ENSO Teleconnections to Subantarctic Mode Water Formation in the Southeast Pacific

The Southern Ocean plays a fundamental role in the global carbon and energy budgets via the uptake of anthropogenic properties in Subantarctic Mode Water (SAMW). SAMW is formed in the wintertime deep mixed layers, with substantial interannual variability and evidence of long-term trends. Even though SAMW is closely related to the magnitude of mixed layer depth formation, relatively little is known about what controls these SAMW variations and trends. Here we use a global 1/10° ocean–sea-ice simulation during 1985-2017 forced by 3-hourly reanalysis atmospheric fields to examine the mechanisms controlling year-to-year variations in Southern Ocean mixed layer depth (MLD). We find that teleconnections from the tropical Pacific via ENSO drives MLD variability, particularly in the southeast Pacific, by modulating the intensity of the Amundsen Sea Low (ASL). The ASL exhibits its largest interannual variability during early winter (May to July), with an intensified ASL forcing an anomalously strong north-westerly airflow of cold continental winds from Antarctica to the South Pacific. These anomalous cold winds drive both net surface and Ekman heat flux anomalies, causing a pronounced sea surface cooling and delayed MLD deepening during August and September. Given the ASL deepening in recent decades, coupled with decadal variability of ENSO via the Interdecadal Pacific Oscillation (IPO) negative phase, our results suggest that, via this atmospheric bridge, deeper Southern Ocean MLD and enhanced SAMW could have been driven by variations in the tropical Pacific.