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Fine et al. (2016, hereafter F16) investigated the potential role of SumatraIsland, as well as the Malay Peninsula and Java, in creating terrain-inducedcirculations over the Indian Ocean (IO) that subsequently develop into tropicalcyclones (TCs). Applying sophisticated vortex tracking software to 2.5 yrs ofmodel analyses, F16 found four regions downstream of topographic features inthe Maritime Continent to be prolific generators of low-level cyclonic vortices(123/yr). This present study extends the limited analyses of F16 by applying asimilar approach to 10 yrs (2008-2017) of ERA5 analyses. While the 2.5-yrperiod which F16 studied was slightly (8%) more active in terms of vortexproduction than the 10-yr period, in general the findings of F16 arerepresentative of the longer-term record with 80% of all shed vorticesoccurring with easterly low-level (925 hPa) flow over the hotspot regions.Additional analysis of the 10-yr record found that vortex counts are highestnear MJO phase 1 when low-level easterlies are strongest over the maritimecontinent region. A secondary peak in vortex counts occurs during MJO phase 4when low-level westerlies were present near the equator west of Sumatra. Thissuggests that low-level westerly surges on the equator impinging on Sumatraassociated with the MJO contribute to an increase in wake vortex development.While the frequency of vortex genesis over the four hotspot regions is stronglytied to the annual cycle of winds, periods with anomalous zonal flow are shownto impact vortex counts. This is most apparent in the region off the southerntip of Sumatra where vortex counts were 4 times higher during periods ofanomalous easterlies compared to periods with anomalous westerlies. In thisregion positive (negative) ENSO and Indian-Ocean Dipole conditions driveanomalous easterlies (westerlies) which impact vortex formation rates.

Extended analysis of the effects of the Sumatra topography on downstream low-level vortex development over the Indian Ocean