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The late spring Caribbean rain-belt: climatology and dynamics
* 1 , 2
1  The International Research Institute for Climate and Society
2  The University of Miami


This study examines the dynamics of late spring rainfall (the Early Rainy Season, ERS) in the Caribbean region, in hopes of identifying mechanistic-based predictors for low-frequency climate modulations of the system. The subtropical Caribbean rain-belt develops in May as seasonal warming proceeds. By July, the rain-belt retreats north apparently following the westerlies and their vigorous synoptic disturbances. Daily climatology data suggest a physical definition of the Caribbean ERS as mid-May to mid-late June. Based on an examination of daily loops for several seasons, we hypothesize that rainfall occurs quasi-randomly throughout tongues of air with sufficiently high (above 45–50 mm) precipitable water (PW). These moist airmasses are brought north from the deep tropics by low-level southerlies, and typically bent over into SW-NE bands by latitudinal shear of the westerlies. The low-level flow that transports PW tongues is partly induced by upper-level synoptic disturbances in the westerlies, but also involves the gentle persistent flow around a geographically anchored Panama Low. While forced ascent is sometimes active ahead of these upper-level troughs, convective and mesoscale processes can produce rain wherever PW is sufficient. In summary, we hypothesize that rainfall hinges largely on the Lagrangian statistics of moist air tongues. Comparison is drawn between the Caribbean rain-belt and its East Asian counterpart (Meiyu-Baiu), and other mechanisms and diagnostics from that literature are discussed. Statistical prediction exercises, based on mechanistically chosen predictors, could both test hypotheses and aid local agricultural interests in the region.

Keywords: Caribbean precipitation; Caribbean rain-belt; mid-summer drought; Caribbean early rainfall season; Caribbean; precipitation
Comments on this paper
Victor .Magaña
the role of transients
What is the role of the mid latitude cyclones in the formation of the Caribbean rain belt, since the role of the mean flow, atmospheric stability and baroclinicity should be appropriate for these systems to penetrate deep into the tropics and amplify as to produce precipitation?

Teddy Allen
We simplify precipitation ingredients into two parts: uplift dynamics and moisture source. The mid-latitude cyclones provide the uplift with their associated jet stream dynamics (about 30 m/s jet streak during the late spring in the Caribbean sub-tropics) and positive vorticity advection. We observe heavy rainfall in the region of the Caribbean rain-belt when these forces are met with adequate moisture levels (about +50mm TPW). It is the accumulation of these heavy rain events (about 3-5 times during the early rain season) that provides the characteristic rain-belt pattern and thus, the first peak of the annual bimodal rainfall pattern.
The jet streaks associated with winter time plunging mid-latitude cyclones are faster than the winter (approx 60m/s), but the moisture levels tend to be much lower then as well. During the mid summer, the moisture levels are higher, but the upper tropospheric mean flow shifts from westerly to easterly and thus limit the influence of any upper tropospheric transient waves.
We continue to study the variability of the Caribbean rain-belt pattern in order to further assess the impact of the late spring mid-latitude cyclones. Thank you for taking the time to read our paper and for posting your question. I am always available to converse more. I am very familiar with your work and I look forward to meeting you one day. (Please tell David Zermano I say hello!)
Teddy Allen
Please accept a correction in the above comment:

The jet streaks associated with winter time plunging mid-latitude cyclones are faster COMPARED TO the SUMMER (approx 60m/s), but the moisture levels tend to be much lower then as well (about 30mm TPA).

Ana Durán-Quesada
precipitable water vapor
Have you consider to directly use a forecasting scheme based on Suominet integrated precipitable water vapor? So far the network for the Caribbean and Mesoamerica captures very well the entrance of the CLLJ-linked moisture transport, it can be a good predictor for the Caribbean considering the moistening of the atmosphere.

Tom Rickenbach
mean and transients
Hi Teddy,

Very nice study! Does your study imply that the transients related to baroclinic systems are equally (or maybe more) important in accounting for the northward seasonal progression of the time-mean ITCZ compared to the seasonal migration of solar forcing? Certainly in monsoon regions with active baroclinic features (like Mei-yu, or SACZ) the summer ITCZ is 'wider' in the north-south direction, likely a time-mean expression of the baroclinic forcing. Is the Caribbean region like that, given the stationary fronts that set up into the tropics even into June? Is there a way for you to separate the precipitation events into those associated with baroclinic systems and those forced by local thermodynamics, to augment your dynamical analysis?

Thanks again for sharing your work here, hope to see you soon, maybe AMS!
- Tom