Wednesday, May 20, 2015

An Evolving El Nino

For a good part of this past Northern Hemisphere winter, above average sea-surface temperatures (SST) persisted near the dateline (180º) in the equatorial Pacific.  For example, the image below shows the weekly SST anomaly (anomaly means departure from a 30-year climatology) for the last week of February with the positive SST anomalies centered on the dateline in the equatorial Pacific.
Source: ESRL/PSD
Since then, positive SST anomalies consistent with weak to moderate El Nino conditions have strengthened and developed across the eastern Pacific and now extend to the coast of South America.

Here's another perspective based on a loop of the SST anomalies since late February, showing very nicely the development of the weak to moderate El Nino.  
Source: CPC
Concurrently, enhanced convection and cloud cover developed over most of the central and eastern tropical Pacific, especially just to the north of the equator along the Intertropical Convergence Zone (ITCZ).  This is reflected in negative outgoing long-wave radiation (OLR) anomalies over the past month.  More cloud cover (especially upper-level clouds) means less outgoing long wave radiation at the top of the atmosphere.

Source: CPC
And, along with this convection, we've seen anomalous upper-level anticyclones to the north and south of the equator, the former associated with the active subtropical jet we've seen across the southwest the past couple of weeks.  

Source: CPC
The latest diagnostic discussion from the Climate Prediction Center calls for a 90% chance of El Nino to continue through the Northern Hemisphere summer and a greater than 80% chance it will last through 2015, but also notes that there is considerable uncertainty regarding the strength of the event.  As we have discussed in the past (e.g., Outlook for the 2013–2014 Ski Season), the potential existence of El Nino does not strongly load the dice one way or the other for snowfall in the Wasatch Range next ski season.  Nevertheless, it would probably mean a different large-scale flow pattern than the one we had last season, which would provide much needed new material for this blog!

1 comment:

  1. I recently looked at some correlations between monthly precipitation and ENSO index for the SLC NWS site as well as Snowbird SNOTEL precipitation. Of course the correlations are generally very weak, but I did notice some subtle seasonal trends. The most consistent results were a slight negative correlation between precipitation and ENSO index during November - January (implying wetter early to mid winter conditions on average during the La Nine phase). For February through August, results were variable month to month and depending on the precipitation site. September stood out as having a consistent positive correlation with ENSO (more precipitation during the El Nino phase). Anyway, thought this might be interesting.