Borderline ENSO-neutral/ weak El Niño conditions are expected to continue into Northern Hemisphere winter 2012-13, possibly strengthening during the next few months.
During September 2012, the trend towards El Niño slowed in several key oceanic and atmospheric indicators. However, the Pacific basin reflects borderline ENSO-neutral/ weak El Niño conditions. Equatorial sea surface temperatures (SST) remained elevated across the Pacific Ocean, although anomalies decreased during the month as indicated by weekly index values in the Niño regions. The oceanic heat content (average temperature in the upper 300m of the ocean) anomalies also weakened, but continued to show large regions of above-average temperatures at depth across the equatorial Pacific. Interestingly, low-level westerly wind anomalies were evident over the equatorial western Pacific Ocean, which may portend possible strengthening of the subsurface anomalies in the coming months. Despite these winds, the atmosphere was still largely ENSO-neutral, as reflected by the Southern Oscillation index and near-average upper-level and lowerlevel winds across much of the Pacific. Tropical convection increased near the Date Line, which is consistent with weak El Niño conditions, but also remained elevated over eastern Indonesia, which is further westward than expected. Thus, the atmosphere and ocean indicate borderline ENSOneutral/ weak El Niño conditions.
Compared to the past few months, the chance is reduced for El Niño to develop during Northern Hemisphere fall/winter 2012-13 (see CPC/IRI consensus forecast). Due to the recent slowdown in the development of El Nino, it is not clear whether a fully coupled El Niño will emerge. The majority of models indicate that borderline ENSO-neutral/ weak El Niño conditions will continue, and about half suggest that El Niño could develop, but remain weak. The official forecast therefore favors the continuation of borderline ENSO-neutral/ weak El Niño conditions into Northern Hemisphere winter 2012-13, with the possibility of strengthening during the next few months.
This discussion is a consolidated effort of the National Oceanic and Atmospheric Administration (NOAA), NOAA’s National Weather Service, and their funded institutions.