Fall/Winter 2017-2018 Update - Walker Circulation Strengthens Suggesting Wet Cold Start and Drier Late Winter

By Paul Huntington

I just wanted to highlight some recent atmospheric and oceanic observations that point toward a "possible" drier than average Winter for Central California with cooler northern storms (Inside Sliders) rotating down from the Aleutian Islands and less atmospheric rivers than last year, however the Madden Julian Oscillation (MJO)--that has been fairly quiet and inactive recently- is showing signs of waking back up along the Intertropical Convergence Zone (ITCZ) in proximity of the Maritime/Indonesia region or West Pacific along with the upwelling directly off Peru (localized El Nino) slowing down allowing warm sea surface temperatures to establish within the far eastern equatorial Pacific Ocean. Meaning things could change late Fall into Winter and recently our wind patterns/oceans have been showing tendencies toward strong variability in the strengths of El Nino indices- and even going from medium strength La Nina to very strong localized coastal El Nino off South America-throughout a single Fall Winter Spring season.

The major observation globally along the equatorial region I'm watching is the strengthening of the Walker Circulation. This circulation or oscillation is basically what an El Nino and La Nina are with the catchy term "El Nino"- that some Peruvian fisherman voiced- winning out over the Walker Circulation, however we should start to all become familiar with the great scientist who discovered it by the name of Gilbert Walker. Walker studied the see saw of atmospheric pressure in numerous climate sites in the Pacific, Atlantic, and Indian Ocean basins along with primarily targeting sites where the region experienced either permanent or seasonal high and low pressures. Walker also broke the spatial analysis up into four groups that represented the seasons with Winter being December-February, Spring March-May and so on. He was able to conclude that oceanic sea surface temperatures had a direct effect on the atmospheric pressure thus the differing sides of the equatorial ocean basins were showing relevant differences in pressure/rainfall. How these warm and cool ocean waters shifted from one side of the ocean basin to the other was influencing the atmospheric pressure, and thermocline depth. Furthermore the difference of these pressures gradients and ocean temperatures was being influenced by the strength of the Trade Winds or Easterlies and his analysis showed that the stronger phase of the Walker Circulation supported La Nina or the pooling of warmer ocean surface waters in the West Pacific and West Atlantic basins and cooler water in the West Indian Ocean (positive phase Indian Ocean Dipole). Whereas, the weak phase is supportive of El Nino with anomalously warmer water in the Eastern Pacific and Atlantic equatorial regions and either Dipole phase of the Indian Ocean (e.g. El Nino of 1997 IOD was in positive and 2015 El Nino was more negative). Recently the Walker cell has strengthened more than I have seen maybe since the late 1990's during the 1998 Fall and is indicative of the La Nina Pattern and this appears to be real similar to what happened in 1984-1985 after the very strong El Nino of 1982-1983 that was followed with a very weak La Nina or neutral El Nino indices in 1983-1984 similar to Winter of 2016-2017. This is not what generally occurs after a very strong El Nino but is something that Climatologist and Meteorologists are studying to see if this pattern of strong La Nina's following strong El Nino's is a consistent trend or only happens every so often like the 1997-1999 period that was a text book cycle of El Nino/La Nina cycle.

In addition this present strong phase of the Walker Circulation or Easterly winds is pushing warmer surface ocean water toward the West Pacific and the winds in affect are shearing apart the thunderstorms and preventing any organized convective circulation or deep low pressures which typically is a perfect environment for extremely strong typhoons (think stronger than Maria that just hit Puerto Rico). These strong Easterlies are weakening the Hadley Cell or re circulation of warm moist air into the prevailing Westerlies which energize the storms moving toward California. Its a very interesting feedback that if not interrupted with some strong thunderstorms and a few typhoons could prevail through the Winter starving the Northern Hemisphere jet stream of warm moist air. It will be real interesting to see how the West Pacific typhoon season plays out this remaining Fall and into early Winter and if it can interrupt this strong Walker Circulation that has taken hold recently. Nevertheless, upon the end of writing this report the Joint Typhoon Center has forecast the MJO to activate and begin to influence the Hadley cell and typhoon potential in the West Pacific which could weaken the Walker circulation and lower the jet stream into California during mid Winter.

If the Walker circulation does show signs of weakening it will allow the Hadley Cell to rebuild strength fueling the Northern Hemispheric Jet stream in the North Pacific and causing the upwelling off Peru to slow which supports warmer sea surface temperatures to establish and a lower pressure to form in the east equatorial Pacific. This pattern generally weakens the North Pacific High off the West Coast of North America opening up the storm door for Central California. The strong phase of the Walker cell/circulation or La Nina pattern can definitely bring decent amounts of rain and snow to Central California, however, the intensity and quasi stationary establishment of the North Pacific High this Summer has me thinking that the ridge could be strong enough to keep the storms more toward Northern California or Southern Oregon until we see signs of this strong Walker circulation getting interrupted or slowed down.

 Graphic shows El Nino/Southern Oscillation (ENSO) values before and after the two strong El Ninos of 1982-83 and 2015-2016. Neutral values (black) mostly dominated after the strong El Ninos other than 5 months of cooler temperatures in the late Summer/Fall. La Nina values(blue) established two years after the strong El Nino (red) and 1984-1985 could be an indicator of what this Winter might resemble; Wet Fall and spotty below average precipitation for the rest of Winter, however  ENSO values, MJO, and West Pacific Typhoon development will be something to watch later into Fall/Winter.

cheers to possible light powder rather than Sierra Cement (at least November and December??)
Paul Huntington