Generation mechanism for the intraseasonal variability in the Indo‐Australian basin

Abstract

In the Indo‐Australian basin the strongest intraseasonal variability occurs during the second half of the year according to satellite altimeter data. This study attempts to identify the generation mechanism of this variability by means of numerical ocean modeling. By separately varying winds and transports through individual straits it is shown that the seasonal cycles of both the wind and the transport through the Lombok Strait play crucial roles in generating mixed barotropic and baroclinic instabilities during July–September. Both the spatial and temporal patterns of the variability are also sensitive to transports through the Ombai Strait and the Timor Passage, though to a lesser degree. The Smagorinsky scheme in the model is essential for these instabilities to reach the observed magnitude of standard deviation in sea level anomaly; constant eddy viscosity, which needs to be sufficiently large for the Somali Current in summer, would damp the eddies in the Indo‐Australian basin severely.

Pathways of intraseasonal variability in the Indonesian Throughflow region

“…On the other hand, the 60-day variations in the SJCU are enhanced by wind forcing over the eastern equatorial Indian Ocean off Sumatra and are mostly captured by the second baroclinic mode [Iskandar et al, 2006]. By separately varying winds and transports through individual straits Yu and Potemra [2006] showed that the seasonal cycles of both the wind and the transport through Lombok Strait play crucial roles in generating mixed barotropic and baroclinic instabilities during July -September. Both the spatial and temporal patterns of the variability are also sensitive to transports through Ombai Strait and, to a lesser degree, Timor Passage.…”

“…The potential predictability of ocean dynamics in the proximity of the ITF has attracted considerable interest in recent years [Qiu et al, 1999;Chong et al, 2000;Potemra et al, 2002;Syamsudin et al, 2004;Wijffels and Meyers, 2004;Iskandar et al, 2005Iskandar et al, , 2006Yu and Potemra, 2006;Atmadipoera et al, 2009]. For example, Wijffels and Meyers [2004] explored observation-based temperature and sea level variability within the Indonesian seas and southeast Indian Ocean.…”

Response to a Steady Poleward Outflow. Part II: Oscillations and Eddies

“…Nof et al (2002) ascribed the IAB variability to a process of eddy shedding from the steady, westwardflowing Timor Passage branch of the ITF as it changes direction to follow the Australian coast southwestward. Eddies are observed to the east of Lombok Strait that may indeed be attributable to the Timor Passage outflow, but the location of maximum variability and the analysis of Yu and Potemra (2006) strongly suggest that the poleward Lombok Strait outflow plays a major role in establishing the IAB variability pattern. Pichevin and Nof (1997, hereafter PN) present a theoretical model of eddy shedding from a steady poleward outflow, and Nof (2005) provides an updated analysis of the model with qualitatively similar results.…”

“…We will follow Yu and Potemra (2006) in referring to this region as the IndoAustralian Basin (IAB). The eddy field is coincident with the South Equatorial Current (SEC), which derives much of its source waters from the Indonesian Throughflow (ITF).…”

“…The most robust periodicity peaks are seen in the second half of 1994, 1995, and 1997, but a peak in the 40-80-day period band can be seen to continue almost unbroken throughout the entire 6-yr record. Yu and Potemra (2006) modeled the IAB intraseasonal variability with a 4 ½-layer model of the Indian Ocean, forced by realistic surface fluxes and by specified ITF transports through Timor Passage, Ombai Strait, and Lombok Strait. They found the barotropic conversion terms to be slightly stronger than the baroclinic conversion terms and concluded that the IAB eddies owe their existence to a mixed instability process.…”