Scientists at Columbia University’s Lamont-Doherty Earth Observatory have found that currents connecting the Indian and Pacific Oceans are colder and deeper than originally believed. This discovery may one day help climate modelers predict the intensity of the Asian monsoon or El Niño with greater accuracy and with more lead-time than is currently possible.
The findings by Arnold L. Gordon, R. Dwi Susanto and Kevin Vranes appear in the October 23 issue of the journal Nature. Their work is the first to combine comprehensive temperature and velocity measurements of an ocean current known as the Indonesian throughflow (ITF) with regional wind data to provide a comprehensive picture of how an important piece of the ocean-atmosphere-climate puzzle works.
“Before now, most people thought the ITF was mostly on the surface,” said Vranes, a former graduate student at Lamont. “Our work shows that the majority of the water flows in the thermocline about 300m below the surface, which makes the overall average flow colder than assumed.”
The ITF is a network of currents that carry tropical Pacific Ocean water into the Indian Ocean through the straits and passages of the Indonesian Archipelago. On average, the ITF flows at a rate of 10 million cubic meters per second (nearly 3 trillion gallons per second), or more than 50 times the average flow of the Amazon River.
The ITF is unique among the world’s inter-ocean currents because it is the only one that exchanges tropical waters between two oceans—all other ocean interchanges occur in the extreme northern or southern latitudes, where the water is already very cold. As a result, the ITF is thought to be an important factor in governing the exchange of heat between the Indian and Pacific Oceans and, consequently, between the oceans and the atmosphere.
“The ocean may very well act as a pace maker to the El Nino and the Asian monsoon,” said Gordon, the lead author on the study. “Which means we might one day be able to predict the intensity of the monsoon a year ahead of time by monitoring the Indonesian throughflow.”
Previously, scientists thought most of the water moving between the Pacific and the Indian Ocean did so on or near the surface, where water temperatures hover around 75°F (24°C). However, using two long-term measuring stations moored in Indonesia’s Makassar Strait, Gordon and the other researchers examined water flow, temperature and salinity from the surface of the strait to the bottom between December 1996 and June 1997. They found that the bulk of the water passing through the strait, which funnels more than 90 percent of the ITF, flowed well below the surface where it could not be warmed by the atmosphere. As a result, it averaged about 59°F (15°C).
Gordon and the others then examined regional wind patterns for the same period. They found that prevailing winds from January to February and again between May and June blow large rafts of buoyant fresh water into areas that effectively block surface water from contributing to the ITF. This, they believe, forces the overall flow to run much deeper and colder than previously thought.
“Before the heat transfer between the Pacific and Indian Oceans was essentially a guess,” said Gordon. “Now we have data.”