Southern Hemisphere Atmospheric Circulation Research Articles

Ocean Model Diagnosis of Interannual Coevolving SST Variability in the South Indian and South Atlantic Oceans

Abstract A global ocean model (ORCA2) forced with 50 yr of NCEP–NCAR reanalysis winds and heat fluxes has been used to investigate the evolution and forcing of interannual dipolelike sea surface temperature (SST) variability in the South Indian and South Atlantic Oceans. Although such patterns may also exist at times in only one of these basins and not the other, only events where there are coherent signals in both basins during the austral summer have been chosen for study in this paper. A positive (negative) event occurs when there is a significant warm (cool) SST anomaly evident in the southwest of both the South Indian and South Atlantic Oceans and a cool (warm) anomaly in the eastern subtropics. The large-scale forcing of these events appears to consist of a coherent modulation of the wavenumber-3 or -4 pattern in the Southern Hemisphere atmospheric circulation such that the semipermanent subtropical anticyclone in each basin is shifted from its summer mean position and its strength is modulated. A relationship to the Antarctic Oscillation is also apparent, and seems to strengthen after the mid-1970s. The modulated subtropical anticyclones lead to changes in the tropical easterlies and midlatitude westerlies in the South Atlantic and South Indian Oceans that result in anomalies in latent heat fluxes, upwelling, and Ekman heat transports, all of which contribute to the SST variability. In addition, there are significant modulations to the strong Rossby wave signals in the South Indian Ocean. The results of this study confirm the ability of the ORCA2 model to represent these dipole patterns and indicate connections between large-scale modulations of the Southern Hemisphere midlatitude atmospheric circulation and coevolving SST variability in the South Atlantic and South Indian Oceans.

The Response of the Southern Hemisphere Atmospheric Circulation to an Enhanced Greenhouse Gas Forcing

Abstract The response of the atmospheric circulation to an enhanced radiative greenhouse gas forcing is investigated. It has been proposed that the response of the climate system to an enhanced forcing projects directly onto the preexisting natural modes of variability. An evaluation of this possibility and in particular of the implications of unchanged flow regimes is performed with a focus on the Southern Hemisphere extratropical atmospheric circulation. Low-pass-filtered mean sea level pressure and geopotential height at 500 and 200 hPa from a transient integration with a coupled global climate model is used. The response to an enhanced forcing projects strongly onto the leading modes of present-day variability, in agreement with other studies. However, the spatial patterns of the leading modes are changed in response to enhanced forcing. The first and second modes of interweekly variability are the Pacific–South American modes, zonal wavenumber-3 wave trains from the central Pacific to the southern Atlantic. In response to the enhanced forcing, the spatial patterns of these modes change, and the wave train extends along a circumpolar path with amplitude also in the Eastern Hemisphere. This change in the spatial patterns is associated with a strengthening of the waveguide for barotropic Rossby waves.

An interhemispheric comparison of the geomagnetic activity signature in the lower atmosphere

The geomagnetic activity signature in the southern hemisphere atmospheric circulation is examined and the results are compared to those of Palamara and Bryant (2004) and other researchers for the northern hemisphere. Sliding correlations are used with monthly data to identify temporal and seasonal aspects of a potential relationship between solar modulated geomagnetic activity and the southern annular mode, while the geomagnetic activity signature in both hemispheres is compared in zonal-mean zonal wind data. The results reveal that apart from a suggestive, but inconclusive, relationship between the geomagnetic AA index and an index of the southern annular mode in March, there is no evidence for a geomagnetic activity signature in the southern hemisphere troposphere.

Variability of seasonal-mean fields arising from intraseasonal variability. part 2, application to nh winter circulations

A study has been made, using the National Centers for Environmental Prediction and National Center for Atmospheric Research re-analysis 500 hPa geopotential height data, to determine how intraseasonal variability influences, or can generate, coherent patterns of interannual variability in the extratropical summer and winter Southern Hemisphere atmospheric circulation. In addition, by separating this intraseasonal component of interannual variability, we also consider how slowly varying external forcings and slowly varying (interannual and longer) internal dynamics might influence the interannual variability of the Southern Hemisphere circulation. This slow component of interannual variation is more likely to be potentially predictable. How sea surface temperatures are related to the slow components is also considered. The four dominant intraseasonal modes of interannual variability have horizontal structures similar to those seen in both well-known intraseasonal dynamical modes and statistical modes of intraseasonal variability. In particular, they reflect intraseasonal variability in the high latitudes associated with the Southern Annular Mode, and wavenumber 4 (summer) and wavenumber 3 (winter) patterns associated with south Pacific regions of persistent anomalies and blocking, and possibly variability related to the Madden-Julian Oscillation (MJO). The four dominant slow components of interannual variability, in both seasons, are related to high latitude variability associated with the Southern Annular Mode, El Nino Southern Oscillation (ENSO) variability, and South Pacific Wave variability associated with Indian Ocean SSTs. In both seasons, there are strong linear trends in the first slow mode of high latitude variability and these are shown to be related to similar trends in the Indian Ocean. Once these are taken into account there is no significant sea surface temperature forcing of these high latitude modes. The second and third ENSO related slow modes, in each season, have high correlations with tropical sea surface temperature variability in the Pacific and Indian Oceans, both contemporaneously and at one season lag. The fourth slow mode has a characteristic South Pacific wave structure of either a wavenumber 4 (summer) or wavenumber 3 (winter) pattern, with strongest loadings in the South Pacific sector, and an association simultaneously with a dipole SST temperature gradient in the subtropical Indian Ocean.

Recent Climate Variability in Antarctica from Satellite-Derived Temperature Data

Recent Antarctic climate variability on month-to-month to interannual time scales is assessed through joint analysis of surface temperatures from satellite thermal infrared observations (TIR) and passive microwave brightness temperatures (TB). Although TIR data are limited to clear-sky conditions and TB data are a product of the temperature and emissivity of the upper ∼1 m of snow, the two datasets share significant covariance. This covariance is largely explained by three empirical modes, which illustrate the spatial and temporal variability of Antarctic surface temperatures. The TB variations are damped compared to TIR variations, as determined by the period of the temperature forcing and the microwave emission depth; however, microwave emissivity does not vary significantly in time. Comparison of the temperature modes with Southern Hemisphere (SH) 500-hPa geopotential height anomalies demonstrates that Antarctic temperature anomalies are predominantly controlled by the principal patterns of SH atmospheric circulation. The leading surface temperature mode strongly correlates with the Southern Annular Mode (SAM) in geopotential height. The second temperature mode reflects the combined influences of the zonal wavenumber-3 and Pacific–South American (PSA) patterns in 500-hPa height on month-to-month time scales. ENSO variability projects onto this mode on interannual time scales, but is not by itself a good predictor of Antarctic temperature anomalies. The third temperature mode explains winter warming trends, which may be caused by blocking events, over a large region of the East Antarctic plateau. These results help to place recent climate changes in the context of Antarctica's background climate variability and will aid in the interpretation of ice core paleoclimate records.

Long-term cosmogenic 3He production rates from 40Ar/39Ar and K–Ar dated Patagonian lava flows at 47°S

Exceptionally well-preserved basaltic lava flows on the cool, arid, eastern side of the southern Andes near Lago Buenos Aires (LBA) at Lat. 47°S provide ideal sites for calibration of in situ produced cosmogenic 3He. We report new 3He measurements and independent age determinations using unspiked K–Ar and 40Ar/39Ar incremental heating measurements from two basalt flows. Both age techniques give concordant ages, with analytical uncertainties of <5% demonstrating their utility for dating surfaces older than ∼35 ka (the limit of 14C dating). The weighted mean age of the Cerro Volcán flow is 109±3 ka and of the Rio Pinturas flow is 68±3 ka. Scaled to sea level and high latitude [D. Lal, Earth Planet. Sci. Lett. 104 (1991) 424–439] and standard atmospheric pressure [J.O. Stone, J. Geophys. Res. 105 (2000) 23 753–23 759], 3He production rates on the Cerro Volcán flow are 130±4 atoms g−1 a−1 in clinopyroxene and 139±4 atoms g−1 a−1 in olivine (±2σ), consistent with a small compositional dependence of the production rate. The 3He production rate in olivine from the younger Rio Pinturas flow is 129±6 atoms g−1 a−1, which is indistinguishable from that obtained on Cerro Volcán. The 3He production rate is at least 11% higher than previous determinations from low-mid latitudes in the Northern Hemisphere. Corrections for magnetic field strength variability lower the scaled LBA production rates by only ∼1%, and do not account for the discrepancy. We infer that the enhanced production rates are caused by lower atmospheric pressure in Patagonia during glacial periods. The Southern Westerlies, which are associated with the steep circumpolar atmospheric pressure gradient, are presently centered on Lat. 50°S, but based on independent marine and terrestrial evidence are thought to have migrated north at least 5° during glacial periods. The inferred paleobarometry provides independent evidence for northward shifts in the patterns of Southern Hemisphere atmospheric circulation.

Temperature variation and its driving forces over the Antarctic coastal regions in the past 250 years

By comparing the oxygen isotopic temperatures recorded by many shallow ice cores from the coastal regions of Antarctica, this paper presents the special characteristics of the temperature variations over the Antarctic coastal regions in the past 50 years, 150 years and 250 years. In the past 50 years, the isotopic temperatures recorded in the ice cores over different sites on the Antarctic coastal regions differ greatly. For instance, although increasing isotopic temperatures have been reported for many sites studied, many sites show decreasing trends, the regional regularity in temperature variations is still insignificant. In the past 150 years, the isotopic temperature trends in the coastal regions of Antarctica show an alternate-distributing pattern. In the past 250 years, all the ice cores from the coastal regions of Antarctica have recorded the so-called Little Ice Age (LIA). The above-mentioned spatial characteristics of the temperature variations over the Antarctic coastal regions are likely to reflect the impacts of the unique Southern Hemisphere atmospheric circulation, the Antarctic Circumpolar Wave (ACW) and the special terrain (such as the large drainage basins) on the coastal regions of Antarctica. Furthermore, the impacting intensity of the unique Southern Hemisphere atmospheric circulation, the Antarctic Circumpolar Wave and the special terrain differs in terms of the temporal scale of the temperature change.

Interannual variation in end‐of‐summer snowlines of the Southern Alps of New Zealand, and relationships with Southern Hemisphere atmospheric circulation and sea surface temperature patterns

AbstractThe end‐of‐summer snowline (EOSS) on 47 glaciers distributed throughout the Southern Alps of New Zealand is related to changes in Southern Hemisphere atmospheric circulation and sea surface temperature patterns over a 23 year period. The EOSS provides an index of the glacier mass balance, as high (low) EOSS elevations relative to the steady‐state mean equilibrium line altitude (ELAm) indicate a negative (positive) glacier mass balance. Southern Hemisphere atmospheric circulation and sea surface temperature anomaly maps are produced for the accumulation season (April–October) and ablation season (November–March) for both composite high and low EOSS years.There is a high correlation between the EOSS for individual glaciers and the mean EOSS for the Southern Alps (EOSSAlps). The highest EOSSAlps occurred in 1978, 1990, 1998, and 1999; the lowest EOSSAlps occurred in 1983, 1992, 1993, and 1995. High (low) EOSSAlps values are associated with anomalous northerly (southerly) airflows and weaker (stronger) westerly airflows over the Southern Alps. These patterns are associated with positive (negative) 700 hPa geopotential anomalies to the southeast of New Zealand, a weaker (stronger) subtropical jet, and negative (positive) 700 hPa height anomalies over the southeast Pacific Ocean. High (low) EOSSAlps values are also associated with warm (cool) sea surface temperature anomalies near New Zealand and cool (warm) sea surface temperature anomalies in the eastern equatorial region of the Pacific Ocean. Copyright © 2002 Royal Meteorological Society

Decreasing trend of temperature in Princess Elizabeth Land, Antarctica in the past 150 years

A 50-m firn core drilled in Princess Elizabeth Land, Antarctica, during the 1996/1997 Chinese First Antarctic Inland Expedition, has been measured for δ18O and major ions. Based on the high quality of the seasonal variations of major ions, the firn core was dated with errors within ±3 years. The features of the temperature change in the past 150 years in the investigated region have first been studied based on the oxygen isotope in the upper 32.93 m of the firn core. Results show that the temperature decreased nearly by 2°C in Princess Elizabeth Land in the past 150 years. On the background of the global, especially the Southern Hemispheric warming in the past 150 years, a temperature decline of 2°C in Princess Elizabeth Land likely reflects the impacts of the unique Southern Hemisphere atmospheric circulation, the Antarctic Circumpolar Wave (ACW) and the special terrain (such as the large drainage basins) on the coastal regions of Antarctica.

Southern Hemisphere Atmospheric Circulation Response to Global Warming

Abstract The response of the Southern Hemisphere (SH), extratropical, atmospheric general circulation to transient, anthropogenic, greenhouse warming is investigated in a coupled climate model. The extratropical circulation response consists of a SH summer half-year poleward shift of the westerly jet and a year-round positive wind anomaly in the stratosphere and the tropical upper troposphere. Along with the poleward shift of the jet, there is a poleward shift of several related fields, including the belt of eddy momentum-flux convergence and the mean meridional overturning in the atmosphere and in the ocean. The tropospheric wind response projects strongly onto the model’s “Southern Annular Mode” (also known as the “Antarctic oscillation”), which is the leading pattern of variability of the extratropical zonal winds.

Teleconnection patterns orographically induced in model results and from observational data in the austral winter of the Southern Hemisphere

Low frequency disturbances in the Southern Hemisphere are investigated through an observational study and numerical model experiments. The orography of the Andes and Antarctica in the Southern Hemisphere are the forcing of numerical experiments, submitted to zonal climatological austral winter conditions. The large-scale pattern forced by the Andes orography shows a high level wavetrain from the southeast of the mountain to the South Atlantic and two jet streams at high levels. The Antarctic case presents a wavenumber three horizontal structure around the hemisphere and three regions of jet stream. Characteristics of this wavenumber three model results are discussed based on analysis of streamfunction, vorticity and temperature, and related to blocking regions in the Southern Hemisphere. Empirical orthogonal function analysis of the low frequency filtered streamfunction at 250 hPa also shows, for the austral winter season, a wavenumber three structure and a wavetrain over the South Pacific and the South Atlantic, similar to the Pacific South American Pattern (PSA). Parts of the low frequency teleconnection patterns are identified in both the observational data and the model results, supporting the influence of the orography on the Southern Hemisphere atmospheric circulation. Analyses of periods with extreme amplitudes of the eigenvectors also show the influence of tropical convection on the anomalies over South America. Copyright © 2000 Royal Meteorological Society

Interdecadal Variation of the Southern Hemisphere Circulation

Abstract Previous studies examining the seasonal contrast and spatial structure of the interdecadal variation in the Southern Hemisphere (SH) atmospheric circulation and the relationship between this interdecadal variation and cyclone activity have been extended using the SH data generated by the Australian Bureau of Meteorology for 1972–92. The major findings of this study are the following: In addition to the deepening of the circumpolar trough and the increase of midlatitude sea level pressure (SLP) in winter, a zonal wavenumber 3 pattern appears in the high middle latitudes. During summer, the interdecadal change is dominated by three latitudinal zones of SLP anomalies with negative values in low and high latitudes and positive values in midlatitudes. A zonal wavenumber 3 pattern that develops during the winter persists into early spring, while the three latitudinal SLP anomaly zones that emerge in late spring and early summer continue on through the summer. An enhancement of cyclone activity is assoc...

Opposing Southern Ocean climate patterns as revealed by trends in regional sea ice coverage

The 16.8 year sea ice record (November 1978 to August 1995) derived from satellite passive microwave data shows evidence of contrasting climate patterns in the Southern Ocean as indicated by persistent opposing trends in regional sea ice coverage. Southern Ocean regions adjoining the south Atlantic, south Indian and southwest Pacific Oceans show increasing trends in sea ice coverage, particularly during non-winter months, while regions adjoining the southwest Pacific Ocean show decreasing trends in sea ice coverage, particularly during summer months. The data are compiled from three successive passive microwave sensors from which two separate time-series are analyzed. The first includes the data originally released by the National Snow and Ice Data Center (NSIDC) which have not been significantly adjusted to account for differences in the successive sensors, while the second includes data recently released by NSIDC which have been rigorously adjusted (Cavalieri et al., 1997) to account for differences between sensors. Although the significance of many of the increasing trends detected in the original time-series decrease in the reanalyzed time-series, the overall pattern of contrasting trends remains evident. These trends have important implications for the southern hemisphere heat budget and surface albedo as well as for marine ecosystems associated with various sea ice habitats. Other evidence of contrasting climate patterns with respect to southern hemisphere atmospheric circulation is explored. Due to the relatively short sea ice record, it still remains to be seen whether these trends are natural decadel variation or indicative of global climate change. However, the persistent opposition in Southern Ocean regional ice coverage is noteworthy and may well be studied using global circulation models in order to better define potential positive and negative feedbacks for global change scenarios.

Paleocene/Eocene boundary changes in atmospheric and oceanic circulation: A Southern Hemisphere record

Deep Sea Drilling Project (DSDP) Site 215 provides an expanded section across the Paleocene/Eocene boundary, the most complete mid-latitude sequence from a Southern Hemisphere location in the Indo-Pacific area. The events of this transition occurred during a span of about 1.2 m.y. Oxygen isotope values derived from benthic foraminiferal calcite decrease by about 1.0{per thousand}, a decrease most likely related to warming of deep ocean waters. Turnovers of benthic foraminifera accompany {delta}{sup 18}O changes and culminate in the predominant extinction event at the end of the Paleocene Epoch. Carbon isotope ratios also shift dramatically toward lighter values near the end of the Paleocene, beginning about 0.45 m.y. after oxygen isotope values start to change. The intensity of Southern Hemisphere atmospheric circulation as recorded by grain sizes of eolian particles shows a large and rapid reduction beginning another 0.45 m.y. later. A significant reduction of zonal wind strength at the Paleocene/Eocene boundary, until now observed only at Northern Hemisphere locations, appears to have been a global phenomenon related to decreased latitudinal thermal gradients occasioned by more effective poleward heat transport via the deep ocean.

Intraseasonal Variations in the Southern Hemisphere Circulation

Abstract Intraseasonal variations in the Southern Hemisphere atmospheric circulation have been examined through the application of a 10–50–day bandpass filter to daily ECMWF analyses for 1980–88. Variations on this time scale contribute more than 40% of the daily variance in 500 hPa geopotential over much of the middle and high latitudes of the Southern Hemisphere. EOF analysis of the unnormalized variance for all seasons shows that 49% of this variance can be explained by zonal wave trains centered on the South Pacific and southern Atlantic/Indian oceans, a high-latitude mode of global extent, and a wavenumber 3 pattern at midlatitudes. These modes are essentially equivalent barotropic but slope westward with height so that the patterns at 100 hPa typically lag those at 1000 hPa by around 1o° longitude. There appears to be little interaction with the low-latitude circulation. All of the leading modes propagate eastward but the most consistent movement is shown by the South Pacific wave train represented ...

Spatial Variability of Antarctic Temperature Anomalies and Their Association With the Southern Hemisphere Atmospheric Circulation

The spatial variability of seasonal mean temperature departures at 26 stations around Antarctica, southern South America, and nearby islands is shown using factor analysis. An opposition in temperature anomalies between mainland stations and those on or near the Ant- arctic Peninsula is a recurring pattern of spatial variability in all seasons but spring. Other factors indicate that temperature anomalies alternate in sign around the continent and especially near the peninsula. The association between the spatial patterns of temperature variability and fea- tures of the atmospheric circulation at middle and higher latitudes of the Southern Hemisphere is also examined. In each season the time series of one factor is significantly associated with temporal variations in the strength of the 500 mb westerlies, measured using height differences across six pairs of middle-latitude and Antarctic stations. In winter and summer the westerlies are linked to the mainlandiPeninsula temperature opposition pattern such that when zonal flow is anomalously strong, mainland stations are anomalously cold. In autumn and spring, temper- ature variability is highly related to meridional flow strength over New Zealand and in the lee of the Andes, and is associated with interannual longitudinal shifts in the positions of the climatological lows near the Antarctic coast. A deep Andes trough is associated with longer- than-usual ice duration at the South Orkneys. Historically, strong troughing and heavy sea-ice conditions in the South Atlantic occurred between 1920 and 1935.

A quasi-biennial standing wave in the Southern Hemisphere and interrelations with sea surface temperature

An empirical orthogonal function analysis is performed on monthly mean sea level pressure (SLP) deviations from normal (anomalies) in the Australasian area and monthly mean sea surface temperature (SST) anomalies in the Tasman Sea for 14 years of data. Only the five components of each that explain most of the variance appear to be significant. Spectral and cross-spectral analysis of the time series of the coefficients reveals significant relations at a quasi-biennial periodicity. An empirical model of the SLP and SST fields for this period is constructed and the amplitudes of the oscillation are found to be about 2 1/2 mb in the monthly mean SLP anomaly field and 0·4 degC in the monthly mean SST anomaly field. Physical and statistical indications are that the SST field is largely caused by the SLP field but some interdependence is present. The quasi-biennial SLP oscillation is related to Pittock's L-index and appears to correspond to a change in wavenumber 3 in the Southern Hemisphere atmospheric circulation. A phase change occurred at the end of 1970, but in even years prior to 1971, and in odd years since 1970, there were increased meridional transports throughout the troposphere and lower stratosphere by the large-scale eddies. A suggestion is made that changes in land–sea heating contrasts may be an important part of the mechanism.

A quasi‐biennial standing wave in the Southern Hemisphere and interrelations with sea surface temperature

AbstractAn empirical orthogonal function analysis is performed on monthly mean sea level pressure (SLP) deviations from normal (anomalies) in the Australasian area and monthly mean sea surface temperature (SST) anomalies in the Tasman Sea for 14 years of data.Only the five components of each that explain most of the variance appear to be significant. Spectral and cross‐spectral analysis of the time series of the coefficients reveals significant relations at a quasi‐biennial periodicity. An empirical model of the SLP and SST fields for this period is constructed and the amplitudes of the oscillation are found to be about 2 1/2 mb in the monthly mean SLP anomaly field and 0·4 degC in the monthly mean SST anomaly field. Physical and statistical indications are that the SST field is largely caused by the SLP field but some interdependence is present.The quasi‐biennial SLP oscillation is related to Pittock's L‐index and appears to correspond to a change in wavenumber 3 in the Southern Hemisphere atmospheric circulation. A phase change occurred at the end of 1970, but in even years prior to 1971, and in odd years since 1970, there were increased meridional transports throughout the troposphere and lower stratosphere by the large‐scale eddies. A suggestion is made that changes in land–sea heating contrasts may be an important part of the mechanism.