Sunday, October 12, 2014

North Atlantic Oscillation (NAO) 2014-2015 Winter Forecast

This is the winter forecast for the 2014-2015 winter season North Atlantic Oscillation, or NAO.

In this sea surface temperature anomaly chart, we'll examine is the presence of warmer than normal waters near Greenland. The presence of above-normal water temperatures near Greenland isn't just some random occurrence; it's a legitimate pattern in its own right. In the winter, warmer than normal SSTAs near Greenland and eastern Canada can allow for persistent ridging/high pressure to form over the land mass. You Northeastern weather junkies may know this phenomenon to be the negative phase of the North Atlantic Oscillation (NAO); the Weather Glossary definition for the NAO can be accessed here

Considering we're likely heading into a weak El Nino, and warm waters are indeed surrounding Greenland, high pressure/ridging over Greenland might be something to watch carefully for. We could even see some upper-latitude 'blocking' scenarios*. 

*Blocking is defined as when strong ridges or upper level lows become entrenched in the atmosphere, literally "blocking" the atmospheric flow from continuing to move west-to-east. This has been known to enhance droughts in areas where blocking high pressure remains for days (or even weeks), as well as flooding in areas where blocking low pressure is found.

The graphic above shows correlations of 500mb height anomalies to the phase of the North Atlantic Oscillation. For example, in the top-left January correlation image, let's say we're in a negative NAO phase. The blues over Greenland means there's a negative correlation, so a negative NAO would bring about positive 500mb height anomalies/high pressure. Similarly, red shadings indicate positive correlations. If the NAO is positive (negative), areas under the red shading are then expected to observe positive (negative) height anomalies, also known as high (low) pressure.

Given that the warmer than normal sea surface temperatures over Greenland may produce ridging/high pressure, which is the negative phase of the NAO (confirmed by the negative correlation in the January panel), and forgiving the unnecessary wordiness of this sentence, the NAO is expected to be predominantly negative this winter.


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