During June 2009, conditions across the equatorial Pacific Ocean transitioned from ENSO-neutral to El Niño conditions. Sea surface temperature (SST) anomalies continued to increase, with the latest weekly departures exceeding +1.0°C along a narrow band in the eastern equatorial Pacific.Sea surface temperatures have steadily increased across all of the monitored regions.
Model forecasts indicate a consensus for further development of El Niño conditions, although there is a range of indications for the maximum intensity. Current conditions and trends imply a "weak-to-moderate strength El Niño into the Northern Hemisphere Fall 2009" and possible further strengthening into the winter.
El Niño usually has weak impacts on the U.S. during summer and fall, but there is some tendency to suppress Atlantic hurricane activity because of increased wind shear. On the other hand, a recently published study shows that the effect on hurricane development may depend on whether the maximum temperature effects occur in the eastern or central Pacific. In years with eastern Pacific maximum warmth, tropical cyclone activity seems to be lower. So far, at least, the current pattern appears to be the eastern Pacific variety. NOAA's hurricane season forecast will be updated along with the next El Niño analysis on August 6.
By increasing the strength of the southern branch of the jet stream, El Niño tends to favor increased winter storminess on the U.S. Pacific Coast, more rainfall in the Southwest, and less wintry weather in the northern portion of the country. As a transitional zone, the Mid Atlantic region can be affected both positively and negatively in the winter depending on the particular pattern of strength and evolution of the El Niño conditions.
Images (click to enlarge): Latest equatorial Pacific sea-surface temperature (SST) departures from average, El Niño region SST history, El Niño model forecasts, all from NOAA/Climate Prediction Center