10.7: Monsoonal Circulation

The Encyclopedia Britannica calls the Indian Ocean “the smallest, geologically youngest, and physically most complex of the world’s three major oceans.” To that I would add that the Indian Ocean is the least known scientifically, a cruel irony given that it’s the one on which humans have sailed since sailing became a thing. Our lack of knowledge of the Indian Ocean strikes home for a more urgent reason: the Indian Ocean strongly influences weather and climate patterns around the world. It connects with the Pacific Ocean through a well-known ocean choke point, the Indonesian Throughflow, that acts as a kind of control valve for exchanges of waters between the Pacific, Indian, and even the Atlantic Oceans. And scientists now recognize that the Indian Ocean spawns the Madden–Julian Oscillation, an intraseasonal (i.e., within a season), eastward-traveling, globe-trotting atmospheric “wave” that influences weather. As described by Gottschalck (2014):

The MJO can modulate the timing and strength of the monsoons, influence tropical cyclone numbers and strength in nearly all ocean basins, and result in jet stream changes that can lead to cold air outbreaks, extreme heat events, and flooding rains over the United States and North America.

At a time when we need as much knowledge about the climate system as we can muster, our incomplete knowledge of the Indian Ocean remains a hindrance. We cannot protect what we don’t understand.

Be that as it may, there is one phenomenon of the Indian Ocean that we understand fairly well: the seasonally reversing wind pattern known as the mausin (alternatively, mausam) in Arabic, better known as the monsoon in English (e.g., Tripati 2017). Characterized by strong winds from the southwest in summer and strong northeast winds in winter, the monsoon brings seasonal changes in rainfall. Given that agricultural productivity in this region feeds a third of the world’s population, the monsoon rains are literally a matter of life and death for billions of people (McPhaden et al. 2009). The monsoon winds also bring seasonal changes in ocean currents. In fact, like the winds, the currents in the Indian Ocean change direction in accordance with the seasonally changing winds.

Left Wind, Right Wind, Left Wind

Since the time of the Harappan Civilization, which spanned 3300 to 1700 BCE in the Indus Valley in modern-day Pakistan (e.g., Shinde 2016), people have known about monsoon weather. This should come as no surprise: the monsoon brings seasonal changes in rainfall, and the tens of thousands of people living in cities in this region at that time depended on the rains to produce their food. It appears that by at least 1 CE and perhaps as early as 2500 BCE, Indian Ocean sailors used the monsoon winds to carry out trading voyages (e.g., Tripati 2017). Whether headed east across the Bay of Bengal and Andaman Sea to Indonesia or west across the Arabian Sea to Africa or the Middle East, these early sailors timed their voyages to coincide with the prevailing winds.

During the summer season, heating of the Indian subcontinent results in a kind of giant sea breeze. Hot air rises above the subcontinent, and cooler air from the ocean rushes in to take its place. Because of the Coriolis force, the predominant direction of these winds is from the southwest. This tropical ocean air holds vast amounts of water vapor. So as it rises above the subcontinent and cools, that water vapor is released as torrential rain. The combination of southwest winds and heavy rains characterizes the summer monsoon, also known as the southwest monsoon.

In the winter the situation reverses. The warmer ocean water (as compared to land) generates a circulation similar to a land breeze (albeit a continental-sized one). Winds blow now from the northeast, from the land to the ocean. The winter monsoon, or northeast monsoon, brings dry weather to the Indian subcontinent and beyond.

These seasonally reversing winds meant that sailors could venture from India to the west toward Africa and the Middle East during the winter (northeast) monsoon and return under favorable winds during the summer (southwest) monsoon. Though scholars still debate the timing of the origins of monsoonal sailing, a considerable body of archaeological and cultural evidence appears to support the earlier dates. It makes for a fascinating study and a rich opportunity to learn about the cultural and economic development of India.

Monsoonal Currents

Changes in wind direction cause changes in ocean currents. During the winter monsoon (November–March), the wind patterns in the equatorial Indian Ocean resemble those in the equatorial Atlantic and Pacific. Winds north of the equator blow from the northeast and winds south of the equator blow from the southeast—a typical trade wind pattern. In this case the currents in the tropical Indian Ocean move toward the west.

Now, the geography of the Indian Ocean and the nature of the currents cause some crossing of the equator by these equatorial currents, but by and large they behave similarly to the equatorial currents in the other ocean basins. A westward-flowing North Equatorial Current is present, though it also goes by the name Northeast Monsoon Current. (Do you ever wonder if these currents are in some kind of witness protection program with all their different names?) A South Equatorial Current and Equatorial Countercurrent also exist.

Things get weird in the summer. The onset of the summer monsoon (June–October) winds blowing from the southwest cause a complete reversal of the ocean currents. The North Equatorial Current and the Equatorial Countercurrent disappear and become the Southwest Monsoon Current—with current speeds in excess of 2 knots (strong for a current). This strong eastward-flowing current sends branches into the Arabian Sea and Bay of Bengal as well. The South Equatorial Current crawls up the northern coast of Africa and southeastern Arabia, where it’s called the Somali Current.

Though our tidy description here makes it all seem quite simple, the circulation of the Indian Ocean is—like some people—complicated. For a deeper look into the details of this circulation, consult Phillips et al. (2021).