Hurricane Formation and Modification in the south-eastern Caribbean

• Main Author: Georgiadis, Alexandros
• Published: University of Sheffield 2007
• Subjects: 551.5
• Online Access: http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.486734

The south-east sector of the Caribbean (core area) exhibits a distinct suppression in cyclogenesis that contrasts sharply with the hurricane climatology of the neighbouring subbasins. Multivariate statistical analysis (Principal Component Analysis) of the basic environmental variables extracted from the ERA-40 reanalysis dataset near the surface and the tropopause reveals three climatologically different regions within the North Atlantic: (i) one located in the south-west Caribbean, (ii) another characteristic for the rest of the Caribbean, and the last (iii) dominating the Atlantic eastwards of the Less<;;r Antilles. Although the thermodynamics are typically conducive for cyclogenesis throughout the analysis area, the major difference between the climatologies of the Caribbean and the Atlantic is due to the effective thermodynamic background, which appears to be less favourable over the Atlantic. The south-west Caribbean, corresponding to the north-most position of the rising limb ofthe Walker circulation over the Americas, is the most favourable of the three climatic 'regimes', characterised by cyclonic relative vorticity and exceptionally low vertical wind shear, while within the eastern Caribbean, (and the Atlantic) these features are reversed. Furthermore, the spatial variation of the mean low-level circulation over the Caribbean leads to a quadrapole pattern across the region where the climatological background of the northern part of the core area is the least favourable for cyclone formation, combining negative relative vorticity with background divergence. Comparative analysis of composites extracted during cyclogenesis/cyclolisis events revealed that the combination of the environmental conditions in the two sub-sectors (south and north) of the western Caribbean renders the whole region conducive to enhanced hurricane formation. On average, the easterlies reach their maximum intensity approximately around IS<W (centre of the Basin), resulting in opposing gradients polewards and equatorwards of the wind maximum, that contribute towards cyclonic (positive) local vorticity in the south and anticyclonic (negative) in the north. The above results in a negative potential vorticity meridional gradient dominating over the central and western Caribbean that satisfies the Charney-Stem condition of instability. This region also satisfies an additional necessary condition for instability: the Fjortoff condition. This combination renders the environmental background conducive for amplification within the west Caribbean of an incoming easterly wave downstream. Other proposed surface convective PV sources in the south Caribbean (consistent with the local PV gradient) are: the convective activity of Lake Maracaibo and the differential friction in the mean easterly wind due to the Venezuelan and Colombian high terrain. An alternative mechanism of cyclone formation is also identified, when the axis of an incoming easterly wave is positioned close to the Lesser Antilles, upstream of a marked ridge in the geopotential heights around 700 hPa located over the south east Caribbean. Near the surface, the height contours ahead of the axis of the atmospheric wave spread apart, entailing a decrease in the speed of the geostrophic wind that triggers an eastwards geostrophic acceleration generating a northward departure from geostrophy. The situation is reversed westwards in the central South Caribbean where the confluent geopotential height contours force an increase in the geostrophic wind (westward geostrophic acceleration) that triggers a southward ageostrophic departure. The two ageostrophic flows connect into an ageostrophic cyclonic circulation over the south east Caribbean, which is associated with evident adiabatic processes, and a weak anticyclonic flow aloft. Since the hurricane formation in the eastern sector is evidently suppressed compared to the western, it emerges implicitly that the first mechanism should be the dominant one, ?robably because it allows weakened easterly waves to reinvigorate. In contrast, cyclogenesis III the south-eastern sector may require stronger (and less common) waves, which are able to maintain their Structure crossing the Atlantic.