Atlantic meridional overturning circulation (AMOC) collapses have punctuated Earth’s climate in the past, and future projections suggest a weakening and potential collapse in response to global warming and high-latitude ocean freshening. Among its most important teleconnections, the AMOC has been shown to influence El Niño–Southern Oscillation (ENSO), although there is no clear consensus on the tendency of this influence or the mechanisms at play. In this study, we investigate the effect of an AMOC collapse on ENSO by adding freshwater in the North Atlantic in a global climate model. The tropical Pacific mean-state changes caused by the AMOC collapse are found to alter the governing ENSO feedbacks, damping the growth rate of ENSO. As a result, ENSO variability is found to decrease by ∼30% due to weaker air–sea coupling associated with a cooler tropical Pacific and an intensified Walker circulation. The decreased ENSO variability manifests in ∼95% less frequent extreme El Niño events and a shift toward more prevalent central Pacific El Niño than eastern Pacific El Niño events, marked by a reduced ENSO nonlinearity and asymmetry. These results provide mechanistic insights into the possible behavior of past and future ENSO in a scenario of a much weakened or collapsed AMOC. Significance Statement The Atlantic meridional overturning circulation (AMOC) has collapsed in the past and a future collapse due to greenhouse warming is a plausible scenario. An AMOC shutdown would have major ramifications for global climate, with extensive impacts on climate phenomena such as El Niño–Southern Oscillation (ENSO), which is the strongest source of year-to-year climate variability on the planet. Using numerical simulations, we show that an AMOC shutdown leads to weaker ENSO variability, manifesting in 95% reduction in extreme El Niño events, and a shift of the ENSO pattern toward the central Pacific. This study sheds light on the mechanisms behind these changes, with implications for interpreting past and future ENSO variability.
Modes of climate variability can drive significant changes to regional climate affecting extremes such as droughts, floods and bushfires. The need to forecast these extremes and expected future increases in their intensity and frequency motivates a need to better understand the physical processes that connect climate modes to regional precipitation. Focusing on east Australia, where precipitation is driven by multiple interacting climate modes, this study provides a new perspective into the links between large-scale modes of climate variability and precipitation. Using a Lagrangian back-trajectory approach, we examine how the El Niño Southern Oscillation (ENSO) modifies the supply of evaporative moisture for precipitation, and how this is modulated by the Indian Ocean Dipole (IOD) and Southern Annular Mode (SAM). We demonstrate that La Niña modifies large-scale moisture transport together with local thermodynamic changes to facilitate local precipitation generation, whereas below average precipitation during El Niño stems predominantly from increased regional subsidence. These dynamic-thermodynamic processes were often more pronounced during co-occurring La Niña/negative IOD and El Niño/positive IOD periods. As the SAM is less strongly correlated with ENSO, the impact of co-occurring ENSO and SAM largely depended on the state of ENSO. La Niña-related processes were exacerbated when combined with +SAM and dampened when combined with -SAM, and vice versa during El Niño. This new perspective on how interacting climate modes physically influence regional precipitation can help elucidate how model biases affect the simulation of Australian climate, facilitating model improvement and understanding of regional impacts from long-term changes in these modes.
<p>The warming of the equatorial Pacific associated with the El Ni&#241;o&#8211;Southern Oscillation (ENSO) causes profound impacts on rainfall and temperature in the tropics and extratropics. El Ni&#241;o drives changes in the Walker and Hadley circulations, warms the tropics and affects the neighboring ocean basins, favoring a short-term rise in global temperatures. We will present an overview of the atmospheric teleconnections driven by ENSO and its diversity focusing on the impacts over land and remote ocean basins. During El Ni&#241;o, dry conditions are generally observed in the Maritime Continent, northern South America, South Asia and South Africa, while wet weather typically occurs in southwestern North America, western Antarctica, and east Africa. Global effects during La Ni&#241;a are overall the opposite to El Ni&#241;o, however this assumption is not true for all regions. ENSO atmospheric teleconnections are non-linear in part due to different locations of the anomalous equatorial warming (Eastern versus Central Pacific events) superimposed on the Pacific mean state, as well as interactions with the annual cycle, off-equatorial regions, remote ocean basins, and other modes of climate variability. Adding to this complex behavior, ENSO teleconnections are non-stationary either due to deterministic low-frequency modulations or stochastic variability, the latter being a factor generally overlooked in the literature.&#160;As the world warms in response to greenhouse gas forcing, ENSO atmospheric teleconnections are expected to change, despite large uncertainties in ENSO projections. We will discuss the limitations of climate models in representing realistic teleconnections from the tropical Pacific to remote regions and some of the challenges for future projections.</p>
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ABSTRACT.Deep convection processes associated with sea surface temperature (SST) is an important mechanism of thermal control, redistributing the sea surface energy to high levels of atmosphere. Tropical areas of warmer SST are usually associated with areas of high precipitation. The present work examines the degree of spatial and temporal correlation, existing between outgoing longwave radiation (OLR), precipitation (PPT) and SST in the Tropical Atlantic Ocean RESUMO. O processo de convecção profunda, associadoà Temperatura da Superfície do Mar (TSM)é um importante mecanismo de controle térmico, redistribuindo a energia da superfície do mar para maiores níveis da atmosfera.Áreas tropicais de elevada TSM normalmente estão associadas aáreas com altosíndices de precipitação.O presente trabalho examina o grau de correlação espacial e temporal existente entre a Radiação de Onda Longa (ROL) emitida, a precipitação (PREC) e a TSM no Oceano Atlântico Tropical (20 • S-20 • N). Verificou-se que aárea de maior correlação espacial entre a PREC, ROL e TSM se situa ao norte do Equador, acompanhando o deslocamento da Zona de Convergência Intertropical (ZCIT). Na região tropical, existe locais onde a correlação entre a TSM e a ROL apresenta valores diferentes, uma possível explicação seria a ação de processos remotos afetando de maneira diferente essas variáveis. Os diagramas de dispersão ROL × TSM para valores significativos de correlação, apresentam uma quebra de tendência nos pontos quando a TSM atinge a faixa de 27à 28 • C, indicando uma transição do estado em que se inicia a convecção profunda. A máxima precipitação ocorre para TSM próxima de 28 • C.Palavras-chave: interação oceano-atmosfera, ROL, TSM, precipitação.
<p>In 2013/14 eastern South America experienced one of its worst droughts, leading to water shortages in S&#227;o Paulo, the world&#8217;s fourth most populated city. This event was also responsible for a dengue fever outbreak that tripled the usual number of fatalities and reduced Brazilian coffee production leading to a global shortages and worldwide price increases. The drought was associated with an anomalous anticyclonic circulation off southeast South America that prevented synoptic systems reaching the region while inhibiting the development of the South Atlantic Convergence Zone and its associated rainfall. A concomitant and unprecedented marine heatwave also developed in the southwest Atlantic. Here we show from observations that such droughts and adjacent marine heatwaves have a common remote cause. Atmospheric blocking triggered by tropical convection in the Indian and Pacific oceans can cause persistent anticyclonic circulation that not only leads to severe drought but also generates marine heatwaves in the adjacent ocean. We show that increased shortwave radiation due to reduced cloud cover and reduced ocean heat loss from weaker winds are the main contributors to the establishment of marine heatwaves in the region. The proposed mechanism, which involves droughts, extreme air temperature over land and atmospheric blocking explains approximately 60% of the marine heatwave events in the western South Atlantic. We also identified an increase in frequency, duration, intensity and extension of marine heatwave events over the satellite period 1982&#8211;2016. Moreover, surface primary production was reduced during these events with implications for regional fisheries.</p>
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