One of the main modes of sea surface temperature variability in the Tropical Atlantic is the Atlantic Equatorial Mode or Atlantic Niño. The region of largest interannual variability, where the ...Atlantic Cold Tongue forms, is also a region of consistent biases in climate models. In this study, we investigate the interannual variability of the Tropical Atlantic and its changes in the recent decades in terms of the Bjerknes Feedback Index (
I
BJ
) in a set of seven ocean reanalyses for the periods 1980–1999 and 2000–2010. Differences were observed among the reanalyses regarding their representation of each term of the Bjerknes Feedback, particularly regarding zonal surface currents, leading to differences in dynamical damping and the zonal advective feedback. However, a consistent negative index (damped) is observed in all reanalysis, with the thermocline feedback being the dominant positive term and thermal damping the dominant negative term. Warming trends are observed in sea surface temperatures in the cold tongue region in all reanalyses, as well as a decrease in interannual variability. These in turn are associated with a weakening in the Bjerknes Feedback in the early XXI century, indicated by a stronger and negative index. This results from a stronger thermal damping and weaker thermocline feedback, associated with a weaker response of equatorial zonal thermocline slope to equatorial zonal wind stress. Despite the spread among the reanalysis, the results obtained are consistent with previous studies.
Stratospheric aerosols from large tropical explosive volcanic eruptions backscatter shortwave radiation and reduce the global mean surface temperature. Observations suggest that they also favour an ...El Niño within 2 years following the eruption. Modelling studies have, however, so far reached no consensus on either the sign or physical mechanism of El Niño response to volcanism. Here we show that an El Niño tends to peak during the year following large eruptions in simulations of the Fifth Coupled Model Intercomparison Project (CMIP5). Targeted climate model simulations further emphasize that Pinatubo-like eruptions tend to shorten La Niñas, lengthen El Niños and induce anomalous warming when occurring during neutral states. Volcanically induced cooling in tropical Africa weakens the West African monsoon, and the resulting atmospheric Kelvin wave drives equatorial westerly wind anomalies over the western Pacific. This wind anomaly is further amplified by air-sea interactions in the Pacific, favouring an El Niño-like response.El Niño tends to follow 2 years after volcanic eruptions, but the physical mechanism behind this phenomenon is unclear. Here the authors use model simulations to show that a Pinatubo-like eruption cools tropical Africa and drives westerly wind anomalies in the Pacific favouring an El Niño response.
Southwestern South America (SWSA) has undergone frequent and persistent droughts in recent decades with severe impacts on water resources, and consequently, on socio-economic activities at a ...sub-continental scale. The local drying trend in this region has been associated with the expansion of the subtropical drylands over the last decades. It has been shown that SWSA precipitation is linked to large-scale dynamics modulated by internal climate variability and external forcing. This work aims at unravelling the causes of this long-term trend toward dryness in the context of the emerging climate change relying on a large set simulations of the state-of-the-art IPSL-CM6A-LR climate model from the 6th phase of the Coupled Model Intercomparison Project. Our results identify the leading role of dynamical changes induced by external forcings, over the local thermodynamical effects and teleconnections with internal global modes of sea surface temperature. Our findings show that the simulated long-term changes of SWSA precipitation are dominated by externally forced anomalous expansion of the Southern Hemisphere Hadley Cell (HC) and a persistent positive Southern Annular Mode (SAM) trend since the late 1970s. Long-term changes in the HC extent and the SAM show strong co-linearity. They are attributable to stratospheric ozone depletion in austral spring-summer and increased atmospheric greenhouse gases all year round. Future ssp585 and ssp126 scenarios project a dominant role of anthropogenic forcings on the HC expansion and the subsequent SWSA drying, exceeding the threshold of extreme drought due to internal variability as soon as the 2040s, and suggest that these effects will persist until the end of the twenty-first century.
Sea surface salinity (SSS) is a major ocean circulation component and indicator of the hydrological cycle. Here we investigate an unprecedented Atlantic SSS compilation from 1896 to 2013 and analyze ...the main modes of SSS decadal variability. Using principal component analysis, we find that the low‐latitude (tropical and subtropical) Atlantic and the subpolar Atlantic have distinct variability. Subpolar and low‐latitude SSS are negatively correlated, with subpolar anomalies leading low‐latitude anomalies by about a decade. Subpolar SSS varies in phase with the Atlantic Multidecadal Oscillation (AMO), whereas low‐latitude SSS varies in phase with the North Atlantic Oscillation (NAO). Additionally, northern tropical SSS is anticorrelated with Sahel rainfall, suggesting that SSS reflects the Intertropical Convergence Zone latitude. The 1896–2013 SSS trend shows amplification of the mean SSS field, with subpolar freshening and low‐latitude salinification. The AMO and NAO have little effect on the long‐term trend but contribute to the trend since 1970.
Key Points
Subpolar Atlantic surface salinity varies with the AMO, while low‐latitude surface salinity lags by a decade and varies with the NAO
Long‐term surface salinity trends show freshening in the subpolar Atlantic and salinification in the tropical and subtropical Atlantic
Northern tropical Atlantic surface salinity is negatively correlated with Sahel rainfall
Volcanic eruptions eject largeamounts of materials into the atmosphere, which can have an impact on climate. In particular, the sulphur dioxide gas released in the stratosphere leads to aerosol ...formation that reflects part of the incoming solar radiation, thereby affecting the climate energy balance. In this review paper, we analyse the regional climate imprints of large tropical volcanic explosive eruptions. For this purpose, we focus on the impact on three major climatic modes, located in the Atlantic (the North Atlantic Oscillation: NAO and the Atlantic Multidecadal Oscillation: AMO) and Pacific (the El Niño Southern Oscillation, ENSO) sectors. We present an overview of the chain of events that contributes to modifying the temporal variability of these modes. Our literature review is complemented by new analyses based on observations of the instrumental era as well as on available proxy records and climate model simulations that cover the last millennium. We show that the impact of volcanic eruptions of the same magnitude or weaker than 1991 Mt. Pinatubo eruption on the NAO and ENSO is hard to detect, due to the noise from natural climate variability. There is however a clear impact of the direct radiative forcing resulting from tropical eruptions on the AMO index both in reconstructions and climate model simulations of the last millennium, while the impact on the ocean circulation remains model-dependent. To increase the signal to noise ratio and better evaluate the climate response to volcanic eruptions, improved reconstructions of these climatic modes and of the radiative effect of volcanic eruptions are required on a longer time frame than the instrumental era. Finally, we evaluate climate models' capabilities to reproduce the observed and anticipated impacts and mechanisms associated with volcanic forcing, and assess their potential for seasonal to decadal prediction. We find a very large spread in the simulated responses across the different climate models. Dedicated experimental designs and analyses are therefore needed to decipher the cause for this large uncertainty.
•Volcanic eruptions can strongly impact key climate variability modes.•Such an impact can deliver valuable climate predictability for society.•A literature review of such potential effect is provided for three modes.•State-of-the-art models and observations show large uncertainty for the response.•A general strategy to improve our knowledge on this topic is sketched.
The South Atlantic subtropical dipole (SASD) has an impact on South American rainfall particular during its negative phase when continental precipitation in the northern part of the continent is ...enhanced. Relying on a series of single forcing transient simulations since the last deglaciation, we differentiate the relative role of meltwater, orbital, ice-sheets and greenhouse gases on the variability of rainfall in South America and links to the SASD. Results indicate that the meltwater forcing is the predominant driver of SASD variability. Wavelet analysis shows that most of the energy for the SASD at lower frequencies (
∼
5 kyr) comes from the meltwater discharge at cold events such as the Heinrich-1 cooling
∼
17 ka and the Younger-Dryas
∼
12.9 ka. Large rainfall changes in Northeastern Brazil can be attributed to changes in the South Atlantic sea surface temperature latitudinal gradient and South Atlantic Northward heat transport driven by the meltwater discharge.
Abstract
Atlantic multidecadal variability is a coherent mode of natural climate variability occurring in the North Atlantic Ocean, with strong impacts on human societies and ecosystems worldwide. ...However, its periodicity and drivers are widely debated due to the short temporal extent of instrumental observations and competing effects of both internal and external climate factors acting on North Atlantic surface temperature variability. Here, we use a paleoclimate database and an advanced statistical framework to generate, evaluate, and compare 312 reconstructions of the Atlantic multidecadal variability over the past millennium, based on different indices and regression methods. From this process, the best reconstruction is obtained with the random forest method, and its robustness is checked using climate model outputs and independent oceanic paleoclimate data. This reconstruction shows that memory in variations of Atlantic multidecadal variability have strongly increased recently—a potential early warning signal for the approach of a North Atlantic tipping point.
While bidecadal climate variability has been evidenced in several North Atlantic paleoclimate records, its drivers remain poorly understood. Here we show that the subset of CMIP5 historical climate ...simulations that produce such bidecadal variability exhibits a robust synchronization, with a maximum in Atlantic Meridional Overturning Circulation (AMOC) 15 years after the 1963 Agung eruption. The mechanisms at play involve salinity advection from the Arctic and explain the timing of Great Salinity Anomalies observed in the 1970s and the 1990s. Simulations, as well as Greenland and Iceland paleoclimate records, indicate that coherent bidecadal cycles were excited following five Agung-like volcanic eruptions of the last millennium. Climate simulations and a conceptual model reveal that destructive interference caused by the Pinatubo 1991 eruption may have damped the observed decreasing trend of the AMOC in the 2000s. Our results imply a long-lasting climatic impact and predictability following the next Agung-like eruption.
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