Sunday, July 05, 2009

Thoughts on Nino and Winter

The developing Nino that we talked about back in early May is now alive and kicking in the Eastern Pacific. While not "officially" classified as such because its duration has not met critieria, the waters off of the Pacific coast of South America are warming above average a fair bit. If this continues, we will probably have an official Nino classified by Fall.

Researchers are wondering if Ninos can come in more than one shape and variety. The prospect of an El Nino Modoki developing in the Pacific has scientists wondering if this could lead to a potential increased risk of hurricane development in the Atlantic. An El Nino Modoki, according to scientists, has its warmest oceanic temperature anamolies in the Central Pacific (generally south or southwest of Hawaii near the Date Line). Compared to classic Nino events where temperature anamolies sometimes are closer to the Ecuador coast or more evenly spread through the eastern half of the Pacific, these Modoki Ninos do not alter the upper atmospheric patterns in the same way that a "classic" Nino event does.

In classic Nino events, the upper winds in the Atlantic become more hostile for hurricane development due to differences in sea surface temperature being a bit further east in the Pacific (which alters things to the east, over the Atlantic). In a Modoki Nino, the thought is that these upper wind changes do not impact the whole of the Atlantic basin, allowing for potentially more tropical development.

The last example of a Modoki Nino was 2002. The hurricane season that summer was a slightly more active than usual season and featured a couple of strong hurricanes (Isidore and Lili). While we are still 'stormless' in 2009 (one tropical depression to date), the peak of the hurricane season is still eight weeks away and anything is possible, although signs do point to a less active than recent average season.

What would a Modoki Nino do to Philadelphia in the winter? Well, the results are mixed. Some of the years that could be classified as a Modoki (1977-8, 1992-3, 2002-3) featured good to great winters while others (1991-2, 1994-5) were not. Strength of the Nino event is one factor to definitely watch...most models suggest that this potentially could be a moderate Nino this winter, which leads to looking at 1994-1995 or 2002-2003 as two completely opposite candidates (unless you consider the Februaries in both years, which were good (1995) or great (2003) for snow in both years).

The wildcard for the other months would probably lie in what happens off our East Coast with the oceanic temperatures in the North Atlantic. Just six weeks ago those waters were pretty cool overall and cold off of Newfoundland. The most recent snapshot shows a changed picture, with warmer than average oceanic temperatures off of much of the East Coast of North America and only one noticable cold pool in the Central Atlantic. This is important as there is a decent connection between the ocean temperatures in this part of the world during the summer and early Fall with NAO status in the winter. Warmer than average temperatures usually sets up a positive NAO in the winter, with colder than average oceanic temperatures yielding a -NAO.

If we hold onto warmer oceanic water going through the remainder of summer and into Fall it might be tough to keep the East Coast cooler than average...and if the Nino event goes strong (as a couple of models suggest) the prospects for massive snow and cold this winter may not be so hot. Still too early to specifically say what will happen this winter but it seems the table is being set slowly for the upcoming winter.