This discussion may be a little long, but there is a lot to discuss about the El Nino & the upcoming winter. Although I have posted this off-season on weather conditions it is only because of their abnormality. I would like to keep these discussions based entirely on snow & how the weather patterns affect our chances of snow. That is main motivation of both the writer & readers of these discussions. I believe that we can continue to learn & hone in upcoming winter predictions immediately following winter seasons if we research it enough. It can give us something to get excited about until we can actually strap our skis or boards back on finally on that first powder day.
There is not a whole lot of research on El Nino that can explain the causes, & the effects are documented over the past 60 years for all to conclude what they can based on their location & what happened with the same conditions in the past at that location. I have & will continue to be researching the past & trying to link it to our future as far as the upcoming winter is concerned in the Sierra.
The basic cause of El Nino is simple. Normally the trade winds will blow East to West along the Equator which piles up the warm water in the Western Pacific & upwelling in the Eastern Pacific keeps it colder. When the trades weaken the Warm water will slide Eastward into the Eastern Pacific warming the water & creating El Nino conditions. An official El Nino is not declared until 5 months in a row the 3 month avg. is at least .5 degrees Celsius above avg. Currently thru June it is only .2 degrees although current readings are near 1.o, so El Nino conditions are present but we don’t have an official El Nino yet. In theory the warmer water in the Eastern equatorial Pacific will keep the tradewinds weak or even reverse them West to East which will increase the warming until conditions reverse, usually within a year.
The change in direction of the winds along with the warmer water to feed on usually enhances the storm track across the Southern U.S. This is what leads to a wetter pattern in CA during El Nino. The wetter pattern normally favors Southern CA, but the stronger the El Nino the greater it’s affect on Northern CA. When you average together all the El Ninos the entire state is above avg. in precip over the winter. If you break them down by strength the weak & moderate El Ninos are evenly split in Northern CA between above & below avg. precip & they avg. barely above average in total. The strong El Ninos are well above average.
Currently a majority of models are predicting that the El Nino becomes moderate or even strong & lasts thru the winter of 09-10. You all know how reliable models can be especially far out. I see some problems with the model predictions for a strong El Nino. First of all, we are in a cold PDO cycle & we are just at the beginning of it. The Pacific & Atlantic Oceans go into warm & cold cycles that avg. about 20-30 years. We are currently over 1 degree below average on the PDO Index. The only time we have ever had a strong El Nino when the PDO was greater than -1 was 1972-73. Even if the El Nino did become strong the cold Pacific waters would limit the strength of storms keeping them weaker than in a normal El Nino with a warm Pacific.
Also, the SOI is the difference in pressure between Tahiti & Darwin. Normally in El Nino the SOI becomes negative with lower pressures in the Eastern Pacific towards Tahiti. Recently the SOI has shot up 18 to +10 which would not line up with a strengthening El Nino. Finally, the Global Atmospheric Angular Momentum (GAAM) would become positive during a strengthening El Nino & it’s currently trending negative. I don’t want to get too into the latter 2 because it’s to confusing & not necessary to the point. The main point is the weak El Nino conditions that I think are hard pressed to strengthen much more, in the midst of a cold Pacific cycle.
I believe that this El Nino is more reactionary to the overall cooling cycle of the Pacific. When looking forward to this upcoming winter I am looking for similar years with El Nino & a cold PDO coming off of a warm PDO. That gives us 51-52, 63-64, 72-73, & 92-93. The only winter in that series not above normal for snowfall was 63-64. We can exclude that winter if we only include winters where El Nino was in reaction to a cooling of the Pacific of more than 2 degrees from its previous warm peak. The only difference with the remaining years are the strength of El Nino & how far below avg. the PDO was. If I am right about the El Nino not becoming strong we can get rid of 72-73. Right now the PDO is greater than 1 degree below avg., so if we only include years that were greater than 1 degree below avg. we can disregard 92-93.
So we are left with 51-52. I believe this is some of the criteria that caused the MEI analog to show 1951-52 as matching closely to this winter. In 1951-52 we had a weak El Nino at the start of the cold 30 year PDO cycle in which the PDO was greater than -1 with a fall of around 2.5 degrees from the previous warm peak possibly causing a reactionary El Nino. That is exactly what we have now. Now I’m not advocating an 800” year by any means, but I just wanted to illustrate how the pattern most closely matches up to 51-52 as compared to any other year in the past 60 years. I believe the odds are finally on our side for an above normal snowfall winter.
It will be interesting to watch this El Nino progress going into the fall to see where it heads. El Ninos normally have the greatest affect on an increase in precip for Northern CA in the late winter & spring, but a normal fall would be a big difference from the drought of the past 3 falls. BA