Remotely and regionally forced pre-monsoon signals over northern India and neighbourhood

A detailed diagnostic examination of the National Centre for Environmental Prediction/National Centre for Atmospheric Research Reanalysis data supplemented by a theoretical study is undertaken with the goal of investigating the dynamics of pre-monsoon stationary wave anomalies occurring over northern India and adjoining region prior to weak Indian summer monsoons. It is seen from the analysis that the rotational wind anomalies preceding weak monsoon years are primarily forced by tropical divergent wind anomalies located over the tropical central-eastern Pacific ocean, south Indian ocean and locally over north-west India. The divergent wind anomalies over the central-eastern Pacific ocean can be linked to anomalous changes in the tropical Pacific sea surface temperature arising due to El Nino/Southern Oscillation (ENSO). The upper level divergent wind forcing overlying the south Indian ocean seems to be locally connected to the warm sea surface temperature anomalies over that region. Results of simple experiments using a forced divergent barotropic vorticity model establish that the maintenance of rotational wind anomalies over north India prior to weak monsoons is attributable to the existence of anomalous divergent wind forcing over the aforementioned regions in the tropics. A normal mode experiment shows that the climatological basic flow alone is not sufficient to support these pre-monsoon vorticity perturbations. Furthermore, calculation of vorticity generation terms suggests that the divergent wind advection and Rossby source terms make significant contributions to the production of rotational anomalies over north India during the pre-monsoon months. Based on the above results, it is conjectured that energy injection into the subtropics via Rossby wave dispersion could be a foremost dynamical mechanism which efficiently sustains rotational wind perturbations over north India and neighbourhood. An assessment of the relative importance of the ENSO and non-ENSO divergent forcings, shows that the latter tend to exert a larger influence on the pre-monsoon signals over north-west India. The implications of using the pre-monsoon signals for long range forecasting of Indian summer monsoon activity are determined by two factors viz., (i) the robustness and intensity of the pre-monsoon signals and (ii) the persistence of the signals during the following months. The first factor in turn depends on the manner in which the different forcing anomalies co-ordinate. The amplitude of the pre-monsoon signals can be large (small) depending on whether the ENSO and non-ENSO boundary forcings act in a mutually co-operative (competitive) manner. The second factor, which is related to the maintenance of anomalous quasi-stationary patterns during the following months, depends crucially on how the different boundary forcings evolve during the summer monsoon season.