Is El Nino causing our warm September?

Posted at 8:26 AM on September 17, 2009 by Paul Huttner

NOAA Sea Surface Temperature animation shows warm water building in the tropical Pacific.

Something has changed in the atmosphere the past few weeks.

After a summer where we had a hard time stringing together consecutive 80 degree days let alone weeks, a stable warm weather pattern is holding firm over the Upper Midwest. The polar front jet stream is bottled way up in Canada. September temperatures are running more than 5 degrees above average in the Twin Cities.

Meanwhile, persistent rains are falling in the south central U.S. into Texas. And the Atlantic hurricane season is nowhere to be found. As you can see from the NHC graphic below, this week marks the statistical peak of Atlantic hurricane activity.

This is a pretty familiar scenario to those of us who've seen El Nino related weather patterns. The funny thing is we usually see the effects as we head into winter.

If it looks like a duck and quacks like a duck, is it an El Nino duck?

There is no way to tell for certain, but it is possible that our unusual spell of mild September weather is the opening salvo from El Nino. The latest ENSO diagnostic discussion (something only a weather geek would read!) forecasts at least a moderate strength El Nino event this winter. That means tropical Pacific SST anomalies of at least +1.0 degrees Celsius. Some of the models forecast SST anomalies of +1.5 to +2.0 degrees Celsius. That would be a strong El Nino.

Historical studies show a high probability of a milder than average winter in the Upper Midwest during El Nino years. It is amazing to note that these correlations really only became known within the past 20 years or so. The strong El Nino event in 1982-83 really opened some eyes in the climate community. Regional atmospheric effects of El Nino years have been refined ever since, and there is still much more to learn.

It is fascinating to be a weather forecaster in these times. When we see a stark pattern change such as the one we've seen this September, we have to ask what has caused this big change in atmospheric circulation pattern over the United States?

The best answer I can come up with right now is that perhaps El Nino is taking control of our short term weather. Stay tuned as developments evolve this fall and winter!

PH

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