Indian Ocean Intermediate Water Masses and Their Simulations by CMIP6 Models

Abstract

Abstract Water masses are carriers of anthropogenic fingerprints in the ocean interior, with their property changes manifesting oceanic thermodynamic responses to climate change. Yet, delimiting ocean water masses remains challenging in either observational atlas or climate models. This study analyzes the distribution of Indian Ocean seawater in the density–spicity space and uses volumetric maxima and minima between σ = 27.1 and 27.4 kg m−3 to track the cores and boundaries of intermediate water masses, respectively. In addition to the well-known Antarctic Intermediate Water (AAIW) and Red Sea–Persian Gulf Intermediate Water (RS-PGIW), two other water masses are identified by the new approach. One is the Indian-AAIW (I-AAIW), as a mixture of the AAIW and the Indonesian Throughflow water, existing in the South Equatorial Current and the Agulhas Current system. The other [equatorial Indian Intermediate Water (EIIW)] exits in the equatorial Indian Ocean and Bay of Bengal, sourced from the RS-PGIW and overlying fresh waters. These waters are corroborated by nutrient and dissolved oxygen data. Around half (26 out of 51) of phase 6 of the Coupled Model Intercomparison Project (CMIP6) models can reasonably simulate these intermediate water masses. Compared with the observed water masses, the intermediate water masses in models are of a smaller thickness and the RS-PGIW is colder and fresher. The former arises from a warm bias in the thermocline, whereas the latter is likely linked to insufficient ventilation in the Red Sea and Persian Gulf in models owing to coarse grid resolution and a surface cold bias. Significance Statement Oceanic water masses are important for understanding climate change. Yet, the definition of water masses is controversial. Here, we use volumetric minima in the density–spicity space to track the boundaries of intermediate (27.1–27.4 kg m−3) water masses in the Indian Ocean. We identify four water masses: AAIW, I-AAIW, EIIW, and RS-PGIW. We delineate the geographical location of each water mass, including two newly identified water masses (I-AAIW and EIIW). We also examined the performance of 51 CMIP6 models in simulating these water masses and found that 26 models can reasonably simulate the distribution of these water masses. There are two systematic model biases emerging from these models. The intermediate water masses in models are of a thinner thickness arising from a warm bias in the thermocline, and the RS-PGIW is colder and fresher owing to insufficient ventilation in the Red Sea and Persian Gulf. These results provide a useful benchmark for understanding water masses in a changing climate and constraining climate models.