Variability in the Indian Ocean circulation and salinity and its impact on SST anomalies during dipole events

Impacts of the IOD-associated temperature and salinity anomalies on the intermittent equatorial undercurrent anomalies

“…The salinity anomalies in the surface over the central Indian Ocean also play a secondary role in enhancing the EUC. As discussed in the previous studies (Thompson et al 2006;Nyadjro and McPhaden 2014;Zhang et al 2014;Chen et al 2016), EUC can feed and intensify the upwelling off the Sumatra-Java coast and provide a positive feedback to the development of IOD events. The present study indicates that subsurface temperature anomalies over the eastern Indian Ocean and the surface salinity anomalies over the central Indian Ocean may favour the development of IOD by intensifying the EUC.…”

“…7a), and strong westward zonal currents driven by anomalous easterlies advect fresh water from the SumatraJava coast and the Bay of Bengal into the central and eastern equatorial Indian Ocean (Fig. 7c) (Thompson et al 2006;Li et al 2016). Note a weak eastward zonal current is visible under the strong westward anomalous surface current above 80 m during pIOD events, which is the autumn EUC with its core within 80-160 m between 75°E and 95°E, as shown in the red boxes in Fig.…”

“…This is because the cooling in the eastern Indian Ocean is associated with a decrease in salinity during a positive IOD event and vice versa in a negative IOD phase. During the pIOD events, the anomalous easterlies drive anomalous zonal currents that advect fresh water from the Sumatra-Java coast and the Bay of Bengal into the central equatorial Indian Ocean, which decrease the salinity (Thompson et al 2006;Zhang et al 2013;Li et al 2016). …”

“…On seasonal time scales, the EUC is most prominent during boreal winter and spring, particularly from February to April, and with a secondary and much weaker peak in boreal late summer to early fall (Knox 1976;McPhaden 1982;Reppin et al 1999;Izumo 2005;Iskandar et al 2009). On interannual time scales, the summer-fall EUC in the central and eastern equatorial Indian Ocean becomes stronger during the pIOD events than during normal period (Reppin et al 1999;Swapna and Krishnan 2008;Iskandar et al 2009), which feeds and intensifies the upwelling off the Sumatra-Java coast, producing a positive feedback to the decrease of SST in this region (Thompson et al 2006;Nyadjro and McPhaden 2014;Zhang et al 2014;Chen et al 2016).…”

“…It was argued that the salinity anomalies may have a very important impact on the EUC during the 1997 IOD event (Masson et al 2004). This is probably because the anomalous zonal currents driven by the easterlies advect the fresh water from the Bay of Bengal and the Sumatra-Java coast into the central equatorial Indian Ocean to change the seawater density distribution (Thompson et al 2006;Grunseich et al 2011;Zhang et al 2013;Li et al 2016). For example, Masson et al (2003Masson et al ( , 2004 designed sensitivity experiments to investigate the impacts of salinity on the pressure gradient and vertical mixing and concluded the importance of the IOD-deduced salinity anomalies to the EUC.…”