Indo-Pacific interbasin exchange
Project leader: Dr Bernadette Sloyan
Staff and associates
Dr Maxim Nikurashin (UTAS)
Dr Beatriz Peña-Molino (CSIRO staff)
Dr Helen Phillips (UTAS)
Dr Océane Richet (CSIRO staff)
Dr Susan Wijffels (WHOI and CSIRO staff)
[Music plays and animation waves appear moving up from the bottom of the screen and text appears above: The Centre for Southern Hemisphere Oceans Research]
[Image shows animation waves moving up the screen and over the text and then the image changes to show Bernadette Sloyan talking to the camera and text appears: Bernadette Sloyan, CSHOR Project Leader, CSIRO]
Bernadette Sloyan: So, the project in CSHOR that I am working on is the Indonesian Throughflow. It’s the exchange from the Pacific to the Indian Ocean and how that water moves through the Indonesian Archipelago.
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The Indonesian Throughflow is the transfer of water, heat and salt from the Pacific to the Indian Ocean.
[Image changes to show Susan Wijffels talking to the camera and text appears: Susan Wijffels, CSHOR Scientist, CSIRO and Woods Hole Oceanographic Institute]
Susan Wijffels: The Indonesian Throughflow is really important for climate all through Australasia.
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It links two very warm bodies of water together so they can exchange heat and energy and then the image changes to show Susan Wijffels talking to the camera.
So, understanding how the two fluids that make up our climate system, the atmosphere and the ocean interact over those warm pools matters a lot and the Indonesian Throughflow is kind of a circuit between the two.
[Image changes to show Bernadette Sloyan talking to the camera]
Bernadette Sloyan: We’ll be able to really, for the first time, understand the dynamics that really control that transfer of water between the Indian and the Pacific Ocean. We’re then able to make good projections of what the climate will look like, whether that’s seasonal or longer term.
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As the only interbasin exchange at low latitudes, the Indonesian Throughflow (ITF) connects two warm pools of global climate significance – the eastern Indian and Western Pacific. The full drivers of ITF transport variability and its impacts on regional and global climate remain poorly understood. Regional ocean and climate models struggle to simulate the region due to complex bathymetry and processes. A dearth of observations, particularly of the flow itself and the internal seas is impeding progress.
We will use observational data and develop a high-resolution model to focus on the following:
1. Response of the ITF and regional seas to intraseasonal – interannual forcing
A number of in situ observational programs provide a variety of observations over a 20 year timeframe, yet there has been few studies that combine these data to investigate the variability of the ITF at various timescales. For example, the INSTANT program, 2004-2005, directly measured the full-depth volume, temperature and salinity transport in the two major inflows – Makassar Strait and Lifamatola Passage- and three major outflows – Timor Passage, Ombai Strait and Lombok Strait – of the Indonesian Throughflow. More recently, the Australian IMOS program monitored the Ombai Strait and Timor Passage between 2011-2015 and there is an ongoing US program to monitor the Makassar Strait (Gordon, pers coms). Additionally, there are ongoing XBTs and satellite SST, and altimetry data. These programs provided valuable observations of the inflow and outflow passages, and the Indonesian seas. We will use these ocean data sets and atmospheric reanalysis products to characterize the response of the ITF to intraseasonal – interannual forcing.
2. Dynamics of the Indonesian Seas
The controlling dynamics of the mean and interannual behaviour of the Indonesian Throughflow remains a major research challenge. Understanding both the advection pathways and mixing patterns are important: Kida and Wijffels (2012) show that the ITF has a profound impact on the pattern of sea surface temperature in the region on seasonal time scales by suppressing seasonal upwelling via warm advection, which in turn will feedback to atmospheric patterns. Jochum and Potemra (2008) show that regional mixing rates can have strong feedbacks to the atmosphere. Investigation of the key ocean dynamics of the ITF will be assessed using both the observational data sets and a high resolution ocean regional model. The observations will be used to guide a number of model perturbation experiments specifically designed to investigate key dynamics that influence ITF characteristics.
3. Strength and spatial patterns of tidally driven mixing and internal wave generation
There have been limited studies of ocean mixing in the region. INDOMIX was one of the first projects dedicated to observations of ocean mixing in the Indonesian Seas, with most of the microstructure measurements taken in the energetic Halmahera sea. INDOMIX confirmed the importance of vertical mixing associated to internal tides in setting the vertical profile of the temperature and salinity of the ITF (Cuypers et al. 2013). Based on the mixing observations, theoretical studies have indicated possible resonance between coastal wave and internal waves which would lead to increase turbulent dissipation leading to enhanced ocean mixing (Reznik and Zeitlin, 2011). However, there is still a paucity of observations of turbulence and mixing and more are needed from this region. To understand the impact of ocean mixing we will include tidal and internal wave generated mixing in our high resolution model of the region. In addition, the project will work with international partners to include ocean mixing observations in an intensive ocean field campaign.
4. Modulation of the ITF by external ocean forcing
Lee et al. (2015) suggests that a stronger than normal ITF has been transferring heat from the Pacific to the South Indian Ocean, helping to account for strong upper ocean heat content growth in that basin since 2003, compared to stasis in Pacific heat content over the same time. The relatively rapid intraseasonal variability (e.g. Madden Julian Oscillation, MJO) affects the evolution and predictability of seasonal signals. The MJO is a key phenomenon that spans and links both the areas in the Indian and Pacific Oceans and has a significant impact on the regions climate. Having a dominant time period near 50 days, and imbedded in the seasonally migrating inter-tropical convergence zones, the MJO is suggested to play a role in the initiation and evolution of ENSO and IOD events (McPhaden et al., 2006), modulation of the Indonesian Throughflow (Sprintall et al, 2009), and partly accounts for the dry break events within the monsoons (Wheeler and McBride 2005). At present, coupled models do not capture this variability very realistically (Lin et al. 2006). Recognition of the importance of the MJO for both numerical weather prediction (Hendon et al, 1999) and seasonal climate forecasting is driving a demand for comprehensive process studies of the ITF –observation and high-resolution modeling- which resolve these phenomena and increased process understanding to improve model simulation of this phenomena. Our comprehensive observational data set and high resolution models will be used to investigate the modulation of the ITF by these external forces, this will provide a more thorough understanding of the connections amongst the Indian, Pacific Oceans and the Indonesian Seas.
Cuypers, Y. S. Pous, S. Wijffels, J. Sprintall, A. Atmadipoera, G. Madec and R. Molcard, 2013. A thermohaline circulation in the Timor basin associated with high values of the vertical eddy. J. Phys. Oceanogr, under revision.
Hendon, H.H., B. Liebmann, M. Newman, J.D. Glick, and J. Schemm, 1999: Medium range forecast errors associated with active episodes of the MJO. Mon. Wea. Rev., 128, 69-86.
Jochum M. and J. Potemra, 2008: Sensitivity of tropoical rainfall to Banda Sea diffusivity in the Community Climate System Model. J. Climate, 21, 6445–6454
Kida, S. and S. Wijffels, 2012: The impact of the Indonesian Throughflow and tidal mixing on the summer-time Sea Surface Temperature in the western Indonesian Seas. J. Geophys. Res. Oceans, 117, C09007 , DOI: 10.1029/2012JC008162
Lee, S-K, W. Park, M. O. Baringer, A. L. Gordon, B. Huber and Y. Liu, 2015: Pacific Origin of the abrupt increase in Indian Ocean heat content during the warming hiatus.
Lin, J.-L., G.N. Kiladis, B.E. Mapes, K.M. Weickmann, K.R. Sperber, M.C. vo, S.D. Schubert, A. Del Genio, L. J. Donner, S. Emori, J.-F. Gueremy, F. Hourdin, P.J. Rasch, E. Roeckner, and J.F. Scinocca, 2006: Tropical intraseasonal variability in 14 IPCC AR4 climate models. Part I: Convective signals. J. Climate, 19, 2665-2690.
McPhaden, M.J., X. Zhang, H.H. Hendon, and M.C. Wheeler, 2006: Large scale dynamics and MJO forcing of ENSO variability. Geophy. Res. Lett., 33, LI6702, doi:10.1029/2006GL026786.
Reznik G. and V. Zeitlin, 2011. Resonant excitation of trapped waves by free inertia-gravity waves in coastal waveguide, and their nonlinear evolution, J. Fluid Mech., 673, 349- 394.Robertson, R., 2010. Tidal Currents and mixing at the INSTANT mooring locations, Dynam. of Atmos. and Oceans, 50, 331-373
Sprintall, J., S., Wijffels, R. Molcard, I. Jaya. , 2009: Direct Estimates of the Indonesian Throughflow Entering the Indian Ocean: 2004-2006. Journal Of Geophysical Research, 114, C07001, doi:10.1029/2008JC005257.
Wheeler, M.C., and J.L. McBride, 2005: Australian-Indonesian monsoon. In: W.K.M. Lau and D.E. Waliser (eds), Intraseasonal Variability in the Atmosphere-Ocean Climate System. Praxis, Springer Berlin Heidelberg, pages 125-173.
CSHOR Science Seminar Project Presentation:
Ma, J., Feng, M. Sloyan, B.M., Lan, J. (2019). Pacific influences on the low-frequency meridional temperature transport of the Indian Ocean. Journal of Climate, 32(4), 1047-1061. https://doi.org/10.1175/JCLI-D-18-0349.1