Séminaire

Previous studies have found a connection between a summer Atlantic Niño (Niña) and the next winter Pacific La Niña (El Niño) after the 1970s, nevertheless a detailed study of the air-sea interactions associated to the Atlantic Niño influence on ENSO phenomena was required. It has been demonstrated that the Atlantic-Pacific Niños connection is carried out through an air-sea coupled mechanism: when an Atlantic Niño takes place during boreal summer, the convection is enhanced over the Atlantic, altering the Walker circulation with anomalous subsidence over the central Pacific. As a consequence, anomalous easterlies along the western equatorial Pacific trigger an upwelling Kelvin wave propagating eastward from summer to winter months. As the Kelvin wave propagates to the east, the thermocline shallows, allowing the cooling of the sea surface through temperature advection associated to anomalous zonal currents and mean vertical velocity. This connection emerges as the leading mode of inter-annual tropical variability during the first and last decades of the XX century, coinciding with negative phases of the Atlantic Multidecadal Oscillation (AMO), which could modulate this inter-basin connection through changes in the variability of the equatorial Atlantic convection and eastern Pacific SST.

According to the relation between the Atlantic and Pacific Niños, a statistical hindcast of ENSO phenomena have been performed using the tropical Atlantic SSTs as the predictor field, obtaining high correlation skill for those periods in which the connection is established. For those decades, negative AMO phases, two different spatial configurations of the Atlantic Niño pattern co-exist in the tropical Atlantic basin. The leading mode, denoted as Basin-Wide (BW) Atlantic Niño, is characterized by anomalous warming covering the entire tropical Atlantic. It is preceded by a weakening of both Azores and Sta Helena High, which produces a weakening of the subtropical trades and an anomalous equatorial wind convergence. On the other hand, the second mode, denoted as Canonical (C) Atlantic Niño, is associated with a previous zonal Sea Level Pressure (SLP) gradient, which intensifies the subtropical trades and produces strong anomalous westerlies at the equator. The development of BW and C- Atlantic Niños differs in its timing and associated oceanic processes.