Large-scale features associated with Arabian Sea cyclonic storms

Relatively speaking few cyclonic storms form over the Arabian Sea. On average only 1.5 storms form each year during either the pre- or post-monsoon season, and very few of them are classified as severe cyclonic storms of stronger (sustained wind speeds of 48 knots or greater). Consequentially, few studies have explicitly examined the conditions associated with Arabian cyclone activity on seasonal to interannual time scales. At the same time, nearly 60% of all Arabian cyclones make landfall as tropical depressions or stronger, affecting Arabian Sea coastal populations that are particularly sensitive to these extreme events. Furthermore, over the last 30 years there has been an increase in annual Arabian Accumulated Cyclone Energy, largely reflecting three Very Severe Cyclonic Storms (sustained wind speeds of 64-120 knots) and one Super Cyclonic Storm (sustained wind speeds of 120 knots or greater) that formed in this basin over the last 10 years, with Super Cyclonic Storm Gonu (2007) making landfall in both Oman and Iran, and causing an estimated four billion in damages and the loss of nearly 100 lives.

Here we examine the large-scale features associated with tropical cyclogenesis and storm intensification in the Arabian Sea, with particular focus on conditions that control the intra-annual and inter-annual characteristics of storm activity. We discuss how changes in onset and withdraw of the southeast monsoon largely dictate the timing and intensity of Arabian Sea storms by changing the regional patterns of vertical wind shear, sea-surface temperature, low-level vorticity, and convergence. We use the so-called genesis potential index to understand how features consistent with storm genesis and intensification coherently change over the course of the cyclone season, and from one year to the next. We also use this genesis index to identify how environmental conditions have changed over the last 30 years in such a way that makes this basin more favorable for cyclonic storm activity.