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John Molinari and David Vollaro

Abstract

An objective definition of monsoon gyres in the northwest Pacific was developed in order to construct a gyre climatology. Over a 31-yr period, 53 gyres were identified with a median formation location at 16.5°N, 135°E. More than 80% formed during July–September. More than half of gyres developed during El Niño periods at a median location 1200 km farther to the east-southeast than during La Niña. Cyclonic winds at 850 hPa extended across a diameter of more than 4000 km, with maximum tangential wind near the 1000-km radius. A precipitation maximum extended westward for several thousand kilometers south of the gyre. Typhoons were most common north and east of the gyre centers. More than 70% of gyres developed during large-amplitude MJO events, with a strong preference for Real-time Multivariate MJO (RMM) phases 5–7. In boreal summer these phases contain circulation and convective anomalies that coincide most closely with those of the climatological monsoon trough. Gyres are most likely to form when an active, large-amplitude MJO event superposes with the monsoon trough in the presence of high sea surface temperature. Gyres exhibited 850-hPa wind, height, and vorticity anomalies and surface latent heat flux anomalies that closely resembled the active Pacific–Japan pattern (PJP). This was especially true during La Niña, even though no attempt was made to isolate the PJP. It is hypothesized that an active MJO modulates gyre formation, and the gyres project onto the active phase of the PJP as they move westward.

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Karen I. Mohr, John Molinari, and Chris D. Thorncroft

Abstract

The characteristics of convective system populations in West Africa and the western Pacific tropical cyclone basin were analyzed to investigate whether interannual variability in convective activity in tropical continental and oceanic environments is driven by variations in the number of events during the wet season or by favoring large and/or intense convective systems. Convective systems were defined from Tropical Rainfall Measuring Mission (TRMM) data as a cluster of pixels with an 85-GHz polarization-corrected brightness temperature below 255 K and with an area of at least 64 km2. The study database consisted of convective systems in West Africa from May to September 1998–2007, and in the western Pacific from May to November 1998–2007. Annual cumulative frequency distributions for system minimum brightness temperature and system area were constructed for both regions. For both regions, there were no statistically significant differences between the annual curves for system minimum brightness temperature. There were two groups of system area curves, split by the TRMM altitude boost in 2001. Within each set, there was no statistically significant interannual variability. Subsetting the database revealed some sensitivity in distribution shape to the size of the sampling area, the length of the sample period, and the climate zone. From a regional perspective, the stability of the cumulative frequency distributions implied that the probability that a convective system would attain a particular size or intensity does not change interannually. Variability in the number of convective events appeared to be more important in determining whether a year is either wetter or drier than normal.

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