Indonesian Throughflow transport in an eddy-resolving climate model

Abstract

The Indonesian Throughflow (ITF) is an important pathway in global circulation, providing a passage for warm and fresh waters from the Pacific to enter the Indian Ocean. Modelling it is a challenge: in order to resolve the narrow straits of the Indonesian seas, relatively high horizontal resolution is required; additionally, as a component of thermohaline circulation, a global or quasi-global model is required. Analysed in this study are the results of the International Laboratory for High-Resolution Earth System Prediction (iHESP) High-Resolution Model Intercomparison Project (HighResMIP) run of the Community Earth System Model version 1.3 (CESM 1.3) with nominal 0.1°, eddy-resolving ocean resolution. In this study, we will investigate if the model captures the Indonesian Throughflow transport, interannual variability, and future trends.

A comparison is made between the modelled and observed transport through four passageways of the throughflow: the Makassar Strait, Timor Passage, Ombai Strait, and Lombok Strait. In comparison with the observations, the model underestimated winter transport; nonetheless, there is a good correspondence in the mean depth-integrated transport except in Ombai Strait for which there are ~1 Sv estimations below 300 m. We also compare the geostrophic transport from observations and total transport from the model at a survey transect called the IX1. This transect is in the Indian Ocean and is often used as a proxy for total ITF transport. The yearly average of modelled transport (-11.51 +/- 6.13 Sv) across the IX1 line for the top 400 m is much higher by approximately 7 Sv than that of observed geostrophic transport (-4.88 +/- 3.50 Sv). This difference might be accounted for by Ekman transport and flow below 400 m.

The interannual variability of the tropical Pacific Ocean has a significant effect on ITF transport. Through lead-lag correlations, the ITF transport correlates significantly with the El Niño-Southern Oscillation (ENSO) from July to October. By compositing the years by ENSO phase, we see the expected weakening of transport during El Niño and strengthening during La Niña. The interannual variations in the Indian Ocean also have an effect but not with statistical significance.