Interhemispheric Sea Surface Temperature Gradient Research Articles

Mid to late 20th century freshening of the western tropical South Atlantic triggered by southward migration of the Intertropical Convergence Zone

In the tropical Atlantic Ocean, the Intertropical Convergence Zone (ITCZ) is an important climate feature controlled by the interhemispheric sea surface temperature (SST) gradient, and greatly influences rainfall patterns over the adjacent continents. To better understand ITCZ dynamics in the context of past and future climate change, long-term oceanic records are needed, but observational data are limited in temporal extent. Shallow-water corals provide seasonally-resolved archives of climate variability over the tropical ocean. Here we present seasonally-resolved records of stable oxygen (δ18O) and carbon (δ13C) isotope values of a Siderastrea stellata coral from northeastern Brazil (Maracajaú, ~5°S). We show that the long-term trends in the record of coral δ18O values are not primarily driven by SST but by hydrological changes at the sea surface. Combining the record of coral δ18O values with instrumental SST, we present the first reconstruction of seawater δ18O changes (δ18Oseawater) in the western tropical South Atlantic back to the early 20th century, a parameter that is related to changes in sea surface salinity. The reconstructed δ18Oseawater changes indicate a prominent freshening between the mid-1940's and mid-1970's, which coincides with a weakening of the Atlantic interhemispheric SST gradient during this time interval. Our results suggest that the weakened Atlantic SST gradient resulted in a southward shift of the thermal equator that was accompanied by a southward migration of the ITCZ, resulting in freshening of the western tropical South Atlantic during the mid to late 20th century.

Earlier onset of the Early Cretaceous Equatorial humidity belt

The long-standing view on the Cretaceous climate zones suggests the inception of an Equatorial Humid Belt (EHB) in the Albian. This inception contrasts with recent sedimentological and palynological evidence of sustained low-latitude, wetter paleoenvironment in the South American and African landmasses during the Aptian. The trigger for this early-stage EHB stands on two hypotheses: [1] the first major marine incursions to the equatorial Gondwana; [2] the southward displacement of a proto-Intertropical Convergence Zone (ITCZ). We provide new palynological, ichnological, and sedimentological data from the Codó Formation obtained from borehole 2-ANP-5-MA, drilled in the São Luís Basin, Brazilian equatorial margin. This integrated analysis allowed recovering well-preserved palynomorph assemblages of the upper Aptian Codó Formation. Based on three palynomorph-based climate indicators (sporomorph genera richness, the relative abundance of reworked palynomorphs, and climate-indicative palynofloras), and independent climate-indicative sedimentological indicators, our results show an interval of increasingly humid climate, which precedes the first major marine incursion. It also confirms that sporomorph genera richness scales directly with precipitation levels. Based on that, we reconstructed coeval late Aptian humidity patterns from thirteen South American and African sedimentary basins. Our findings support the establishment of the EHB in the late Aptian and challenge the view of a dominantly arid Equatorial Gondwana before the Albian. We argue that decreasing Proto-North Atlantic/South Atlantic sea-surface temperature (SST) gradient may have triggered a southward EHB displacement, leading to wetter, low-latitude biomes with broad and complex ecosystems. For the first time, a plausible mechanism is raised to explain the linkage between changing interhemispheric SST gradient and the dynamics of a proto-ITCZ in the Early Cretaceous.

Spatial modeling of rainfall patterns and groundwater on the coast of northeastern Brazil

The objective of this study was to characterize the patterns of monthly rainfall in the mesoregions of Zona da Mata and Metropolitan of Recife in the state of Pernambuco, Brazil, based on geostatistical techniques and multivariate analysis, in addition to establishing rain flow strategies in urban centers and alerting of possible impacts on local aquifers. The meteorological data are from a 20-year time series (2000–2019). Maps of the projection of aquifers, hydrochemical and hydrogeological for the study region were established. The pluviometric data were submitted to descriptive, geostatistical, and multivariate analysis. The Gaussian model was the one that best fitted (R2 ≥ 0.84). The Atlantic interhemispheric sea surface temperature gradient together with Easterly Wave Disturbances, Intertropical Convergence Zone, sea breezes circulations, and trade winds interfered in the dynamics of coastal rains, especially on the Northeast Brazilian coast. The hydrogeological formation of groundwater on the coast, of the porous type, justified the higher flows of groundwater associated with the greater rainfall. Kriging maps were consistent in the spatialization of rains on the coast of Pernambuco and, thus, making it possible to assess the dynamics of rains and the creation of socio-economic strategies, to minimize the impacts during the period of heavy rains and major floods.

Forcing of western tropical South Atlantic sea surface temperature across three glacial-interglacial cycles

Abstract The western tropical Atlantic (WTA) supplies warm and saline waters to the upper-limb of the Atlantic Meridional Overturning Circulation (AMOC) and may store excess heat and salinity during periods of AMOC slowdown. Since previous sea surface temperature (SST) reconstructions from the WTA typically focus on the Last Glacial Maximum and the last deglaciation, additional long-term records spanning multiple glacial-interglacial transitions are needed in order to elucidate the drivers of long-term WTA SST variability. We performed Mg/Ca analyses on the surface-dwelling planktic foraminifera Globigerinoides ruber (pink) from a sediment core raised from the southern WTA to reconstruct SST changes over the past 322 kyr. We evaluate the relative importance of atmospheric pCO2, AMOC strength and trade-wind intensity in driving the thermal evolution of the WTA across three glacial-interglacial cycles. Our results indicate a lack of pronounced glacial-interglacial variability in the SST record, prompting us to exclude atmospheric pCO2 as a direct driver of SST variations in the southern WTA. Similarly, we conclude that variations in AMOC strength also likely did not have a strong influence on long-term WTA SST, based on the low and relatively stable interhemispheric SST gradient over the past 322 kyr. Our results reveal high-amplitude variability in zonal SST gradients within the (sub)tropical South Atlantic and similarities between the long-term patterns of the intrahemispheric meridional SST gradient and our SST record. Based on these findings, we propose that changes in the intrahemispheric meridional SST gradient modulate southeast trade wind intensity, which in turn drives variations in zonal SST gradients and southern WTA SSTs.

Dynamics of Asian Summer Monsoon Response to Anthropogenic Aerosol Forcing

Anthropogenic aerosols partially mask the greenhouse warming and cause the reduction in Asian summer monsoon precipitation and circulation. By decomposing the atmospheric change into the direct atmospheric response to radiative forcing and sea surface temperature (SST)-mediated change, the physical mechanisms for anthropogenic-aerosol-induced changes in the East Asian summer monsoon (EASM) and South Asian summer monsoon (SASM) are diagnosed. Using coupled and atmospheric general circulation models, this study shows that the aerosol-induced troposphere cooling over Asian land regions generates anomalous sinking motion between 20° and 40°N and weakens the EASM north of 20°N without SST change. The decreased EASM precipitation and the attendant wind changes are largely due to this direct atmospheric response to radiative forcing, although the aerosol-induced North Pacific SST cooling also contributes. The SST-mediated change dominates the aerosol-induced SASM response, with contributions from both the north–south interhemispheric SST gradient and the local SST cooling pattern over the tropical Indian Ocean. Specifically, with large meridional gradient, the zonal-mean SST cooling pattern is most important for the Asian summer monsoon response to anthropogenic aerosol forcing, resulting in a reorganization of the regional meridional atmospheric overturning circulation. While uncertainty in aerosol radiative forcing has been emphasized in the literature, our results show that the intermodel spread is as large in the SST effect on summer monsoon rainfall, calling for more research into the ocean–atmosphere coupling.

The Effects of External Forcing and Internal Variability on the Formation of Interhemispheric Sea Surface Temperature Gradient Trends in the Indian Ocean

Abstract A striking trend of the Indian Ocean interhemispheric gradient in sea surface temperatures (SSTs) developed during the recent global warming hiatus. The contributions of external forcing and internal variability to this trend are examined in forced climate model experiments. Results indicate that the observed negative trend was strong by historical standards and most likely due to internal variability rather than to external forcing. Anthropogenic aerosol forcing favors negative gradient trends, but its effects are countered by greenhouse gas forcing, and both are weak relative to internal variability. The observed interhemispheric gradient trend occurred in parallel with a negative phase of the interdecadal Pacific oscillation (IPO), a linkage that is also found in climate models. However, the physical mechanisms responsible for these gradient trends in models differ from those in ocean reanalysis products. In particular, oceanic processes via an increased Indonesian Throughflow (ITF) transport into the Indian Ocean forced by stronger Pacific trade winds are the principal cause of the observed negative SST gradient trend during 2000–13. In contrast, atmospheric processes via changing surface wind stress over the southern Indian Ocean remotely forced by the IPO appear to play a dominant role in changing the interhemispheric SST gradients in climate models. The models underestimate the magnitude of the IPO and produce changes in the ITF that are too weak owing to their coarse spatial resolution. These model deficiencies may account for the differences between the simulations and observations.

Rainfall variability over Alagoas under the influences of SST anomalies

The rainfall variability for the state of Alagoas, Northeast of Brazil, was evaluated based on the Standardized Precipitation Index (SPI). Harmonic decomposition was applied to 31 years-long (1960–1990) series of SPI, from 33 stations, to relate their modes of variability to El Nino-Southern Oscillation (ENSO) and the Atlantic Ocean sea surface temperature (SST). The most important harmonics identified by the spectral analysis had periods of 10–15 and 2–3 years, followed by other oscillations with smaller periods. The 10–15 years harmonic was associated with the Atlantic interhemispheric SST gradient (AITG), a cross-equatorial dipole which impacts the northeast region of Brazil by influencing the position of the intertropical convergence zone (ITCZ), leading to dry or wet conditions. The 2–3 years harmonic was consistent with the variability of ENSO events. The harmonic analysis is a powerful tool to identify the principal modes of variability of SPI. Although the magnitude of SPI is underestimated in some cases, this tool significantly increases the knowledge of the main drivers of rainfall and droughts in the region.

Fast and Slow Responses to Global Warming: Sea Surface Temperature and Precipitation Patterns

Abstract The time-dependent response of sea surface temperature (SST) to global warming and the associated atmospheric changes are investigated based on a 1% yr−1 CO2 increase to the quadrupling experiment of the Geophysical Fluid Dynamics Laboratory Climate Model, version 2.1. The SST response consists of a fast component, for which the ocean mixed layer is in quasi equilibrium with the radiative forcing, and a slow component owing to the gradual warming of the deeper ocean in and beneath the thermocline. A diagnostic method is proposed to isolate spatial patterns of the fast and slow responses. The deep ocean warming retards the surface warming in the fast response but turns into a forcing for the slow response. As a result, the fast and slow responses are nearly opposite to each other in spatial pattern, especially over the subpolar North Atlantic/Southern Ocean regions of the deep-water/bottom-water formation, and in the interhemispheric SST gradient between the southern and northern subtropics. Wind–evaporation–SST feedback is an additional mechanism for the SST pattern formation in the tropics. Analyses of phase 5 of the Coupled Model Intercomparison Project (CMIP5) multimodel ensemble of global warming simulations confirm the validity of the diagnostic method that separates the fast and slow responses. Tropical annual rainfall change follows the SST warming pattern in both the fast and slow responses in CMIP5, increasing where the SST increase exceeds the tropical mean warming.

Long-Term Behavior of the Atlantic Interhemispheric SST Gradient in the CMIP5 Historical Simulations

Abstract Multidecadal and longer changes to the Atlantic interhemispheric sea surface temperature gradient (AITG) in phase 5 of the Coupled Model Intercomparison Project (CMIP5) historical simulations are investigated. Observations show a secular trend to this gradient over most of the twentieth century, with the southern lobe warming faster relative to its northern counterpart. A previous study of phase 3 of the CMIP (CMIP3) suggests that this trend is partially forced by anthropogenic sulfate aerosols. This analysis collectively confirms the partially forced trend for the CMIP5 and by anthropogenic aerosols. Like the CMIP3, the CMIP5 also simulates a reversal in the AITG trend in the late 1970s, which was attributed to a leveling off of the anthropogenic aerosol influence and increased influence of greenhouse gases in the late twentieth century. Two (of 25) CMIP5 models, however, systematically simulate a twentieth-century trend opposite to observed, leading to some uncertainty regarding the forced nature of the AITG trend. The observed AITG also exhibits a pronounced multidecadal modulation on top of the trend, associated with the Atlantic multidecadal oscillation (AMO). Motivated by a recent suggestion that the AMO is a forced response to aerosols, the causes of this multidecadal behavior were also examined. A few of the CMIP5 models analyzed do produce multidecadal AITG variations that are correlated to the observed AMO-like variation, but only one, the Hadley Centre Global Environmental Model, version 2 (HadGEM2), systematically simulates AMO-like behavior with both the requisite amplitude and phase. The CMIP5 simulations thus point to a robust aerosol influence on the historical AITG trend but not to the AMO-like multidecadal behavior.

The response of the large‐scale ocean circulation to 20th century Asian and non‐Asian aerosols

Multidecadal trends in the large‐scale ocean circulation are influenced by changes in radiative forcings such as long‐lived greenhouse gases, volcanic aerosols, and solar irradiance. Model simulations suggest that anthropogenic aerosols can also force circulation changes, including delaying the weakening of the North Atlantic thermohaline circulation, altering the interhemispheric sea surface temperature gradient, and inducing a pan‐oceanic heat redistribution. The extent to which aerosols from different regions contribute to these oceanic changes is currently unknown. Using specifically designed 20th century coupled climate model experiments that separate Asian and non‐Asian aerosol impacts, it is shown that the non‐Asian aerosol component, predominantly sulfate aerosols, accounts for much of the simulated aerosol‐induced oceanic changes. These include delaying the weakening of the global meridional circulation, increasing the northward heat transport across the equatorial Atlantic, and inducing a subsurface cooling in the subtropical southern Indian Ocean. As global sulfate aerosol levels peaked in the 1980s, these trends may be starting to reverse. This study highlights the importance of Northern Hemisphere non‐Asian anthropogenic aerosols in driving remote changes in Southern Hemisphere subtropical and extratropical oceans.

Exploring the climate problems of Brazil’s Nordeste: a review

This article reviews the exploration of the climate dynamics of Brazil’s Nordeste in the course of a century. The drought-prone and semi-arid northern Nordeste of Brazil experiences a short rainy season around March–April, when the interhemispheric gradient of sea surface temperature (SST) in the tropical Atlantic is weakest and the Intertropical Convergence Zone (ITCZ) reaches its southernmost position in the course of the annual cycle. The recurrent Secas (droughts) have a severe socio-economic impact. During drought years, the interhemispheric SST gradient in the tropical Atlantic is steep and the ITCZ stays far North, while the waters of the eastern equatorial Pacific tend to be anomalously warm. Based on the extensive diagnostic exploration of the circulation mechanisms of interannual climate variability, empirical methods have been developed for the forecasting of the Nordeste rainy season. These have been applied in the real-time prediction of seasonal rainfall anomalies throughout the last decade of the 20th century.

Seasonal anomalous rainfall in the central and eastern Amazon and associated anomalous oceanic and atmospheric patterns

Previous papers have shown that the rainfall anomalies over the Amazon region result from the independent or combined effects of the eastern tropical Pacific and Atlantic Basins. Here, the monthly rainfall data of Itaituba, Manaus, Parintins, and Santarem for the 1931–1996 period has been used to classify its dry and rainy seasons into dry, very dry, wet, and very wet categories. The sea surface temperature (SST) and sea level pressure (SLP) patterns associated with these climate conditions in the central and eastern Amazon are discussed in terms of the relative influence of the tropical Atlantic and Pacific sectors, as well as their seasonal differences. These seasonal differences are, in part, determined by the seasonal phase locking of the El Nino-Southern Oscillation (ENSO) and Atlantic equatorial modes. The rainy season cases are mostly driven by the ENSO and the inter-hemispheric SST gradient mode in the tropical Atlantic, and the dry season cases, by the Atlantic equatorial mode (AEM). A new result here is the role of the inter-Pacific-Atlantic anomalous SST gradient mode on the rainfall anomalies. This mode acts concordantly with the Atlantic modes on the rainfall over the central and eastern Amazon, and is responsible for precipitation extreme cases during the dry season and for the very wet rainy season case. In other words, the intense inter-Pacific-Atlantic mode for these cases is the differential factor in relation to the moderate cases. The results here, especially in relation to the role of the inter-Pacific-Atlantic gradient in the central and eastern Amazon precipitation, have not been discussed before and should be taken into consideration in the diagnostic activities. Copyright © 2011 Royal Meteorological Society

The Role of Oceanic Heat Advection in the Evolution of Tropical North and South Atlantic SST Anomalies*

Abstract The role of horizontal oceanic heat advection in the generation of tropical North and South Atlantic sea surface temperature (SST) anomalies is investigated through an analysis of the oceanic mixed layer heat balance. It is found that SST anomalies poleward of 10° are driven primarily by a combination of wind-induced latent heat loss and shortwave radiation. Away from the eastern boundary, horizontal advection damps surface flux–forced SST anomalies due to a combination of mean meridional Ekman currents acting on anomalous meridional SST gradients, and anomalous meridional currents acting on the mean meridional SST gradient. Horizontal advection is likely to have the most significant effect on the interhemispheric SST gradient mode through its impact in the 10°–20° latitude bands of each hemisphere, where the variability in advection is strongest and its negative correlation with the surface heat flux is highest. In addition to the damping effect of horizontal advection in these latitude bands, evidence for coupled wind–SST feedbacks is found, with anomalous equatorward (poleward) SST gradients contributing to enhanced (reduced) westward surface winds and an equatorward propagation of SST anomalies.

Circulation and teleconnection mechanisms of Northeast Brazil droughts

The Northern Nordeste of Brazil has its short rainy season narrowly concentrated around March–April, when the interhemispheric southward gradient of sea surface temperature (SST) is weakest and the Intertropical Convergence Zone (ITCZ), which is the main rainbearing system for the Nordeste, reaches its southernmost position in the course of the year. The recurrent Secas (droughts) have a severe socio-economic impact in this semi-arid region. In drought years, the pre-season (October–January) rainfall is scarce, the interhemispheric SST gradient weakened and the basin-wide southerly (northerly) wind component enhanced (reduced), all manifestations of an anomalously far northward ITCZ position. Apart from this ensemble of Atlantic indicators, the Secas also tend to be preceded by anomalously warm equatorial Pacific waters in January. During El Niño years, an upper-tropospheric wave train extends from the equatorial eastern Pacific to the northern tropical Atlantic, affecting the patterns of upper-tropospheric topography and divergence, and hence of vertical motion over the Atlantic. The altered vertical motion leads to a weaker meridional pressure gradient on the equatorward flank of the North Atlantic subtropical high, and thus weaker North Atlantic tradewinds. The concomitant reduction of evaporation and wind stirring allows for warmer surface waters in the tropical North Atlantic and thus steeper interhemispheric meridional thermal gradient. Consequently, the ITCZ stays anomalously far North and the Nordeste rainy season becomes deficient.

Mechanisms Controlling Variability of the Interhemispheric Sea Surface Temperature Gradient in the Tropical Atlantic

Abstract The seasonal evolution of sea surface temperature (SST) fields in the tropical Atlantic is explored for composites of extremely STRONG and WEAK northward SST gradients, because these are known to control the basinwide pressure gradient, latitude position of the intertopical convergence zone (ITCZ), and regional rainfall. Aimed at the origin and maintenance of anomalies in the interhemispheric SST gradient, and using surface and subsurface marine observations, differences in forcing of SST patterns from surface heat fluxes and entrainment are scaled by calendar-monthly mixed layer depth, and compared to the observed evolution of SST. For years with a STRONG as compared to WEAK gradient in boreal winter, a substantial portion of anomalous SST evolution from September to February can be attributed to departures in SST forcing by latent heat transfer: in the North Atlantic, strengthened northeast trades associated with a stronger North Atlantic high promote enhanced evaporative cooling, while weakene...

Decadal‐scale trends in mechanisms controlling meridional sea surface temperature gradients in the tropical Atlantic

The meridional gradient of sea surface temperature (SST) in the tropical Atlantic hydrostatically controls the atmospheric pressure gradient, thereby steering the latitude position of the Intertropical Convergence Zone (ITCZ), which affects rainfall in adjacent land areas. This study investigates the mechanisms controlling long‐term changes in meridional SST gradients, using surface and subsurface ship observations from 1951–1990. SST forcing from surface heat fluxes and entrainment are scaled by monthly climatic‐mean mixed‐layer depth. Observed SST tendency and contributions from various forcing mechanisms are computed for the 1951–1990 mean and trend. A substantial warming trend in the surface waters of the tropical South Atlantic is concentrated in the austral summer months of February–March, while SST in the tropical North Atlantic increased in boreal summer (August–September) and decreased in winter. As a consequence of the strengthened interhemispheric SST gradient in February–March, the position of the ITCZ was displaced southward, and rainfall in northeast Brazil increased over 1951–1990. A southward displacement of the ITCZ during boreal summer, accompanying warming between the equator and the ITCZ contributed to the decrease of rainfall over the west African Sahel. Strongest warming trends in the respective summer hemisphere imply an amplification of both seasonal cooling during winter and warming during summer in each hemisphere. During September–February a trend toward intensified northeast trade winds results in increased seasonal cooling due to latent heat transfer, whereas weakened southeast trades reduce latent heat loss, intensifying seasonal warming. For March–August a dominant mechanism forcing enhanced seasonal cooling in the South Atlantic is associated with entrainment as mixed‐layer depth increases during the transition to winter; this process acts to dilute warm anomalies developed during summer.

Forcing of anomalous sea surface temperature evolution in the tropical Atlantic during Pacific warm events

The seasonal evolution of anomalous interhemispheric sea surface temperature (SST) gradients in the tropical Atlantic from January to April is studied by composites of the 10 warmest (warm) and 10 coldest (cold) Januaries during 1948–1993 in the equatorial Pacific using Comprehensive Ocean‐Atmosphere Data Set ship observations. In the warm as compared with the cold years, an anomalous weakening of the northward SST gradient develops, mainly due to anomalous warming in the tropical North Atlantic. This stems from the combination of three forcings all related to the weakened North Atlantic trade winds during Pacific warm events. Most important are the reduced latent heat flux in much of the tropical North Atlantic and anomalous downwelling equatorward of 20°N, with a further contribution from increased net radiation resulting from the reduced cloudiness due to the diminished convergence in the downstream portion of the North Atlantic trades. In response to the development of warm anomalies in the tropical North Atlantic during January–March, the cross‐equatorial northward winds accelerate to April, and this leads south of the equator not only to Ekman downwelling, enhanced divergence, reduced cloudiness, and increased net radiation, but also to enhanced wind speed and evaporation. The result is a modest anomalous warming in the western tropical South Atlantic. Increased air temperature over the entire basin, presumably due to atmospheric advection from the Pacific, has little effect on the Atlantic SST pattern. The anomalous interhemispheric SST gradient, controlled primarily by the warm anomaly in the North Atlantic, has a pivotal role to play in steering the late boreal winter atmospheric circulation in the tropical Atlantic sector and thus regional climate anomalies.