Weekly Weather Event -Week of October 11thOctober 15, 2021
For the second winter in a row, a La Nina phase of the ENSO cycle has emerged in the Pacific Ocean. Cooler than average temperatures have been detected across the central and eastern Pacific Ocean for the month of September, and models are in agreement for sea-surface temperatures to remain cooler than average as the Northern Hemisphere moves further into fall and winter. Finally, the trade winds near the equator are stronger than usual, which is a sign that the atmosphere is responding to ocean conditions. This is a sign that the Walker circulation—a “loop” of air in the atmosphere over the equatorial Pacific—is stronger than average, which is the expected response from the La Nina phase.
Unlike the El Nino phase of the ENSO cycle, La Nina years tend to occur in back-to-back years. This is largely because there is less dispersal of La Nina temperature anomalies compared to El Nino temperature anomalies. As a result, it is easier for the ENSO cycle to slip back into a La Nina phase the following year.
During a La Nina phase, the Atlantic hurricane season is more active, which has been seen in the 2021 season and the record breaking 2020 season. As a La Nina event moves into the winter months, the Southern U.S. is typically drier than average while the Pacific Northwest and the Ohio Valley is wetter than usual. This will continue to exacerbate the drought conditions in the Southwest U.S. that were worsened last year by a La Nina event.