Investigating the Causes of Poleward Shift in Monsoon Low-level Jet

Recent studies using coupled model simulations and observation datasets suggest a poleward shift and overall weakening of Indian Summer Monsoon (ISM) circulation (Sandeep and Ajayamohan, 2015). Their investigation using experiments from the fifth phase of coupled model Inter-comparison project (CMIP5) indicate a poleward migration of the monsoon low-level jet (LLJ), with the magnitude of shift  linked to the degree of warming. 

Here we investigate the changes in monsoon LLJ in multiple reanalysis datasets as well as historical and future scenario simulations of the sixth phase of coupled model Inter-comparison project (CMIP6). The latitudinal location of LLJ is defined as the latitude of zero absolute vorticity over the Arabian Sea, following Tomas and Webster (1997). Although all reanalysis datasets show a poleward shift in LLJ since late 1970s, the magnitude of shift varies among them. The multi model ensemble of CMIP6 historical simulations show a northward shift of 0.4 degrees in LLJ. The ensemble mean of SSP585 simulations show a northward shift in LLJ by 0.8 degrees in the 2081 - 2100 period. The changes in the latitudinal position of LLJ and the land-sea temperature difference are significantly correlated, with a Pearson correlation coefficient of 0.81 and 0.67 for the ensemble means of historical and SSP585 runs, respectively. This suggests that the underlying dynamics of the monsoon circulation is changing in a warming climate.

References

Sandeep, S., & Ajayamohan, R. S. (2015). Poleward shift in Indian summer monsoon low level jetstream under global warming. Climate Dynamics, 45(1), 337-351.

Tomas, R. A., & Webster, P. J. (1997). The role of inertial instability in determining the location and strength of near‐equatorial convection. Quarterly Journal of the Royal Meteorological Society, 123(542), 1445-1482.