ENSO REMAINS IN NEUTRAL PHASE:
SSTs WARM ACROSS THE CENTRAL EQUATORIAL PACIFIC







 Sea-Surface Temperatures (SSTs) and Mixed-Layer Conditions:
Equatorial Pacific Ocean surface and subsurface temperatures have been near- or cooler-than-average over the past several months, as the phase of ENSO was neutral. During June, the SSTs warmed across the equatorial Pacific region, as a weak Kelvin wave propagated across the basin. However, water temperatures in the mixed-layer remained below normal, with an area of -2.0°C (-3.6°F) and cooler temperature anomalies between 100-200 meters depth in the central equatorial Pacific. Warmer-than-average upper ocean temperatures remained in the far western equatorial Pacific west of the Date Line in June.

For the month, the SST anomaly in the Niño 3.4 Index region was +0.09°C (+0.16°F), which was an increase of +0.46°C (+0.83°F) compared to the May value. The SSTs in the Niño 4 Index region of the western equatorial Pacific also warmed slightly during June, which resulted in a monthly anomaly of 0.19°C (0.34°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 2007.

The cooling of the SST anomalies in the Niño 3.4 index region over the past several months decreased the 3-month running mean below the long-term average during June. (NOTE: For NOAA's official ENSO classification scheme, please see NOAA's El Niño/La Niña Index Definition).

The Climate Prediction Center's most recent ENSO Diagnostic Discussion indicated that the current phase of ENSO was neutral, while the latest ENSO update from the Australian Bureau of Meteorology (BoM) also reflected the ENSO-neutral conditions in the equatorial Pacific basin. Both CPC and the BoM have indicated an elevated probability of a La Niña cold event developing over the next 1-3 months (see the Australian BoM ENSO Wrap-Up).




Equatorial Zonal Winds (U-Component Winds) and Sea-Level Topography:
The easterly Trade winds were above normal across the equatorial Pacific during June. Any observed increase in easterly Trade winds along the equatorial zone increases upwelling in the mixed-layer, and is therefore one important factor that will likely influence the development of a La Niña event in the coming months.

Over the past month, significant week-to-week variability in the near-surface winds has been observed along the equatorial region of the Pacific, as shown in the animation of June zonal winds. Periods of anomalous westerly flow occurred across parts of the eastern equatorial Pacific region during June, as the easterly Trade winds were near-average for the month off the South American coast.

Pacific sea levels measured by the NASA/JPL Jason-1 satellite remained below average across the central equatorial Pacific, reflecting the cooling ocean surface temperatures in this region (see the most recent image of 18 June 2007 sea level anomalies).


Outgoing Longwave Radiation (OLR):
The map to the left shows the spatial pattern of global OLR (in W m-2) measured by satellite during June. A region of positive OLR anomalies was measured in the eastern Pacific just north of the equator along the Inter-Tropical Convergence Zone (ITCZ), illustrating the suppression of tropical convection due to the below-normal SSTs observed in this region. This pattern of positive OLR anomalies along the equator in the eastern Pacific was observed during the April-June period as well.

The monthly OLR index for June was +0.6 W m-2 averaged across an area in the western Pacific near the Date Line between 160° E and 160° W. Therefore, the June value of the OLR Index increased significantly, reflecting that a transition from neutral ENSO toward La Niña conditions has likely started.

Note: high frequency variability in OLR is typically associated with the Madden-Julian Oscillation (MJO), which is an intra-seasonal oscillation in 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.
Southern Oscillation Index (SOI):
The standardized SOI was +0.2 in June. This shift in sign from a negative to a positive index value followed 6 consecutive months with negative indices [consistently negative (positive) values of the SOI are typical of El Niño (La Niña) conditions].

Note, the monthly SOI values will likely remain positive over the next several months, as NOAA's Climate Prediction Center (CPC) has forecasted the potential development of La Niña (cold event) conditions during July-September 2007.

Citing This Report

NOAA National Centers for Environmental Information, El Nino/Southern Oscillation for June 2007, published online July 2007, retrieved on April 23, 2024 from https://www.ncei.noaa.gov/access/monitoring/monthly-report/enso/200706.