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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.

Key points

  • 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.

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Fig. 1: Mean precipitation and surface ocean temperature and salinity conditions.
Fig. 2: Indo-Pacific trends.
Fig. 3: Time series of key heat and freshwater metrics in the Indo-Pacific warm pool.
Fig. 4: Impacts of anthropogenic warming and multi-decadal variability associated with the Interdecadal Pacific oscillation on Indo-Pacific climate.
Fig. 5: Time series of sea surface temperature and coral proxy reconstructions over the Indo-Pacific warm pool.

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Acknowledgements

This work was supported by the US National Science Foundation under AGS-2002083 (to S.A.M. and C.C.U.), ICER-1663704 (to C.C.U.) and OCE-1851316 (to J.S.). C.C.U. also acknowledges support from the Andrew W. Mellon Foundation Award for Innovative Research and the James E. and Barbara V. Moltz Fellowship for Climate-Related Research, S.A.M. from the WHOI Postdoctoral Scholar Program and N.J.A. from the Australian Research Council through the Centre of Excellence for Climate Extremes (CE170100023) and a Future Fellowship (FT160100029). Part of this research was carried out at the Jet Propulsion Laboratory, California Institute of Technology, under a contract with the National Aeronautics and Space Administration. Graphic support from N. Renier (WHOI Graphics) is gratefully acknowledged.

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C.C.U. and S.A.M. conducted the analyses and produced the figures. C.C.U., S.A.M., J.S., T.L. and N.J.A. wrote sections within the manuscript. All authors contributed to the discussion and commented on the manuscript.

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Correspondence to Caroline C. Ummenhofer.

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Supplementary information

Glossary

Indo-Pacific warm pool

(IPWP). Region at the intersection of the Indian and Pacific oceans, defined as the area with annual sea surface temperature above 28 °C (Fig. 1b), coinciding with the rising branch of the Walker circulation.

Ocean heat content

(OHC). The quantity of heat stored in the ocean, proportional to temperature integrated vertically over a prescribed depth range.

Internal variability

Climate variability that arises due to natural processes or interactions between various components of the climate system, as opposed to anthropogenic or external forcing.

Modes of variability

Natural, recurrent climate phenomena with an underlying space-time structure that displays a preferred spatial pattern and temporal variation in components of the climate system (e.g. ocean, atmosphere and cryosphere).

Indian Ocean Dipole

(IOD). Coupled ocean–atmosphere phenomenon in the tropical Indian Ocean peaking in boreal fall, with its positive phase characterized by anomalous cooling (warming) in the tropical south-east (western) Indian Ocean.

El Niño–Southern Oscillation

(ENSO). Strong year-to-year climate variability originating in the equatorial Pacific Ocean through coupled ocean–atmosphere interactions. El Niño–Southern Oscillation manifests itself in anomalous surface warming (El Niño) or cooling (La Niña) that typically peaks in boreal winter.

Interdecadal Pacific oscillation

(IPO). Decadal mode of Pacific variability (similar to Pacific decadal oscillation) but with a meridionally broader tropical El Niño-like warm temperature anomaly pattern and cool extratropical Pacific during its positive phase.

Thermocline

Zone of maximum vertical temperature gradient, separating warm and cold layers of water. The 20 °C isotherm is often used as an indicator of thermocline depth in the equatorial Indo-Pacific.

Indonesian throughflow

(ITF). Ocean currents from the Pacific Ocean to the Indian Ocean through the passages of the Indonesian archipelago.

Walker circulation

Thermally driven tropical zonal overturning atmospheric circulation associated with rising (sinking) air over the Indo-Pacific warm pool (eastern Pacific), undergoing substantial longitudinal shifts in location in response to the El Niño–Southern Oscillation, Indian Ocean Dipole, and Interdecadal Pacific Oscillation.

Ekman transport

Lateral movement of water in the frictional boundary layer of a fluid, directed to the right or left of the wind in the Northern or Southern Hemisphere, respectively, because of the Coriolis force.

Leeuwin Current

Poleward-flowing eastern boundary current off the west coast of Western Australia that transports relatively warm and fresh waters southward.

Teleconnections

Changes in atmospheric or oceanic circulation over widely separated, geographically fixed spatial locations; often a consequence of large-scale wave motions, whereby energy is transferred from source regions along preferred atmospheric/oceanic paths.

La Niña

The cold phase of the El Niño–Southern Oscillation, characterized by anomalous surface cooling and stronger trade winds in the equatorial Pacific Ocean.

El Niño

The warm phase of the El Niño–Southern Oscillation, characterized by anomalous surface warming and weaker trade winds in the equatorial Pacific Ocean.

Argo

International programme that collects subsurface ocean property measurements using a fleet of robotic instruments that profile between the surface and a mid-depth level (1,000–2,000 m) and then drift with the ocean currents.

Geostrophic

Resulting from a balance between pressure gradients and the Coriolis force.

Pycnocline

Layer in the ocean in which water density increases rapidly with depth.

δ18O

Oxygen isotope composition in ‘delta’ notation, referring to relative departure of sample oxygen isotopic ratios 18O/16O compared with a standard. Coral calcium carbonate δ18O reflects combined sea surface temperature and seawater δ18O influences.

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Ummenhofer, C.C., Murty, S.A., Sprintall, J. et al. Heat and freshwater changes in the Indian Ocean region. Nat Rev Earth Environ 2, 525–541 (2021). https://doi.org/10.1038/s43017-021-00192-6

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