ENSO Update

It's been a while since we looked at the current phase of ENSO (the El Niño - Southern Oscillation), so let's do an update.  As I'm sure readers know, last winter's intense El Niño episode is long gone now, as equatorial Pacific sea surface temperatures dropped back to normal in the spring.  In the past couple of months temperatures have become modestly lower than normal in the central and eastern equatorial Pacific Ocean, so we're close to marginal La Niña conditions, but La Niña has not yet been declared.  Here's the latest weekly SST anomaly map, showing rather feeble cool anomalies along the equator.  Note the extraordinary warmth all around Alaska, a result (and a cause) of the exceptionally persistent warmth all year.

As last winter's El Niño event unfolded, there was much speculation that the next winter, i.e. 2016-2017, would be a La Niña winter, because a quick reversal into La Niña conditions has occurred before after El Niño; for example, this happened in 2009-2011 and in 1997-1999.  For a while the seasonal forecast models were showing this kind of scenario, with a significant La Niña developing this autumn, but the models have now backed off.  Compare the two charts below, the top taken from CFSv2 runs initialized in late April, and the bottom from the most recent runs.  The latest forecast shows the key Niño3.4 region getting no colder than at present, but actually warming as we go through winter.

The latest IRI/CPC forecast for ENSO still shows a greater chance of La Niña than ENSO-neutral conditions until late winter, but the outlook is quite different from a few months ago, when the chance of La Niña was believed to be above 75%.

So given that La Niña is on hold if not cancelled this winter, we'll be reverting to the PDO as a seasonal forecast tool for Alaska this winter, correct?  Unfortunately, perhaps not - because the PDO index crashed about a month ago and near-neutral conditions have prevailed since then.

With near-neutral ENSO and PDO conditions at the present time, and ENSO-neutral looking like a reasonable bet for the coming months, we would not expect to be able to say much about what might unfold this winter in Alaska.  It is interesting to note, however, that Papineau (2001) showed results indicating that ENSO-neutral winters are warmer on average than El Niño winters from Barrow down the west coast to Cold Bay; the rest of the state tends to be cooler than during El Niño, but nowhere near as cold as during La Niña.  On this blog we've seen that the warmest January's in Fairbanks have tended to occur during near-neutral ENSO conditions; so it would not be a surprise to see warmth persist this winter, especially if surface temperatures don't cool off relative to normal in the Gulf of Alaska and Bering Sea.

The latest CPC forecast for December-February expects neither warm nor cold conditions, relative to normal, over the southern half of the state, and they also show equal chances for precipitation over the interior.  Recall that this doesn't mean CPC is forecasting near-normal; they just don't feel able to make a call one way or the other.  Hopefully a clearer picture of likely anomalies will emerge in the coming months.