What follows are extracts from the US NOAA’s own blog. They report: ‘After twelve months of El Niño Watches, we are issuing an El Niño Advisory.’ But the conditions are ‘extremely weak’ at this time [note: the maps shown are NOT current conditions]:
Over the last several months, we’ve seen warmer-than-average sea surface temperatures (SSTs) in the tropical Pacific, including the Niño3.4 region, which we track as one indicator of El Niño. The seasonal Niño3.4 Index has been at or above 0.5°C since September, and the most recent weekly Niño3.4 index was +0.6°C.
The El Niño/Southern Oscillation (ENSO) is a coupled phenomenon, though, so we also monitor the atmosphere for signs that it is responding to those positive SST anomalies. For the last few months, we’ve been seeing some suggestions of borderline atmospheric El Niño conditions, but until this month [March] we were below that borderline. This month, we’ve finally crept above it, and thus NOAA is declaring the onset of El Niño conditions.
But what does it mean for North American and global weather?
At this time, these are extremely weak El Niño conditions, during a time of year when the influence of El Niño on weather patterns in North America or other locations outside of the Tropics is weakening. For example, historical precipitation patterns associated with El Niño show that only about 3 of the past 10 El Niño years exhibited above-average rainfall in California during March-April-May (map pair below). Another way of looking at the historical relationships shows that ENSO has very little correlation to precipitation over North America during the spring.
The current warm SST anomalies are greatest in the Central Pacific (see map below). ENSO events centered in this area tend to be weaker (see Michelle’s post on ENSO flavors for more on this).
Especially since the rainy season in the West is winding down by March, it is unlikely that these current El Niño conditions will lead to substantial, drought-breaking rains.
What’s to come?
The CPC/IRI consensus forecast calls for an approximately 50-60% chance that El Niño conditions will continue through the spring. In the ocean-atmosphere coupling indicative of El Niño, warmer waters lead to atmospheric changes, and those atmospheric changes in turn help maintain the warmer water. So, it’s possible that even the weak coupling we’re seeing now will support the continuation of the positive SST anomalies. As I mentioned above, the recent westerly wind anomalies and the downwelling Kelvin wave will also help to keep the SSTs above average for the next few months.
Dynamical climate models are mostly forecasting a slow increase in the Niño3.4-region SST anomalies throughout 2015. Springtime is a difficult time of year for forecasting, as models traditionally have some difficulty seeing beyond the so-called “spring barrier,” and so the CPC/IRI consensus forecast probabilities decrease somewhat going into the summer, as forecast confidence decreases. That said, probabilities remain at or above 50% that El Niño conditions will continue through the fall.
After twelve months of El Niño Watches, we are issuing an El Niño Advisory. However, what it really represents is an incremental crossing of the borderline. If we follow the “Is it El Niño Conditions” flowchart, the warmer SST conditions, our anticipation that they will continue for the next several seasons, and signs of weak atmospheric coupling over the past month, mean we arrive at “yes!” From an impacts perspective, this is not particularly momentous, as El Niño impacts are weak in the spring and summer. Still, after months of hovering under the threshold, we can now say that El Nino conditions have arrived.
NOAA’s Climate Prediction Center synopsis says:
‘There is an approximately 50-60% chance that El Niño conditions will continue through Northern Hemisphere summer 2015.’
Nothing so far to suggest any unusual temperature movements might be on the way.