Abstract
The study of Equatorial Undercurrent (EUC) has attracted a broad attention in recent years due to its strong response and feedback to the Indian Ocean Dipole. In this paper, we first produce a high-quality simulation of three-dimensional temperature, salinity and zonal current simulation from 1982 to 2014, using a high-resolution ocean general circulation model. On this basis, with two sensitivity experiments, we investigate the role of temperature and salinity anomalies in driving and enhancing the EUC during the positive IOD events by examining the variation of the EUC seasonal cycle and diagnosing the zonal momentum budget along the equatorial Indian Ocean. Our results show that during January–March, the EUC can appear along the entire equatorial Indian Ocean in all years, but during August–November, the EUC can appear and reach the eastern Indian Ocean only during the positive IOD events. The zonal momentum budget analysis indicates that the pressure gradient force contributes most to the variation of the eastward acceleration of zonal currents in the subsurface. During the positive IOD events, strong negative subsurface temperature anomalies exist in the eastern Indian Ocean, with negative surface salinity anomalies in the central and eastern Indian Ocean, resulting in a large pressure gradient force to drive EUC during the August–November. Further, the results of two sensitivity experiments indicate that the temperature anomalies significantly impact the pressure gradient force, playing a leading role in driving the EUC, while the surface salinity anomalies can secondarily help to intensify the eastward EUC through increasing the zonal density gradient in the eastern Indian Ocean and impacting the vertical momentum advection in the subsurface.
Highlights
The Indian Ocean Dipole (IOD) is a major ocean–atmosphere coupled mode of inter-annual climate variability in the tropical Indian Ocean (Saji et al 1999; Webster et al 1999), characterized by opposite sea surface temperature (SST) anomalies in the eastern (90°E–110°E, 10°S–0°S) and western (50°E–70°E, 10°S–10°N) tropical Indian Ocean
We focus on the transient variation of the Equatorial Undercurrent (EUC) in the equatorial Indian Ocean
Two sensitivity experiments are conducted to investigate the role played by the temperature and salinity anomalies in driving and enhancing the eastward EUC, which intermittently appears in the subsurface during the positive IOD (pIOD) events
Summary
The Indian Ocean Dipole (IOD) is a major ocean–atmosphere coupled mode of inter-annual climate variability in the tropical Indian Ocean (Saji et al 1999; Webster et al 1999), characterized by opposite sea surface temperature (SST) anomalies in the eastern (90°E–110°E, 10°S–0°S) and western (50°E–70°E, 10°S–10°N) tropical Indian Ocean. 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). A further examination of the impact of temperature and salinity anomalies on the EUC in such a way is a necessary step towards a fully understanding to the EUC dynamics involving transient surface zonal winds, zonal pressure gradient, monsoon, equatorial Kelvin and Rossby waves Such a study can allow us to shed light on the dominant factor of EUC anomalies.
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