) using the methodology of wavelet coherence. During the second half of the twentieth century, in-phase or quasi-in-phase links in oscillations of 2–6 years, a lack of relation in oscillations of 8–12 years and time lagged links in the longest periodicities are detected in the observed dataset. Some of the analysed models are able to reproduce the observed relationships in oscillations shorter than 6 years and few of them also reproduce the lacks of significant relation in the band of 8–12 years. On the other hand, all models have serious problems representing the observed time lagged links in the longest waves. The analysis shows that climate models are still unable to reproduce specific features of tropical Pacific interactions with potential effect on the Southern Hemisphere atmospheric circulation and, in particular, the South American climatic variability.

Estimating components of covariance between two climate variables using model ensembles

The seasonal mean of a climate variable is considered to consist of: (a)~slow-external; (b)~slow-internal; and (c)~intraseasonal components. Using an Analysis of Variance-based method, the interannual variability of the seasonal mean from an ensemble of coupled atmosphere-ocean general circulation model realisations is separable into these components. Here, we propose a method for analysing the covariability of these components between pairs of climate variables. In particular, the method allows for an estimate of the covariability of the projected time series of the modes of variability of one climate variable with the time series of another. To illustrate this, the relationship between time series of the modes of variability of 500\,hPa geopotential height and sea surface temperature is examined for an ensemble of coupled general circulation model realisations. The method is applicable to other atmospheric climate variables and datasets. References C. S. Frederiksen and X. Zheng. Coherent structures of interannual variability of the atmospheric circulation: the role of intraseasonal variability. Frontiers in Turbulence and Coherent Structures, World Scientific Lecture Notes in Complex Systems , Vol. 6, Eds Jim Denier and Jorgen Frederiksen, World Scientific Publications, 87--120, 2007. C. E. Leith. The standard error of time-average estimates of climatic means. J. Appl. Meteor. , 12:1066--1069, 1973. 2.0.CO;2>doi:10.1175/1520-0450(1973)012 2.0.CO;2 X. Zheng and C. S. Frederiksen. Variability of seasonal-mean fields arising from intraseasonal variability. Part 1, methodology. Climate Dynamics , 23:177--191, 2004. doi:10.1007/s00382-004-0428-7 X. Zheng and C. S. Frederiksen. Validating interannual variability in an ensemble of AGCM simulations. J. Climate , 12:2386--2396, 1999. 2.0.CO;2>doi:10.1175/1520-0442(1999)012 2.0.CO;2 C. S. Frederiksen and X. Zheng. Variability of seasonal-mean fields arising from intraseasonal variability. part 2, application to nh winter circulations. Climate Dynamics , 23:193--206, 2004. doi:10.1007/s00382-004-0429-6 C. S. Frederiksen and X. Zheng. Variability of seasonal-mean fields arising from intraseasonal variability. Part 3: Application to SH winter and summer circulations. Climate Dynamics , 28:849--866, 2007. doi:10.1007/s00382-006-0214-9 S. Grainger, C. S. Frederiksen and X. Zheng. A method for evaluating the modes of variability in general circulation models. ANZIAM J. , 50:C399--C412, 2008. http://anziamj.austms.org.au/ojs/index.php/ANZIAMJ/article/view/1431 X. Zheng, M. Sugi and C. S. Frederiksen. Interannual variability and predictability in an ensemble of climate simulations with the MRI-JMA AGCM. J. Meteor. Soc. Jap. , 82:1--18, 2004. doi:10.2151/jmsj.82.1 S. Grainger, C. S. Frederiksen, X. Zheng, D. Fereday, C. K. Folland, E. K. Jin, J. L. Kinter, J. R. Knight, S. Schubert and J. Syktus. Modes of variability of Southern Hemisphere atmospheric circulation estimated by AGCMs. Climate Dynamics , 36:473--490, 2011. doi:10.1007/s00382-009-0720-7 H. von Storch and F. W. Zwiers. Statistical Analysis in Climate Research. Cambridge University Press , 484pp, 1999. G. A. Meehl, C. Covey, T. Delworth, M. Latif, B. McAvaney, J. F. B. Mitchell, R. J. Stouffer and K. E. Taylor. The WCRP CMIP3 multimodel dataset: A new era in climate change research. Bull. Amer. Meteor. Soc. , 88:1383--1394, 2007. doi:10.1175/BAMS-88-9-1383 E. Kalnay, M. Kanamitsu, R. Kistler, W. Collins, D. Deaven, L. Gandin, M. Iredell, S. Saha, G. White, J. Woollen, Y. Zhu, A. Leetmaa, R. Reynolds, M. Chelliah, W. Ebisuzaki, W. Higgins, J. Janowiak, K. C. Mo, C. Ropelewski, J. Wang, R. Jenne and D. Joseph. The NCEP/NCAR 40-year reanalysis project. Bull. Amer. Meteor. Soc. , 77:437--471, 1996. 2.0.CO;2>doi:10.1175/1520-0477(1996)077 2.0.CO;2 N. A. Rayner, D. E. Parker, E. B. Horton, C. K. Folland, L. V. Alexander, D. P. Rowell, E. C. Kent and A. Kaplan. Global analyses of sea surface temperature, sea ice, and night marine air temperature since the late nineteenth century. J. Geophys. Res. , 108(D14):4407, 2003. doi:10.1029/2002JD002670 J. M. Arblaster and G. A. Meehl. Contributions of external forcings to Southern Annular Mode trends. J. Climate , 19:2896--2905, 2006. doi:10.1175/JCLI3774.1

Large cancellation, due to ozone recovery, of future Southern Hemisphere atmospheric circulation trends

Received 7 January 2011; accepted 26 January 2011; published 25 February 2011. [1] The role of stratospheric ozone recovery in the Southern Hemisphere climate system, in the coming decades, is examined by contrasting two 10‐member ensembles of Community Atmospheric Model (CAM3) integrations, over the period 2000–2060. Model integrations in the first ensemble are conducted with a complete set of forcings: greenhouse gas concentrations from the A1B scenario, SSTs from corresponding ocean‐atmosphere coupled model integrations, and ozone starting with severe depletion over the South Pole and recovering by mid‐century. The integrations in the second ensemble are very similar to the first, except that only the transient ozone forcing is specified, and all other forcings are kept at year 2000 levels. Specifying ozone recovery in isolation allows us to determine unambiguously how it impacts the atmospheric circulation. We find that, in DJF, most key indices of atmospheric circulation show significant trends in the second ensemble, due to the closing of the ozone hole. In the first ensemble, however, trends are found to be statistically insignificant for nearly all key circulation indices. This suggests that ozone recovery will result in a nearly complete cancellation (and possible reversal) of the atmospheric circulation effects associated with increasing greenhouse gases, in Southern Hemisphere summer, over the coming half century. Citation: Polvani, L. M., M. Previdi, and C.Deser(2011),Largecancellation,duetoozonerecovery,offuture Southern Hemisphere atmospheric circulation trends, Geophys. Res. Lett., 38, L04707, doi:10.1029/2011GL046712.

Sensitivity of South American summer rainfall to tropical Pacific Ocean SST anomalies

[1] A suite of idealised ensemble experiments are used to investigate the sensitivity of the Southern Hemisphere atmospheric circulation to the location of SST anomalies over the tropical Pacific Ocean. Of particular interest is the response of South American rainfall during austral summer. The experiments reveal an approximately opposite response in rainfall over South America when the tropical SST forcing is applied over the western and eastern Pacific Ocean, respectively. The contrasting tropical rainfall conditions are due primarily to the displaced Walker circulation anomalies. Further south, the atmospheric circulation over the South American subtropics is modulated by teleconnection patterns that appear as a wave train. The resulting circulation manifests as an anomalous cyclone over central-eastern South America which in turn leads to a northward displacement of the westerly moisture transport when the SST forcing is located further west. The opposite pattern occurs when the SST forcing is applied to the eastern equatorial Pacific.

Spatial fields of Antarctic sea-ice concentration anomalies for summer–autumn and their relationship to Southern Hemisphere atmospheric circulation during the period 1979–2009

AbstractSummer–autumn monthly sea-ice concentration anomaly (SICA) fields in Antarctica obtained from satellite data for the period 1979–2009 were analysed with Varimax-rotated T-mode principal component analysis (PCA). the first three PCA scores described the SICA spatial behaviour and explained 38.07% of the total variance. the related atmospheric circulation characteristics were analysed using 850 hPa height and surface air-temperature anomalies for the months clustered by the corresponding SICA composites, which were based on PCA loadings above a ±0.3 threshold. the principal characteristics of SICA can be seen between the Ross and Weddell Seas, areas that remained ice-covered during the analysis period. Elsewhere around Antarctica, small distinct characteristics occur mostly in embayments. the leading summer–autumn SICA pattern shows a structure with two centres of equal sign located one over the Weddell and the other over the Ross Sea–southwest Pacific Ocean sector and a centre of opposite sign over the Bellingshausen and Amundsen Seas. the second SICA pattern is represented by a dipole over the Weddell Sea as a result of an increase (decrease) in sea-ice concentration in the northern sector (positive phase) and a decrease (increase) in the southern region, together with a positive (negative) centre over the Ross and Amundsen Seas. the latter pattern is characterized by equal-sign anomalies on both sides of the Antarctic Peninsula and opposite-sign centres all around Antarctica with the highest intensity over the Ross Sea.

Antarctic lakes suggest millennial reorganizations of Southern Hemisphere atmospheric and oceanic circulation

The phasing of millennial-scale oscillations in Antarctica relative to those elsewhere in the world is important for discriminating among models for abrupt climate change, particularly those involving the Southern Ocean. However, records of millennial-scale variability from Antarctica dating to the last glacial maximum are rare and rely heavily on data from widely spaced ice cores, some of which show little variability through that time. Here, we present new data from closed-basin lakes in the Dry Valleys region of East Antarctica that show high-magnitude, high-frequency oscillations in surface level during the late Pleistocene synchronous with climate fluctuations elsewhere in the Southern Hemisphere. These data suggest a coherent Southern Hemisphere pattern of climate change on millennial time scales, at least in the Pacific sector, and indicate that any hypothesis concerning the origin of these events must account for synchronous changes in both high and temperate latitudes.

Southern Hemisphere atmospheric circulation response to the El Chichón and Pinatubo eruptions in coupled climate models

We study the response of the Southern Hemisphere circulation to the 1982 eruption of El Chichon and 1991 eruption of Pinatubo volcanoes in a suite of up-to-date coupled climate models. We find a significant response in austral spring and autumn in the years following the eruptions, which consists of a stronger stratospheric polar vortex and lowered sea-level pressure over the Antarctic, both consistent with the positive phase of the Southern Annular Mode. The seasonality of the response may be explained in terms of zonal flow-planetary wave interactions. This dynamical response is inconsistent with the observational reanalyses in the polar stratosphere in spring, but not in the troposphere where the internal variability is large compared to the magnitude of the response.

Interannual modes of variability of Southern Hemisphere atmospheric circulation in CMIP3 models

The atmospheric circulation acts as a bridge between large-scale sources of climate variability, and climate variability on regional scales. Here a statistical method is applied to monthly mean Southern Hemisphere 500hPa geopotential height to separate the interannual variability of the seasonal mean into intraseasonal and slowly varying (time scales of a season or longer) components. Intraseasonal and slow modes of variability are estimated from realisations of models from the Coupled Model Intercomparison Project Phase 3 (CMIP3) twentieth century coupled climate simulation (20c3m) and are evaluated against those estimated from reanalysis data. The intraseasonal modes of variability are generally well reproduced across all CMIP3 20c3m models for both Southern Hemisphere summer and winter. The slow modes are in general less well reproduced than the intraseasonal modes, and there are larger differences between realisations than for the intraseasonal modes. New diagnostics are proposed to evaluate model variability. It is found that differences between realisations from each model are generally less than inter-model differences. Differences between model-mean diagnostics are found. The results obtained are applicable to assessing the reliability of changes in atmospheric circulation variability in CMIP3 models and for their suitability for further studies of regional climate variability.

Modes of variability of Southern Hemisphere atmospheric circulation estimated by AGCMs

The seasonal mean variability of the atmospheric circulation is affected by processes with time scales from less than seasonal to interannual or longer. Using monthly mean data from an ensemble of Atmospheric General Circulation Model (AGCM) realisations, the interannual variability of the seasonal mean is separated into intraseasonal, and slowly varying components. For the first time, using a recently developed method, the slowly varying component in multiple AGCM ensembles is further separated into internal and externally forced components. This is done for Southern Hemisphere 500 hPa geopotential height from five AGCMs in the CLIVAR International Climate of the Twentieth Century project for the summer and winter seasons. In both seasons, the intraseasonal and slow modes of variability are qualitatively well reproduced by the models when compared with reanalysis data, with a relative metric finding little overall difference between the models. The Southern Annular Mode (SAM) is by far the dominant mode of slowly varying internal atmospheric variability. Two slow-external modes of variability are related to El Nino-Southern Oscillation (ENSO) variability, and a third is the atmospheric response to trends in external forcing. An ENSO-SAM relationship is found in the model slow modes of variability, similar to that found by earlier studies using reanalysis data. There is a greater spread in the representation of model slow-external modes in winter than summer, particularly in the atmospheric response to external forcing trends. This may be attributable to weaker external forcing constraints on SH atmospheric circulation in winter.

Influence of boundary conditions on the Southern Hemisphere atmospheric circulation during the last glacial maximum

Based upon coupled climate simulations driven by present day and glacial boundary conditions, we demonstrate that although the ice sheet topography modifications during the glacial period are primarily placed in the Northern Hemisphere (NH), a climate simulation that employs the ICE-5G glacial topography delivers significantly enhanced climate anomalies in the Southern Hemisphere (SH) as well. These conditions, in association with climate anomalies produced by the modification of the atmospheric CO² concentration characteristic of the Last Glacial Maximum (LGM) interval, are shown to be the primary forcing of the SH climate during this epoch. Climate anomalies up to -6°C over the Antarctic region and -4°C over South America are predicted to occur in respect to present day conditions. Accompanying the SH cooling in the LGM simulation there exists a remarkable reduction in the specific humidity, which in turn enforces the overall Southern Hemisphere cooling due to the weaker greenhouse capacity of the dry atmosphere.

Impacts of increasing anthropogenic aerosols on the atmospheric circulation trends of the Southern Hemisphere: An air‐sea positive feedback

A recent model study shows that in response to an increasing aerosol forcing, the Southern Hemisphere (SH) ocean circulation, including the Agulhas outflow, its retroflection, and the entire subtropical gyre circulation intensify and shift polewards. Are these oceanic responses manifested in the SH atmospheric circulation? We demonstrate that as a result of the poleward shift, maximum sea surface temperature (SST) gradients, midlatitude storms and the westerly jet shift southward, intensifying the trend of the southern annular mode (SAM). Because the response of the atmospheric circulation to the underlying oceanic anomalies is equivalent barotropic, a poleward shift and intensification in zonal wind and vertical velocities are generated from the atmosphere‐ocean interface to the middle troposphere. These atmospheric circulation responses, in turn, reinforce the ocean circulation changes. This constitutes an air‐sea positive feedback. Our results illustrate an impact of Northern Hemisphere (NH) aerosols on the SH atmospheric circulation trends through a SH ocean circulation response.

Southern Hemisphere atmospheric circulation effects of the 1991 Mount Pinatubo eruption

Global average cooling and Northern Hemisphere winter warming are well‐known climatic responses to the June 15, 1991 eruption of the Mount Pinatubo volcano in the Philippines. Here we investigate the Southern Hemisphere response. Using National Centers for Environmental Prediction/National Center for Atmospheric Research Reanalysis, European Centre for Medium‐Range Weather Forecasting Reanalysis, and simulations with the National Aeronautics and Space Administration Goddard Institute for Space Studies ModelE climate model, we find that, in contrast to the Northern Hemisphere, there were no strong significant anomalies in atmospheric circulation in the Southern Hemisphere. We examined 50 mb and 500 mb circulation patterns, as well as the Southern Hemisphere Annular Mode index, and found no consistent significant anomalies associated with the volcanic eruption, or the previous large volcanic eruptions of the past 50 years, the 1963 Agung and 1982 El Chichón eruptions. The few anomalies that occurred after Pinatubo are consistent with patterns found during an El Niño event, which took place that same year.

Teleconnections between Andean and New Zealand glaciers

Retreat and advance of glaciers in the Southern Alps of New Zealand have occurred over two distinct 20-yr climate periods (1954–1974) and (1974–1994). Changes in tropical and southern Andean glaciers are compared over these same periods. Behaviour of glaciers in the tropical Andes are out of phase with the Southern Alps glaciers, but some glaciers in Patagonia appear to be in phase. Southern Hemisphere atmospheric circulation using 700 hPa geopotential height anomalies and sea surface temperature patterns are examined for these periods. Glacier response on inter-decadal timescales is linked with distinctive shifts in atmospheric circulation patterns around the Southern Hemisphere. Retreat (advance) of glaciers in the Southern Alps and southern Andean glacier and advance (retreat) of glaciers in the tropical Andes are all associated with weaker (stronger) westerlies, blocking events in the South-east Pacific, negative (positive) geopotential height anomalies over Southern Africa and higher latitudes of the Southern Hemisphere. These glacier changes are also linked with the negative (positive) phase of the Inter-decadal Pacific Oscillation, a higher frequency of La Niña (El Niño) events, and warm (cool) sea surface temperatures in the New Zealand region and cool (warm) sea surface temperatures in the equatorial eastern region of the Pacific Ocean off the coast of Peru.

Atmospheric Circulation in Southern Hemisphere and Summer Rainfall over Yangtze River Valley

The relationship between Southern Hemisphere (SH) atmospheric circulation and the summer rainfall over the Yangtze River Valley using the composition analysis is discussed in this paper. The results show Antarctic Oscillation (AAO) in boreal spring (MAM) is much remarkable than that in boreal summer (JJA). The SH Subtropical Highs have good seasonal persistence from MAM to JJA. Therefore, SH circulation anomaly in MAM is one of important short-term climate predictors for summer rainfall over Yangtze River Valley. The inter-hemispheric high latitudes interaction impacts on summer rainfall over Yangtze River Valley. One possible physical mechanism is barotropical wave train distributed from high latitude in SH to coast of the East Asia. Second possible physical mechanism is the inter-hemispheric temperature gradient decrease due to SH temperature increase in the whole troposphere from MAM to JJA, consequently, causing the weak summer Asian monsoon and more summer rainfall over Yangtze River Valley.