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A little moisture in atmosphere can change weather

Variations in the moisture content of the atmosphere can influence large-scale winds blowing in the higher reaches of the atmosphere, finds a study by the Indian Institute of Science (IISc) researchers. Called “Rossby waves”, such winds circulating in the upper echelons of the Earth's atmosphere play a major role in the development of weather.

Rossby waves are known to scientists since 1939, and a lot of research has gone into understand how they behave, and affect weather. But, the influence of a commonplace phenomenon like moisture on the Rossby waves was not studied before. The IISc team was convinced that Rossby waves were not so 'dry', and it developed equations that included the effect of moisture.

“Until recently, all existing theories of Rossby wave dynamics assumed that the atmosphere is "dry", i.e. water vapor does not play an important role in the propagation and stability of these waves. Therefore, our work tried to look at under what conditions Rossby waves can interact with water vapor and the associated latent heat release”, said Joy Monteiro, a PhD student who was part of the study led by Prof Jai Sukhatme. Both of them belong to the Center for Atmospheric and Oceanic Sciences, IISc.

Using a theoretical approach, Jai Sukhatme and Joy Monteiro studied the dynamics of “Rossby waves” in the presence of moisture gradients, which are regions of increasing or decreasing moisture content in the North/South or East/West directions. These findings have far-reaching consequence of enabling more accurate weather predictions in the future (weather prediction models incorporate these effects, since they are brute force integrations of the model equations, which include moisture. Our contribution is towards the basic physical understanding of a phenomenon, which these models probably already simulate).

Atmosphere is a complicated chaotic system influenced by various factors including the position of the sun relative to earth, and variations in temperature, pressure and humidity. Small changes to one part can manifest themselves as bigger changes affecting the whole system: a phenomenon popularly called the Butterfly effect. Such delicate processes pose a great challenge while modeling the atmospheric phenomena, and influence our ability to predict weather accurately. Rotation of the earth also affects the weather phenomenon, and one such effect is the presence of “Rossby waves”. (Indeed, one of the reasons why weather is predictable for 5-7 days is that regularities like waves are present)

“We showed that, for conditions that are representative of the extratropical atmosphere (atmosphere outside the tropics), if there is an increase or decrease of mean water vapor in the North/South or East/West directions, they can significantly change the character of the Rossby waves and in some cases even make them unstable”, explains Joy Monteiro. The IISc study specifically found that the presence of moisture gradients from North to South slowed down Rossby waves.

In addition to this, the study also found something else: under certain conditions (in the presence of east-west gradients), these waves behaved like another well-known phenomenon - the Madden Julian Oscillation (MJO) - a traveling pattern found in tropical atmospheres.

Accurate and timely weather forecasts can indeed save lives, property and crops, and avert disasters. Understanding Rossby waves, though a small element in a complex system, is definitely a step towards achieving the larger goal of understanding the atmosphere.

Contact Information:

Prof. Jai Sukhatme

Assistant Professor

Centre for Atmosphere & Ocean Sciences, IISc, Bangalore.

Email: jai at caos.iisc.ernet.in

About the authors:

Joy M Monteiro is a PhD. Scholar at the Center for Atmospheric and Oceanic Sciences, IISc, Bangalore and Prof. Jai Sukhatme is an Assistant Professor at the Center for Atmospheric and Oceanic Sciences, IISc, Bangalore.

About the Paper:

More information on the paper can be found here.