Changes in tropical Atlantic interannual variability from a substantial weakening of the meridional overturning circulation

Abstract

In response to a substantial weakening of the Atlantic Meridional Overturning Circulation (AMOC)-from a coupled ocean-atmosphere general circulation model experiment-significant changes in the interannual variability are found over the tropical Atlantic, characterized by an increase of variance (by ~150%) in boreal late spring-early summer and a decrease of variance (by ~60%) in boreal autumn. This study focuses on understanding physical mechanisms responsible for these changes in interannual variability in the tropical Atlantic. It demonstrates that the increase of variability in spring is a consequence of an increase in the variance of the El Niño-Southern Oscillation, which has a large impact on the tropical Atlantic via anomalous surface heat fluxes. Winter El Niño (La Niña) affects the eastern equatorial Atlantic by decreasing (increasing) cloud cover and surface wind speed which is associated with anomalous downward (upward) short wave radiation and reduced (enhanced) upward latent heat fluxes, creating anomalous positive (negative) Sea Surface Temperature (SST) anomalies over the region from winter to spring. On the other hand, the decrease of SST variance in autumn is due to a deeper mean thermocline which weakens the impact of the thermocline movement on SST variation. The comparison between the model results and observations is not straightforward owing to the influence of model biases and the lack of a major MOC weakening event in the instrumental record. However, it is argued that the basic physical mechanisms found in the model simulations are likely to be robust and therefore have relevance to understanding tropical Atlantic variability in the real world, perhaps with modified seasonality.

Predicción multianual del Niño Atlántico: Una primera aproximación desde ENSEMBLES

“…Our results suggest that ocean dynamics play a role in reproducing the Atlantic Niño evolution beyond the thermal inertia of the ocean. This is in agreement with the idea of changes in the Atlantic meridional overturning circulation impacting the mean thermocline and thus Atlantic Niño mode (Chang et al 2008;Haarsma et al 2008;Polo et al 2013). It also implies that ATL3-related climate phenomena such as rainfall variability in Northeastern Brazil or the WAM may have some predictability at 2-5 years lead time, and could be skilfully predicted on those time scales if the models can reproduce the relevant climate processes.…”

Intensified impact of tropical Atlantic SST on the western North Pacific summer climate under a weakened Atlantic thermohaline circulation

“…The climatological SST changes exhibit as a cooling in the North Atlantic and a warming in the South Atlantic, because of the pronounced decrease of northward ocean heat transport under the weakened THC (e.g., Dong and Sutton 2002;Dahl et al 2005;Zhang and Delworth 2005;Timmermann et al 2007). In addition, the interannual variability of SST over the Atlantic Ocean can be modulated by the weakened THC (Haarsma et al 2008;Polo et al 2013). Polo et al (2013) suggested an enhancement of tropical Atlantic variability in boreal late spring-early summer, which is due to an increase in the variance of wintertime ENSO through affecting anomalous surface heat fluxes.…”

“…The weakened THC significantly influences both the mean states and interannual variability of SST over the Atlantic (e.g., Polo et al 2013). The climatological SST changes exhibit as a cooling in the North Atlantic and a warming in the South Atlantic, because of the pronounced decrease of northward ocean heat transport under the weakened THC (e.g., Dong and Sutton 2002;Dahl et al 2005;Zhang and Delworth 2005;Timmermann et al 2007).…”

Seasonal to decadal variations of sea surface pCO2 and sea‐air CO2 flux in the equatorial oceans over 1984–2013: A basin‐scale comparison of the Pacific and Atlantic Oceans

“…However, some studies have characterized the profound seasonality in the physical and biogeochemical fields of the equatorial Atlantic Ocean [Busalacchi and Picaut, 1983;Christian and Murtugudde, 2003;Foltz and McPhaden, 2004;Grodsky et al, 2008]. Moreover, there have been reports of similarities and differences in decadal variability in physical fields between the two basins [Polo et al, 2013]. These differences would have implications for the carbon cycle in the two basins.…”