Suitability of ocean reanalyses for monitoring of oceanic exchanges through the Indonesian Throughflow

The Indonesian Seas are characterized by numerous narrow channels connecting basins and seas of varying sizes and depths that serve as a transition between the Pacific and the Indian Ocean, known as the Indonesian Throughflow (ITF). The interaction between the ITF and important climate anomalies such as the El Niño Southern Oscillation (ENSO), the Indian Ocean Dipole (IOD), or the Australian-Indonesian monsoon indicates the high relevance for monitoring the ITF region. In situ observations of ITF transports are highly valuable but are temporally and spatially limited. Hence, near real-time monitoring is only possible with reanalyses, yet their quality needs to be evaluated. Here we present an assessment of oceanic transports in the ITF diagnosed from the Copernicus Marine Service (CMEMS) Global Reanalysis Ensemble Product (GREP) and the higher-resolution product GLORYS12V1. Validation data comes from several moorings in Makassar strait, Lombok strait, Ombai strait, and Timor passage, obtained as part of the well-known INSTANT (2004-2006) and MITF (2006-2011 and 2013-2017 in Makassar) campaigns. The campaigns provide a total of 11.5 years of in situ observations in Makassar, therefore allowing the assessment of the mean seasonal cycle of ITF transport and a thorough investigation of the shorter sampled outflow passages. The results showcase that reanalysis-based volume transports agree reasonably well with in situ observations, however, some aspects, such as asymmetries in the flow through each strait, are more accurately represented by GLORYS12V1. Also, in terms of mean integrated transports, the increased horizontal resolution of GLORYS12V1 leads to a better performance in the narrower straits of Lombok and Ombai. Furthermore, we draw attention to an apparent one-month lag between reanalyses and observations in Makassar strait transports, which we assess by studying the influence of the monsoon-driven (vertically varying) pressure gradient on the ITF.