How alike are diabatic processes in the tropical Atlantic to the Pacific?

The Atlantic Niño is the main mode of interannual sea surface temperature (SST) variability in the tropical Atlantic. It bears resemblance to the SST variability associated with the El Niño Southern Oscillation (ENSO) in the Pacific. Like its counterpart in the Pacific, this oscillating mode of variability is a source of weather and climate predictability for bordering countries and beyond. The Atlantic Niño is widely thought of as an air-sea coupled mode, though the ratio of atmospheric versus oceanic forcing leading to the SST anomalies is still under discussion. A recently developed novel water mass transformation analysis can shed light on this question. This method investigates the physical processes behind sea surface cooling and heat uptake into the ocean and relates them to diabatic equatorial upwelling in time and space. This analysis has successfully been applied to the tropical Pacific, where it highlighted the role of ocean processes in creating and enhancing SST variability. During El Niño, for example, subsurface ocean mixing, which drives the bulk of diabatic upwelling in the thermocline shuts down almost entirely, enhancing the warm SST anomaly. During La Niña, on the other hand, SST are strongly cooled by diabatic upwelling driven by anomalously strong vertical mixing. 

 

This method can be readily applied to the Atlantic, to investigate the role of ocean subsurface processes in driving SST variability. The water mass transformation calculations are applied on output of a 0.1° horizontal resolution forced ocean and sea ice simulation. This simulation exhibits realistic SST variability related to the Atlantic Niño. The resulting diabatic velocities and physical processes show that there are distinct differences between the diabatic upwelling in the Atlantic compared to the Pacific, promising additional insights into the mechanism of SST variability behind the Atlantic Niño as compared to the Pacific ENSO.