A Climatology of Central American Gyres

Monsoon gyres, commonly found over the western Pacific Ocean, are characterized by broad low-level cyclonic circulations that occur at a variety of spatial scales ranging from 1500-3000 km. Low-level cyclonic gyre circulations, while less frequent and occupying a smaller scale, are also observed over Central America (CA) during the tropical cyclone (TC) season. A noteworthy gyre observed during the 2010 PREDICT field project served as a “collector” of TC Matthew and a source for TC Nicole. From 25-30 September, heavy rainfall produced devastating flooding across CA, Jamaica, and a large portion of the eastern United States as the gyre interacted with a mid-latitude trough. These gyre occurrences, their apparent links to TC activity, and their association with high-impact weather motivates this presentation.

An analysis of CA gyres suggests that their spatial scales vary between 1000-2000 km in diameter. These large-scale circulations develop from the combination of enhanced low-level westerly flow in the EPAC, northerly flow in the western/southern Gulf of Mexico (GoM), and easterly flow across the Caribbean and northern GoM. CA gyres also tend to be co-located with reservoirs of deep moisture characterized by high precipitable water values (>50 mm) and embedded deep convection on their southern and eastern sides. Catastrophic flooding can occur when gyre cyclonic circulations interact with the topography of CA.

A CA gyre climatology including gyre frequency over the TC season will be presented. This climatology is used to craft a series of gyre composites using objectively identified gyre cases from 1980-2010. Particular attention will be given to the synoptic flow that precedes and take place during gyre formation. Gyre events will be organized into different categories based upon their genesis type and characteristics during gyre evolution. This study will also investigate the Madden-Julian Oscillation (MJO) and its ability to modulate the low-level flow over the EPAC, resulting in a more favorable synoptic pattern for gyre formation. TC genesis and tracks within gyre circulations will also be highlighted by analyzing TCs on a gyre-relative coordinate system.