In this subsurface equatorial Pacific image, notice how the cool tongue of anomalies has faded dramatically this month, and warm anomalies are now developing where it used to be. Remember, a La Nina consists of cool temperatures, and it is necessary to have a substantial subsurface cool tongue for one to develop.
Even the surface equatorial Pacific SSTAs have jumped back up over the past few weeks, especially in Nino 4 region. This is in association with increased easterly winds in the western equatorial Pacific, something that isn't found in La Ninas either.
Another crucial ENSO factor is the Southern Oscillation Index. Simply a 30-day average measure of pressures at Tahti/Darwin Austrailia, positive SOI values tend to correlate with La Nina, and vice versa. The values over the past few days have been around -14...VERY low. Although it can be partially explained by a wet MJO over the region, for a developing La Nina you'd need consistent normal or above normal values.
Even the CPC and BOM have both backed off on calling for La Nina development.
http://www.cpc.noaa.gov/products/predic ... xus05.html
http://www.bom.gov.au/climate/enso/
Does this mean the probabilities of La Nina this season are 0? Nope. But they've certainly decreased. Actually, I still think there's a fair chance that we WILL see a weak La Nina form in late summer or early fall. Let's take the year 1995 (not TC activity wise, but ENSO wise)...during the Spring, El Nino dissipated and a cool tongue looked to be well established. However, during June and July, the cool tongued weakened, and the SSTAs shot up temporarily. Kinda similar to what's happening right now. Later in the summer, La Nina ended up developing. A similar scenario *could* happen this year. The only possible advantage that 1995 had was the stratospheric QBO winds were westerly, which aids in La Nina formation. This year, it's easterly. Whether that will make a huge difference or not, I do not know.
But my point is, we'll need to see some cooling of the SSTAs in the equatorial Pacific (surface and subsurface both) within the next few weeks. Indeed, the positive MJO passing through the area will be supportive for this to happen. So there is still plenty of hope I suppose. But, if we don't see cooling soon, then La Nina probably won't arrive in time to have an enhancing influence on the Atlantic hurricane season. And since TWW and I were the first to predict weak La Nina way back in November...we'll be the first to say BUST if it does not come.
Very interesting that La Nina was predicted to become a factor this season, and how things can change so dramatically that now we are looking at a non-La Nina season! :o
