El Niño/Southern Oscillation - June 2006
NEUTRAL ENSO CONDITIONS PERSIST
ACROSS THE EQUATORIAL PACIFIC
ACROSS THE EQUATORIAL PACIFIC
SSTs increased across most of the equatorial Pacific Ocean during June. The observed SST anomalies this past month were slightly above average in the western and central parts of the tropical Pacific basin. Water temperatures in the mixed-layer warmed during June and were slightly above normal at the end of the month, with the warmest anomalies in the eastern Pacific near 120°W.
For the month, the SST anomaly in the Niño 3.4 Index region was +0.25°C (+0.45°F). The SSTs in the Niño 4 Index region of the western equatorial Pacific were also warmer-than-normal, with a monthly index of +0.39°C (+0.7°F) above the mean (map of Niño regions). For the most recent global ocean surface temperatures, please see the loop of satellite-derived weekly SST anomalies for June 2006.
Below average SST anomalies persisted throughout the latter half of 2005 and into the first few months of 2006 in the central equatorial Pacific basin, and this can be seen in the data from NCDC's Extended Reconstructed Sea Surface Temperature dataset (ERSST version 2). Due to the warming of SSTs and mixed-layer temperatures in the central tropical Pacific, the 3-month running mean of the Niño 3.4 index region increased above -0.5°C in April, and has continued to warm during June. (NOTE: For NOAA's official ENSO classification scheme, please see NOAA's El Niño/La Niña Index Definition and also see the CPC ENSO Diagnostic Discussion for NOAA's latest official assessment of ENSO conditions.)
Winds (U-Component Winds) and Sea-Level Topography:
The monthly averaged easterly trade winds were slightly above-normal across the eastern and central equatorial Pacific during June. Weaker than normal easterly winds were observed in the western equatorial Pacific, where the trade winds were below average during this past month. However, there were several periods during June when the trade winds weakened substantially, especially in the western Pacific region at the beginning of the month (see the loop of June zonal winds).
The most recent Pacific overpass of the NASA/JPL Jason-1 satellite measured an area with large positive sea level anomalies in the southwest equatorial Pacific, which was associated with enhanced tropical convection along the South Pacific Convergence Zone (SPCZ).
Sea levels were mostly near-average throughout the equatorial Pacific region during June, as the sea level heights reflected the near-normal SST conditions at the end of the month (see the 11 June 2006 overpass).
Longwave Radiation (OLR):
The map to the left shows the spatial pattern of global OLR (in W m-2) observed by satellite during June. Positive OLR anomalies (typically associated with La Niña) that persisted over the past several months primarily in the equatorial western Pacific region dissipated in June. Tropical convection increased over Indonesia and along the equatorial region west of the Date Line in the western Pacific this past month. Despite the changes observed in June, the 3-month averaged OLR anomalies remained positive over sections of both the eastern and western Pacific along the equator, reflecting the reduced cloudiness that was observed during the latter stages of 2005/2006 La Niña.
The monthly-averaged OLR Index switched to a negative index during June, although it remained near-neutral. For the month, the OLR index was -0.3 W m-2 averaged across an area centered over the Date Line in the western Pacific between 160° E and 160° W (note that negative OLR Index values are typically associated with El Niño conditions in this region).
Note that high frequency variability in OLR is typically associated with the Madden-Julian Oscillation (MJO, which is convective activity that propagates west to east in the near-equatorial region from the Indian Ocean into the Pacific Ocean approximately every 30-60 days). The latest MJO activity can be seen in CPC's graphs of Daily MJO Indices.
Oscillation Index (SOI):
The standardized SOI was negative in June, with a monthly averaged value of -0.7 for the month. The SOI was inconsistent during the relatively weak 2005/2006 La Niña event, fluctuating several times between near-neutral values and positive indices that were more indicative of the weak cold event. The shift to a negative value during May and June coincided with the development of warmer than average SSTs across a majority of the equatorial Pacific.