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Climate Change

Indian Ocean keeps the Pacific cool

ITFArticle

Lee S.-K., W. Park, M.O. Baringer, A.L. Gordon, B. Huber, and Y. Liu (2015), Pacific origin of the abrupt increase in Indian Ocean heat content during the warming hiatus, Nature Geoscience, 8, 445-449, doi:10.1038/ngeo2438

Cover image by lead author Lee Sang-Ki. Image shows the dynamics contributing to increased heat transport from the Pacific to Indian Ocean via Indonesia’s archipelago.

Background

Since the end of the 20th century, a slowdown in global mean surface warming has been observed despite net positive radiation at the top of the atmosphere caused by greenhouse gasses. This energy imbalance has many questioning where the residual heat is being stored, and many studies have alluded to the Pacific’s role in sequestering the missing heat below the sea surface, a process occurring over the past 15 years.

This new study finds that the excess heat in the Pacific doesn’t add up. Ocean temperature data from the National Oceanographic and Atmospheric Administration’s World Ocean Atlas shows that Pacific Ocean heat content has been decreasing since 2003, suggesting that the initial heat uptake by the Pacific must be redistributed through ocean circulatory pathways. The authors analyzed observed and simulated ocean heat transports from a suite of computer models, which suggested that thermohaline circulation may play a role (Figure 1).

Figure 1. Global thermohaline circulation (or ocean conveyor belt) helps to redistribute temperature and compositional properties throughout the global ocean.

Figure 1. Global thermohaline circulation (or ocean conveyor belt) helps to redistribute ocean temperature and compositional properties throughout the global ocean. (Image: Geological Society, London)

Thermohaline circulation is a global ocean pathway that transports oceanic heat over large geographic distances. Often referred to as the ocean conveyor belt, a once-around trip takes approximately 1000 years to complete. An important throughway in thermohaline circulation is the Indonesian passage, specifically the Makassar Strait between the islands of Borneo and Sulawesi. The authors of this study find that the Indonesian throughflow may be an important accomplice in the redistribution of heat from the Pacific to Indian Oceans. This study compares the rate of change in ocean heat content in the upper 700 meters of the Pacific, Indian, and global oceans and identifies what component of Earth’s energy imbalance has contributed to elevated ocean heat content in the Indian Ocean.

Findings

This study finds that the heat content in the Indian Ocean accounts for more than 70% of total global ocean heat gain in the upper 700 meters over the past decade. These results suggest that the missing heat from the warming hiatus in atmospheric surface temperatures has since made its way from the Pacific, through the Indonesian passage, and now resides in the Indian Ocean following the path of thermohaline circulation. Furthermore, the authors attribute the abrupt Indian Ocean heat gain after 2003 to ocean currents rather than absorption from the atmosphere. These findings imply that the mechanisms for heat storage in the Pacific Ocean is not the same for the Indian Ocean, and that inter-ocean thermohaline circulation may play a key role.

Figure 2. Sea surface heights (cm) and heat transport in the upper 700 meters shown by wind stress arrows during 2003-2012.

Figure 2. Sea surface heights (cm) and heat transport in the upper 700 meters shown by wind stress arrows during 2003-2012 (Lee et al., 2015)

Is ENSO the culprit?

Ocean circulation and volume transport through the Indonesian throughflow undergoes year-to-year variability closely associated to the El Niño-Southern Oscillation. Strengthened trade winds along the equatorial Pacific associated with La Niña pile warm water anomalies across the Pacific basin towards the west. Above average sea surface heights sets up a strong pressure gradient from the tropical northwestern Pacific to the tropical South Indian Ocean (Figure 2). Surface waters flow from regions of high to low pressure, thus increasing volume and heat transport through the Indonesian throughflow into the Indian Ocean. The persistence of long-lasting La Niña events during the warming hiatus makes the ocean very efficient in absorbing heat from the atmosphere in the Pacific and enhancing heat transport to the Indian Ocean. This study emphasizes the role of ocean current and heat transport during the global warming hiatus and is yet another piece to the puzzle in finding the missing heat and solving Earth’s energy imbalance.

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