Direct evidence of the South Java Current system in Ombai Strait (original) (raw)
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The throughflow within Ombai Strait
Deep Sea Research Part I: Oceanographic Research Papers, 2001
A current meter mooring was deployed for one year in December 1995 in Ombai Strait, one of the deep connections between the Paci"c Ocean and the Indian Ocean. Depending on the horizontal extrapolation, the mean transport was estimated to be between 4 and 6 Sv towards the Savu Sea. Succession of intense events of one or two months duration nearly hides the expected annual variability with maximum in August}September. Although the mean currents in the upper 200 m were "ve times higher than that below, the deep and wide strait section leads to a signi"cant deep transport. Analysis of the hydrological characteristics of the concerned water masses corroborates the circulation given by the current measurements. The east-north-east current in December in the upper layer is thought to be related to the arrival of a Kelvin wave originating in the equatorial Indian Ocean and trapped along the coasts of the Sunda Islands before entering the Savu Sea between Sumba and Flores Islands.
Observations of intraseasonal variability in the Sunda Strait throughflow
Journal of Oceanography
Using velocity profiles observed by bottom-mounted ADCPs, we identified strong intraseasonal variability in the Sunda Strait throughflow. This intraseasonal variability, with typical periods of 20-40 days and the strongest energy occurring in the boreal spring, can reverse the Sunda Strait throughflow. Further analysis showed this intraseasonal variability to be closely related to local zonal wind and the sea level gradient along the strait. These observations confirm for the first time the existence of Kelvin-wave-like signals in the Sunda Strait, propagating from the equatorial Indian Ocean. This study also provides new insights into the effects of Kelvin waves on the Sunda Strait throughflow.
2020
Intraseasonal variability (20-90 days) of the Indonesian Throughflow (ITF), which is primarily forced by the Madden-Julian Oscillation (MJO), is investigated using a high-resolution global ocean reanalysis and satellite altimeter data. Previous studies show that during the MJO active phase, downwelling Kelvin waves generated in the central equatorial Indian Ocean propagate along the coast of Sumatra and Java islands, affecting the ITF transport at exit passages in the Indonesian Seas and Makassar Strait. However, the intraseasonal variation of the ITF transport through these straits over the MJO life cycle, especially during the suppressed phase, has not been quantified. To quantify the ITF transport associated with the MJO, composites of ITF transport through major straits in the Indonesian Seas are constructed using a 0.08 • global HYbrid Coordinate Ocean Model (HYCOM) reanalysis. A prominent reduction of ITF transport through major straits is found during the MJO active phase, and a transport enhancement comparable to the reduction is evident during the suppressed phase. As a result, the net effect of the MJO on the mean ITF transport is very small due to the cancellation of the enhancement and reduction. The magnitude of the MJO-associated ITF transport variation through all ITF exit passages is about 6 Sv, which is about 50% of the total transport. While the propagation of coastal Kelvin waves during the MJO active phase and their impact on ITF transport is clearly evident in the composite, such Kelvin wave influence on ITF transport is not clearly detected during the suppressed phase. This suggests that anomalous winds over the Maritime Continent (MC) area are mostly responsible for the ITF variation during the suppressed phase in many MJO events. Yet, during some MJO events, remotely-forced upwelling Kelvin waves and their significant impact on the ITF transport are evident during the MJO suppressed phase.
Journal of Geophysical Research: Oceans, 2014
Ongoing acoustic Doppler current profilers (ADCP) observation in the eastern equatorial Indian Ocean and a recent International Nusantara Stratification and Transport (INSTANT) experiment in the Indonesian Throughflow (ITF) straits have shown coherent intraseasonal oceanic variations in this region. The intraseasonal variations are dominated by 30-70 day variations, with a tendency for the observed currents in the eastern equatorial Indian Ocean to lead those at the Lombok and Ombai Straits. Phase speed of these eastward propagating signals estimated using lag correlation analysis does not correspond to one particular baroclinic mode, though it is in the range expected for the first two baroclinic modes. In this study, the dynamics underlying this intraseasonal coherency is evaluated using output from a high-resolution ocean general circulation model developed for the Earth Simulator (OFES). The results from model simulation of January 2001 through December 2007 show that the first two baroclinic modes dominate the intraseasonal variations in this region. While the first and second baroclinic modes have comparable contribution to the intraseasonal oceanic variations in the eastern equatorial Indian Ocean and in the Ombai Strait, the intraseasonal oceanic variations in the Lombok Strait are dominated by the first baroclinic mode. Moreover, the analysis reveals that the intraseasonal variability at all mooring sites is mostly confined in the upper layer above 100 m depth. Both equatorial wind from the Indian Ocean and alongshore winds off Sumatra and Java play important roles in generating intraseasonal variations in the Lombok and Ombai Straits.
Indian Ocean Kelvin waves in the Indonesian Throughflow Exit Passages
2008
Equatorial Kelvin waves generated by westerly wind anomalies over the central Indian Ocean propagate eastward to Indonesia, where they can enter the outflow passages of the Indonesian Throughflow (ITF) and affect transports of mass and heat. This has potential consequences for local thermohaline properties and may also be related to larger scale climate modes such as the Indian Ocean Dipole (IOD) and Madden-Julian Oscillation (MJO). Moorings were deployed in Lombok and Ombai Straits, two ITF exit passages, as part of the three-year International Nusantara STratification ANd Transport (INSTANT) program. These data provide the first comprehensive, full-depth, high-resolution measurements of velocity and temperature in the exit passages and have allowed us to thoroughly characterize the properties of Kelvin waves in the throughflow region. Here, we discuss the relationship between ITF Kelvin waves and the structure and timing of the wind forcing over the Indian Ocean. Specifically, we focus on the partitioning of Kelvin wave energy between the ITF outflow passages, mode-1 versus mode-2 vertical structure of the Kelvin waves, and the connection between wind forcing, MJO, IOD, and monsoon dynamics.
Pathways of intraseasonal variability in the Indonesian Throughflow region
Dynamics of Atmospheres and Oceans, 2010
1 Abstract The recent INSTANT measurements in the Indonesian archipelago revealed a broad spectrum of time scales that influence Throughflow variability, from intraseasonal (20-90 days) to interannual. The different time scales are visible in all transport and property fluxes and are the result of remote forcing by both the Pacific and Indian Ocean winds, and local forcing generated within the regional Indonesian seas. This study focuses on the time-dependent three-dimensional intraseasonal variability (ISV) in the Indonesian Throughflow (ITF) region, in particular at the locations of the INSTANT moorings at the Straits of Lombok, Ombai and Timor. The Bluelink ocean reanalysis in combination with the observations from the INSTANT observing program reveals that deep-reaching sub-surface ISV in the eastern Indian Ocean and ITF is closely linked with equatorial wind-stress anomalies in the central Indian Ocean. Having travelled more than 5000km in about 14 days, the associated Kelvin waves can be detected as far east as the Banda Sea. ISV near the Straits of Ombai and Timor is also significantly influenced by local wind forcing from within the ITF region. At the INSTANT mooring sites the ocean reanalysis agrees reasonably well with the observations. Intraseasonal amplitudes are about ±1.0 o C and
Journal of Physical Oceanography, 1999
Seasonal variations of upper-ocean mass transport between the Pacific and Indian Oceans via the Indonesian Throughflow (ITF) are examined using numerical experiments with a 1½-layer, reduced-gravity model forced with specific climatological winds. The model ITF transport, computed as a sum of through-strait transport, has an annual range of more than 8 Sv (an annual harmonic of amplitude 4.2 Sv and a smaller, semiannual harmonic amplitude of 0.5 Sv (Sv ϵ 10 6 m 3 s Ϫ1), with peak transport from mid-April through July and minimum transport in November and December. Limited long-term observations make it difficult to validate these results, but they are consistent with current theory. Experiments with time-varying winds in specific regions show that most of the annual throughflow signal is due to equatorial winds (from 10ЊS to 10ЊN); ITF transport anomalies generated by off-equatorial winds account for less than 1 Sv and are mostly out of phase with the baseline throughflow signal. For the particular wind data used in this study, effects of remote wind forcing in the equatorial Indian Ocean are countered by local winds in the Indonesian seas, and the annual cycle of through-strait transport derived from the model forced only by equatorial Pacific winds is nearly equivalent to that of the baseline run. In this model, the specified wind stress causes annual Rossby waves to be formed in the eastern Pacific by Ekman pumping. These Rossby waves propagate to the western boundary of the Pacific, then form coastal Kelvin waves that propagate through the Indonesian seas. In northern spring, a downwelling wave brings elevated sea level to the Pacific side of the Indonesian seas, and the ITF is maximum. In northern fall an upwelling wave reduces the sea level on the Pacific side, and ITF transport is minimum. In the Indian Ocean, monsoon winds produce equatorial Kelvin waves that propagate eastward and form coastal waves along the southern coasts of Sumatra and Java. A downwelling (upwelling) wave increases (decreases) sea level on the Indian Ocean side of Indonesia in northern spring (winter), thus acting in opposition to the baseline ITF variability. The effect of local Ekman pumping is in the opposite sense. In northern winter, when remote Indian Ocean winds create an upwelling coastal wave, the local wind stress provides downward Ekman pumping. In northern spring, locally forced, upward Ekman suction counters the remotely forced, downwelling Kelvin wave. * SOEST Contribution Number 4738.
Seasonal variations in the equatorial Indian Ocean and their impact on the Lombok throughflow
Journal of Geophysical Research, 1996
In order to identify the origin of the nonequilibrated pressure field which introduces semiannual variations to the Lombok throughflow, one of the major components of the Indonesian throughflow, seasonal sea level records along the eastern boundary of the Indian Ocean and subsurface temperature records in the equatorial Indian Ocean are compared with the results from ocean general circulation models forced by climatological winds.
Journal of Geophysical Research, 2001
An array of shallow pressure gauge pairs is used to determine shallow geostrophic flow relative to an unknown mean velocity in the five principal straits that separate the eastern Indian Ocean from the interior Indonesian seas (Lombok Strait, Sumba Strait, Ombai Strait, Savu/Dao Straits, and Timor Passage). Repeat transects across the straits over several tidal cycles with a 150-kHz acoustic Doppler current profiler were made during three separate years, and provide a first look at the lateral and vertical structure of the upper throughflow in these straits as well as a means of "leveling" the pressure gauge data to determine the mean shallow velocity and provide transport estimates. We estimate a total 2-year average transport for 1996 -1997 through Lombok, Ombai, and Timor Straits as 8.4 Ϯ 3.4 Sv toward the Indian Ocean. The flow structure in the upper 200 m is seen to be similar in Lombok, Sumba, and Ombai Straits, with a division into two layers, governed by different dynamics, where the upper layer episodically flows away from the Indian Ocean. Laterally, flow tends to be strongest in the deepest parts of the channel, with the exception of Lombok Strait which shows a consistent intensification of flow toward the western side. Eastward flowing northern boundary currents in Sumba and Ombai Straits suggest that the South Java Current may penetrate to the Banda Sea, farther eastward than previously documented. Although additional observations are required for a conclusive comparison, the estimated transport time series suggest differences in timing of outflow into the Indian Ocean relative to inflow from the Pacific of a size that could significantly impact the Banda Sea thermocline structure.
Journal of Climate, 2016
Previous studies indicate that equatorial zonal winds in the Indian Ocean can significantly influence the Indonesian Throughflow (ITF). During the Cooperative Indian Ocean Experiment on Intraseasonal Variability (CINDY)/Dynamics of the Madden–Julian Oscillation (DYNAMO) field campaign, two strong MJO events were observed within a month without a clear suppressed phase between them, and these events generated exceptionally strong ocean responses. Strong eastward currents along the equator in the Indian Ocean lasted more than one month from late November 2011 to early January 2012. The influence of these unique MJO events during the field campaign on ITF variability is investigated using a high-resolution (1/25°) global ocean general circulation model, the Hybrid Coordinate Ocean Model (HYCOM). The strong westerlies associated with these MJO events, which exceed 10 m s−1, generate strong equatorial eastward jets and downwelling near the eastern boundary. The equatorial jets are realis...