For something as chaotic as wind, meteorologists tend to have a pretty good grasp of the kinds of airflow patterns we might expect to see storms around the world.
One, it seems, has so far slipped under the radar. To better understand ocean temperature contrasts in the Indian Ocean, scientists have discovered a new type of tropical cyclone that occurs several times a year off Sumatra.
Single, short-lived storms occur cyclically in the southeast Indian Ocean. They tend to start swirling in winter and spring (in the southern hemisphere), when equatorial westerly winds meet northwesterly winds.
Researchers are quite familiar with the sloshing of large volumes of air near westerly equatorial winds during the summer months, more accurately known as the intraseasonal boreal summer oscillation.
But the periodicity of these oscillations didn’t quite match the patterns of the storms, prompting scientists to dig deeper.
According to daily atmospheric data, only three significant cooling events have occurred off Sumatra since 1988, and these anomalies appear to follow a lack of cyclonic activity.
The results are actually quite similar to the cooling force of the El Niño-Southern Oscillation (ENSO), the researchers say, which occurs in the eastern equatorial Pacific.
In the equatorial Indian Ocean, the difference in sea surface temperatures in the western and eastern tropics is known as the Indian Ocean Dipole (IOD), and it appears to be closely related to cyclone activity. near Sumatra.
As a warming water cyclone forms in the Southeast Tropical Indian Ocean (SETIO), northwesterly winds intensify, while equatorial westerly winds weaken .
The entire event takes place over approximately 10 days, and during each IOD season (July to September), approximately 5 SETIO storms occur on average – a cumulative duration of approximately 50 days. During certain seasons, however, only one cyclone may occur.
“Dramatic changes occur in some years when SETIO cyclones do not develop, and ambient winds trigger the appearance of cold sea water over a wide area strongly disrupting winds and precipitation over the Indian Ocean,” explains oceanographer Jochen Kaempf of Flinders University.
The westerly winds that blow around Sumatra have been noticed by scientists before, but where they came from has never been understood.
“This frequent occurrence of SETIO cyclones explains their control over mean equatorial zonal winds in the eastern Indian Ocean, which are almost absent otherwise,” the authors write.
In fact, cyclones around the world make up most of the force of the wind that circulates around the equator.
During a SETIO cyclone, upwellings of cold water in the southeast Indian Ocean are suppressed by northwesterly winds – in the same way that winds off the coast of Peru control upwellings. La Niña water.
“Consequently, wind disturbances strongly modulate the intensity of coastal upwelling and [sea surface temperature] anomalies in both oceans,” the authors write.
“However, the intensity of the equatorial upwelling is markedly different. In the Pacific Ocean, the easterly trade winds induce continuous equatorial upwelling which is enhanced near the Peruvian coast. Due to the effect of the SETIO cyclones, equatorial winds in the eastern Indian Ocean tend to be generally westerly and therefore work to suppress equatorial upwelling.
Given the uncertainty of Earth’s future, the researchers say a better understanding of SETIO cyclones is of the utmost importance.
These storms influence the Indian Ocean Dipole, which in turn has a large impact on climate and rainfall in surrounding countries, including Australia.
A better understanding of atmospheric and ocean currents in this part of the world will help improve local climate models, allowing experts to better predict where rapid global warming is heading and how we can better prepare for future disaster.
The study was published in the Journal of Southern Hemisphere Earth Systems Science.
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