Nature Reviews Earth & Environment | 2021
Heat and freshwater changes in the Indian Ocean region
Abstract
Across the Indo-Pacific region, rapid increases in surface temperatures, ocean heat content and concomitant hydrological changes have implications for sea level rise, ocean circulation and regional freshwater availability. In this Review, we synthesize evidence from multiple data sources to elucidate whether the observed heat and freshwater changes in the Indian Ocean represent an intensification of the hydrological cycle, as expected in a warming world. At the basin scale, twentieth century warming trends can be unequivocally attributed to human-induced climate change. Changes since 1980, however, appear dominated by multi-decadal variability associated with the Interdecadal Pacific oscillation, manifested as shifts in the Walker circulation and a corresponding reorganization of the Indo-Pacific heat and freshwater balance. Such variability, coupled with regional-scale trends, a short observational record and climate model uncertainties, makes it difficult to assess whether contemporary changes represent an anthropogenically forced transformation of the hydrological cycle. Future work must, therefore, focus on maintaining and expanding observing systems of remotely sensed and in situ observations, as well as extending and integrating coral proxy networks. Improved climate model simulations of the Maritime Continent region and its intricate exchange between the Pacific and Indian oceans are further necessary to quantify and attribute Indo-Pacific hydrological changes. The Indian Ocean has undergone substantial heat and freshwater changes. This Review uses various data sources to examine the causes of contemporary and longer-term hydrological changes, revealing that trends over the twentieth century are linked to anthropogenic forcing, but that those since 1980 are related to the Interdecadal Pacific oscillation. At the basin scale, the Indian Ocean sustained robust twentieth century surface warming exceeding that of other tropical ocean basins. Yet, substantial variability exists regarding the magnitude and confidence in trends at regional scales, especially in the subsurface, due to the sparse observational network. Indian Ocean heat content has risen rapidly since the 2000s and concomitant freshening occurred over the eastern Indian Ocean and Maritime Continent (MC). Broad-scale warming and MC freshening trends are consistent with expected changes of an intensifying hydrological cycle in a warming world; however, the rate of observed change since the 1980s likely results from natural multi-decadal variability associated with the Interdecadal Pacific oscillation. Disentangling the effects of multi-decadal natural variability and anthropogenic change on heat and freshwater changes in the Indian Ocean and MC region — of importance for climate risk assessments for vulnerable societies in Indian Ocean rim countries — require sustained and enhanced observations. Centennial trends based on coral proxies indicate robust warming and freshening since the 1850s over the Indian Ocean and broader MC region. However, the reconstructed century-scale trend magnitude is much lower than the rapid trends observed since 1980, which were most likely exacerbated by recent acceleration of anthropogenic climate warming and natural multi-decadal variability associated with Interdecadal Pacific oscillation phase shifts. Quantifying change in the Indian Ocean heat and freshwater balance warrants a multi-pronged approach that capitalizes on a systematic integration of in situ observations, remote sensing, numerical modelling efforts and palaeo proxy networks across temporal and spatial scales. At the basin scale, the Indian Ocean sustained robust twentieth century surface warming exceeding that of other tropical ocean basins. Yet, substantial variability exists regarding the magnitude and confidence in trends at regional scales, especially in the subsurface, due to the sparse observational network. Indian Ocean heat content has risen rapidly since the 2000s and concomitant freshening occurred over the eastern Indian Ocean and Maritime Continent (MC). Broad-scale warming and MC freshening trends are consistent with expected changes of an intensifying hydrological cycle in a warming world; however, the rate of observed change since the 1980s likely results from natural multi-decadal variability associated with the Interdecadal Pacific oscillation. Disentangling the effects of multi-decadal natural variability and anthropogenic change on heat and freshwater changes in the Indian Ocean and MC region — of importance for climate risk assessments for vulnerable societies in Indian Ocean rim countries — require sustained and enhanced observations. Centennial trends based on coral proxies indicate robust warming and freshening since the 1850s over the Indian Ocean and broader MC region. However, the reconstructed century-scale trend magnitude is much lower than the rapid trends observed since 1980, which were most likely exacerbated by recent acceleration of anthropogenic climate warming and natural multi-decadal variability associated with Interdecadal Pacific oscillation phase shifts. Quantifying change in the Indian Ocean heat and freshwater balance warrants a multi-pronged approach that capitalizes on a systematic integration of in situ observations, remote sensing, numerical modelling efforts and palaeo proxy networks across temporal and spatial scales.