ENSO Update: Weak La Nina Likely for the Winter

Good afternoon! We continue to monitor developments with La Nina this weekend, as it will one important features will be considering in our monthly and winter forecasts. Sea-surface temperatures this past week, have cooled over more of ENSO regions in the Central and Eastern Pacific, as anticipated in our last major update. The thermocline (sea-surface temperature gradient) has been rising this week, with an easterly trade wind bust causing more upwelling cooler sub-surface waters into the Central Tropical Pacific. The latest 30-day moving SOI is now at +13.03. So La Nina conditions have been strengthening this week.

Latest model forecast show the zonal or trade winds will be still more easterly that will result in more upwelling of cooler waters over the Tropical Pacific into next week. Then these trade winds weaken and reverse to more westerly direction after the MJO passage by the end of this month, This will likely result in warmer waters return over Eastern Pacific. So we anticipate the SOI or La Nina conditions will stabilize or decrease somewhat going into November.

 

GFS forecast zonal winds over the Tropical Pacific to weaken or reversal near the end of month.

GFS forecast zonal winds over the Tropical Pacific to weaken or reversal near the end of month.

Thereafter, a majority of the latest models are now showing that a weak La Nina for rest of the fall and much of the upcoming winter, with ONI values somewhere between 0.5 and 1.0  There are fewer models that forecast La Nina will approach moderate intensity at 1.0 or will remain neutral negative ENSO state, tri-monthly ONI values less 0.5. So at this time, it appears La Nina will mostly likely, remain weak going into winter months. But we can’t completely rule out a neutral or near moderate La Nina event, at this time.

A La Nina event during winter typically support a fast jet over Northern Pacific that carves a deeper trough over the Northern Plains and Great Lakes and a stronger ridge near or over Southeast United States. This pattern could result in warmer than normal temperatures for much East Coast, with a storm track largely over the Ohio Valley. However, during weaker La Nina event, is southeast ridge is often more mitigated by some episodes of high-latitude blocking. This results more sustained cold air and a storm track closer to the East Coast. To sum up–how much cold air and snowfall this winter, will likely come down to the amount of high-latitude blocking, especially for the major I-95 cities and coastal plain in the Northeast and Mid-Atlantic. 

Latest ENSO model forecast for this month, show a weak La Nina on average

Latest ENSO model forecast for this month, show a weak La Nina on average

There are a number of contributor factors to the magnitude of high-latitude blocking that will be observing as we put together our winter forecast for next two weeks. One factor is the QBO, which is currently in more easterly or negative phase from 30mb and higher. A negative QBO with values -10 or lower, typically leads to more stratospheric warming events, that weaken the polar vortex in both the stratosphere and troposphere during the winter. Thus, there is more room for a ridge to build over the North Pole or Greenland regions, resulting more episodes of negative AO/NAO associated with high-latitude blocking.

However, the easterly QBO has yet to fully downwell into the lower stratsophere at 50mb. If winds remain just weaker at this level, it may more difficult for the polar vortex to fullybreakdown this winter. Also solar activity is also another contributing factor that could affect how much of –AO/NAO we this winter. We are still couple winters away from the solar activity minimum, and there recent solar spike last month. Even more importantly, we are still seeing more geomagnetic storms the past few years, than was observerd during previous solar cycle. Solar winds or flares could result in geomagnetic storms that could help the stratosphere polar vortex re-intensify or re-consolidate, after a period of weakening or displacement.

So these are things we need to watch, along with the La Nina, as we put together our weekly long-range and monthly outlooks, our winter forecast, over the next few weeks. So stay tuned more ENSO updates over next few weeks, as new data comes in from the Pacific.