Friday, October 16, 2015

Precipitation and El Ninos: West Coast Rains Not Always A Lock

Not all El Ninos are alike. We've heard this time and time again. Sea surface temperatures are never exactly alike. Jet stream position varies. Overall winter temperatures vary too as does the location of above and below normal precipitation. We remember the El Nino of 1997-98 that produced heavy rains out west.  But that's not always the case.

We examine the top El Ninos according to the ONI Index from this site. We only examined the El Ninos that are ranked MODERATE, STRONG AND VERY STRONG.

THE STRONGEST EL NINOS (1997-98 and 1982-83)  

Storm track kept heaviest precipitation along the west coast and in the southeast.  At or below normal precipitation around the Great Lakes.


Storm track is still strong in the deep south with a southerly shift out over the Gulf of Mexico with above normal precipitation.  Aside from the winter of 1957-58, precipitation along the west coast is less in these years. Still below normal precipitation across much of the Great Lakes.


Southern storm track is still strong but weaker during the winters of 1986-87 and 1963-64. Storm track shifted much further north into the Ohio Valley and Mid-Atlantic during the winters of 2002-03 and 2009-10 with heavy wet snows. During many of these winter, well below normal precipitation was observed along the west coast especially during the winter of 1963-64

Contrary to what you hear, its not a guarantee that El Ninos always produce heavy precipitation events across the west coast and the Pacific Northwest. It does appear that the chances are greater with stronger El Ninos per the ONI Index.  The southern storm track seems to be more consistent--with varying intensities--in each of top El Ninos. 


A few side notes:

* Another modulator of the El Nino and storm track is the mode of the PDO (Pacific Multidecadal Oscillation). The winters of 1957-58, 86-87, 87-88, 97-98, 2002-03 and this year so far all have PDO values strongly positive.

* Yet another area of the Pacific that may have ties to the evolution of El Nino is the Pacific Meridional Mode located between the equator and Baja California. The link between the two is being studied. Here is a paper published in 2014.  Looking at the data from late summer through fall during early El Ninos, late 1957, 1986, 2009, 2015 (and partly 1982 although it was trending cooler) all had positive values (above normal water temperatures).

Do we see some matches above for this winter?  Hmmmm.