Guest Post by Bob Tisdale
NOAA’s weekly sea surface temperature anomalies for the NINO regions (based on Reynolds OI.v2 data) are furnished on Mondays. This week’s update for the week centered on July 1, 2015 shows the surface temperature anomalies have reached 2.0 deg C in the NINO3 region of the eastern equatorial Pacific (5S-5N, 150W-90W). NINO3 sea surface temperature anomalies are used in the JMA’s El Niño outlooks.
On the other hand, the weekly sea surface temperature anomalies of the NINO3.4 region (5S-5N, 170W-1250W), which NOAA uses to define an El Niño and its strength in its Oceanic NINO Index, is still at 1.4 deg C, which is below the 1.5 deg C threshold of a strong El Niño. But it’s early in the development of an “average” El Niño, which typically peaks in December.
The weekly sea surface temperature anomalies for the easternmost NINO1+2 have recently exceeded the values reached during the 1991/92 El Niño, but they are lagging behind the weekly values for the 1997/98 El Niño. And at the westernmost NINO4 region, surface temperatures are slowly dropping, seeming to be the response as more warm water is pushed eastward.
And there was another westerly wind burst at the end of June/early July, which, even though it wasn’t very strong, should help to strengthen the El Niño in coming months.
NINO REGION SEA SURFACE TEMPERATURE ANOMALIES
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. 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 for the four NINO regions 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 (pentad centered on July 2)at the NOAA GODAS website includes the 12-month Hovmoller of wind stress (not anomalies) along the equator through July 2nd. 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.