Remote influences on the Indian monsoon low-level jet intraseasonal variations
Résumé
A strong low-level jet (LLJ), also known as the Findlater jet, develops over the Arabian Sea during the Indian summer monsoon. This jet is an essential source of moisture for monsoonal rainfall over the densely-populated Indian subcontinent and is a key contributor to the Indian Ocean oceanic productivity by sustaining the western Arabian Sea upwelling systems. The LLJ intensity fluctuates intraseasonally within the 20- to 90-day band, in relation with the northward-propagating active and break phases of the Indian summer monsoon. Our observational analyses reveal that these large-scale regional convective perturbations only explain about half of the intraseasonal LLJ variance, the other half being unrelated to large-scale convective perturbations over the Indian Ocean. We show that convective fluctuations in two regions outside the Indian Ocean can remotely force a LLJ intensification, four days later. Enhanced atmospheric deep convection over the northwestern tropical Pacific yields westerly wind anomalies that propagate westward to the Arabian Sea as baroclinic atmospheric Rossby Waves. Suppressed convection over the eastern Pacific/North American monsoon region yields westerly wind anomalies that propagate eastward to the Indian Ocean as dry baroclinic equatorial Kelvin waves. Those largely independent remote influences jointly explain 40% of the intraseasonal LLJ variance that is not related to convective perturbations over the Indian Ocean (i.e. 20% of the total), with the northwestern Pacific contributing twice as much as the eastern Pacific. Taking into account these two remote influences should thus enhance the ability to predict the LLJ.