Enhanced rainfall over warm ocean eddies in the eastern tropical Pacific

In situ, radar, and satellite observations of upper ocean structure and precipitation showed anomalously high rainfall over a warm-core ocean eddy. A warm eddy was identified from both sea surface height (SSH) anomalies in Pathfinder satellite data, sea surface temperature (SST) anomalies in TMI satellite data, and in-situ temperature, salinity and acoustic Doppler current profiler measurements made during EPIC2001. Satellite data showed a ~10 cm positive SSH anomaly that moved westward from the Central American coast to the EPIC observational site. The diameter of the eddy was about 200 km, and it moved at a speed of about 13 cm/s prior to the EPIC survey period. The relative vorticity, estimated from ADCP velocity, was –0.27f, which is comparable to vorticity estimates based on SSH anomalies. The eddy had a warm SST anomaly in excess of 0.5C for August and 1.5C for September. SST anomalies were computed by subtracting 6-year monthly-averaged means from monthly-averaged SST for 2001. Relatively high rainfall along the path of the eddy was observed. The maximum in rainfall associated with the eddy exceeded 30 mm/day during August, and 40 mm/day during September. A two-day deep-convective event in September produced 0.2 to 0.3 m of rainfall, which accounted for most of the rainfall observed during the 20-day EPIC ship observations. There was good agreement between the locations of high rainfall and the warmest SST prior to deep convection. In addition to the heavy rainfall over the warm eddy observed during the EPIC experiment, an analysis was carried out to determine the relationship between anomalies of SSH, SST, and TMI rainfall in the region for six years of August and September satellite observations. Six-year monthly averaged rainfall anomalies for August and September range from -5 mm/da for highly cyclonic eddies (SSH ~ -15 cm) to +5 mm/da for highly cyclonic eddies (SSH ~ +15 cm). The anomalies may be compared to a mean rainfall rate of about 20 mm/day over the region for the two-month period. Mean monthly SST anomalies range from -0.4C for SSH = -15 cm to +0.4C for SSH = + 15 cm. The analysis supports the conclusion that warm anticyclonic eddies, which form near the Central American coast during the summer and move westward, are preferential sites for heavy rainfall.