Buoy and Satellite Observations of Boreal Summer Intraseasonal Variability in the Tropical Northeast Pacific

Eastern north Pacific intraseasonal variability during June-September of 2000-2003 is analyzed using satellite and buoy observations. QuikSCAT ocean vector winds and TRMM precipitation indicate that periods of anomalous surface westerly flow over the east Pacific warm pool during an intraseasonal oscillation (ISO) lifecycle are generally associated with an enhancement of convection to the east of 120W. A notable exception includes a narrow east-west band of suppressed convection that is centered near 8N to the south of the strongest wind anomalies. Widespread suppressed convection occurs to the west of 120W during the westerly phase. Periods of surface easterly anomalies are generally associated with suppressed convection over the warm pool to the east of 120W.

Anomalous ISO westerly flow is generally accompanied by enhanced wind speed to the east of 120W, while anomalous easterly flow is associated with suppressed wind speed. During the westerly phase, southwesterly intraseasonal vector wind anomalies combined with climatological southwesterly flow account for the bulk of the wind speed enhancement in the warm pool, with a much weaker contribution from enhanced eddy activity on timescales of less than 20 days. Suppression of wind speed during the ISO easterly phase has strong contributions from both northeasterly intraseasonal vector wind anomalies and suppression of eddy activity.

The increase in wind speed during periods of ISO westerly anomalies implies an increase in the wind-induced component of latent heat flux. TAO buoys along the 95W line, and the buoy at 8N, 110W, are used to examine the relationship between intraseasonal precipitation and latent heat flux. Buoy flux anomalies are primarily wind-induced. A statistically significant correlation of 0.59 between intraseasonal latent heat flux and precipitation occurs at the 12N, 95W buoy, consistent with previous studies suggesting a role for wind-evaporation feedback in supporting intraseasonal convection over the east Pacific warm pool. Correlations between precipitation and latent heat flux at the 10N, 95W and 8N, 95W buoys are positive, but not statistically significant, due to the short data record at these buoys.

A negative, but not statistically significant correlation (-0.34) exists between intraseasonal precipitation and latent heat flux at the 8N, 110W buoy. Satellite observations show that the buoy is located in a region of suppressed convection during periods of enhanced wind speed, with anomalous frictional divergence and anomalously low column-integrated water vapor (SSM/I). In contrast, surface convergence and cyclonic vorticity anomalies occur over the warm pool to the north of 10N during convectively enhanced/westerly phases of the ISO.

Wind jets in the Gulf of Tehuantepec and Gulf of Papagayo appear to be active during periods of ISO easterlies and suppressed convection. The composite total wind vector in the Gulf of Papagayo and Gulf of Tehuantepec during ISO westerly phases is close to that in the June-September mean.