Friday, February 11, 2011

El Nino, Snowfall and Air Quality

It is often observed that the air quality for a month or season is better when there is above average precipitation which removes the pollutants from the lower atmosphere.  The converse would be true as well with low precipitation associated with poorer air quality.  The problem is that quantitative precipitation forecasts are difficult to get right for more than a few days in advance.  For month or season ahead projections, the best that can be done is an estimate of the anomaly - and even this is only of marginal accuracy.   On the positive side, during El Nino years, changes produced in the atmospheric circulation by prolonged sea surface temnperature anomalies also persist and these have quantitative impacts on regional temperature and precipitation over North America - and typically winter temperatures are above normal.  This year, a relatively strong El Nino is in progress and its impact on the coming winter weather can be estimated, however crudely.

The most recent measurements of sea surface temperatures for the last week of November  from NOAA is shown here (notice the band of above normal temperatures at the equator stretching form the dateline to Peru):

Reference:  El Niño/La Niña Home (Climate Prediction Center, NOAA)

A recent article  Trends in Twentieth-Century U.S. Extreme Snowfall Seasons (Journal of Climate, Dec, 2009), looks at the relationship between El Nino and extreme winter snowfall across the USA over the last century.

Key Quotes:

"In almost all regions of the United States, temperature during November–March is more highly correlated than precipitation to the occurrence of extreme snowfall years"

" El Niño events are strongly associated with an increase in low-extreme snowfall years over the United States as a whole, and in the northwest, northeast, and central regions. "

Going out on a limb,  one might expect to see warmer conditions than normal across North America this coming winter, along with extremely low snowfall in locations where the temperature anomalies are greatest- and as a result, poorer air quality can be expected in the same locations.

Let's check back next spring and see how it turned out.

Meanwhile here is the latest outlook from Environment Canada:

Forecast predicts less snow in Canada this year (CTV news and video)
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