15 OCTOBER 2002 INOUE ET AL. 2897 Bimodal Distribution of Tropical Cyclogenesis in the Caribbean: Characteristics and Environmental Factors MASAMICHI INOUE Coastal Studies Institute, and Department of Oceanography and Coastal Sciences, Louisiana State University, Baton Rouge, Louisiana ITSUKI C. HANDOH AND GRANT R. BIGG School of Environmental Sciences, University of East Anglia, Norwich, Norfolk, United Kingdom (Manuscript received 15 August 2001, in ®nal form 22 April 2002) ABSTRACT Tropical cyclogenesis critically depends on the presence of warm water at the sea surface. For the North Atlantic basin as a whole, the tropical storm season starts in May, peaks in September, and then declines, generally following the seasonal warming and cooling of sea surface temperature. In the Caribbean, in contrast, there is a distinct bimodal distribution in the number of tropical storms formed, with peaks in June and October separated by a signi®cant minimum in July. The timing of the observed minimum in tropical cyclogenesis appears to be related to the strengthening of the easterly trade winds over the Caribbean associated with the onset of the so-called veranillo, or midsummer drought (MSD), previously recognized over south-central Mexico, Central America, and parts of the Caribbean. It appears that the observed minimum in cyclogenesis is caused by a combination of environmental factors related to the strengthening of the easterly trade winds across the Caribbean Basin. The strengthening easterly trade winds and their associated changes in wind stress curl give rise to enhanced upwelling in the southwestern Caribbean. This appears to trigger an enhanced local atmosphere±ocean coupling, giving rise to very unfavorable conditions in several environmental variables including cooler sea surface temperature (SST), higher sea level pressure (SLP), increase in outgoing longwave radiation (OLR), and decrease in precipitable water content (PRW). Moreover, strengthening trade winds result in increases in tro- pospheric vertical wind shear (VSH). Except for OLR, these environmental variables become least favorable for southwestern Caribbean cyclogenesis in July. In contrast, the transition from weak to intense convective activity in the eastern Paci®c results in weaker trade winds in the Caribbean in October. The resulting westerly wind anomalies lead to weakening upwelling, warmer SST, enhanced convection, and moist air coupled with weaker VSH in the southwestern Caribbean. All variables, except OLR, then become most favorable for cy- clogenesis. In the rest of the Caribbean, some of the conditions, primarily SST related, are not fully met. Nevertheless, the southwestern Caribbean appears to dominate the rest of the Caribbean in terms of setting the stage for either favorable or unfavorable conditions for cyclogenesis in the whole Caribbean Basin. Therefore, ocean±atmosphere interaction over the southwestern Caribbean appears to play an integral role in both suppressing and enhancing tropical cyclogenesis in the Caribbean on an annual basis. 1. Introduction referred to collectively as tropical storms). In the North Atlantic as a whole, the tropical storm season starts in Cyclogenesis of tropical storms critically depends on May, peaks in September, and then declines. Peaking of the presence of warm water at the sea surface (Malkus and Riehl 1960; Carlson 1971; Wendland 1977; Shapiro cyclogenesis in September also coincides with maxi- 1982; Gray 1984; Shapiro and Goldenberg 1998). Con- mum tropical wave activity emanating from West Africa sequently, the tropical storm season generally follows (Thorncroft and Hodges 2001). The importance of west- the annual movement of the sun. The annual cycle of ward-propagating disturbances emanating from West the number of tropical storms (NTS) covering the period Africa (often referred to as tropical, African, or easterly January 1886±December 1999 for the whole Atlantic is waves), in the tropical cyclogenesis over the North At- shown in Fig. 1. It is noted that tropical storms in this lantic and eastern Paci®c Oceans, has been recognized study include tropical storms and hurricanes (hereafter since the 1940s (Dunn 1940). Satellite images have been very useful in tracking tropical waves across the At- lantic. During their westward propagation across the Corresponding author address: Dr. Masamichi Inoue, Coastal North Atlantic Ocean, tropical waves encounter variable Studies Institute, Dept. of Oceanography and Coastal Sciences, Lou- isiana State University, Baton Rouge, LA 70803. environmental conditions. Under favorable conditions, E-mail: [email protected] those tropical waves can turn into tropical storms in- q 2002 American Meteorological Society Unauthenticated | Downloaded 09/30/21 04:00 AM UTC 2898 JOURNAL OF CLIMATE VOLUME 15 velopment regions has been attributed to the seasonal changes of position and intensity of the intertropical convergence zone (ITCZ) and vertical wind shears (Gray 1968). This annual shifting of major development regions could give rise to the bimodal distribution in cyclogenesis in the Caribbean and in the Gulf of Mex- ico. However, this does not explain the uniqueness of the distinct bimodality for Caribbean cyclogenesis in comparison to the neighboring Gulf of Mexico. Previous studies have identi®ed several environmen- tal variables in addition to warm sea surface tempera- tures that are necessary for tropical cyclogenesis (e.g., Gray 1968; Landsea et al. 1998). Those variables in- clude low sea level pressure (SLP; Gray 1968; Shapiro 1982; Gray et al. 1993; Knaff 1997); low outgoing long- wave radiation (OLR), which is an indicator of deep atmospheric convection (Zhang 1993) necessary for FIG. 1. Climatology of the number of tropical storms formed in the whole North Atlantic (solid line) and in the Caribbean Basin tropical cyclogenesis (Gray 1968); low tropospheric (dotted line). Based on the period 1886±1999. vertical wind shear (VSH; Gray 1968; Goldenberg and Shapiro 1996; Landsea et al. 1998); and high precipi- table water content (PRW; Miller 1958; Malkus and cluding the strongest Atlantic hurricanes (e.g., Landsea Riehl 1960; Emanuel 1986). It should be noted that most 1993). In fact, it appears that tropical waves initiate most of these variables are not independent but closely in- Atlantic tropical cyclones (e.g., Landsea et al. 1998). terrelated. These variables have been identi®ed as cru- In various subbasins within the North Atlantic, how- cial in providing favorable conditions for hurricane for- ever, deviations from the general pattern of cyclogen- mation in the Caribbean Basin (Landsea et al. 1998), esis for the whole North Atlantic have been noted pre- which lies within the critical 108±208N latitude belt viously (e.g., Cry and Haggard 1962). One prominent known as the main development region of tropical example is the Caribbean Basin, where the observed storms in the North Atlantic (Goldenberg and Shapiro occurrences of tropical storms show a distinct bimodal 1996). The objective of this study is to examine the distribution with peaks in June and October separated climatological annual cycle in the relevant environmen- by a distinct minimum in July (Fig. 1). The temporal tal variables that might explain the observed bimodal dip in July is statistically signi®cant (paired t test of distribution in tropical cyclogenesis in the Caribbean. June and July, p , 0.05, n 5 114 yr), while the number reaches its seasonal maximum in October. Within the 2. Data and analysis whole North Atlantic, the Caribbean Basin is the only subbasin with such a distinct bimodal distribution of We examined climatological monthly mean data of NTS (Ballenzeig 1959; Cry and Haggard 1962); NTS, SST, SLP, OLR, VSH, and PRW. Additionally, though, a similar but much less distinct bimodal dis- surface wind vectors (SWV) are examined in order to tribution of NTS is found in the Gulf of Mexico (Cry explore a possible atmosphere±ocean link through wind and Haggard 1962; Fig. 2a). Curiously, its uniqueness stress acting on the sea surface. NTS are derived from has not attracted much attention in the published lit- all the tropical storms formed in the North Atlantic for erature. the period 1886±1999, reported by the National Hur- It is well known that there is a seasonal shift of de- ricane Center in Miami, Florida. Both tropical storms velopment regions for tropical storms in the North At- and hurricanes are included primarily to increase the lantic (Cry and Haggard 1962; Gray 1968). During the number of samples; however, the general outcome of early part of the hurricane season (June), major devel- this study is still applicable even if only hurricanes were opment regions are located in the western Caribbean considered (not shown here). The climatological month- and in the Gulf of Mexico (Cry and Haggard 1962; Cry ly mean data used here include the 1982±99 SST from 1965). In July the favored genesis region shifts to the the National Centers for Environmental Prediction east, namely the Lesser Antilles and the southwestern (NCEP) optimal interpolation (Reynolds and Smith North Atlantic (Cry and Haggard 1962). In August and 1994); the long-term SLP, VSH, and PRW taken from the ®rst half of September, activity is noted in all regions the NCEP±NCAR (National Center for Atmospheric with principal contributions coming from the Lesser An- Research) reanalysis of 1958±97 (Kalnay et al. 1996); tilles and the southeastern North Atlantic (Cry and Hag- and the 1978±95 series of the National Oceanic