Interaction between the Indonesian Seas and the Indian Ocean in Observations and Numerical Models*

Abstract

Recent measurements from six bottom-mounted gauges are used with numerical model results to study the exchange of water between the Indonesian seas and the Indian Ocean via the Lesser Sunda Islands known collectively as Nusa Tengarra. The observations are approximately three years in length, from late 1995 to early 1999, and include measurements of bottom pressure, temperature, and salinity. The locations of the gauges are at the boundaries of three straits connecting the southern Indonesian seas with the eastern Indian Ocean: the Lombok Strait, the Ombai Strait, and the Timor Passage. The magnitude of intraseasonal variations in the pressure data dominates over that of the seasonal cycle. Intraseasonal variability appears most frequently and largest in magnitude at the westernmost strait (Lombok) and decreases along the coastline to the Timor Passage. Comparison to wind data shows these intraseasonal variations to be due to Kelvin wave activity in the Indian Ocean, forced by two distinct wind variations: semiannual monsoon reversals and Madden-Julian oscillation (MJO) activity. Sea level variations from both forcing mechanisms are then adjusted by local, alongshore winds. Longer-duration model results show the observation period (1996 through early 1999) to be a time of increased ENSO-related interannual variability and of suppressed annual cycle. MJO activity is also increased during this time. These factors explain the dominance of the higher frequency signals in the pressure data and the relative lack of a distinct annual cycle. An optimal fit of model sea level variations to model through-strait transport variations is used to estimate transport variability from the observed pressure records. At each strait the optimal fit is consistent with a cross-strait geostrophic balance for transport variations in the upper 250 m.

Vertical Structure of Kelvin Waves in the Indonesian Throughflow Exit Passages

“…In the Indian Ocean, anomalous westerly winds occur at both semiannual and intraseasonal frequencies (Potemra et al 2002). The mechanisms underlying the forcing at these frequencies are fundamentally different: during the monsoon transition seasons, roughly in May and November, westerly wind anomalies over the central part of the Indian Ocean force a semiannual Kelvin wave, commonly referred to as the Wyrtki jet (Wyrtki 1973).…”

“…1). Theoretical (Durland and Qiu 2003;Johnson and Garrett 2006) and modeling (Qiu et al 1999) studies have demonstrated that nearly all of the incoming intraseasonal Kelvin wave energy should enter Lombok Strait, but in situ observations have shown Kelvin wave energy in Ombai Strait (Molcard et al 2001;Potemra et al 2002;Sprintall et al 2009, hereafter S09) and a recent modeling effort by Qu et al (2008) demonstrated the same. Syamsudin et al (2004) used altimetric data to estimate that 56% of incoming semiannual Kelvin wave energy enters Lombok Strait, but the amount of intraseasonal Kelvin wave energy entering Lombok Strait has not been well quantified with observations.…”

“…The variability in the intraseasonal band (30-90 days) is particularly striking. Although some intraseasonal energy in the ITF exit passages originates from local and remote Pacific Ocean winds (Qiu et al 1999;Schiller et al 2010), the majority of the energy is forced remotely by wind anomalies over the Indian Ocean, which excite Kelvin waves that propagate eastward along the equator to the Indonesian coast and then on into the ITF region as coastally trapped Kelvin waves (Potemra et al 2002;Wijffels and Meyers 2004).…”

“…Syamsudin et al (2004) used altimetric data to estimate that 56% of incoming semiannual Kelvin wave energy enters Lombok Strait, but the amount of intraseasonal Kelvin wave energy entering Lombok Strait has not been well quantified with observations. Few in situ measurements have captured Kelvin waves in the Indonesian archipelago, and most of these have focused on the semiannual Kelvin waves forced during the monsoon transition seasons in May and November (Arief and Murray 1996;Sprintall et al 1999Sprintall et al , 2000Hautala et al 2001;Potemra et al 2002). Little is known about the vertical profile of velocity associated with Kelvin waves (Wijffels and Meyers 2004), particularly in the Indonesian archipelago, which has many sills, islands, and other topographic features that likely affect local Kelvin wave dynamics (S09).…”

Pathways of intraseasonal variability in the Indonesian Throughflow region

“…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.…”