14 Mar 2023
 | 14 Mar 2023
Status: this preprint is open for discussion.

Assessment of Indonesian Throughflow transports from ocean reanalyses with mooring-based observations

Magdalena Fritz, Michael Mayer, Leopold Haimberger, and Susanna Winkelbauer

Abstract. The transport of heat and freshwater from the Pacific to the Indian Ocean via the Indonesian Seas is commonly referred to as the Indonesian Throughflow (ITF). The interaction between the ITF and large scale phenomena occurring from intraseasonal to decadal time scales reflects its connection to the global ocean and the climate system, indicating the need for monitoring the ITF region. In situ observations in this region are highly valuable, but they are temporally and spatially insufficient for near real-time monitoring. Ocean reanalyses have the potential to serve as near-realtime monitoring tools but also to extended time series backward in time, yet a comprehensive quality assessment of their realism in this region with challenging bathymetry has been lacking so far. We focus on oceanic transports diagnosed from the Copernicus Marine Service (CMEMS) Global Reanalysis Ensemble Product (GREP) and the higher-resolution product GLORYS12V1, totalling six reanalysis products. They are validated against in situ observations taken from two different monitoring programs, namely International Nusantara Stratification and Transport (INSTANT 2004–2006) and Monitoring the Indonesian Throughflow (MITF 2006–2011 and 2013–2017), resulting in a total time series of about 11.5 years in the major inflow passage of Makassar Strait and shorter sampled time series in Lombok Strait, Ombai Strait, and Timor Passage. Results show that there is reasonable agreement between reanalysis-based transports and observations in terms of means, seasonal cycles, and variability, although some shortcomings stand out. The lower resolution products do not represent the spatial structure of the flow accurately. They also tend to underestimate the integrated net flow in the narrower straits of Lombok and Ombai, an aspect that is improved in GLORYS12V1. Reanalyses tend to underestimate the effect of seasonal Kelvin waves on the transports, which leads to errors in the mean seasonal cycle. Interannual variations of reanalysed transports agree well with observations, but uncertainties are much larger on sub-annual variability. Finally, as an application of physically consistent and observationally constrained fields provided by ocean reanalyses, we study the impact of the vertically varying pressure gradient on the vertical structure of the ITF to better understand an apparent two-layer regime of the flow.

Magdalena Fritz et al.

Status: open (until 09 May 2023)

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Magdalena Fritz et al.

Magdalena Fritz et al.


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Short summary
The interaction between the Indonesian Throughflow and regional climate phenomena indicate the high relevance for monitoring the ITF. Observations remain temporally and spatially limited, hence near real-time monitoring is only possible with reanalyses. We asses how well ocean reanalyses depict the intensity of the ITF by comparing them to observations. The results showcase that reanalyses agree reasonably well with in situ observations, however, some aspects require higher-resolution products.