Moisture transport associated with southwest monsoon rainfall over Sri Lanka in relatively wet and dry rainfall years

Abstract

Atmospheric moisture transportation associated with the occurrence of relatively wet and dry southwest monsoon (SWM) years over Sri Lanka is still not fully understood.This study focused on investigating the role of moisture transport in contrast SWM years. We selected seven wet (SWM Wet ) and nine dry (SWM Dry ) years for 1985-2015 and found that the whole country experiences above-average (below average) rainfall in SWM Wet (SWM Dry ) years. In SWM Wet years, strengthening moisture-laden low-level jets (LLJ) from the Arabian Sea bring a large amount of moisture towards Sri Lanka. In contrast, the weakening of the LLJ from the Arabian Sea direction is observed in SWM Dry years. As a consequence, the climatological mean of net moisture flux (4.35 ×10 5 kg s -1 ) over the study domain is increased (5.33×10 5 kg s -1 ) and decreased (3.98 ×10 5 kg s -1 ) in SWM Wet and SWM Dry years, respectively. With respect to long-term Vertically Integrate Moisture Flux Divergence (VIMFD, -3.28×10 -5 kg m -2 s -1 ), negative anomalous VIMFD (-1.78×10 -5 kg m -2 s -1 ) in SWM Wet years and positive anomalous VIMFD (1.44×10 -5 kg m -2 s -1 ) in SWM Dry years are recorded, which ascribed above-average and below-average rainfall over the country. Furthermore, strong moisture convergence (divergence) center in the western/ southwestern part of Sri Lanka during the SWM Wet (SWM Dry ) years explain why strong positive and negative SWM rainfall anomalies are concentrated in these two regions. Furthermore, results highlighted a strong relationship between net moisture flux availability and SWM rainfall (r= 0.63) that may explain the observed SWM rainfall variability over the country.