The Impact of Indonesian Throughflow Constrictions on Eastern Pacific Upwelling and Water‐Mass Transformation

Abstract The Indonesian Throughflow (ITF) is a key link in the global ocean overturning circulation. Heat gained from upwelling and watermass transformation in the eastern equatorial Pacific Ocean (PO) is transported through the ITF into the Indian Ocean, ultimately connecting to high‐latitude regions of heat loss such as the North Atlantic. However, state‐of‐the‐art General Circulation Models (GCMs) often poorly simulate upwelling, particularly in the eastern Pacific boundary regions. Recent studies have hypothesized that these difficulties may be related to biases in downstream ITF volume and heat transport, whereby insufficient warm surface water from the western PO passes through the ITF, a complex region that is hard to accurately represent in models. Here, we test this hypothesis using a coarse‐resolution ocean model by artificially widening the ITF and allowing greater quantities of warm water to exit the western PO. This reduces the Indo‐Pacific sea‐surface height and pressure gradient biases. Using diagnostics in temperature coordinates, we trace the source of increased ITF warm‐water transport to enhanced watermass transformation of cold‐to‐warm waters in the eastern tropical PO. Additional cool water is brought to the surface in the Cold Tongue region, resulting in an enhancement of surface flux‐driven warming. However, we find only limited evidence for changes in upwelling at lower temperatures along the eastern PO margins, considerably less than hypothesized in a previous study. Our experiment provides useful insights to help understand simulated ITF mass and heat transport biases and their links with the tropical PO in GCMs.