Latitudinal variation of air sea fluxes in the western Indian ocean during austral summer and fall

Daily and zonal (latitudinal belt) averages of heat and momentum fluxes were computed using bulk aerodynamic formulae, from the meteorological parameters measured onboard M. S. Thuleland during the sixth Indian scientific expedition to Antarctica (26th November, 1986 to 22nd March, 1987). Both estimates showed significant variations, the momentum flux showing the largest variation. The maximum values of sensible and latent heat fluxes were observed over the 30°–40° S and 10°–20° S zones during the southern summer and fall respectively while the minimum values of latent heat flux were observed in the 60°–70° S zone for both seasons. The sensible heat flux minimum was observed in the 50°°60° S and 60°–70° S zones for summer and fall, respectively. Higher momentum flux values over the 40°–50° S zone in summer shifted to the 50°–60° S zone during fall.     

The latitudinal distribution of carbon monoxide across the pacific from California to Antarctica

The results of a research study of the carbon monoxide concentration from California to 90° S, Antarctica are presented. The data both extend and support other research studies of the latitudinal distribution of carbon monoxide in that higher concentrations are evident over the Northern Hemisphere than over the Southern Hemisphere. Carbon monoxide concentrations range between 50 to 60 ppb with a few peaks into the 60s in the latitudinal area south of the ITCZ and values of 80 ppb or higher at latitudes north of Hawaii. A comparison is also made of carbon monoxide and ozone concentrations along the flight tract between California and Antarctica, over the Ellsworth Mountains of Antarctica, and between 78° S and the South Pole. These ozone-carbon monoxide data show statistically significant negative correlations in the upper troposphere and lower stratosphere over Antarctica. It is believed that this is a good indication of mixing across the tropopause.     

Snow accumulation and its moisture origin over Dome Argus, Antarctica

The spatial and temporal variability of snow accumulation near Dome Argus, Antarctica, is assessed using new snow pit and stake measurement data together with existing snow pit, ice core and automatic weather station records. Snow accumulation rate shows large inter-annual variations, but stable multi-decadal levels over the last seven centuries. Spatial variations in snow accumulation within the space of 50 km of Dome Argus are relatively small, probably thanks to the smooth topography. A comparison of theses accumulation observations with ECMWF reanalyses (ERA-40 and ERA-Interim) suggests ECMWF reanalysis captures the seasonal variations, but underestimates the overall snow accumulation at Dome Argus by ~50 %. The moisture sources for precipitation over Dome Argus are examined by means of a Lagrangian moisture source diagnostic, based on the tracing of specific humidity changes along air parcel trajectories, for the period 2000–2004 using operational ECMWF analysis data. Dome Argus mainly receives moisture from the mid-latitude (46 ± 4°S) South Indian Ocean, with a seasonal latitudinal shift of about 6°. Compared to other central East Antarctic deep ice core sites such as Dome F, Dome C, Vostok, and EPICA Dronning Maud Land, Dome Argus has a more southerly moisture origin, probably due to topographic influences on the moisture transport paths. These results have important implications for the interpretation of future ice cores at Dome Argus.     

Joining Australia to Antarctica GCM implications for the Cenozoic record of Antarctic glaciation

A previous GCM study concerning the formation and maintenance of Antarctic glaciation is expanded to include the joining of Australia to Antarctica; the two continents were physically connected prior to about 40 million years ago. It has been proposed that the increased continentality resulting from the enlarged landmass inhibited glaciation by increasing the degree of summer heating. However, simulations with the NCAR CCM1 suggest little change in the net Antarctic snow accumulation when Australia is joined to Antarctica, even under extreme variations in SST and topography. If anything, there is a slight increase in the net accumulation with the larger landmass. The climate of Australia does change markedly, consistent with the roughly 30° poleward shift in latitude. These results may not be inconsistent with paleoclimatic data from the early Cenozoic and the Cretaceous, with temperate flora and fauna along the coast, and large ice sheets inland.     

Mid latitude winter climate variability in the South Indian and southwest Pacific regions since 1300 AD

Mid-latitude winter atmospheric variability in the South Indian Ocean and southwest Pacific Ocean regions of the circum-Antarctic are reconstructed using sea-salt aerosol concentrations measured in the high resolution Law Dome (DSS) ice core from East Antarctica. The sea-salt aerosol concentration data, as sodium (Na), were measured at approximately monthly resolution spanning the past 700 years. Analyses of covariations between Na concentrations in Law Dome ice, and mean sea-level pressure (MSLP) and wind field data were conducted to define the mid-latitude and sub-Antarctic atmospheric circulation patterns associated with variations in Na delivery. High Na concentrations in Law Dome snow are associated with increased meridional aerosol transport from mid-latitude sources. The seasonal average Na concentration for early winter (May, June, July (MJJ)) is strongly correlated to the mid-latitude MSLP field in the South Indian and southwest Pacific Oceans, and southern Australian regions. In addition, the average MJJ Na concentrations display a strong association with the stationary Rossby wave number 3 circulation, and are anti-correlated to the Southern Annular Mode (SAM) index of climate variability: high (low) Na concentrations occurring during negative (positive) SAM phases. This observed relationship is used to derive a proxy record for early-winter MSLP anomalies and the SAM in the South Indian and southwest Pacific Ocean regions over the period 1300–1995 AD. The proxy SAM index from 1300 to 1995 AD shows pronounced decadal-scale variability throughout. The period after 1500 AD is marked by a tendency toward slower variations and a weakly-positive mean SAM (enhanced westerlies in the 50° to 65°S zone) compared to the early part of the record.     

The sensitivity of the south chilean snowline to climatic change

Inferred climatic changes in southern Chile during the Last Glacial Maximum are modelled to investigate the role of the southern Westerlies on the region's glacial history. This is accomplished with a numerical model of the surface energy balance which derives glacial mass balance profiles from existing climatic stations. This provides an independent measure of the regional snowline which is compared with palaeoecological evidence of former snowlines.The modelled snowline mirrors the latitudinal trend of current glacier equilibrium line altitudes. It is most sensitive to temperature changes in regions with high precipitation (46°–50° S) and to precipitation changes in regions with lower precipitation totals (south of 50° and north of 40°). This differential sensitivity with latitude implies that glacial expansion in the region depends on a delicate interplay between cooling induced by the equatorward movement of the oceanic Antarctic Polar Front and access to precipitation comparable to or greater than that of today. The main conclusion is that glacial expansion in southern Chile is associated with the migration of the southern Westerlies towards the equator. The importance of migrating precipitation belts in permitting glacier growth carries the implication that maximum depression of the snowline is unlikely to have been contemporaneous from latitude to latitude.     

Holocene hydrological cycle changes in the Southern Hemisphere documented in East Antarctic deuterium excess records

Four Holocene-long East Antarctic deuterium excess records are used to study past changes of the hydrological cycle in the Southern Hemisphere. We combine simple and complex isotopic models to quantify the relationships between Antarctic deuterium excess fluctuations and the sea surface temperature (SST) integrated over the moisture source areas for Antarctic snow. The common deuterium excess increasing trend during the first half of the Holocene is therefore interpreted in terms of a warming of the average ocean moisture source regions over this time. Available Southern Hemisphere SST records exhibit opposite trends at low latitudes (warming) and at high latitudes (cooling) during the Holocene. The agreement between the Antarctic deuterium excess and low-latitude SST trends supports the idea that the tropics dominate in providing moisture for Antarctic precipitation. The opposite trends in SSTs at low and high latitudes can potentially be explained by the decreasing obliquity during the Holocene inducing opposite trends in the local mean annual insolation between low and high latitudes. It also implies an increased latitudinal insolation gradient that in turn can maintain a stronger atmospheric circulation transporting more tropical moisture to Antarctica. This mechanism is supported by results from a mid-Holocene climate simulation performed using a coupled ocean-atmosphere model. Received: 7 July 1999 / Accepted: 21 July 2000     

Emission and flux of DMS from the Australian Antarctic and Subantarctic Oceans during the 1988/89 summer

DMS emissions and fluxes from the Australasian sector of the Antarctic and Subantarctic Oceans, bound by 46–68° S and 65.5–142.6° E, were determined from a limited number of samples (n=32) collected during three summer resupply voyages to Australian Antarctic continental research bases between November 1988 and January 1989 (a 92 day period). The maximum DMS emission from this sector of the Antarctic Ocean was in an area near the Antarctic Divergence (60–63° S) and the minimum DMS emission was from the Antarctic coastal and offshelf waters. The greatest emission of DMS from this sector of the Southern Ocean was from the Subantarctic waters. DMS flux from the Australasian Antarctic Ocean was 64.3×10⁶ (±115) mol d^–1 or 5.9 (±10.6)×10⁹ mol based on an emission of 10.9 (±19.5) µmol m^–2 d^–1 (n=26). The flux of DMS from the Australasian sector of the Subantarctic Ocean was probably twice the flux of DMS from the adjacent Antarctic Ocean.     

Coupled variability and air-sea interaction in the South Atlantic Ocean

A total of 52 years of data (1949–2000) from the NCEP/NCAR reanalysis are used to investigate mechanisms involved in forcing and damping of sea surface temperature (SST) variability in the South Atlantic Ocean. Organized patterns of coupled ocean–atmosphere variability are identified using EOF and SVD analyses. The leading mode of coupled variability consists of an SST pattern with a strong northeast–southwest gradient and an SLP monopole centered at 15°W, 45°S. The anomalous winds associated with this monopole generate the SST pattern through anomalous latent heat flux and mixed layer deepening. Other heat flux components and anomalous Ekman transport play only a secondary role. Once established, the SST pattern is attenuated through latent heat flux. The higher SST modes are also induced by anomalous winds and destroyed by latent heat flux. It thus appears that the coupled variability in the South Atlantic Ocean consists of atmospheric circulation anomalies that induce SST anomalies through anomalous latent heat fluxes and wind-induced mixed layer deepening. These SST anomalies are destroyed by latent heat flux with no detectable systematic feedback onto the atmospheric circulation. Atmospheric variability in the South Atlantic is found to be largely independent of that elsewhere, although there is a weak relation with ENSO (El Niño-Southern Oscillation).     

Interannual and interdecadal oscillation patterns in sea level

Relative sea-level height (RSLH) data at 213 tide-gauge stations have been analyzed on a monthly and an annual basis to study interannual and interdecadal oscillations, respectively. The main tools of the study are singular spectrum analysis (SSA) and multi-channel SSA (M-SSA). Very-low-frequency variability of RSLH was filtered by SSA to estimate the linear trend at each station. Global sea-level rise, after postglacial rebound corrections, has been found to equal 1.62±0.38 mm/y, by averaging over 175 stations which have a trend consistent with the neighboring ones. We have identified two dominant time scales of El Niño-Southern Oscillation (ENSO) variability, quasi-biennial and low-frequency, in the RSLH data at almost all stations. However, the amplitudes of both ENSO signals are higher in the equatorial Pacific and along the west coast of North America. RSLH data were interpolated along ocean coasts by latitudinal intervals of 5 or 10 degrees, depending on station density. Interannual variability was then examined by M-SSA in five regions: eastern Pacific (25°S–55°N at 10° resolution), western Pacific (35°S–45°N at 10°), equatorial Pacific (123°E–169°W, 6 stations), eastern Atlantic (30°S, 0°, and 30°N–70°N at 5°) and western Atlantic (50°S–50°N at 10°). Throughout the Pacific, we have found three dominant spatio-temporal oscillatory patterns, associated with time scales of ENSO variability; their periods are 2, 2.5–3 and 4–6 y. In the eastern Pacific, the biennial mode and the 6-y low-frequency mode propagate poleward. There is a southward propagation of low-frequency modes in the western Pacific RSLH, between 35°N and 5°S, but no clear propagation in the latitudes further south. However, equatorward propagation of the biennial signal is very clear in the Southern Hemisphere. In the equatorial Pacific, both the quasi-quadrennial and quasi-biennial modes at 10°N propagate westward. Strong and weak El Niño years are evident in the sea-level time series reconstructed from the quasi-biennial and low-frequency modes. Interannual variability with periods of 3 and 4–8 y is detected in the Atlantic RSLH data. In the eastern Atlantic region, we have found slow propagation of both modes northward and southward, away from 40–45°N. Interdecadal oscillations were studied using 81 stations with sufficiently long and continuous records. Most of these have variability at 9–13 and some at 18 y. Two significant eigenmode pairs, corresponding to periods of 11.6 and 12.8 y, are found in the eastern and western Atlantic ocean at latitudes 40°N–70°N and 10°N–50°N, respectively.     

Climate condition of the significant precipitation decrease over the middle-eastern region of Inner Mongolia, China in recent 10 years (2001–2010)

Compared to the 50-year mean climatological value (1961–2010), the precipitation of middle-eastern Inner Mongolia exhibited a significant decrease during the past 10 years (2001–2010). To identify the climatic causes, a comprehensive investigation was conducted by inspecting climatic factors from this 50-year period, which appear to work together in connecting closely to the precipitation. Significant positive correlations with precipitation were found in sea level pressure (SLP) difference between the area of (30° N–20° S; 50–160° E) and the northeastern Pacific Ocean, between the Northern Atlantic and the northeastern Pacific Oceans, and sea surface temperature difference between the northeastern and northwestern Pacific in the previous year, while negative connections were found in the 500-hPa temperature difference between the Antarctic and the belt region around 60° S. During the period of 2001–2010, East Asia was prevailingly controlled by a huge high, which was regarded as one of unfavorable factors for producing rain or snow. Other factors were the enlarged 500 hPa temperature differences between the Antarctic and the zones around 60° S and the Equator, the negative SLP difference between the East Asia, northern Atlantic, and Pacific Oceans. Finally, the unique wind flows and associated moisture transports also played a key role in the precipitation reduction for the first decade of the twenty-first century.     

Temporal and spatial variability of SSTs in the tropical Atlantic and Indian Oceans

Summary The objective of this study is to describe spatial and temporal patterns of sea-surface temperature (SST) variability in the Atlantic and Indian Oceans. The analysis domain extends from 40°S to 25°N and 50°W to 80°E, hence the tropical and most of the South Atlantic and central and western Indian Oceans. The investigation, covering the years 1948 to 1979, utilizes the COADS marine data set. Empirical orthogonal functions and spectral analysis are used to analyze SST fields.A major finding of this investigation is that SSTs vary coherently throughout most of the analysis domain. The greatest coherence is evident from 10°N to 30°S in the Atlantic and from 20°N to 35°S in the western Indian Ocean. Spectral analysis of regional time series shows that throughout this region the time scale of 5–6 years is the dominant one in the fluctuations; this is also the case for the Southern Oscillation and for equatorial rainfall. SST variations are roughly in-phase within each ocean and the two oceans are roughly in-phase with each other, i.e., the lags which exist are much smaller than the dominant time scale of the fluctuations. The SST anomalies appear to propagate eastward from NE Brazil; the eastern Atlantic lags the western by two to six months and the Indian Ocean lags the western Atlantic by four to eight months.With 15 Figures     

Ozone variations in the Northern polar region

Summary All total ozone observations ever made in the Northern polar region, including some from the 1930's, have been corrected and the basic climatology presented. The long-term ozone changes were considered in relation to the stratospheric temperatures. For each deviation from the monthly normal of the 100 hPa temperature by 1°C, there was found to be a corresponding 5–6 m atm-cm change in the monthly ozone deviation. A distinction between the ozone regimes over the Scandinavian, Canadian and East Siberian sectors of the polar region was noted. The strong appearance of the QBO (Quasi Biennial Oscillation) in the interannual ozone fluctuations was obvious. It is demonstrated that for the past three decades the total ozone experienced a few periods with positive and a few periods with negative deviations. In view of this, trends in ozone must obviously be based on greater than 10 years of data. During 1964–86, the weighted trend over the polar stations was (–0.9±0.4)% per decade. There have been, however, three periods (1958–64, 1968–76 and 1979–86), coinciding with the declining phase of the 11 year sunspot cycle, during which the ozone at all polar stations has been declining by about 0.5% per year (or less if the QBO component is filtered out). Some of the differences with Antarctic ozone are mentioned and the dominant role of the stratospheric circulation for the ozone variations is discussed. In general the Arctic ozone observations show no evidence of a major ozone decline similar to that over Antarctica.With 9 Figures     

Southeastern pacific blocking episodes and their effects on the South American weather

Summary Southeastern Pacific blocking episodes are studied using 17 years of reanalyzed daily data from the National Centers for Environmental Prediction (NCEP). The anomalous sea level pressure (SLP) within the area bounded by the longitudes of 130° W and 100° W and the latitudes of 50° S and 70° S is used as the base variable to determine periods with 7 or more sequential days with positive anomalies in this domain. Using these periods, composites are calculated for the SLP and its anomalies, 500-hPa geopotential height anomalies and the 250-hPa and 925-hPa wind vectors in the western southern hemisphere (SH). Composites for austral winter and summer exhibit atmospheric circulation features quite similar to those associated with the blocking episodes in the southeastern Pacific. The corresponding composite patterns of the precipitable water (Pw) and 925-hPa temperature anomalies for the South American sector are also discussed. For both seasons blocking episodes in the southeastern Pacific change the distributions of these thermodynamic variables over South America, in particular in its southern and southeastern regions by reducing (increasing) the Pw and low-level temperature in the southern South America (the central part of the continent). Therefore, monitoring the southeastern Pacific circulation patterns may lead to improved weather forecast for the South American sector.With 9 Figures     

Evolution and variability of the ITCZ in the SW Indian Ocean: 1988–90

Summary The structure and variability of the inter-tropical convergence zone (ITCZ) in the SW Indian Ocean in the austral summer is investigated. The ITCZ is identified by satellite microwave (SSMI) precipitable water (PW) values > 5 g cm^–2, minimum outgoing longwave radiation (OLR) values < 220 W m^–2 and the pattern of convergence in the low level (850 hPa) winds. According to OLR climatology, the ITCZ lies over 15°S latitude to the west of Madagascar (40–50°E), but near 10°S to the east of 60°E. Inter-annual and intra-seasonal variability is induced by the interaction of the convective NW monsoon and subsident easterly trades. Symptoms of the structure and variability are presented using tropical cyclone (TC) tracks, axes of PW exceedences and OLR, 850hPa wind and PW fields in the period 1988–1990. The shape and intensity of the ITCZ is modulated by the strength of the NW monsoon off east Africa and by standing vortices in the SW Indian Ocean. The topography of Madagascar imparts a distinctive break in convective characteristics, and distinguishes the SE African ITCZ from its maritime counterpart.With 6 Figures     

Recent precipitation trends over the polar ice sheets

Summary Meteorological and glaciological analyses are integrated to examine the precipitation trends during the last three decades over the ice sheets covering Antarctica and Greenland. For Antarctica, the best data source is provided by glaciologically-measured trends of snow accumulation, and for limited sectors of East Antarctica consistency with precipitation amounts calculated from the atmospheric water balance equation is obtained. For Greenland, precipitation rates parameterized from atmospheric analyses yield the only comprehensive depiction. The precipitation rate over Antarctica appears to have increased by about 5% over a time period spanning the accumulation means for the 1955–65 to 1965–75 periods, while over Greenland it has decreased by about 15% since 1983 with a secondary increase over the southern part of the ice sheet starting in 1977. At the end of the 10-year overlapping period, the global sea-level impact of the precipitation changes over Antarctica dominates that for Greenland and yields a net ice-sheet precipitation contribution of roughly 0.02 mm yr^–1. These changes are likely due to marked variations in the cyclonic forcing affecting the ice sheets, but are only weakly reflected in the temperature regime, consistent with the episodic nature of cyclonic precipitation. These conclusions are not founded on high quality data bases. The importance of such changes for understanding global sea-level variations argues for a modest research effort to collect simultaneous meteorological and glaciological observations in order to describe and understand the current precipitation variations over both ice sheets. Some suggestions are offered for steps that could be taken.With 8 Figures     

Past Antarctic Peninsula climate (1850–1980) deduced from an ice core isotope record

A detailed climatic study of the Antarctic Peninsula from 1850 to 1980 has been carried out through the analysis of deuterium content in the snow layers of Dalinger Dome (James Ross Island, Antarctic Peninsula). It is based on the high correlation found between mean deuterium contents at this site and temperature data from stations within this region going back as far as April 1903 for the Argentine Orcadas station.The strong correlation between isotopes and temperatures first reveals a 1956 isotope reference for the region considered. Secondly, the isotope-temperature gradient is estimated at 4.5%. °C^–1 for deuterium.After checking that the major temperature anomalies on the Antarctic Peninsula recorded since 1904 (according to available data) correspond to annual mean stable isotope peaks at Dalinger Dome, the amplitude of four prior anomalies are estimated in °C. Finally, a cooling of about 2 °C since 1850 is suggested for the region.     

Extraordinary blowing snow transport events in East Antarctica

In the convergence slope/coastal areas of Antarctica, a large fraction of snow is continuously eroded and exported by wind to the atmosphere and into the ocean. Snow transport observations from instruments and satellite images were acquired at the wind convergence zone of Terra Nova Bay (East Antarctica) throughout 2006 and 2007. Snow transport features are well-distinguished in satellite images and can extend vertically up to 200 m as first-order quantitatively estimated by driftometer sensor FlowCapt?. Maximum snow transportation occurs in the fall and winter seasons. Snow transportation (drift/blowing) was recorded for ~80% of the time, and 20% of time recorded, the flux is >10^?2 kg m^?2 s^?1 with particle density increasing with height. Cumulative snow transportation is ~4 orders of magnitude higher than snow precipitation at the site. An increase in wind speed and transportation (~30%) was observed in 2007, which is in agreement with a reduction in observed snow accumulation. Extensive presence of ablation surface (blue ice and wind crust) upwind and downwind of the measurement site suggest that the combine processes of blowing snow sublimation and snow transport remove up to 50% of the precipitation in the coastal and slope convergence area. These phenomena represent a major negative effect on the snow accumulation, and they are not sufficiently taken into account in studies of surface mass balance. The observed wind-driven ablation explains the inconsistency between atmospheric model precipitation and measured snow accumulation value.     

Large-Scale Temperature Changes across the Southern Andes: 20th-Century Variations in the Context of the Past 400 Years

Long-term trends of temperature variations across the southern Andes (37–55° S) are examined using a combination of instrumental and tree-ring records. A critical appraisal of surface air temperature from station records is presented for southern South America during the 20th century. For the interval 1930–1990, three major patterns in temperature trends are identified. Stations along the Pacific coast between 37 and 43° S are characterized by negative trends in mean annual temperature with a marked cooling period from 1950 to the mid-1970s. A clear warming trend is observed in the southern stations (south of 46°S), which intensifies at higher latitudes. No temperature trends are detected for the stations on the Atlantic coast north of 45° S. In contrast to higher latitudes in the Northern Hemisphere where annual changes in temperature are dominated by winter trends, both positive and negative trends in southern South America are due to mostly changes in summer (December to February) temperatures. Changes in the Pacific Decadal Oscillation (PDO) around 1976 are felt in summer temperatures at most stations in the Pacific domain, starting a period with increased temperature across the southern Andes and at higher latitudes.Tree-ring records from upper-treeline were used to reconstruct past temperature fluctuations for the two dominant patterns over the southern Andes. These reconstructions extend back to 1640 and are based on composite tree-ring chronologies that were processed to retain as much low-frequency variance as possible. The resulting reconstructions for the northern and southern sectors of the southern Andes explain 55% and 45% ofthe temperature variance over the interval 1930–1989, respectively. Cross-spectral analysis of actual and reconstructed temperatures over the common interval 1930–1989, indicates that most of the explained varianceis at periods >10 years in length. At periods >15 years, the squaredcoherency between actual and reconstructed temperatures ranges between 0.6 and 0.95 for both reconstructions. Consequently, these reconstructions are especially useful for studying multi-decennial temperature variations in the South American sector of the Southern Hemisphere over the past 360 years. As a result, it is possible to show that the temperatures during the 20thcentury have been anomalously warm across the southern Andes. The mean annual temperatures for the northern and southern sectors during the interval 1900–1990 are 0.53 °C and 0.86 °C above the1640–1899 means, respectively. These findings placed the current warming in a longer historical perspective, and add new support for the existence of unprecedented 20th century warming over much of the globe. The rate of temperature increase from 1850 to 1920 was the highest over the past 360 years, a common feature observed in several proxy records from higher latitudes in the Northern Hemisphere.Local temperature regimes are affected by changes in planetary circulation, with in turn are linked to global sea surface temperature (SST) anomalies. Therefore, we explored how temperature variations in the southern Andes since 1856 are related to large-scale SSTs on the South Pacific and South Atlantic Oceans. Spatial correlation patterns between the reconstructions and SSTs show that temperature variations in the northern sector of the southern Andes are strongly connected with SST anomalies in the tropical and subtropical Pacific. This spatial correlation pattern resembles the spatial signature of the PDO mode of SST variability over the South Pacific and is connected with the Pacific-South American (PSA) atmospheric pattern in the Southern Hemisphere. In contrast, temperature variations in the southern sector of the southern Andes are significantly correlated with SST anomalies over most of the South Atlantic, and in less degree, over the subtropical Pacific. This spatial correlation field regressed against SST resembles the `Global Warming' mode of SST variability, which in turn, is linked to the leading mode of circulation in the Southern Hemisphere. Certainly, part of the temperature signal present in the reconstructions can be expressed as a linear combination of four orthogonal modes of SST variability. Rotated empirical orthogonal function analysis, performed on SST across the South Pacific and South Atlantic Oceans, indicate that four discrete modes of SST variability explain a third, approximately, of total variance in temperature fluctuations across the southern Andes.     

A study of climate and weather variability over the tropical southwest Indian Ocean

Summary The climatology and variability of summer convection and circulation over the tropical southwest Indian Ocean is investigated using satellite imagery, routine synoptic observations, outgoing longwave radiation (OLR) data, sea surface temperatures (SST) and areal averaged rainfall departures. OLR has a –0.90 correlation with rainfall departures and the OLR minimum (ITCZ) in January and February lies across the 10°S latitude, extending further south near Madagascar. The intensity of ITCZ convection is greatest in the longitudes 20–35°E over northern Zambia and is considerably reduced over the SW Indian Ocean. Spatial correlations are analyzed for standardized departures of OLR, rainfall and SST. The correlations change sign in a coherent fashion, creating a climatic dipole between southern Africa and the SW Indian Ocean. Interannual trends are examined through analysis of January–February zonal and meridional wind indices constructed from significantly correlated variables at Zimbabwe, Madagascar and Mauritius. Circulation variability is dominated by quasi-decadal cycles and a trend of inereasing westerly winds. Zonal wind shear alternates from easterly (barotropic) to westerly and together with SST appears to regulate the frequency and intensity of tropical cyclogenesis. Areally averaged rainfall departures exhibit 6.25 year cycles in NE Madagascar and 12.5 and 18.75 year cycles in SW Madagascar and Zimbabwe, respectively. Summer rainfall and meridional winds in NE Madagascar and Zimbabwe are out of phase and negatively correlated in most summers. The presence of synoptic weather systems is assessed using daily Hovmoller-type satellite imagery composites. Convective structure is dominated by transient waves in the 10°–20°S latitude band, with periods of 15–20 days common. The waves are more prominent in summers with increased easterly shear and contribute to fluctuations in rainfall over SE Africa.With 8 Figures