Vertical Structure of Kelvin Waves in the Indonesian Throughflow Exit Passages

Abstract

The subsurface structure of intraseasonal Kelvin waves in two Indonesian Throughflow (ITF) exit passages is observed and characterized using velocity and temperature data from the 2004-06 International Nusantara Stratification and Transport (INSTANT) project. Scatterometer winds are used to characterize forcing, and altimetric sea level anomaly (SLA) data are used to trace the pathways of Kelvin waves east from their generation region in the equatorial Indian Ocean to Sumatra, south along the Indonesian coast, and into the ITF region.During the 3-yr INSTANT period, 40 intraseasonal Kelvin waves forced by winds over the central equatorial Indian Ocean caused strong transport anomalies in the ITF outflow passages. Of these events, 21 are classed as ''downwelling'' Kelvin waves, forced by westerly winds and linked to depressions in the thermocline and warm temperature anomalies in the ITF outflow passages; 19 were ''upwelling'' Kelvin waves, generated by easterly wind events and linked to shoaling of the thermocline and cool temperature anomalies in the ITF. Both downwelling and upwelling Kelvin waves have similar vertical structures in the ITF outflow passages, with strong transport anomalies over all depths and a distinctive upward tilt to the phase that indicates downward energy propagation. A linear wind-forced model shows that the first two baroclinic modes account for most of the intraseasonal variance in the ITF outflow passages associated with Kelvin waves and highlights the importance of winds both in the eastern equatorial Indian Ocean and along the coast of Sumatra and Java for exciting Kelvin waves.Using SLA as a proxy for Kelvin wave energy shows that 37% 6 9% of the incoming Kelvin wave energy from the Indian Ocean bypasses the gap in the coastal waveguide at Lombok Strait and continues eastward. Of the energy that continues eastward downstream of Lombok Strait, the Kelvin waves are split by Sumba Island, with roughly equal energy going north and south to enter the Savu Sea.

Intraseasonal Kelvin wave in Makassar Strait

“…Furthermore, a normal mode approximation is also applicable to explain the link between the vertical structure of v 0 and stratification. We suggest that these characteristics pertinent to the 2-to 3-month variability [36] The 0 , u 0 , and v 0 data in Lombok Strait also demonstrate two Kelvin wave attributes in the semi-geostrophic balance between v 0 and d 0 /dx and in enhanced d 0 /dx, which corroborates those reported by Drushka et al [2010]. The v 0 data in Lombok Strait and those in Makassar Strait are coherent and yield a Kelvin wave dispersion relationship between the wavenumber and wave frequency.…”

“…[37] Drushka et al [2010] Unlike the study of Zhou and Murtugudde [2010], which suggested that the MJO affected the intraseasonal variability only at the ITF outflow passages such as Lombok Strait, we propose that the MJO-related oceanic Kelvin wave is also observed in Makassar Strait.…”

“…Drushka et al [2010] proposed that the vertical structure of the observed Lombok Strait throughflow at intraseasonal timescales was best explained by a baroclinic Kelvin wave. We will be discussing two other Kelvin wave attributes measured at Lombok Strait: semigeostrophic balance and reflected Kelvin wave.…”

“…Drushka et al [2010], focusing their analyses on the vertical structure of along-strait flow at the outflow passages of the ITF, indicated that the intraseasonal along-strait motions at Lombok Strait had a vertical structure of a remotely wind-forced Kelvin wave.…”

Interocean and Interbasin Exchanges

“…The semiannual signal in ITF transport is much more dominant than previously thought (relative to the annual signal). These Kelvin waves can also occur on intraseasonal timescales in response to Madden Julian Oscillation wind fluctuations and correspondingly drive transport, thermocline, and temperature anomalies in the ITF exit passages (Drushka et al, 2010) and up into Makassar Strait (Pujiana et al, 2010). During the northwest monsoon of November-January, the surface transport relaxes in Timor Strait and weakly reverses in Ombai, Lombok, and Makassar Strait, so the main core of the ITF is subsurface.…”