Ocean Sciences Research Spotlight

Mysterious Intraseasonal Oscillations in Monsoons

The unpredictable cycles of rainfall during India's summerlong monsoon have stymied scientists for decades.

Source: Journal of Geophysical Research: Oceans

By Sarah Witman

India’s summer monsoon is a major event, single-handedly supplying water for agriculture across all of southern Asia. Because of its widespread effect on the region’s environmental and socioeconomic health, the monsoon has long been studied by meteorologists, climatologists, and oceanographers hoping to understand and forecast its behavior.

One phenomenon in particular, the monsoon intraseasonal oscillations (MISOs), has captured scientists’ interest. MISOs are alternating periods of heavy and minimal rainfall, each lasting for about a month or so and tending to follow a cyclical, northward shifting pattern from the equator to southern Asia. Although they were once believed to be a function of the tropical atmosphere, more recent studies have suggested that MISOs come from some kind of powerful atmosphere-ocean interaction.

For one such study, Li et al. examined the pathways of MISOs traveling across the Bay of Bengal, a region where the monsoon undergoes changes in intensity and frequency.

The same team has previously examined the role of ocean salinity stratification (layering of seawater with different salt contents) in variations in sea surface temperature and the atmospheric processes that produce rainfall. Here, however, they addressed the contribution from two other variables: the depth of the mixed layer, which lies just below the sea surface, and the thickness of the barrier layer, which forms the bottom of the mixed layer.

Using an ocean model with data from 2000 to 2014, the researchers investigated how these upper ocean processes affect sea surface temperature (and how variations in sea surface temperature, in turn, affect rain formation in the MISOs).

An influx of freshwater from the monsoon, the researchers found, creates a shallow mixed layer and a thick barrier layer, causing dramatic fluctuations in sea surface temperature over the course of the season. What’s more, these air-sea interactions lead to the highly irregular rainfall patterns seen as the MISOs reach Asia.

Using advanced models and checking them against satellite data allowed the researchers to pinpoint these driving forces behind MISOs with more precision than in previous studies. Their efforts promise to improve the accuracy of future simulations and forecasts of the MISOs that occur during India’s summer monsoon. (Journal of Geophysical Research: Oceans, https://doi.org/10.1002/2017JC012692, 2017)

—Sarah Witman, Freelance Writer

Citation: Witman, S. (2017), Mysterious intraseasonal oscillations in monsoons, Eos, 98, https://doi.org/10.1029/2017EO071313. Published on 12 April 2017.
© 2017. The authors. CC BY-NC-ND 3.0