The Indian Easterly Jet During the Pre‐Monsoon Season in India

• Published in: Geophysical research letters 2023-12, Vol.50 (23), p.n/a
• Main Authors: Croad, Hannah L., Shonk, Jonathan K. P., Chevuturi, Amulya, Turner, Andrew G., Hodges, Kevin I.
• Format: Article
• Language: English
• Subjects: Atmosphere; Baroclinic instability; Barotropic mode; Convection; Earth rotation; Heat waves; IEJ; India; Indian Easterly Jet; jet; Lower atmosphere; Moist convection; Monsoon onset; Monsoons; Potential vorticity; pre‐monsoon; Severe storms; Spring; Spring (season); Storms; Summer monsoon; Surface temperature; Temperature; Temperature gradients; thermal wind; Thermal winds; Troposphere; Vorticity; Wind; Winds
• ISSN: 0094-8276; 1944-8007
• DOI: 10.1029/2023GL105400

We identify for the first time an Indian Easterly Jet (IEJ) in the mid‐troposphere during the pre‐monsoon using reanalysis data. The IEJ is weaker and smaller than the African Easterly Jet over West Africa, with a climatological location of 10°N, 60–90°E, 700 hPa, and strength 6–7 m s−1 during March–May. The IEJ is a thermal wind associated with low‐level meridional gradients in temperature (positive) and moisture (negative), resulting from equatorward moist convection and poleward dry convection. The IEJ is associated with a negative meridional potential vorticity gradient, therefore satisfying the Charney‐Stern necessary condition for instability. However, no wave activity is detected, suggesting that the potential for combined barotropic‐baroclinic instability is not often realized. IEJ strong (weak) years feature increased (reduced) near‐surface temperatures and drier (wetter) conditions over India. This study provides an introduction to the IEJ's role in pre‐monsoon dynamics, and a platform for further research. Plain Language Summary Jets, or concentrated regions of fast‐moving winds, can form due to temperature gradients working together with the Earth's rotation. Typically these form in the east/west direction, such as the African Easterly Jet, which is well known over tropical West Africa. Using observation‐based data of the atmosphere, we set out to explore whether such a jet exists near India during spring‐time (the “pre‐monsoon”). The Indian Easterly Jet (IEJ), like its counterpart in West Africa, forms in the lower atmosphere between a south‐to‐north increasing low‐level temperature gradient and a reversed temperature gradient at higher altitudes. But unlike in West Africa, the IEJ breaks down once the summer monsoon begins. We use established methods to show that the IEJ winds have the potential to amplify small cyclonic disturbances, which can develop into larger storms. However, we find that this does not occur commonly. Meanwhile, years in which the IEJ is unusually strong tend to be associated with spring‐time conditions in India that are warmer and drier than normal. This first research into the IEJ paves the way for further analysis on its role in the development of heatwaves, severe storms and the monsoon onset. Key Points The Indian Easterly Jet is a thermal wind in the mid‐troposphere over India during the pre‐monsoon, identified in reanalysis data The jet resembles the African Easterly Jet, but is smaller and weaker, and is not asso