The oceans primarily absorb the Earth's energy imbalance caused by anthropogenic greenhouse gases. However, current measurements of ocean heat content (OHC) don't fully close the global energy budget. The marginal seas, especially in the Indo-Pacific Convergence Zone - a critical link between the tropical Pacific and Indian Oceans - remain underexplored as heat reservoirs.

A research team led by Prof. WANG Fan from the Institute of Oceanology of the Chinese Academy of Sciences (IOCAS) has quantified the changes in the OHC down to 2,000 meters deep in the Maritime Continent region since the 1990s, and in revealing the patterns and mechanisms of these changes.

The study was published in Geophysical Research Letters on Mar. 26.

To address the challenges of scarce temperature observations and complex terrain in marginal seas, the researchers introduced a novel volume correction method for calculating OHC. Using high-resolution bathymetric data from Earth's Topography and Bathymetry (ETOPO1) and applying a "nearest neighbor" approach, they accurately estimated the volume of complex terrain grid boxes.

"This advancement allows for more accurate estimates of OHC changes, thereby improving our understanding of ocean warming," said JIN Yuchun, first author of the study.

This analysis, which covered the period from 1990 to 2015, showed that the OHC in the Indo-Pacific Convergence Zone increased significantly, surpassing the global and Pacific rates for the same period. The increase in upper ocean heat content was mainly influenced by surface wind anomalies, with easterly anomalies causing warm water to accumulate and penetrate into the Maritime Continent region.

In particular, the Sulu Sea was identified as a critical heat storage "hotspot", with more pronounced warming signals below 500 meters than in other regions, supported by winter observations.

 "This study enhances our understanding of the role of the Maritime Continent in global climate change and provides important insights for the protection of marine ecosystems in the context of global warming," said Prof. WANG.

(a) Bathymetry of the Maritime Continent (MC) region; (b-f) Time series of Ocean Heat Content Anomalies (OHCAs) for the MC, Sulu Sea, Sulawesi Sea, Banda Sea, and Java Sea. (Image by IOCAS)

(a) Climatology of the 0-2000 m OHC. (b-d) △OHC between 1990-2000 and 2005-2015 periods for 0-2000 (b), 0-500 (c), and 500-2000 m (d). (e) Changes in winds (vectors; m s-1 yr^-1) and SSH (color; in cm yr^-1). (Image by IOCAS)