Zonal Circulation Cell Research Articles

Mechanisms of early and late summer precipitation in Southwest China: dynamic and thermodynamic processes

This study investigates dynamic and thermodynamic components of moisture flux convergence in Southwest China (SW-MFC) and their underlying physical mechanisms during early and late summer. Using precipitation observation and CRA-40 reanalysis datasets from 1979 to 2023, the results show that both dynamic and thermodynamic processes modulate the SW-MFC in early summer (May-June), with dynamics playing a pivotal role. In contrast, the precipitation anomaly in late summer (July-August) is predominantly driven by the dynamic factors. Meanwhile, the large-scale circulation over the northern Indian Peninsula significantly modulates the SW-MFC. In early summer, anomalous convection around the Maritime Continent with the tripole sea surface temperature (SST) mode in the tropical Indo-Pacific can trigger the formation of “double ring” vertical zonal circulation cells. A large-scale westerly anomaly at the lower troposphere over the northern Arabian Sea foster cyclone strengthening over the northern Indian Peninsula, enhancing southerly moisture transport and increasing precipitation over Southwest China. During the late summer, large-scale dipole SST pattern between the subtropical central-eastern Pacific and the Indo-Pacific warm pool generates significant easterly anomalies towards the Maritime Continent. The SST gradient stimulates an extensive anticyclonic shear zone over the western equatorial Pacific, with an intensified low-pressure zone to its north. This atmospheric pattern over Southwest China and Indian Peninsula can form a vertical circulation circle that largely intensifies widespread precipitation. Numerical model experiments can reproduce the mechanisms of tropical Indo-Pacific joint effects on the Southwest precipitation in both early and late summer, providing a theoretical basis for understanding and forecasting summer precipitation over Southwest China.

Atmospheric and Oceanic Factors Related to the Increasing Summer High Temperature Extremes on the Tibetan Plateau under the Background of Global Warming

Abstract The related atmospheric and oceanic factors are investigated in this analysis to understand the natural attributes responsible for the significant increase of the high temperature extremes (HTEs) on the Tibetan Plateau (TP) in summer. It is found that a stronger-than-normal South Asian high (SAH) and corresponding weaker-than-normal East Asian jet, an anomalous anticyclone and intensified midlevel westerly wind over the TP, and a more extensive, stronger, farther westward- and northward-stretching western Pacific subtropical high motivate more occurrences of HTEs over the TP on the interannual time scale. From 1961 to 2021, these crucial circulation patterns show a significant changing trend favorable for the occurrence of HTEs and thus contribute to its great increase. Further, the significant warmings of sea surface temperature (SST) in the tropical western Indian, northern North Pacific, and western North Atlantic Oceans make great contributions through different air–sea interactive processes as the Matsuno–Gill response, zonal vertical circulation cell, and mid- to high-latitude teleconnection wave train, respectively. Meanwhile, the interdecadal variability plays an important role. A breakpoint at the early twenty-first century is detected in the occurrence of summer HTEs on the TP. Both the crucial circulation patterns and the SST anomalies in the key oceanic regions experienced significant interdecadal transition to favor the occurrence of HTEs. In particular, the Atlantic multidecadal oscillation (AMO) is significantly and positively correlated with the interdecadal variation of summer HTEs on the TP. The zonal teleconnection wave train triggered by AMO forms a stronger-than-normal SAH and strengthened midlevel westerly airflow over the TP, conducive to the increase of summer HTEs on the TP.

The change in convection over the Indo-Pacific warm pool in the mid-Holocene and its influence on South Asian precipitation

This study investigates how convection over the Indo-Pacific warm pool (IPWP) differed during the mid-Holocene compared to the preindustrial period. The research employs modeling data from the Paleoclimate Modelling Intercomparison Project phases 3 and 4 combined with published hydroclimate records from the Maritime Continent and a new primary productivity reconstruction in the western Celebes Sea. While there is large variability amongst models in terms of simulating precipitation changes, the model AWI-ESM-1-1-LR has the highest agreement with proxy data. This model indicates a dipole pattern consisting of strengthened convection over the eastern equatorial Indian Ocean and Sumatra and weakened convection over the western equatorial Pacific and Borneo-Sulawesi-New Guinea. The reduced convection over the latter region is supported by the primary productivity record illustrating lower precipitation over Borneo and a deepened nutricline in the western Celebes Sea driven by anomalous equatorial easterly winds. This dipole pattern demonstrates an anomalous trans-ocean zonal circulation cell that links the western equatorial Pacific and equatorial Indian Oceans. This cell emerging in summer and peaking in autumn, is proposed to arise from a trans-ocean zonal imbalance of equatorial precipitation change induced by altered summer insolation, and higher equatorial insolation in autumn. The increased precipitation in South Asia during the mid-Holocene is partially attributed to moisture transported from the western equatorial Pacific associated with this anomalous zonal cell. Thus, the dipole pattern of altered IPWP convection is tightly linked to dynamic precipitation changes and can influence regions outside the IPWP.

Pacific and Atlantic multidecadal variability relations to the El Niño events and their effects on the South American rainfall

AbstractThis paper examines the oceanic mean states associated with the Pacific decadal oscillation (PDO) and the Atlantic multidecadal oscillation (AMO) and their relations to the El Niño (EN). The mean states refer to the simultaneous occurrences of warm (W) and cold (C) phases of these oscillations: WAMO/CPDO, CAMO/WPDO and CAMO/CPDO. In general, the sea surface temperature (SST) anomaly patterns of the mean states show a combination of the PDO and AMO related anomaly patterns in the Pacific Ocean, and the strongest AMO‐related anomaly signals in the Atlantic Ocean with almost antisymmetric SST anomaly patterns in the northern and southern extratropical sectors of this Ocean. One of the most important results of this paper concerns the contrasting features between the CAMO/WPDO and the WAMO/CPDO mean states, which are noticeable for the SST, zonal circulation cell and precipitation during the austral autumn. Besides the opposite inter‐basin equatorial Atlantic/eastern equatorial Pacific cells, the CAMO/WPDO and WAMO/CPDO mean states also feature, respectively, a weakened and strengthened Walker cell in the tropical Pacific during the austral autumn. The results here indicate that the inter‐basin east–west cell between the equatorial Atlantic and eastern Pacific contributes to strengthen (weaken) the Walker cell during the austral autumn of the WAMO/CPDO (CAMO/WPDO) mean state. We also found that the mean states alter the EN features. The EN‐related largest positive SST anomalies occur in the central tropical Pacific during the WAMO/CPDO, and in the eastern tropical Pacific during the CAMO/WPDO. The Atlantic and Pacific oceanic mean states play an important role in modulating the EN features and their effects on the South American rainfall. The results presented in this paper might be relevant for climate monitoring and modelling studies.

The Eurasia–North Pacific Oscillation in atmospheric mass variations independent of both IHO and AO and its possible impacts on winter climate

Using NCEP/NCAR reanalysis, we have investigated the features of migrations of atmospheric mass (AM) between land and ocean in Eurasia–North Pacific domain in boreal winter after having both signals of Inter-hemispheric Oscillation and Arctic Oscillation removed from the physical quantities. It is found that there is a Eurasia–North Pacific Oscillation (ENPO) in surface air pressure anomalies. This ENPO pattern characterizes with two oppositely signed anomalous surface pressure centers over Eurasia and North Pacific respectively, indicating strong connections between Siberian high and Aleutian low during period 1979–2012. The maintenance of this ENPO teleconnection is significantly associated with three factors including the anomalous AM flows and zonal circulation cell over Eurasia–North Pacific domain, the Rossby wave energy propagations, and the thermal forcing contrasts near the surface between Eurasia and North Pacific during boreal winter. The variations of both wintertime rainfall and temperature over Eurasia may be strongly affected by ENPO. When the ENPO index is positive (negative), there occurs the AM accumulation (depletion) over Eurasia with simultaneous depletion (accumulation) over mid-latitude North-Pacific. Correspondingly, this anomalous surface pressure pattern along with the related circulation anomalies at different isobaric levels possibly results in winter precipitation decreases (increases) over Siberian Plain and East China, whereas increases (decreases) over southeastern Europe, Xinjiang of China, and the west coast of Sea of Okhotsk. On the other hand, surface air temperature decreases (increases) over large areas of Eurasia. These results are helpful for our better understanding the mechanisms behind circulation and winter climate variations over Eurasia–North Pacific region.

A late Holocene paleoclimate reconstruction from Boqueirão Lake sediments, northeastern Brazil

Lake level fluctuations and environmental changes during the late Holocene were inferred from changes in sedimentology, bulk and isotope organic geochemistry, and a diatom based transfer function in a sediment core from Boqueirão Lake, northeast Brazil. The age-depth model was established using fifteen AMS 14C dates. Lake water level began increasing about AD 400, and reached its maximum during the Medieval Climate Anomaly (MCA), AD ~900–1100. Low lake water level was recorded during the Little Ice Age (LIA), AD ~1400–1820, which allowed macrophyte development in the littoral zone that was recorded by high Corg/Ntotal ratios. Considerable lake level variability was evident during the Current Warm Period (CWP). Humid/dry conditions in northeast Brazil during MCA/LIA, respectively, are related to the southward shift of Inter-Tropical Convergence Zone (ITCZ) during austral summer and fall. Those conditions contrast with records of a decrease/increase in the South American Summer Monsoon (SASM) during these periods. This observation also contrasts with paleoclimate inferences from the circum-Caribbean region indicating a northward shift of the ITCZ northern hemisphere summer position during the MCA and a southward shift during the LIA. We suggest that these shifts in ITCZ seasonality were higher during the MCA and smaller during the LIA. Our aim was to answer whether the zonal atmospheric circulation cell between the Amazon and northeast Brazil was responsible for antiphasing with the SASM. A strong monsoon over South America during the LIA reinforced convection upon Amazon, increasing the northeast low in the upper troposphere and large-scale subsidence over northeast Brazil and the Atlantic Ocean leading to a northward repositioning of the South Atlantic Subtropical Anticyclone. These factors in combination may have limited the southward seasonal shift of ITCZ, and they must have been responsible for drier conditions in northeast Brazil during the LIA.

The eastward shift of the Walker Circulation in response to global warming and its relationship to ENSO variability

This study investigates the global warming response of the Walker Circulation and the other zonal circulation cells (represented by the zonal stream function), in CMIP3 and CMIP5 climate models. The changes in the mean state are presented as well as the changes in the modes of variability. The mean zonal circulation weakens in the multi model ensembles nearly everywhere along the equator under both the RCP4.5 and SRES A1B scenarios. Over the Pacific the Walker Circulation also shows a significant eastward shift. These changes in the mean circulation are very similar to the leading mode of interannual variability in the tropical zonal circulation cells, which is dominated by El Nino Southern Oscillation variability. During an El Nino event the circulation weakens and the rising branch over the Maritime Continent shifts to the east in comparison to neutral conditions (vice versa for a La Nina event). Two-thirds of the global warming forced trend of the Walker Circulation can be explained by a long-term trend in this interannual variability pattern, i.e. a shift towards more El Nino-like conditions in the multi-model mean under global warming. Further, interannual variability in the zonal circulation exhibits an asymmetry between El Nino and La Nina events. El Nino anomalies are located more to the east compared with La Nina anomalies. Consistent with this asymmetry we find a shift to the east of the dominant mode of variability of zonal stream function under global warming. All these results vary among the individual models, but the multi model ensembles of CMIP3 and CMIP5 show in nearly all aspects very similar results, which underline the robustness of these results. The observed data (ERA Interim reanalysis) from 1979 to 2012 shows a westward shift and strengthening of the Walker Circulation. This is opposite to what the results in the CMIP models reveal. However, 75 % of the trend of the Walker Circulation can again be explained by a shift of the dominant mode of variability, but here towards more La Nina-like conditions. Thus in both climate change projections and observations the long-term trends of the Walker Circulation seem to follow to a large part the pre-existing dominant mode of internal variability.

Diagnosing Kenya Rainfall in Boreal Autumn: Further Exploration

Abstract A powerful zonal vertical circulation cell along the Indian Ocean equator controls the boreal autumn rains in Kenya, with a tight negative correlation between surface westerlies (UEQ) and rainfall. UEQ is favored by a steep eastward pressure gradient (PWE) and slow winds in the downstream portion of the South Indian Ocean trade winds (SIW). The high phase of the Southern Oscillation (SO) favors weak SIW, lower pressure in the east, and thus steeper PWE, but that is also affected by pressure in the west. In 1958–97 circulation causalities were most distinct in the regime of abundant rain and slow UEQ, with the SO in the low phase. In the regime of deficient rain and fast UEQ, relationships were less distinct, although the SO was in the high phase almost throughout. In the decade 2001–10, UEQ had a weaker (stronger) correlation with PWE (SIW), the SO was more active and contributing in the proper sense to pressure in the east, but PWE was mainly controlled by pressure in the west. UEQ strongly controlled the rainfall in 1958–97 and 2001–10.

Circulation Mechanisms of Kenya Rainfall Anomalies

Abstract Expanding earlier studies on the boreal spring and autumn rainy seasons in equatorial East Africa, pending challenges on the mechanisms of rainfall variability, are investigated. Eastward pressure gradient and slack south Indian Ocean trade winds allow surface equatorial westerlies in spring and autumn. Complementing that, upper-tropospheric easterlies are required for the development of a zonal vertical circulation cell along the Indian Ocean equator. Because of the summer warming and high stand of upper-tropospheric topography over South Asia, strong upper-tropospheric easterlies over the tropical northern and equatorial Indian Ocean persist from summer into autumn, thus allowing the development of a zonal vertical circulation cell. By contrast, the winter cooling entails low stand of upper-tropospheric topography in the north, thus hindering easterlies over the equator. Consequently, an equatorial zonal circulation cell does not develop in boreal spring. The equatorial zonal circulation cell, with subsidence over East Africa, strongly controls the boreal autumn rains, as evidenced in their tight correlation with the equatorial westerlies. In a related vein, rain gauge stations show much shared variance in boreal autumn as compared to spring. Plausibly consistent with this, boreal autumn rather than spring has brought the extreme flood and drought disasters in the course of the past half-century.

Diagnosing the 2005 Drought in Equatorial East Africa

Abstract Equatorial East Africa suffered severe drought during its 2005 “short rains,” centered on October–November. The circulation mechanisms of such precipitation anomalies are examined, using long-term upper-air and surface datasets, and based on diagnostic findings from earlier empirical investigations. The steep eastward pressure gradient is conducive to fast westerlies over the central-equatorial Indian Ocean, surface manifestation of a powerful zonal circulation cell with subsidence over East Africa, and ascending motion over Indonesia. With fast westerlies, rainfall in East Africa is deficient and they tend to be accompanied by anomalously cold waters in the northwestern and warm anomalies in the southeastern extremity of the equatorial Indian Ocean Basin, without any seesaw between these domains. In October–November 2005, pressure in the west was anomalously high, entailing a steep eastward pressure gradient along the equator, conducive to fast westerlies and, further symptomatic of the zonal circulation cell, subsidence in the west and ascending motion in the east were enhanced. Overall, the chain of causalities can be traced to anomalously high pressure in the west.

Circulation mechanisms of climate anomalies in East Africa and the equatorial Indian Ocean

The “Short Rains” at the coast of equatorial East Africa, concentrated around October–November, are highly variable and have an extremely tight concurrent correlation with the westerlies over the central equatorial Indian Ocean. The equatorial westerlies drive the Wyrtki Jet in the upper ocean and enhance the westward temperature gradient, and they are the surface manifestation of a powerful zonal-vertical circulation cell along the Indian Ocean Equator at this time of the year. With a strong equatorial circulation cell, the enhanced subsidence and cool surface waters in the West serve to reduce the rainfall over East Africa, and conversely the enhanced ascending motion and warm waters in the East favor precipitation in Indonesia. In the interannual variability over the second half of the 20th century, the years 1961, 1994 and 1997 stand out with particularly weak westerlies over the central equatorial Indian Ocean and disastrous floods in East Africa. At longer time scales, there is evidence of an enhancement of the equatorial westerlies accompanying the drop of lake levels and onset of glacier recession in East Africa in the last two decades of the 19th century. Despite the tight concurrent correlations, the prospects for seasonal forecasting are modest. Predictability decreased from the 1958–1977 to the 1978–1997 period, although the boreal autumn equatorial zonal circulation cell remained strong throughout.

Equatorial zonal circulations: Historical perspectives

The changing perceptions on zonal circulations in the equatorial belt are traced for (a) stratospheric wind regimes, and (b) vertical-zonal circulation cells in the troposphere. (a) Observations from the Krakatoa eruption 1883 and Berson's 1908 expedition to East Africa, along with later soundings over Batavia (Jakarta) led to the notion of “Krakatoa easterlies” around 30 km (10 mb) and “Berson westerlies” around 20 km (50 mb). Prompted by contrary observations since the late 1950s, this dogma was replaced by the notion of easterlies alternating with westerlies in the equatorial stratosphere at a rhythm of about 26 months. (b) Stimulated by Bjerknes’ postulate of a “Walker circulation” along the Pacific Equator, a multitude of such cells have been hypothesized at other longitudes, in part from zonal contrasts of temperature and cloudiness. Essential for the diagnosis of equatorial zonal circulation cells is the continuity following the flow between the centers of ascending and subsiding motion. Evaluation of the recent NCEP-NCAR and ECMWF Reanalysis upper-air datasets reveals equatorial zonal circulation cells over the Pacific all year round, over the Atlantic only in boreal winter, and over the Indian Ocean only in autumn, all being seasons and oceanic longitudes with strong zonal flow in the lower troposphere.

Interannual and Interdecadal Rainfall Variations in the Hawaiian Islands*

Abstract Hawaii rainfall has exhibited both interannual and interdecadal variations. On the interannual time scale, Hawaii tends to be dry during most El Niño events, but low rainfall also occurred in the absence of El Niño. On the interdecadal time scale, Hawaii rainfall is negatively and significantly correlated with the Pacific decadal oscillation (PDO) signal; an epoch of low rainfall persists from the mid-1970s to 2001, which is preceded by an epoch of high rainfall lasting for nearly 28 yr. Difference patterns in winter [November–December–January–February–March (NDJFM)] rainfall are investigated for composites of extremely dry and wet winters during the dry and wet epochs, respectively. These patterns (i.e., DRY minus WET) are then compared to the difference in constructive match conditions of El Niño and PDO (i.e., El Niño/+PDO minus La Niña/−PDO). Relative to the El Niño/PDO stage, the magnitude of dryness during the rainfall-based stage is enhanced. The corresponding large-scale atmospheric circulation composites are studied. Similar patterns are revealed between these two stages. However, anomalously stronger and deeper sinking motions over Hawaii are revealed in the height–latitude section of the rainfall-based analysis compared to the El Niño/PDO stage. Moreover, an anomalous zonal circulation cell is well established over the subtropical North Pacific with a pronounced descending branch over Hawaii in the rainfall-based stage. The band of anomalous surface westerlies to the north of Hawaii, and the deep sinking motion as well as the anomalously vertically integrated moisture flux divergence over Hawaii are all unfavorable for rainfall in Hawaii.

Mechanisms of climate anomalies in the equatorial Indian Ocean

Building on earlier work on the interannual variability of the boreal autumn equatorial westerlies (UEQ) over the Indian Ocean and concomitant rainfall anomalies at the coast of East Africa and in Indonesia, the inherent circulation mechanisms are here explored further from long‐term surface and upper air data. Fast UEQ and deficient East African rainfall come with positive sea level pressure (P) and negative sea surface temperature (T) departures in a domain (W) at the northwestern extremity and opposite departures in a domain (E) at the southeastern extremity of the equatorial Indian Ocean. However, there is no seesaw between W and E in either P or T and no indication of local forcing of T on P. The large‐scale pressure field, in particular the zonal pressure gradient along the equator and the South Indian Ocean pressure and southern tradewinds, control the evolution of UEQ. Fast UEQ steepens the zonal temperature gradient, thus tightening the inverse relationships between the zonal gradients of pressure and temperature. The rainfall anomalies associated with the interannual variability of UEQ, surface manifestation of a zonal circulation cell along the Indian Ocean equator, are favored by the kinematic and thermodynamic conditions in W and E. Thus, with fast UEQ the domain W features departure lower tropospheric divergence and subsidence and, favored by the cold T and subsidence, reduced precipitable water, all conducive to deficient precipitation. By contrast, E has departure lower tropospheric convergence and ascending motion and, favored by the warm T and ascending motion, enhanced precipitable water, in conjunction conducive to abundant rainfall. The interannual variability of the boreal autumn equatorial westerlies, dominated as it is by the large‐scale pressure field, is crucial in the climate dynamics of the equatorial Indian Ocean region. This leads to the question: What controls the pressure pattern over the Indian Ocean basin?

Zonal circulations over the Indian and Pacific Oceans and the level of lakes Victoria and Tanganyika

AbstractLevel records of two East African Great Lakes, Lake Victoria and Lake Tanganyika, which are considered as hydro‐climatic proxies, are analysed. Comparisons between the two lake signals show synchronisms, which can only be accounted for by large‐scale mechanisms. Lake‐level variations associated with the short rains season (October–January) appear to have a prominent effect on the annual lake levels. The relations between lake‐level variations and atmospheric circulation indexes are then investigated. Over the period 1946–2000, four indexes are selected to characterize the October–December zonal circulation over the Pacific and Indian Oceans. Over the Indian Ocean two surface zonal wind indexes (ZWIs) are used. For the Pacific, the southern oscillation index (SOI) and the Niño3 index are held to account for the El Niño–southern oscillation (ENSO). It is shown that significant overall negative correlations of level fluctuations are preferentially obtained with Indian Ocean circulation indexes. Although ZWI is highly correlated with the ENSO indexes, the latter display weaker relations with East African lake levels. It is shown that, for the 1946–2000 period, the October–December zonal circulation cell over the Indian Ocean plays a key role in the equatorial lake‐level anomalies, thus demonstrating their influence on the hydro‐climatic interannual variability of a large region. However, lake‐level variation is a function not only of regional hydro‐climatic conditions, but also of the initial (October) absolute lake level. Higher correlations are evident in a multiple correlation approach taking into account the initial lake level status in addition to the ZWI and ENSO indexes. Copyright © 2004 Royal Meteorological Society

Exploring the predictability of the ‘Short Rains’ at the coast of East Africa

AbstractThe boreal autumn ‘Short Rains’ at the coast of East Africa are deficient when there is weak development of a zonal circulation cell along the Indian Ocean equator, an anomalously low sea‐surface temperature in the western portion of the basin, and in the high phase of the southern oscillation. Such large‐scale circulation departures and their precursors are described by compact indices. September values of these indices for the period 1958–96 are used to explore the predictability of an index (RON) of October–November rainfall at the coast of East Africa. Regressions with cross‐validation over the entire 1958–96 period are evaluated for the early (1958–77) and late (1978–96) halves of the record. In complementary experiments, the entire record is separated into 1958–77 as a training period and 1978–96 as a verification period. In all experiments, correlation of calculated versus observed rainfall is high for the early record and low for the late half of the record, a behaviour not noted in cross‐validation over the entire 39 year time span. The 11‐year sliding correlations of the indicated circulation indices with RON all reveal a drastic deterioration of relationships from the early to the late half of the record, although the equatorial zonal circulation cell appears to remain strong throughout. Copyright © 2004 Royal Meteorological Society

Dynamics of the surface wind field over the equatorial Indian Ocean

AbstractThe annual cycle and interannual variability of the equatorial westerlies over the Indian Ocean are examined in the context of the equations of motion, using long‐term surface ship observations. Eastward pressure gradient along the equator may accelerate equatorial westerlies, but the process is more intricate. The south Indian Ocean trade winds can recurve in the southern hemisphere only with an eastward pressure gradient. This is counteracted by the Coriolis acceleration directed at right angles to the left of the motion. With weak trades and strong eastward pressure gradient, the flow can recurve at relatively high southern latitudes, thus allowing a broad equatorial zone in which westerlies can develop. In the annual cycle, the southern Indian Ocean high and trade winds are closest to the equator and strongest in austral winter when, however, the eastward pressure gradient in the equatorial zone is also steepest. Accordingly, the latitude of flow recurvature in the southern trade winds stays farthest away from the equator in boreal spring and autumn. Consequently, the equatorial westerlies peak in these short transition seasons. The boreal autumn equatorial westerlies are the surface manifestation of a powerful zonal circulation cell along the Indian Ocean equator. The interannual variability of boreal autumn is characterized by weak westerlies accompanying a slack zonal pressure gradient and strong southern trade winds with recurvature near the equator. Such an ensemble of circulation departures is characteristic of abundant rainfall in East Africa and deficient precipitation in Indonesia. Copyright © 2004 Royal Meteorological Society

Circulation mechanisms of climate anomalies in the equatorial Indian Ocean

The circulation mechanisms of climatic anomalies in the equatorial Indian Ocean are diagnosed from long-term surface and upper-air data for the October-November core of the East African Short Rains and season of zonal circulation cell along the Indian Ocean Equator. Climatic variability is dominated by variations in the intensity of the zonal circulation cell, manifested in subsidence in the West, ascending motion in the East, divergent westward flow in the upper troposphere, and equatorial surface westerlies (UEQ). The UEQ requires steep zonal pressure gradient, with low pressure in the East. Disastrous floods in East Africa and drought in Indonesia occur in years with weak development of the zonal circulation cell, characterized by high surface pressure anomaly in the East, which is common but not limited to the low phase of the Southern Oscillation (SO). A deficient Indian summer monsoon, common in the low SO phase, leaves behind an anomalously warm western Indian Ocean. With high pressure anomaly in the East during the months preceding October-November the Indonesian waters tend to be anomalously cold. Accordingly, with weak zonal circulation cell and in the low SO phase, the westward gradient of sea surface temperature in the equatorial Indian Ocean tends to be reduced. Weak equatorial zonal circulation in October-November is preceded by flow departures of same sense in the near-equatorial atmosphere evolving since boreal summer, and this is evidenced in the 1958-97 record as a whole, as well as for the individual extreme years 1961, 1994 and 1997. Peculiar to the 1961 extreme event is that it was preceded by an extremely good Indian summer monsoon yet anomalously warm waters in the West of the basin, and that the functionally important surface high pressure anomaly in the East did not coincide with the low SO phase.

Spatial/temporal patterns in Weddell gyre characteristics and their relationship to global climate

We examine the spatiotemporal variability of the upper ocean sea ice system of the Atlantic sector of the Southern Ocean subpolar seas (Weddell gyre) and the nature of its covariability with extrapolar climate. To systematically evaluate the sporadic and sparse distribution of subpolar data, we employed an optimal analysis involving empirical orthogonal functions (EOFs). The EOFs reveal that the pattern of spatial covariability of Weddell gyre characteristics is dominated by high interannual variability near the northern (circumpolar) rim of the gyre and lesser variability (10–20% of the variance) in the gyre's central core region. We find considerable, statistically significant teleconnections between subpolar and extrapolar climate. The dominant link is with El Niño–Southern Oscillation. Our analysis of the gyre characteristics suggest that during El Niño, the Atlantic subpolar (Weddell) gyre spins up while the eastern Pacific subpolar gyre spins down; the opposite occurs for La Niña. This regional subpolar response is consistent with recent GCM modeling analyses. These show that equatorial warm anomalies drive changes in the meridional and zonal atmospheric circulation cells. This results in opposite shifts of the subpolar jets in the Pacific and Atlantic oceans that lead to a change in the interaction (especially in cyclones) between the subtropical and polar front jets in those basins, and ultimately in the cyclonic forcing of the subpolar gyres. We also find that the Weddell gyre interior upper ocean characteristics covary positively with sea ice extent in the Atlantic region and negatively with the sea ice extent in the Amundsen/Bellingshausen regions, reflecting a strong Antarctic dipole.

Atmospheric Circulation Cells Associated with the El Niño–Southern Oscillation

Abstract Atmospheric circulation cells associated with the El Nino–Southern Oscillation (ENSO) are described and examined using the NCEP–NCAR reanalysis field and the NCEP sea surface temperature (SST) from January 1950 to December 1999. The divergent wind and pressure vertical velocity are employed for the identification of atmospheric circulation cells. The warm phase of ENSO shows positive SST anomalies in the equatorial eastern Pacific and along the east coast of Asia and the west coast of North America, and negative SST anomalies in the off-equatorial western Pacific and in the central North Pacific. Associated with this SST anomaly distribution are variations of atmospheric zonal and meridional circulation cells over the Pacific. The equatorial zonal Walker circulation cell is weakened, consistent with previous schematic diagrams. The anomalous meridional Hadley circulation cell in the eastern Pacific shows the air rising in the Tropics, flowing poleward in the upper troposphere, sinking in the subt...