Authors:

Abstract

This study revealed the influence of Madden Julian oscillation (MJO) and cold surge (CS) on the extreme rainfall events that occurred in Sri Lanka from 19th to 28th December 2014. During this period, the Northeast Monsoon winds over the Bay of Bengal were strengthened by a CS, which intensified convective activity over the Bay of Bengal. Simultaneously, convective phase 3 of the MJO, which was located over the eastern Indian Ocean is associated with a positive rainfall anomaly over Sri Lanka. Westerly wind anomalies associated with MJO phase 3 shifted southward due to the strengthening of northeasterly winds by the CS, forming an incipient vortex to the southeast of Sri Lanka. The low level moisture transport was enhanced by this vortex circulation together with strong cross-equatorial moisture flux transportation. The convectively active phase of MJO provided favourable conditions such as low level convergence, cyclonic relative vorticity, and deep convection for the intensification of vortex into a tropical depression over the southwest Bay of Bengal. Strong upward motion and combination of strong low level convergence with upper-level divergence allowed evacuation of the ascending air mass, aiding the intensification of deep convection. This persistent vertical coupling structure allowed deep convection of moist warm air, which led to heavy rainfall during the period. Combination of a number of factors, such as local terrain effects, CS at low levels, MJO wet phase and a vortex to southeast of Sri Lanka led to an intensification of the tropical depression that caused an extreme rainfall event over the Eastern and Northeastern parts of Sri Lanka.

Keywords: Cold surge,  convection,  convergence,  divergence,  MJO