Abstract
The subtropical South Indian Ocean (SIO) has been described as one of the world's largest heat accumulators due to its remarkable warming during the past two decades. However, the relative contributions of the remote (of Pacific origin) forcing and local wind forcing to the variability of heat content and sea level in the SIO have not been fully attributed. Here, we combine a general circulation model, an analytic linear reduced gravity model, and observations to disentangle the spatial and temporal inputs of each forcing component on interannual to decadal timescales. A sensitivity experiment is conducted with artificially closed Indonesian straits to physically isolate the Indian and Pacific Oceans, thus, intentionally removing the Indonesian throughflow (ITF) influence on the Indian Ocean heat content and sea level variability. We show that the relative contribution of the signals originating in the equatorial Pacific versus signals caused by local wind forcing to the interannual variability of sea level and heat content in the SIO is dependent on location within the basin (low vs. mid latitude; western vs. eastern side of the basin). The closure of the ITF in the numerical experiment reduces the amplitude of interannual-to-decadal sea level changes compared to the simulation with a realistic ITF. However, the spatial and temporal evolution of sea level patterns in the two simulations remain similar and correlated with El Nino Southern Oscillation (ENSO). This suggests that these patterns are mostly determined by local wind forcing and oceanic processes, linked to ENSO via the ‘atmospheric bridge’ effect. We conclude that local wind forcing is an important driver for the interannual changes of sea level, heat content, and meridional transports in the SIO subtropical gyre, while oceanic signals originating in the Pacific amplify locally-forced signals.
Highlights
The Indian Ocean has been characterized as one of the major heat accumulators in the ocean with a significant increase of ocean heat content (OHC) and sea level during the past 2 decades, in particular in the subtropical southern Indian Ocean (SIO)
Before exploring the ocean processes related to sea level changes in the Indonesian Throughflow (ITF)-on and ITF-off experiments, we first validate the model’s performance by comparing the modeled upper-ocean ITF transport with that obtained from moored velocity measurements across the Makassar Strait, which is the primary ITF gateway (Fig. 2a; Gordon et al, 1999, 2010; Pujiana et al, 2019)
The data–model comparisons demonstrated that the model performs reasonably well in simulating the circulation of the Indian Ocean, including the ITF transport
Summary
The Indian Ocean has been characterized as one of the major heat accumulators in the ocean with a significant increase of ocean heat content (OHC) and sea level during the past 2 decades, in particular in the subtropical southern Indian Ocean (SIO) The decade-long increase of the upper-ocean heat content in the SIO in 2005–2013 (Fig. 1a) has been attributed to an enhanced heat transport from the equatorial western Pacific carried by the Indonesian Throughflow (ITF; Lee et al, 2015). The ITF is driven by the inter-ocean pressure gradient that exists between the western equatorial Pacific Ocean and the eastern Indian Ocean (Wyrtki, 1987) and serves as an important component of the global thermohaline circulation
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