Southern Hadley Cell Research Articles

Revisiting the Seasonal Cycle of Rainfall over Central Africa

Abstract The intertropical convergence zone (ITCZ), with its twice-annual passage over central Africa, is considered as the main driver of the rainfall seasonality. In this ITCZ paradigm, high rainfall occurs over regions of large low-level convergence. But recently, this paradigm was challenged over central Africa. Here, we show that a shallow meridional overturning circulation—driven by surface conditions—plays a thermodynamical control on the rainfall seasonality over central Africa. Indeed, due to the local evaporative cooling effect, the foot of the ascending branch of Hadley cells occurs where the temperature is the warmest, indicating a thermal low. This distorts the southern Hadley cell by developing its bottom-heavy structure. As result, both shallow and deep Hadley cells coexist over central Africa year-round. The deep mode is associated with the poleward transport of atmospheric energy at upper levels. The shallow mode is characterized by a shallow meridional circulation, with its moisture transport vanishing and converging in the midtroposphere rather than at lower troposphere. This midtropospheric moisture convergence is also the dominant component that shapes the vertically integrated moisture flux convergence, with little contribution of African easterly jets. This convergence zone thus controls the precipitating convection. Its meridional migration highlights the interhemispheric rainfall contrast over central Africa and outlines the unimodal seasonality. On the other hand, forced by the Congo basin cell, the precipitable water regulates the deep convection from the vegetated surface of Congo basin, acting as a continental sea. This nonlinear mechanism separates the rainfall into three distinct regimes: the moisture-convergence-controlled regime, with convective rainfall exclusively occurring in the rainy season; the local evaporation-controlled regime with drizzle in the dry season; and the precipitable-water-controlled regime, with exponential rainfall increase in the dry season.

Deep Time Conservation Paleobiology of the Atlantic Jigsaw Puzzle and the Future of the Southwestern Angolan Coast

The puzzle-like fit of Africa and South America reflects the tectonically driven opening of the South Atlantic Ocean beginning over 130 mya. By 90 Ma, the North and South Atlantics were conjoined. The introduction of Cretaceous marine reptiles into the central South Atlantic from the north coincides with through-flow in the Equatorial Atlantic Gateway and with increased productivity and upwelling of the Benguela Current. The K-Pg extinction saw the demise of most marine reptiles, but upwelling apparently persisted, evidenced by a growing Cenozoic fossil record of sea turtles and marine mammals from the Angolan coast. Convergent similarities between the Cretaceous marine reptile vertebrate community and the modern vertebrate community of the Benguela Large Marine Ecosystem suggest essentially continuous productivity related to upwelling along the southwest African coast since Cretaceous time. Paleolatitude reconstructions show that predicted positions of coastal upwelling of the Benguela Current have moved south along the coast as Africa drifted northward through the descending limb of the southern Hadley Cell. The Cretaceous and modern faunas were both adapted to a productive upwelling zone. The Cretaceous relict Welwitschia mirabilis is consistent with coastal aridity alongside upwelling. Thus, the sediments of coastal Angola and the fossils they entomb are relevant to conservation paleobiology because they provide a baseline through deep time. Comparisons underscore the resilience of the Benguela Current on the one hand and emphasize human-driven threats to the Benguela Large Marine Ecosystem on the other. Solutions are being sought; for instance, through the evaluation of Ecologically or Biologically Significant Marine Areas (EBSA) in the Benguela Current Large Marine Ecosystem. In Angola, the geologic record of the opening of the South Atlantic, the fossils, public interest, and the value for sustainable development are positive indications for the future.

Metrics of the Hadley circulation strength and associated circulation trends

Abstract. This study compares trends in the Hadley cell (HC) strength using different metrics applied to the ECMWF ERA5 and ERA-Interim reanalyses for the period 1979–2018. The HC strength is commonly evaluated by metrics derived from the mass-weighted zonal-mean stream function in isobaric coordinates. Other metrics include the upper tropospheric velocity potential, the vertical velocity in the mid-troposphere, and the water vapour transport in the lower troposphere. Seven known metrics of HC strength are complemented here by a metric of the spatially averaged HC strength, obtained by averaging the stream function in the latitude–pressure (φ–p) plane, and by the total energy of zonal-mean unbalanced circulation in the normal-mode function decomposition. It is shown that metrics, which rely on single-point values in the φ–p plane, produce unreliable 40-year trends in both the northern and southern HCs, especially in ERA-Interim; magnitudes and even the signs of the trends depend on the choice of the HC strength metric. The two new metrics alleviate the vertical and meridional inhomogeneities of the trends in HC strength. The unbalanced energy metric suggests a positive HC trend in both reanalyses, whereas the metric based on averaging the stream function finds a significant positive trend only in ERA5.

Slowing of Caribbean through-flow

Trends in near-surface currents passing through the southeastern Caribbean are described using ocean reanalysis and a coupled model projection. The decline of Caribbean through-flow is linked with tropical and polar forcing. In the upstream zone, the North Brazil Current shows increasing retroflection as the southern Hadley Cell expands across the equatorial Atlantic. This inhibits northwestward transport next to the Amazon River plume. In the downstream zone, the Labrador Current feeds low saline Arctic waters into the Gulf Stream, inducing an equatorward shift and reduced flow.Evidence is growing for spin-down of the North Atlantic gyre and its feeder currents, that inhibit poleward heat exchange and contribute to climatic shifts. Eastward trends of winds and currents in the equatorial Atlantic is projected to bring a drier climate to the lower Amazon basin and southeastern Caribbean, hence greater salinity with knock-on effects for regional sea temperatures.

Northward excursion of the ITCZ across the inter-Americas during boreal summer

The study describes northward excursions of the ITCZ across the inter-Americas during boreal summer, using observational datasets at multi-year, annual and daily time-scales. The annual cycle is felt across the region via evenly distributed SST warming in the summer hemisphere. Low-level air is propelled northward from June to October by the southern Hadley cell. Discharges from the Amazon and Orinoco Rivers spread into the Atlantic Ocean at that time of year. A N–S dipole pattern in satellite outgoing longwave radiation (OLR) is calculated and its temporal character reveals northward ITCZ excursions at 4–6 years period corresponding with a warm east Pacific and intensified diabatic heating near Panama. Composite analysis reveals evidence of a standing atmospheric Rossby wave pattern embedded in westerly flow anomalies which bifurcate on passing the Andes and slant across the inter-Americas in Jun–Oct season: lowering SLP and energizing Caribbean hurricanes. Variability at intra-seasonal time scale is underpinned by the southern jet stream and a ridge over Brazil that pushes the ITCZ northward, as seen in statistical regression and case study analysis of satellite OLR and NCEP2 reanalysis fields. The southern Hadley cell plays a prominent role, transmitting signals from winter to summer hemisphere at all time scales.

Coupled Interannual Variability of the Hadley and Ferrel Cells

This work investigates the covariability in the strength of the Hadley and Ferrel cells on interannual time scales using reanalysis data. A significant correlation is found in both hemispheres only during boreal winter. For other seasons, only the outermost (subtropical) part of the Hadley cell is correlated with changes in the extratropical eddy momentum fluxes, as the eddies are unable to penetrate into the deep tropics. During boreal winter, the northern Hadley cell variability is driven by extratropical planetary momentum fluxes, but the mean meridional circulation response is primarily found below the level of maximum climatological outflow. Instead, at upper levels, changes in the zonal wind dominate the response to the anomalous eddy forcing. During austral winter, the southern Hadley cell is shielded from the extratropical eddy fluxes and its variability displays some of the characteristics of the angular momentum–conserving solution.

The G4Foam Experiment: global climate impacts of regional ocean albedo modification

Abstract. Reducing insolation has been proposed as a geoengineering response to global warming. Here we present the results of climate model simulations of a unique Geoengineering Model Intercomparison Project Testbed experiment to investigate the benefits and risks of a scheme that would brighten certain oceanic regions. The National Center for Atmospheric Research CESM CAM4-Chem global climate model was modified to simulate a scheme in which the albedo of the ocean surface is increased over the subtropical ocean gyres in the Southern Hemisphere. In theory, this could be accomplished using a stable, nondispersive foam, comprised of tiny, highly reflective microbubbles. Such a foam has been developed under idealized conditions, although deployment at a large scale is presently infeasible. We conducted three ensemble members of a simulation (G4Foam) from 2020 through to 2069 in which the albedo of the ocean surface is set to 0.15 (an increase of 150 %) over the three subtropical ocean gyres in the Southern Hemisphere, against a background of the RCP6.0 (representative concentration pathway resulting in +6 W m−2 radiative forcing by 2100) scenario. After 2069, geoengineering is ceased, and the simulation is run for an additional 20 years. Global mean surface temperature in G4Foam is 0.6 K lower than RCP6.0, with statistically significant cooling relative to RCP6.0 south of 30° N. There is an increase in rainfall over land, most pronouncedly in the tropics during the June–July–August season, relative to both G4SSA (specified stratospheric aerosols) and RCP6.0. Heavily populated and highly cultivated regions throughout the tropics, including the Sahel, southern Asia, the Maritime Continent, Central America, and much of the Amazon experience a statistically significant increase in precipitation minus evaporation. The temperature response to the relatively modest global average forcing of −1.5 W m−2 is amplified through a series of positive cloud feedbacks, in which more shortwave radiation is reflected. The precipitation response is primarily the result of the intensification of the southern Hadley cell, as its mean position migrates northward and away from the Equator in response to the asymmetric cooling.

Mechanisms of Forced Tropical Meridional Energy Flux Change

Abstract Anthropogenically forced changes to the mean and spatial pattern of sea surface temperatures (SSTs) alter tropical atmospheric meridional energy transport throughout the seasonal cycle—in total, its partitioning between the Hadley cells and eddies and, for the Hadley cells, the relative roles of the mass flux and the gross moist stability (GMS). The authors investigate this behavior using an atmospheric general circulation model forced with SST anomalies caused by either historical greenhouse gas or aerosol forcing, dividing the SST anomalies into two components: the tropical mean SST anomaly applied uniformly and the full SST anomalies minus the tropical mean. For greenhouse gases, the polar-amplified SST spatial pattern partially negates enhanced eddy poleward energy transport driven by mean warming. Both SST components weaken winter Hadley cell circulation and alter GMS. The Northern Hemisphere–focused aerosol cooling induces northward energy flux anomalies in the deep tropics, which manifest partially via strengthened northern and weakened southern Hadley cell overturning. Aerosol-induced GMS changes also contribute to the northward energy fluxes. A simple thermodynamic scaling qualitatively captures these changes, although it performs less well for the greenhouse gas simulations. The scaling provides an explanation for the tight correlation demonstrated in previous studies between shifts in the intertropical convergence zone and cross-equatorial energy fluxes.

How Does El Niño Affect the Interannual Variability of the Boreal Summer Hadley Circulation?

Abstract Analyses of 30-yr four reanalysis datasets [NCEP–NCAR reanalysis (NCEP1), NCEP–Department of Energy reanalysis (NCEP2), Japanese 25-year Reanalysis Project (JRA-25), and Interim ECMWF Re-Analysis (ERA-Interim)] reveal remarkably interannual variability of the Hadley circulation (HC) in boreal summer (June–August). The two leading modes of interannual variability of boreal summer HC are obtained by performing empirical orthogonal function (EOF) analysis on the mass streamfunction. A general intensification of boreal summer HC is seen in EOF-1 mode among NCEP1, NCEP2, and JRA-25 but the corresponding EOF-2 mode in ERA-Interim, while a weakened northern Hadley cell and remarkable regional variation of a southern Hadley cell are captured by the EOF-2 mode (from NCEP1, NCEP2, and JRA-25) and EOF-1 mode (from ERA-Interim), as evidenced by the enhanced (decreased) southern Hadley cell in the southern tropics (the northern tropics and southern subtropics). Both modes are driven by El Niño–like SST forcing in boreal summer, but are relevant to different phases of El Niño events. The EOF-1 (or EOF-2 derived from ERA-Interim) [EOF-2 (or EOF-1 derived from ERA-Interim)] mode is driven by SST anomalies in developing (decaying) El Niño summers. The interannual variations of the northern Hadley cell in both modes are driven by El Niño through modulating the interannual variations of the East Asian summer monsoon, while anomalous local Hadley circulation (LHC) in the regions 30°S–20°N, 110°E–180° and 30°S–20°N, 160°E–120°W in response to El Niño forcing largely determine the interannual variations of southern Hadley cell in both modes, respectively. The different behaviors of the southern Hadley cell between two leading modes can be well explained by the southward shift of the tropical heating center from north of 10°N in developing El Niño summers to south of 10°N in decaying El Niño summers.

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

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

Seasonal to decadal modulation of the impact of El Niño-Southern Oscillation on New Caledonia (SW Pacific) rainfall (1950-2010)

[1] New Caledonia (NC; ∼166°E, 22°S) rainfall anomalies are more sensitive to central Pacific (CP) El Nino and La Nina events than to those exhibiting highest sea surface temperature (SST) anomalies in the eastern Pacific (EP). The linear relationship between NC rainfall anomalies and CP SST indices peaks from September to March (S–M). The seasonal S–M atmospheric anomalies observed in the South West (SW) Pacific during the warm CP events are highly dissimilar to the EP ones, while there are more similarities during the cold events with a higher amplitude during the CP ones. The warm CP events strengthen the southern Hadley cell around NC longitudes, with positive rainfall anomalies in the equatorial Pacific leading to an anomalous release of latent heat in the upper troposphere and an increased subsidence in the SW Pacific. Atmospheric anomalies are strongest in September–November because of a combination of a rather strong zonal SST gradient with the warmest SST in the equatorial Pacific just west of the dateline. The cold CP and EP events are associated with a southwestward shift of the South Pacific Convergence Zone with strongest atmospheric anomalies during the CP events. Squared wavelet coherence between NC rainfall and Nino 4 SST index shows that their negative correlations are mostly carried by two distinct timescales: the classical El Nino–Southern Oscillation (i.e., 3–6 years) variability and a quasi-decadal one (i.e., 10–12 years). The high-frequency (>1/8 cycle per year) correlations peak around Christmas and are quasi-stationary since 1950, whereas the low-frequency ones (<1/8 cycle per year) peak from the austral autumn to the austral spring and have strengthened from ∼1975 to 1980 onward with a subtle warming trend in the equatorial Pacific near the dateline.

Environmental Patterns Associated with Active and Inactive Caribbean Hurricane Seasons

Abstract This study of hurricanes passing through the Caribbean in the 1950–2005 period reveals that seasons with more intense hurricanes occur with the onset of Pacific La Niña events and when Atlantic SSTs west of Africa are above normal. Composites of NCEP reanalysis fields with regard to Caribbean hurricanes reveal development of an anomalous equatorial Atlantic zonal overturning circulation (upper easterly/lower westerly) that shifts toward the Caribbean coincident with a westward spread of the cold tongue in the east Pacific. Ocean–atmosphere coupling is promoted through interaction of the southern Hadley cell and the Atlantic ITCZ. A heat budget analysis suggests that evaporation governs SSTs in the major development region (MDR) and near Venezuela, but the signal is weak prior to May. Using the knowledge gained, statistical algorithms are developed to predict Caribbean hurricanes at seasonal lead times. These make use of equatorial Pacific SST, subtropical Atlantic SST, and the zonal Walker cell over the Atlantic.

The transition from the present-day climate to a modern Snowball Earth

We use the coupled atmosphere–ocean general circulation model ECHAM5/MPI-OM to investigate the transition from the present-day climate to a modern Snowball Earth, defined as the Earth in modern geography with complete sea-ice cover. Starting from the present-day climate and applying an abrupt decrease of total solar irradiance (TSI) we find that the critical TSI marking the Snowball Earth bifurcation point is between 91 and 94% of the present-day TSI. The Snowball Earth bifurcation point as well as the transition times are well reproduced by a zero-dimensional energy balance model of the mean ocean potential temperature. During the transition, the asymmetric distribution of continents between the Northern and Southern Hemisphere causes heat transports toward the more water-covered Southern Hemisphere. This is accompanied by an intensification of the southern Hadley cell and the wind-driven subtropical ocean cells by a factor of 4. If we set back TSI to 100% shortly before the transition to a modern Snowball Earth is completed, a narrow band of open equatorial water is sufficient for rapid melting. This implies that for 100% TSI the point of unstoppable glaciation separating partial from complete sea-ice cover is much closer to complete sea-ice cover than in classical energy balance models. Stable states can have no greater than 56.6% sea-ice cover implying that ECHAM5/MPI-OM does not exhibit stable states with near-complete sea-ice cover but open equatorial waters.

A quasi‐decadal cycle in Caribbean climate

Climatic variables in the period from 1951 to 2000 are analyzed across the tropics from the East Pacific to Africa. A quasi‐decadal mode is isolated using singular value decomposition (SVD) applied to monthly smoothed and detrended rainfall, sea surface temperature (SST), sea level pressure (SLP), and 200 hPa zonal (U) wind anomalies. Seven‐ to 10‐year cycles in Caribbean rainfall are revealed as the dominant mode (50% variance) and are found to be related to a mode of tropical variability that is distinct from previously known global signals. Sources of this signal include east Atlantic SST and the northern subtropical ridge, which modulate upwelling off Venezuela. SVD analysis of daily rainfall suggests interaction between annual and quasi‐decadal signals, with northern summer convection as a driver. The second mode of Caribbean rainfall variability derives from the east Pacific El Niño Southern Oscillation (ENSO, 21% variance) that is expressed as an east‐west dipole of convection across the Caribbean. Composite analysis of rainfall for high and low phases of the quasi‐decadal cycle reveals a corresponding signal that extends from the eastern Pacific Ocean across the Caribbean and West Africa to India. The southern Hadley cell spins up during wet phase, and the ITCZ migrates northward. This hemispheric‐scale anomaly brings pulses of convection to the Caribbean. Impacts of the quasi‐decadal cycle on socioeconomic resources are investigated.

An idealized two‐dimensional approach to study the impact of the West African monsoon on the meridional gradient of tropospheric ozone

An idealized vertical‐meridional zonally symmetrical version of the Méso‐NH model is used to study the response of tropospheric ozone to the dynamics of the West African monsoon, as well as surface emissions and NOx‐production by lightning (LNOx). An O3‐NOx‐VOC chemical scheme has been added to the dynamical model, including surface emissions and a parameterization of the LNOx production. The model shows that the ozone precursors emitted at the surface are uplifted by deep convection and then advected in the upper branches of the Hadley cells on both sides of the Inter Tropical Convergence Zone (ITCZ). The NOx produced by lightning promotes chemical ozone production in the middle and upper troposphere from the oxidation of CO and VOCs. The analysis of the convective and chemical tendencies shows that the ozone minimum at the ITCZ is induced by venting of ozone‐poor air masses into the upper troposphere. The bi‐dimensional model suffers from limitations due to the absence of exchange with the higher latitudes and ventilation in the zonal direction. Despite of these restrictions, sensitivity simulations show that the LNOx source and biogenic VOCs are necessary to create the meridional gradient of ozone observed by the Measurements of OZone and water vapor by in‐service AIrbus airCraft (MOZAIC) aircrafts in the southern Hadley cell. The LNOx source is also required to maintain the meridional ozone gradient up to 24°N in the northern Hadley cell. The modeled meridional gradient of O3 in the upper troposphere ranges from 0.22 to 0.52 ppbv/deg without the LNOx source and from 0.60 to 1.08 ppbv/deg with the LNOx source, in the southern and the northern cells respectively.

Atmospheric electric conductivity measurements over the Indian Ocean during the Indian Antarctic Expedition in 1996–1997

The atmospheric electric conductivity measurements made over the Indian Ocean with a Gerdien's apparatus mounted aboard MV Polarbird during the XVI Indian Scientific Expedition in 1996–1997 are reported. Simultaneous three‐hourly measurements of aerosol concentrations of 13–1000 nm size and some meteorological parameters are also reported. Latitudinal variation of conductivity along the cruise route shows a minimum at ∼28°S. Further, the variations in conductivity in the 10°N–20°S and 60°–70°S latitudinal belts show opposite trends on the outward and return cruises, which fall near to the onset and withdrawal phases of the northeast monsoon season, respectively. The results are explained on the basis of the well‐known northward shift of the subsidence leg of the southern Hadley cell and of the position of the Intertropical Convergence Zone during the months of March–April in this region and the observations of the cyclonic systems near the continent of Antarctica during the period of outward cruise.

The Hadley circulation: assessing NCEP/NCAR reanalysis and sparse in-situ estimates

We present a comparison of the zonal mean meridional circulations derived from monthly in situ data (i.e. radiosondes and ship reports) and from the NCEP/NCAR reanalysis product. To facilitate the interpretation of the results, a third estimate of the mean meridional circulation is produced by subsampling the reanalysis at the locations where radiosonde and surface ship data are available for the in situ calculation. This third estimate, known as the subsampled estimate, is compared to the complete reanalysis estimate to assess biases in conventional, in situ estimates of the Hadley circulation associated with the sparseness of the data sources (i.e., radiosonde network). The subsampled estimate is also compared to the in situ estimate to assess the biases introduced into the reanalysis product by the numerical model, initialization process and/or indirect data sources such as satellite retrievals. The comparisons suggest that a number of qualitative differences between the in situ and reanalysis estimates are mainly associated with the sparse sampling and simplified interpolation schemes associated with in situ estimates. These differences include: (1) a southern Hadley cell that consistently extends up to 200 hPa in the reanalysis, whereas the bulk of the circulation for the in situ and subsampled estimates tends to be confined to the lower half of the troposphere, (2) more well-defined and consistent poleward limits of the Hadley cells in the reanalysis compared to the in-situ and subsampled estimates, and (3) considerably less variability in magnitude and latitudinal extent of the Ferrel cells and southern polar cell exhibited in the reanalysis estimate compared to the in situ and subsampled estimates. Quantitative comparison shows that the subsampled estimate, relative to the reanalysis estimate, produces a stronger northern Hadley cell (∼20%), a weaker southern Hadley cell (∼20–60%), and weaker Ferrel cells in both hemispheres. These differences stem from poorly measured oceanic regions which necessitate significant interpolation over broad regions. Moreover, they help to pinpoint specific shortcomings in the present and previous in situ estimates of the Hadley circulation. Comparisons between the subsampled and in situ estimates suggest that the subsampled estimate produces a slightly stronger Hadley circulation in both hemispheres, with the relative differences in some seasons as large as 20–30%. 6These differences suggest that the mean meridional circulation associated with the NCEP/NCAR reanalysis is more energetic than observations suggest. Examination of ENSO-related changes to the Hadley circulation suggest that the in situ and subsampled estimates significantly overestimate the effects of ENSO on the Hadley circulation due to the reliance on sparsely distributed data. While all three estimates capture the large-scale region of low-level equatorial convergence near the dateline that occurs during El Nino, the in situ and subsampled estimates fail to effectively reproduce the large-scale areas of equatorial mass divergence to the west and east of this convergence area, leading to an overestimate of the effects of ENSO on the zonal mean circulation.

ベンガル湾と北東太平洋の夏季モンスーンの差異

Structure of the July mean local Hadley circulation is investigated by utilizing wind, surface pressure, sea surface temperature (SST), and outgoing longwave radiation (OLR) data along 90°E and 110°W. The objective is to compare two types of monsoon, one is totally driven by land-sea contrast (Asiatic summer monsoon) and the other primarily induced by SST contrast over the ocean (eastern North Pacific summer monsoon).In July, there exists a large-scale Hadley circulation covering both hemispheres along 90°E. Superimposed upon this are distinctly separate regional-scale Hadley cells, one each over either hemisphere. The northern hemisphere Hadley cell is associated with southerly surface winds accelerating from the equator toward the monsoon trough as a low-level branch, vigorous summer monsoon rains near the head Bay of Bengal representing an updraft leg, and 200mb northerly winds as an upper-level return flow. Finally, the northern cell is completed by an ill-defined downdraft leg due to divergent southerly surface winds over the equatorial Bay of Bengal. Continentality with high mountains is the major contributor to enhancement of the summer monsoon over the Bay of Bengal, while the contribution due to SST is minimal. The southern hemisphere Hadley cell is supported by low-level southerly surges bursting out of the midlatitude anticyclone and subjected to considerable warming and air mass modification due to strong SST gradients between 20°S and 10°S. Modified air converges into the near-equatorial trough and causes winter-time rains near 2°-8°S, where an updraft leg of the southern hemisphere Hadley cell is located. The southern Hadley cell interacts with the northern counterpart via low-level cross-equatorial southerlies that are generated as a response to an overall heat contrast between the winter and summer hemispheres.Continentality appears to be insignificant along 110°W. The eastern Pacific is characterized by a zonally oriented cold SST tongue along the equator. This exceptionally strong south-to-north SST gradient causes a prominent poleward surface pressure gradient, which tends to accelerate southerly surface winds to the north of the equator. A large amount of energy is likely to be furnished to these southerlies due to a rapid increase of SST between the equator and about 10°N. Convection becomes strongest near 12°N, two to three degrees south of the thermal equator where SST is maximum (28.3°C). No convection occurs over the tropical South Pacific because of cold SST (less than 25°C). Throughout the year, the inter-tropical convergence zone (ITCZ) remains in the tropical North Pacific. The summer monsoon occurs when the ITCZ (updraft leg of the Hadley circulation) reaches it's northern most latitude of about 14°N in July. Over the equatorial eastern Pacific, the annual variation of surface meridional winds (low-level Hadley circulation) is much more pronounced than the annual variation of surface zonal winds (low-level Walker circulation). This favors the development of the summer monsoon over the eastern North Pacific.

Energy Transfer and Tropical Cell Structure Over the Central Pacific

Abstract The meridional circulation over the equatorial central Pacific Ocean in April 1967 was found to consist of a shallow northern Hadley cell, an extensive equatorial cell centered south of the equator with maximum subsidence over the equator, and a strong southern Hadley cell. Part of the high-level outflow of the southern Hadley cell subsides across the equator and decreases the activity of the northern convergence zone. The divergence of the equatorial easterlies and the resulting subsidence is increased by a low value of the shearing stress which was observed over the cool upwelling equatorial waters. Transfer of energy by radiation, turbulence, precipitation and advection is determined from observations and calculation. Energy budgets are constructed for three regions between 13N and 13S to obtain as a residual the transport of energy by cumulus and cumulonimbus convection. The budget shows that the mid-tropospheric layer of the low energy is maintained by the efficient vertical transfer of ener...