Guest Post by Bob Tisdale
The post provides a look at the most recent weekly sea surface temperature anomalies for the equatorial Pacific. It also includes a Hovmoller diagram of the wind stress (not anomalies) along the equator…to confirm that there was another westerly wind burst at the beginning of last month.
NOAA’s weekly sea surface temperature anomalies for the NINO regions (based on Reynolds OI.v2 data) are furnished on Mondays. Today’s update for the week centered on July 29, 2015 shows the sea surface temperature anomalies of the NINO3.4 region (5S-5N, 170W-120W), which NOAA uses to define an El Niño and its strength, is at 1.7 deg C, just above the 1.5 deg C threshold of a strong El Niño…where it’s been hovering for a few weeks. But it’s still early in the development of an “average” El Niño, which typically peaks in December.
The weekly NINO region sea surface temperature anomaly data for Figures 1 and 2 are from the NOAA/CPC Monthly Atmospheric & SST Indices webpage, specifically the data here. The base years for anomalies for the NOAA/CPC data are referenced to 1981-2010.
Figure 1 includes the weekly sea surface temperature anomalies of the 4 most-often-used NINO regions of the equatorial Pacific. They start in January 1990. From west to east they include:
- NINO4 (5S-5N, 160E-150W)
- NINO3.4 (5S-5N, 170W-120W)
- NINO3 (5S-5N, 150W-90W)
- NINO1+2 (10S-0, 90W-80W)
And for Figure 2, the evolutions of the sea surface temperature anomalies in 2015 are compared to 1997 as a reference for a very strong El Niño and compared to 2014 as a reference for a very weak El Niño. Keep in mind that 2015 started the year at or near El Niño conditions, where that was not the case in 1997 and 2014.
ANOTHER WESTERLY WIND BURST
In the post ENSO Basics: Westerly Wind Bursts Initiate an El Niño, we discussed how westerly wind bursts prompt the downwelling Kelvin waves that appear early in the development of an El Niño. Later in the process of El Niño evolution, westerly winds bursts also help to push more warm surface water than normal eastward along the Pacific Equatorial Countercurrent. So they too help to strengthen an El Niño.
The most recent update at the NOAA GODAS website includes the 12-month Hovmoller of wind stress (not anomalies) along the equator through July 27th. See Figure 3. It shows yet another westerly wind burst in late June/early July. That westerly wind burst should be the response to the two tropical depressions that recently straddled the equator in the west-central tropical Pacific.
That’s all for now. I’ll try to provide the full update next week when the data for July are available. I’ll also try to provide an update on The Blob next week.