Atlantic Sea Surface Temperature Dipole Research Articles

How the two nodes of the tropical Atlantic sea surface temperature dipole relate the climate of the surrounding regions during austral autumn

The relations of both nodes of the tropical Atlantic (TA) cross‐equatorial sea surface temperature (SST) gradient (GRAD) mode to the climate of northeast Brazil (NEB) during the 1901–2012 period are examined separately. The GRAD mode consists of a SST anomalous pattern anti‐symmetric about the equator with centres in the region of the northeasterly and southwesterly trade winds. The positive GRAD (PGRAD) (negative GRAD [NGRAD]) modes defined by their northern or southern nodes are considered separately, as well as, if they are preceded or not by an El Niño (EN) (a La Niña, LN). The PGRAD mode defined by the northern (southern) node and preceded by an EN is indicated by PGRAD_TNA_EN (PGRAD_TSA_EN), and that not preceded by an EN is indicated by PGRAD_TNA (PGRAD_TSA). The corresponding NGRAD modes are indicated by NGRAD_TNA_LN, NGRAD_TSA_LN, NGRAD_TNA and NGRAD_TSA. Our analysis is limited to the NEB rainy season (March–May), when the GRAD mode is more pronounced and the NEB rainfall is more closely related to the TA SST variations. In general, the South Atlantic subtropical high is crucial in defining the cross‐equatorial flow, responsible for the anomalous meridional positioning of the Atlantic inter‐tropical convergence zone. When a positive (negative) dipole‐like SST anomaly pattern is established in the TA, the negative (positive) rainfall anomalies occupy most of NEB. This is the case of the PGRAD_TSA_EN, PGRAD_TNA, NGRAD_TSA_LN and NGRAD_TNA_LN. Most of the GRAD modes defined by TNA are also related to the El Niño‐Southern Oscillation (ENSO), in such a way that an EN (a LN) precedes a PGRAD (NGRAD) event, and most of the GRAD modes defined by TSA do not depend on the ENSO. Also, the PGRAD events are well distributed during both Atlantic multidecadal oscillation phases and the NGRAD events occur preferably during its cold phase.

Analogous seasonal evolution of the South Atlantic SST dipole indices

AbstractTwo variants of sea‐surface temperature (SST) dipole indices for the South Atlantic Ocean (SAO) has been previously described representing: (1) the South Atlantic subtropical dipole (SASD) supposedly peaking in austral summer and (2) the SAO dipole (SAOD) in winter. In this study, we present the analysis of observational data sets (1985–2014) showing the SASD and SAOD as largely constituting the same mode of ocean–atmosphere interaction reminiscent of the SAOD structure peaking in winter. Indeed, winter is the only season in which the inverse correlation between the northern and southern poles of both indices is statistically significant. The observed SASD and SAOD indices exhibit robust correlations (P ≤ 0.001) in all seasons and these are reproduced by 54 of the 63 different models of the Coupled Models Intercomparison Project analysed. Their robust correlations notwithstanding the SASD and SAOD indices appear to better capture different aspects of SAO climate variability and teleconnections.

How the AMOC affects ocean temperatures on decadal to centennial timescales: the North Atlantic versus an interhemispheric seesaw

This study investigates how variations of the Atlantic meridional overturning circulation (AMOC) affect sea surface temperature (SST) within the simulations of the coupled model intercomparison project phase 5. In particular, we explore whether the SST response is interhemispheric in nature, specifically as reflected in the Atlantic SST Dipole index, or whether the response is localized more in the North Atlantic Ocean. In the absence of direct observational data, this Dipole index has been proposed to approximate AMOC variations over the duration of the instrumental temperature record. We find that typically, on timescales between decadal and centennial, the SST Dipole index correlates with the AMOC with coefficients ranging from 0.2 to 0.7, typically with a 0–6 year lag, and thus explains less than half of the AMOC variance. In just two models this value slightly exceeds 50 %. Even for the models with the highest correspondence between the AMOC and the Dipole index, the correlation between the two variables is controlled mainly by SST variations in the North Atlantic, not the South Atlantic, both for the model control and historical simulations. Consequently, in nearly all models, the North Atlantic SST provides a better indicator of AMOC variations than the Atlantic Dipole. Thus, on decadal to centennial timescales AMOC variability affects mainly the North Atlantic Ocean, with the sensitivity of the North Atlantic SST between 40 and 60°N, given by the multi-model average, of about 0.3 °C per 1 Sv of AMOC change, explaining roughly one third of the SST variance.

Predictability of the mid-latitude Atlantic meridional overturning circulation in a multi-model system

Assessing the skill of the Atlantic meridional overturning circulation (AMOC) in decadal hindcasts (i.e. retrospective predictions) is hampered by a lack of observations for verification. Models are therefore needed to reconstruct the historical AMOC variability. Here we show that ten recent oceanic syntheses provide a common signal of AMOC variability at 45°N, with an increase from the 1960s to the mid-1990s and a decrease thereafter although they disagree on the exact magnitude. This signal correlates with observed key processes such as the North Atlantic Oscillation, sub-polar gyre strength, Atlantic sea surface temperature dipole, and Labrador Sea convection that are thought to be related to the AMOC. Furthermore, we find potential predictability of the mid-latitude AMOC for the first 3–6 year means when we validate decadal hindcasts for the past 50 years against the multi-model signal. However, this predictability is not found in models driven only by external radiative changes, demonstrating the need for initialization of decadal climate predictions.

Intraseasonal and Interannual Variability of Extreme Dry and Wet Events over Southeastern South America and the Subtropical Atlantic during Austral Summer

Abstract Intraseasonal and interannual variability of extreme wet and dry anomalies over southeastern Brazil and the western subtropical South Atlantic Ocean are investigated. Precipitation data are obtained from the Global Precipitation Climatology Project (GPCP) in pentads during 23 austral summers (December–February 1979/80–2001/02). Extreme wet (dry) events are defined according to 75th (25th) percentiles of precipitation anomaly distributions observed in two time scales: intraseasonal and interannual. The agreement between the 25th and 75th percentiles of the GPCP precipitation and gridded precipitation obtained from stations in Brazil is also examined. Variations of extreme wet and dry anomalies on interannual time scales are investigated along with variations of sea surface temperature (SST) and circulation anomalies. The South Atlantic SST dipole seems related to interannual variations of extreme precipitation events over southeastern Brazil. It is shown that extreme wet and dry events in the continental portion of the South Atlantic convergence zone (SACZ) are decoupled from extremes over the oceanic portion of the SACZ and there is no coherent dipole of extreme precipitation regimes between tropics and subtropics on interannual time scales. On intraseasonal time scales, the occurrence of extreme dry and wet events depends on the propagation phase of extratropical wave trains and consequent intensification (weakening) of 200-hPa zonal winds. Extreme wet and dry events over southeastern Brazil and subtropical Atlantic are in phase on intraseasonal time scales. Extreme wet events over southeastern Brazil and subtropical Atlantic are observed in association with low-level northerly winds above the 75th percentile of the seasonal climatology over central-eastern South America. Extreme wet events on intraseasonal time scales over southeastern Brazil are more frequent during seasons not classified as extreme wet or dry on interannual time scales.

Interannual variability of the tropical Atlantic independent of and associated with ENSO: Part I. The North Tropical Atlantic

AbstractThe interannual variability of the tropical Atlantic ocean–atmosphere system is examined using 50 years of sea‐surface temperature (SST) and re‐analysis data, and satellite data when available. A singular value decomposition analysis of 12‐ to 72‐month bandpass filtered SST and zonal wind stress reveals two dominant modes of interannual variability. The SST anomalies are confined to the North Tropical Atlantic (NTA) in the first mode and extend over the equatorial and South Tropical Atlantic in the second mode. No evidence is found for an Atlantic SST dipole. The structure of the first (NTA) mode is examined in detail here, while the second mode has been described in a companion paper. In particular, the relationship of the NTA mode with El Niño–Southern Oscillation (ENSO) is investigated. There are 12 NTA events (seven warm and five cold) that are associated with ENSO, and 18 NTA events (seven warm and 11 cold) that are independent of ENSO.The ENSO‐associated NTA events appear to be a passive response to remote ENSO forcing, mainly via a Pacific‐North America (PNA)‐like wave train that induces SST anomalies over the NTA through changes in the surface wind and latent heat flux. The NTA anomalies peak four months after ENSO. There does not appear to be an atmospheric response to the NTA SST anomalies as convection over the Atlantic is suppressed by the anomalous Walker circulation due to ENSO.The ENSO‐independent NTA events also appear to be induced by an extratropical wave train from the Pacific sector (but one that is independent of Pacific SST), and forcing by the North Atlantic Oscillation (NAO) also contributes. As the event matures, the atmosphere does respond to the NTA SST anomalies, with enhanced convection over the Caribbean and a wave train that propagates northeastward to Europe. Copyright © 2006 Royal Meteorological Society

The effect of Atlantic sea surface temperature dipole mode on hurricanes: Implications for the 2004 Atlantic hurricane season

Results from this study indicate that the dipole mode of tropical Atlantic Ocean sea surface temperature (SST) anomalies is correlated with the overall activity of hurricanes as well as with the annual hurricane landfall frequency along the southeast coast of the United States. The tropical Atlantic SST dipole mode could affect hurricanes in at least three ways: 1) modulating the weather in West Saharan Africa; 2) influencing the local SST and hence the atmosphere‐ocean environment in the hurricane main development region; 3) coupling with the tropical and subtropical atmospheric circulation that controls the steering of hurricanes. The warm tropical North Atlantic and cool tropical South Atlantic waters are likely one of the main causes for Florida's hurricane havoc in 2004.

Thermodynamic air-sea interactions and tropical atlantic SST dipole pattern

Climate variability of the Tropical Atlantic is examined from the point of view of the air-sea thermodynamic interactions. Data from the reanalyses of the ECMWF and NCEP are used to investigate the physical mechanism of tropical Atlantic Sea Surface Temperature (SST) dipole pattern. Results suggest that the well-known positive feedback between the SST and the wind-induced evaporation may not be a sufficient mechanism to make the SST oscillate or maintain the dipole pattern since the thermal damping effect may also be important.

Climate change and mass movements in the NW Argentine Andes

The chronology of multiple landslide deposits and related lake sediments in the eastern Argentine Cordillera suggests that major mass movements cluster in two time periods during the Quaternary: between 35 000 and 25 000 14C yr BP and after 5000 14C yr BP. The older cluster may correspond to the Minchin wet period (40 000 and 25 000 14C yr BP) identified in tropical and subtropical South America, suggesting a causal relation between enhanced landslide activity and climate change. The younger cluster predates the Titicaca wet period that began at about 3900 14C yr BP which also affected other regions in the Andes and the Amazon Basin. No landslide and associated lake sediments are documented during the Tauca wet period (between 16 000 and 8000 14C yr BP). However, the two clusters correspond to periods where it assumed that the El Niño/Southern Oscillation (ENSO) and tropical Atlantic sea surface temperature dipole (TAD) were active. The analysis of the present-day precipitation patterns in NW Argentina indicates significant spatial and temporal differences between the intra-Andean part of the study area and the Andean foreland. Whereas the TAD seems to consistently increase rainfall, the intensity of precipitation during the El Niño phase of the ENSO is reduced to only 25% of the mean annual average in the intra-Andean basins, whereas the regions east of the Andes receive more than 125%. Similar results, but with an opposite sign, characterize La Niña events. The comparison of this pattern with paleo-precipitation data as inferred from varved lake sediments suggests that increased interannual climate variability and, therefore, increased fluctuations in rainfall and river discharge in narrow valleys may reduce landsliding thresholds.

Formation of landslide-dammed lakes during a wet period between 40,000 and 25,000 yr B.P. in northwestern Argentina

New AMS radiocarbon data from the deposits of landslide-dammed lakes in the Santa Maria Basin (26°S, 66°W) and the Quebrada del Toro (25°S, 66°W) in the eastern Argentine Cordillera show that important mass movements occurred during a wet period between 40,000 and 25,000 yr B.P. In addition to increased humidity during this time, detailed analysis of annual-layered lacustrine strata suggests enhanced inter- and intra-annual fluctuations in precipitation as compared to the present. Variations in sediment characteristics and diatom assemblages with mean periodicities of 2 to 5 and 10 to 14 years, respectively, suggest an influence of the El Niño/Southern Oscillation (ENSO) and the Atlantic sea-surface temperature dipole. The Atlantic dipole slightly increased rainfall every 10–14 years as at present. In contrast, the weak modern ENSO influence in this area differs from the past situation where rainfall increased significantly with ENSO-type periodicities. Both increased humidity and seasonality are believed to have reduced thresholds for the generation of catastrophic mass movements in northwestern Argentina.

A linkage between decadal climate variations in the Labrador Sea and the tropical Atlantic Ocean

According to observations, the Labrador Sea Water (LSW) thickness varies significantly on decadal time scales. Sea surface temperature (SST) in the tropical Atlantic also exhibits considerable decadal changes, including the so‐called tropical Atlantic SST dipole—a cross‐equatorial pattern. In this study, we propose that the SST dipole and variations of LSW thickness are linked through the Meridional Overturning Circulation (MOC). Southward transport of LSW along the deep western boundary must be compensated by northward flow in the upper ocean. Once the LSW pulse enters the tropics, it affects the cross‐equatorial heat transport and the upper ocean response generates a dipole pattern in the SST field. The correlation between observed LSW thickness and SST with a lag of 5 years is significant at 95% significance level. Results from an ocean model indicate that the time lag between LSW and tropical SST is set primarily by coastally‐trapped waves.

A Pilot Research Moored Array in the Tropical Atlantic (PIRATA)

The tropical Atlantic Ocean is characterized by a large seasonal cycle around which there are climatically significant interannual and decadal timescale variations. The most pronounced of these interannual variations are equatorial warm events, somewhat similar to the El Niño events for the Pacific, and the so-called Atlantic sea surface temperature dipole. Both of these phenomena in turn may be related to El Niño-Southern Oscillation variability in the tropical Pacific and other modes of regional climatic variability in ways that are not yet fully understood. PIRATA (Pilot Research Moored Array in the Tropical Atlantic) will address the lack of oceanic and atmospheric data in the tropical Atlantic, which limits our ability to make progress on these important climate issues. The PIRATA array consists of 12 moored Autonomous Temperature Line Acquisition System buoy sites to be occupied during the years 1997–2000 for monitoring the surface variables and upper-ocean thermal structure at key locations in the tropical Atlantic. Meteorological and oceanographical measurements are transmitted via satellite in real time and are available to all interested users in the research or operational communities. The total number of moorings is a compromise between the need to put out a large enough array for a long enough period of time to gain fundamentally new insights into coupled ocean–atmosphere interactions in the region, while at the same time recognizing the practical constraints of resource limitations in terms of funding, ship time, and personnel. Seen as a pilot Global Ocean Observing System/Global Climate Observing System experiment, PIRATA contributes to monitoring the tropical Atlantic in real time and anticipates a comprehensive observing system that could be operational in the region for the 2000s.