Research

Seasonal Balances of Kinetic Energy in the Somali Jet

Somali jet, a cross-equatorial flow in the western Indian ocean that turns eastward off the coast of Somalia, is the harbinger of Indian summer monsoon rainfall. Different balances persist at various sections of the jet. These balances can be quantified by a non-dimensional parameter, the local Rossby number (Ro).

A peculiar advective balance and subsequent acceleration of the flow occurs where the magnitude of Ro is greater than 1. The flow exhibits a rapid intensification in this region

Details of this can be found here.

Rapid Intensification of Somali Jet

The kinetic energy (KE) of the Somali jet intensifies rapidly at the start of the monsoon. It increases from its minimum to its close to its maximum value in the span of two weeks. This nonlinear intensification of the jet can not be attributed to the seasonal march of the sun, which has a near linear evolution.

This abrupt increase in KE can be explained by the nonlinear (quadratic) dependence of the KE generation term on the meridional geopotential gradient. This quadratic dependence is a consequence of the fact that in the advective boundary layer, the cross-isobaric meridional winds are strongly associated with meridional geopotential gradients.

For further details visit our Journal of the Atmospheric Sciences article [Link].

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LF-ISO of rainfall anomalies filtered using MSSA along with 850hPa wind anomalies for 1998-1999 monsoon seasons. The northward march of rainfall signal is evident in the animation.

Monsoon Intraseasonal Oscillations:

The Indian monsoon has variability in a lot of different scales. The subseasonal variability is dominated by low-frequency intraseasonal oscillations (LF-ISOs). These LF-ISOs of rainfall propagate northward from the equator to central India rainbands on 30-60 days timescales and modulate the active-break cycle of a monsoon season. The vorticity and divergence are shown to play a crucial role in the initiation and propagation of these LF-ISOs. In our work, we study the spatiotemporal structure of low-frequency vorticity and divergence with the help of data-adaptive filtering technique namely multichannel singular spectrum analysis (MSSA).

Zonal (70E-100E) and pentad mean TRMM rainfall climatology (1998-2018). There is an abrupt increase in the rainfall and rapid shift in the location of rainfall aggregation (Intratropical convergence zone (ITCZ)) at the start of June, corresponding to the onset of Indian summer monsoon.

Abrupt Monsoon Onset:

The Indian summer monsoon onset is characterized by the rapid shift of the intratropical convergence zone (ITCZ) along with an abrupt increase in the low-level zonal winds (westerlies) and kinetic energy. These changes occur within a few days whereas the march of the sun (solar insolation) to the summer hemisphere is gradual of the order of a month. Thus the sudden commencement of the Indian monsoon can not be completely attributed to the solar cycle.

In our work, we try to explore the possible reasons for this abrupt onset of monsoon with a specific focus on the rapid increase of kinetic energy of the Somali jet at the outset of monsoon.

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