A Climatology of Central American Gyres

Monsoon gyres (MGs) and monsoon depressions (MDs), 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, but have rarely been studied, over Central America (CA) during the tropical cyclone (TC) season. A noteworthy gyre observed during the 2010 PREDICT field project served as a “sink” 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 this gyre interacted with a mid-latitude trough. The lack of prior research on 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 often develop from the combination of zonal low-level westerly flow in the east Pacific (EPAC), and zonal easterly flow across the Caribbean and northern Gulf of Mexico (GoM). CA gyres also tend to be co-located with reservoirs of deep moisture that are 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, which includes frequency over the TC season, will be presented. This climatology is used to craft a series of composites using objectively identified gyre cases occurring from May-November between 1980-2010. In this period, 42 gyre cases were identified with a bimodal peak in gyre occurrence: an early season peak (May-June) with a more pronounced late season peak in the latter half of the TC season (Sep.-Oct.). Earth-relative composites are used in diagnosing the role of anomalous low-level zonal flow aiding in gyre genesis. Supplementary work involved using the real-time multivariate Madden-Julian Oscillation (RMM) index to assess the role that tropical intraseasonal oscillations have on gyre frequency. Results show that over 75% of gyre genesis events occur in RMM phases 8,1,2, which represent the convectively active MJO signal over the western hemisphere in association with anomalous westerly low-level zonal winds in the EPAC. TC genesis and tracks within gyre circulations will also be highlighted from an analysis of TCs in a gyre-relative coordinate system. Much of this research represents the first known objective climatology of these types of disturbances in the Atlantic and EPAC basins.