Intraseasonal Variability of Equatorial Indian Ocean Zonal Currents

Abstract

New satellite and in situ observations show large intraseasonal (10-60 day) variability of surface winds and upper-ocean current in the equatorial Indian Ocean, particularly in the east. An ocean model forced by the Quick Scatterometer (QuikSCAT) wind stress is used to study the dynamics of the intraseasonal zonal current. The model has realistic upper-ocean currents and thermocline depth variabilities on intraseasonal to interannual scales. The quality of the simulation is directly attributed to the accuracy of the wind forcing. At the equator, moderate westerly winds are punctuated by strong 10-40-day westerly wind bursts. The wind bursts force swift, intraseasonal (20-50 day) eastward equatorial jets in spring, summer, and fall. The zonal momentum balance is between local acceleration, stress, and pressure, while nonlinearity deepens and strengthens the eastward current. The westward pressure force associated with the thermocline deepening toward the east rapidly arrests eastward jets and, subsequently, generates (weak) westward flow. Thus, in accord with direct observations in the east, the spring jet is a single intraseasonal event, there are intraseasonal jets in summer, and the fall jet is long lived but strongly modulated on an intraseasonal scale. The zonal pressure force is almost always westward in the upper 120 m, and changes sign twice a year in the 120-200-m layer. Transient eastward equatorial undercurrents in early spring and late summer are associated with semiannual Rossby waves generated at the eastern boundary following thermocline deepening by the spring and fall jets. An easterly wind stress is not necessary to generate the undercurrents. Experiments with a single westerly wind burst forcing show that apart from the intraseasonal response, the zonal pressure force and current in the east have an intrinsic 90-day time scale that arises purely from equatorial adjustment.

Large-Scale Oceanic Variability Associated with the Madden-Julian Oscillation during the CINDY/DYNAMO Field Campaign from Satellite Observations

“…Since strong equatorial currents on the subseasonal timescale are often generated in the central Indian Ocean, e.g., [24,[50][51], the upper ocean salinity could fluctuate largely on the subseasonal time scale.…”

Ariosoma-type Leptocephali (Congridae: Bathymyrinae) in the Mentawai Islands region off western Sumatra, Indonesia

“…All of these species could be present in the eastern Indian Ocean, considering that these larvae grow for many months reaching large sizes, and they could be transported long distances by ocean currents. It is well documented that water from the Indonesian Seas flows into the eastern Indian Ocean (Hautala et al 2001;Vranes et al 2002), and equatorial current jets can transport water in both directions across the northern Indian Ocean (Thompson et al 2006;Sengupta et al 2007;Kantha et al 2008). This makes it likely that some species reported to be present as juveniles or adults in western Indonesia, the South China Sea, or the western Indian Ocean could also be present off western Sumatra.…”

“…Upwelling sometimes occurs off the coast of western Sumatra that would transport surface water offshore (Susanto et al 2001). In addition, strong eastward equatorial jets lasting about 10 to 20 days typically occur during May to the northwest of the study area (Thompson et al 2006;Sengupta et al 2007;Kantha et al 2008), which could have a major effect on the variability of currents and eddies around the Mentawai Islands. These current flows and eddies likely transport leptocephali in a variety of directions during their larval periods in this area.…”

A SEEK filter assimilation of sea surface salinity from Aquarius in an OGCM: Implication for surface dynamics and thermohaline structure

“…As already mentioned, Masson et al [2003] studied this impact in a numerical experiment by conveniently changing the salinity in the two terms in the zonal momentum equation, which are affected by salinity, namely the pressure gradient and the vertical mixing. Interestingly, these are the two terms which dominate zonal momentum balance [Sengupta et al, 2007] in the equatorial Indian Ocean. The study by Masson et al [2003] is undoubtedly an interesting one and probably the first of its kind.…”