Makassar Strait Intraseasonal Variability

The intraseasonal variability (ISV; 20-90 days) in Makassar Strait, the primary pathway for Pacific water flow into the Indian Ocean and a waveguide for transmitting subinertial energy from the tropical Indian Ocean to Indonesian seas, is investigated, using the 2004-2006 International Nusantara Stratification and Transport (INSTANT) observations. The INSTANT current and temperature timeseries in the Labani Channel, a narrow constriction in Makassar Strait, are used to identify the ISV. Additionally, insitu current measurements along with satellite-derived wind and sea level anomaly data in the region are employed to track the transmission of ISV from their likely origin. We find that the Makassar Strait ISV can be classified as locally or remotely forced features. Local winds and shear flow instability-generated eddies within Makassar Strait control the locally forced ISV component, while the remotely forced part is linked to equatorial Indian Ocean Kelvin waves and Sulawesi Sea eddies. The oceanic response to the local wind stress varying at periods of 45-90 days, with 60-day oscillation showing the strongest coherence, is constrained to along-strait flow primarily within the upper 50 m of the water column. At depths greater than 50 m, we observe that the 20-40 day variability reflects locally generated eddy signatures, while the 60-90 day variability agrees with remotely forced Kelvin wave characteristics. Moreover the Sulawesi Sea ISV, signifying eddy signatures, and along-strait flow across the Makassar Strait pycnocline of 50-450 m display significant coherence at periods of 45-90 days. The observed 60-90 day variability at depths of 100-450 m, coinciding with weaker Makassar Strait throughflow, exhibits Kelvin wave signatures including vertical energy propagation, energy equipartition, non-dispersive relationship and semi-geostrophic balance. Current meters at 750 m and 1500 m further provide evidence that the vertical structure of the Kelvin waves resembles that of a second baroclinic mode. We propose that the intraseasonal Kelvin waves emanate from the tropical Indian Ocean as wind-forced equatorial Kelvin waves and propagate along a waveguide which extends from the southwestern coasts of the Indonesian archipelago to Makassar Strait, via Lombok Strait. From Lombok Strait to Makassar Strait, the Kelvin waves navigate along the 100-m isobath. The intraseasonal Kelvin waves induce increased vertical shear of the along-strait flow across the pycnocline, which potentially leads to instability with a vertical mixing rate of 1-5x 10^-5 m^2s. Moreover the intraseasonal Kelvin waves also force the ITF transport anomalies in Makassar Strait. The 20-40 day variability is most evident in the across-strait flow, and in the across-strait gradient of the along-strait flow as well as in the vertical displacements of isotherms observed at depths of 100-300 m. The flow fields at 20-40 days are approximated by a vortex velocity structure, and the corresponding isotherm displacements signify potential vorticity conservation. We propose that southward-advected eddies, generated in the northern Makassar Strait at latitudes of 0.5-2S due to background flow instability, likely explain the 20-40 day variability observed in the Labani Channel

Makassar Strait Intraseasonal Variability

The intraseasonal variability (ISV; 20-90 days) in Makassar Strait, the primary pathway for Pacific water flow into the Indian Ocean and a waveguide for transmitting subinertial energy from the tropical Indian Ocean to Indonesian seas, is investigated, using the 2004-2006 International Nusantara Stratification and Transport (INSTANT) observations. The INSTANT current and temperature timeseries in the Labani Channel, a narrow constriction in Makassar Strait, are used to identify the ISV. Additionally, insitu current measurements along with satellite-derived wind and sea level anomaly data in the region are employed to track the transmission of ISV from their likely origin. We find that the Makassar Strait ISV can be classified as locally or remotely forced features. Local winds and shear flow instability-generated eddies within Makassar Strait control the locally forced ISV component, while the remotely forced part is linked to equatorial Indian Ocean Kelvin waves and Sulawesi Sea eddies. The oceanic response to the local wind stress varying at periods of 45-90 days, with 60-day oscillation showing the strongest coherence, is constrained to along-strait flow primarily within the upper 50 m of the water column. At depths greater than 50 m, we observe that the 20-40 day variability reflects locally generated eddy signatures, while the 60-90 day variability agrees with remotely forced Kelvin wave characteristics. Moreover the Sulawesi Sea ISV, signifying eddy signatures, and along-strait flow across the Makassar Strait pycnocline of 50-450 m display significant coherence at periods of 45-90 days. The observed 60-90 day variability at depths of 100-450 m, coinciding with weaker Makassar Strait throughflow, exhibits Kelvin wave signatures including vertical energy propagation, energy equipartition, non-dispersive relationship and semi-geostrophic balance. Current meters at 750 m and 1500 m further provide evidence that the vertical structure of the Kelvin waves resembles that of a second baroclinic mode. We propose that the intraseasonal Kelvin waves emanate from the tropical Indian Ocean as wind-forced equatorial Kelvin waves and propagate along a waveguide which extends from the southwestern coasts of the Indonesian archipelago to Makassar Strait, via Lombok Strait. From Lombok Strait to Makassar Strait, the Kelvin waves navigate along the 100-m isobath. The intraseasonal Kelvin waves induce increased vertical shear of the along-strait flow across the pycnocline, which potentially leads to instability with a vertical mixing rate of 1-5x 10^-5 m^2s. Moreover the intraseasonal Kelvin waves also force the ITF transport anomalies in Makassar Strait. The 20-40 day variability is most evident in the across-strait flow, and in the across-strait gradient of the along-strait flow as well as in the vertical displacements of isotherms observed at depths of 100-300 m. The flow fields at 20-40 days are approximated by a vortex velocity structure, and the corresponding isotherm displacements signify potential vorticity conservation. We propose that southward-advected eddies, generated in the northern Makassar Strait at latitudes of 0.5-2S due to background flow instability, likely explain the 20-40 day variability observed in the Labani Channel

Intraseasonal Kelvin Wave in Makassar Strait

Time series observations during 2004-2006 reveal the presence of 60-90 days intraseasonal events that impact the transport and mixing environment within Makassar Strait. The observed velocity and temperature fluctuations within the pycnocline reveal the presence of Kelvin waves including vertical energy propagation, energy equipartition, and nondispersive relationship. Two current meters at 750 and 1500 m provide further evidence that the vertical structure of the downwelling Kelvin wave resembles that of the second baroclinic wave mode. The Kelvin waves derive their energy from the equatorial Indian Ocean winds, including those associated with the Madden-Julian oscillations, and propagate from Lombok Strait to Makassar Strait along the 100-m isobath. The northward propagating Kelvin waves within the pycnocline reduce the southward Makassar Strait throughflow by up to 2 Sv and induce a marked increase of vertical diffusivity

Intraseasonal Variability in the Makassar Strait Thermocline

Intraseasonal variability [ISV] in the Makassar Strait thermocline is examined through the analysis of along-channel flow, regional sea level anomaly and wind fields from January 2004 through November 2006. The dominant variability of 45-90 day in the Makassar Strait along-channel flow is horizontally and vertically coherent and exhibits vertical energy propagation. The majority of the Makassar ISV is uncoupled to the energy exerted by the local atmospheric ISV: instead the Makassar ISV is due to the combination of a remotely forced baroclinic wave radiating from Lombok Strait and deep reaching ISV originating in the Sulawesi Sea. Thermocline depth changes associated with ENSO influence the ISV characteristics in the Makassar Strait lower thermocline, with intensified ISV during El Nin˜o when the thermocline shallows and weakened ISV during La Nin˜a

Intraseasonal Sea Surface Temperature Variability across the Indonesian Seas

Sea surface temperature (SST) variability at intraseasonal time scales across the Indonesian Seas during January 1998–mid-2012 is examined. The intraseasonal variability is most energetic in the Banda and Timor Seas, with a standard deviation of 0.4°–0.5°C, representing 55%–60% of total nonseasonal SST variance. A slab ocean model demonstrates that intraseasonal air–sea heat flux variability, largely attributed to the Madden–Julian oscillation (MJO), accounts for 69%–78% intraseasonal SST variability in the Banda and Timor Seas. While the slab ocean model accurately reproduces the observed intraseasonal SST variations during the northern winter months, it underestimates the summer variability. The authors posit that this is a consequence of a more vigorous cooling effect induced by ocean processes during the summer. Two strong MJO cycles occurred in late 2007–early 2008, and their imprints were clearly evident in the SST of the Banda and Timor Seas. The passive phase of the MJO [enhanced outgoing longwave radiation (OLR) and weak zonal wind stress) projects on SST as a warming period, while the active phase (suppressed OLR and westerly wind bursts) projects on SST as a cooling phase. SST also displays significant intraseasonal variations in the Sulawesi Sea, but these differ in characteristics from those of the Banda and Timor Seas and are attributed to ocean eddies and atmospheric processes independent from the MJO.Keywords: Circulation, Variability, Dynamics, Intraseasonal variability, Atmosphere-ocean interaction, Oceanic variabilit

Distinguishing ichthyogenic turbulence from geophysical turbulence

Measurements of currents and turbulence beneath a geostationary ship in the equatorial Indian Ocean during a period of weak surface forcing revealed unexpectedly strong turbulence beneath the surface mixed layer. Coincident with the turbulence was a marked reduction of the current speeds registered by shipboard Doppler current profilers, and an increase in their variability. At a mooring 1 km away, measurements of turbulence and currents showed no such anomalies. Correlation with the shipboard echo sounder measurements indicate that these nighttime anomalies were associated with fish aggregations beneath the ship. The fish created turbulence by swimming against the strong zonal current in order to remain beneath the ship, and their presence affected the Doppler speed measurements. The principal characteristics of the resultant ichthyogenic turbulence are (i) low wave number roll-off of shear spectra in the inertial subrange relative to geophysical turbulence, (ii) Thorpe overturning scales that are small compared with the Ozmidov scale, and (iii) low mixing efficiency. These factors extend previous findings by Gregg and Horne (2009) to a very different biophysical regime and support the general conclusion that the biological contribution to mixing the ocean via turbulence is negligible.KEYWORDS: ichthyogenic turbulence, geophysical turbulenceThis is the publisher’s final pdf. The article is copyrighted by American Geophysical Union and published by John Wiley & Sons Ltd. It can be found at: http://agupubs.onlinelibrary.wiley.com/agu/jgr/journal/10.1002/%28ISSN%292169-9291

Makassar Strait Throughflow Seasonal and Interannual Variability: An Overview

The Makassar Strait throughflow of ~12–13 Sv, representing ~77% of the total Indonesian Throughflow, displays fluctuations over a broad range of time scales, from intraseasonal to seasonal (monsoonal) and interannual scales. We now have 13.3 years of Makassar throughflow observations: November 1996 to early July 1998; January 2004 to August 2011; and August 2013 to August 2017. Strong southward transport is evident during boreal summer, modulated by an ENSO interannual signal, with weaker southward flow and a deeper subsurface velocity maximum during El Niño; stronger southward flow with a shallower velocity maximum during La Niña. Accordingly, the southward heat flux, a product of the along‐channel current and temperature profiles, is significantly larger in summer and slightly larger during La Niña. The southward flow relaxed in 2014 and more so in 2015/2016, similar though not as extreme as during the strong El Niño event of 1997. In 2017, the throughflow increased to ~20 Sv. Since 2016, the deep layer, 300‐ to 760‐m southward transport increases, almost doubling to ~7.5 Sv. From mid‐2016 into early 2017, the transports above 300 m and below 300 m are about equal, whereas previously, the ratio was about 2.7:1. Near zero or northward flow occurs in the upper 100 m during boreal winter, albeit with interannual variability. Particularly strong winter reversals were observed in 2014/2015 and 2016/2017, the latter being the strongest winter reversal revealed in the entire Makassar time series

Detecting change in the Indonesian Seas

© The Author(s), 2019. This article is distributed under the terms of the Creative Commons Attribution License. The definitive version was published in Sprintall, J., Gordon, A. L., Wijffels, S. E., Feng, M., Hu, S., Koch-Larrouy, A., Phillips, H., Nugroho, D., Napitu, A., Pujiana, K., Susanto, R. D., Sloyan, B., Yuan, D., Riama, N. F., Siswanto, S., Kuswardani, A., Arifin, Z., Wahyudi, A. J., Zhou, H., Nagai, T., Ansong, J. K., Bourdalle-Badie, R., Chanuts, J., Lyard, F., Arbic, B. K., Ramdhani, A., & Setiawan, A. Detecting change in the Indonesian Seas. Frontiers in Marine Science, 6, (2019):257, doi:10.3389/fmars.2019.00257.The Indonesian seas play a fundamental role in the coupled ocean and climate system with the Indonesian Throughflow (ITF) providing the only tropical pathway connecting the global oceans. Pacific warm pool waters passing through the Indonesian seas are cooled and freshened by strong air-sea fluxes and mixing from internal tides to form a unique water mass that can be tracked across the Indian Ocean basin and beyond. The Indonesian seas lie at the climatological center of the atmospheric deep convection associated with the ascending branch of the Walker Circulation. Regional SST variations cause changes in the surface winds that can shift the center of atmospheric deep convection, subsequently altering the precipitation and ocean circulation patterns within the entire Indo-Pacific region. Recent multi-decadal changes in the wind and buoyancy forcing over the tropical Indo-Pacific have directly affected the vertical profile, strength, and the heat and freshwater transports of the ITF. These changes influence the large-scale sea level, SST, precipitation and wind patterns. Observing long-term changes in mass, heat and freshwater within the Indonesian seas is central to understanding the variability and predictability of the global coupled climate system. Although substantial progress has been made over the past decade in measuring and modeling the physical and biogeochemical variability within the Indonesian seas, large uncertainties remain. A comprehensive strategy is needed for measuring the temporal and spatial scales of variability that govern the various water mass transport streams of the ITF, its connection with the circulation and heat and freshwater inventories and associated air-sea fluxes of the regional and global oceans. This white paper puts forward the design of an observational array using multi-platforms combined with high-resolution models aimed at increasing our quantitative understanding of water mass transformation rates and advection within the Indonesian seas and their impacts on the air-sea climate system. IntroductionJS acknowledges funding to support her effort by the National Science Foundation under Grant Number OCE-1736285 and NOAA’s Climate Program Office, Climate Variability and Predictability Program under Award Number NA17OAR4310257. SH was supported by the National Natural Science Foundation of China (Grant 41776018) and the Key Research Program of Frontier Sciences, CAS (QYZDB-SSW-SYS023). HP acknowledges support from the Australian Government’s National Environmental Science Programme. HZ acknowledges support from National Science Foundation under Grant No. 41876009. RS was supported by National Science Foundation Grant No. OCE-07-25935; Office of Naval Research Grant No. N00014-08-01-0618 and National Aeronautics and Space Administration Grant No. 80NSSC18K0777. SW, MF, and BS were supported by Center for Southern Hemisphere Oceans Research (CSHOR), which is a joint initiative between the Qingdao National Laboratory for Marine Science and Technology (QNLM), CSIRO, University of New South Wales and University of Tasmania

INVESTIGATION OF THE COASTALLY TRAPPED WAVES IN THE SOUTH OF INDONESIAN ARCHIPELAGO

Analysis of sea level data derived from Jason-1 altimetry satellite reveals the basic characteristics of a coastally trapped wave along the waveguide in the south of Indonesian archipelago. The most robust signatures of the trapped wave are recorded recurrently in the months of May-June. Hovmoller and coherence analysis synonymously agree that the wave propagates at a speed of 2.8-2.9 m/s towards the eastern end of the waveguide. The trapped wave is dependent upon the stratification regime, and a Wentzel-Kramers-Brillouin (WKB) analysis on the stratification profile inferred from several CTD casts indicates that the trapped wave may be classified as a first mode baroclinic wave