Variability of zonal currents in the eastern equatorial Indian Ocean on seasonal to interannual time scales

Abstract

This study examines equatorial zonal current variations in the upper layers of eastern Indian Ocean in relation to variations in the Indian Ocean Dipole (IOD). The analysis utilizes data from the Research Moored Array for African-Asian-Australian Monsoon Analysis and Prediction (RAMA) and the European Centre for Medium-Range Weather Forecasts-Ocean Reanalysis System 4 (ECMWF-ORAS4). Surface currents are characterized by semiannual eastward flowing Wyrtki jets along the equator in boreal spring and fall, forced by westerly monsoon transition winds. The fall jet intensifies during negative IOD (NIOD) events when westerlies are anomalously strong but significantly weakens during positive IOD (PIOD) events when westerlies are anomalously weak. As zonal wind stress weakens during PIOD events, sea surface height becomes unusually low in the eastern basin and high in the west, setting up an anomalous pressure force that drives increased eastward transport in the thermocline. Opposite tendencies are evident during NIOD events in response to intensified equatorial westerlies. Current transport adjustments to anomalous zonal wind forcing during IOD events extend into the following year, consistent with the cycling of equatorial wave energy around the basin. A surface layer mass budget calculation for the eastern sea surface temperature (SST) pole of the IOD indicates upwelling of 2.960.7 Sv during normal periods, increasing by 40-50% during PIOD events and reducing effectively to zero during NIOD events. IOD-related variations in Wyrtki jet and thermocline transports are major influences on these upwelling rates and associated water mass transformations, which vary consistently with SST changes.

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

“…4a-d reveals that their leading EOF modes at the two latitudes are very similar to each other. In agreement with previous study (Nyadjro and McPhaden 2014), EOF1 shows a dominant pattern like Wyrtki Jets that occur in the upper 80 m (Fig. 4a, c) whereas the EOF2 shows a EUC-like structure, with weaker eastward subsurface currents appearing at about 80 m, peaking at 100 m, and disappearing at 160 m (Fig.…”

“…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.…”

“…The eastward subsurface zonal currents between 80 and 160 m, are defined as EUC in this study. It should be noted that the depth of EUC is not exactly defined in a range of 60-170 m or 16-26 °C equivalently, with some differences for different researchers (Swapna and Krishnan 2008;Iskandar et al 2009;Krishnan and Swapna 2009;Nyadjro and McPhaden 2014;Zhang et al 2014;Chen et al 2015;McPhaden et al 2015). Our definition can separate the EUC from the surface Wyrtki Jets and also capture the EUC maximum at the subsurface.…”

“…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).…”

Ocean Response to CINDY/DYNAMO MJOs in Air-Sea-Coupled COAMPS

“…They were explained as a wind-driven response to zonal winds by O'Brien and Hurlburt (1974), who applied the theory of Yoshida (1959). The WJs have been observed to appear in the spring and the fall of every year on a regular basis (e.g., Nagura and McPhaden 2010;Nyadjro and McPhaden 2014), although the spring jets have been found to be weak in 2006 through 2009 (Joseph et al 2012). The strong response of the WJ to semi-annual forcing has been explained by resonant basin modes (Jensen 1993;Han et al 1999) based on the theoretical work of Cane and Moore (1981).…”