Multidecadal variations in North Atlantic SSTs modulate the relationship between ENSO and the South Atlantic Subtropical Dipole since 1900

Abstract

This study investigates the long-term variability of the relationship between the El Niño South/Oscillation (ENSO) and the South Atlantic Subtropical Dipole (SASD), and their connection to multidecadal variations in the North Atlantic sea surface temperature (MDV-SST). Using a century’s worth of SST and atmospheric data and the Community Atmosphere Model version 5 (CAM5), the study found significant interdecadal variability in the correlation between the Niño3.4 index and the SASD index. This variability is closely linked to the North Atlantic MDV-SST, often interpreted as Atlantic Multidecadal Oscillation (AMO). This is demonstrated by significant ENSO-SASD correlations during warm (positive) phases of MDV-SST (1930-1960 and 2001-2020) and insignificant correlations during cold (negative) phases (1900-1929 and 1961-2000). Through a Gill-type response, MDV-SST excites Rossby wave over the tropical Pacific Ocean, influencing surface wind, SST, convective activities and upper-level zonal wind over the Pacific Ocean. During warm MDV-SST phases, the more eastward positioned Rossby wave source, triggered by SST and precipitation anomalies over the South Pacific Ocean, along with a stronger, more northward subtropical jet stream, propagates the wavetrain more eastwards into the South Atlantic Ocean, thereby strengthening the SST anomalies in the SASD. Conversely, during the cold MDV-SST phases, the more westward-positioned ENSO-related Rossby wave source and a stronger mid-latitude jet stream guide the wavetrain southeastwards into the southeastern Pacific Ocean, exerting less influence on the SST anomalies in the SASD. The Ekman pumping caused by anomalous surface pressure and the associated surface wind field as well as surface turbulent heat flux also affect the SST anomalies in the SASD and the ENSO-SASD relation.