Responses of the Southern Ocean mixed layer depth to the eastern and central Pacific El Niño events during austral winter

Based on the Ocean Reanalysis System version 5 (ORAS5) and the fifth-generation reanalysis datasets derived from European Centre for Medium-Range Weather Forecasts (ERA5), we investigate the different impacts of the central Pacific (CP) El Ni & ntilde;o and the eastern Pacific (EP) El Ni & ntilde;o on the Southern Ocean (SO) mixed layer depth (MLD) during austral winter. The MLD response to the EP El Ni & ntilde;o shows a dipole pattern in the South Pacific, namely the MLD dipole, which is the leading El Ni & ntilde;o-induced MLD variability in the SO. The tropical Pacific warm sea surface temperature anomaly (SSTA) signal associated with the EP El Ni & ntilde;o excites a Rossby wave train propagating southeastward and then enhances the Amundsen Sea low (ASL). This results in an anomalous cyclone over the Amundsen Sea. As a result, the anomalous southerly wind to the west of this anomalous cyclone advects colder and drier air into the southeast of New Zealand, leading to surface cooling through less total surface heat flux, especially surface sensible heat (SH) flux and latent heat (LH) flux, and thus contributing to the mix layer (ML) deepening. The east of the anomalous cyclone brings warmer and wetter air to the southwest of Chile, but the total heat flux anomaly shows no significant change. The warm air promotes the sea ice melting and maintains fresh water, which strengthens stratification. This results in a shallower MLD. During the CP El Ni & ntilde;o, the response of MLD shows a separate negative MLD anomaly center in the central South Pacific. The Rossby wave train triggered by the warm SSTA in the central Pacific Ocean spreads to the Amundsen Sea, which weakens the ASL. Therefore, the anomalous anticyclone dominates the Amundsen Sea. Consequently, the anomalous northerly wind to the west of anomalous anticyclone advects warmer and wetter air into the central and southern Pacific, causing surface warming through increased SH, LH, and longwave radiation flux, and thus contributing to the ML shoaling. However, to the east of the anomalous anticyclone, there is no statistically significant impact on the MLD.