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Moving from El Niņo to La Niņa


2007 image of sea surface temperatures showing shift from El Niņo to La Niņa. Click to see more. Courtesy Nasa
2007 image of sea surface temperatures showing shift from El Niņo to La Niņa. Click to see more. Courtesy Nasa

Laurissa Anyas-Weiss, content producer

August 30, 2010 — Sea surface temperatures in the Pacific Ocean have started to cool off, an indication that El Niņo is shifting to La Niņa. What will that mean for Canadians?

Meteorologists tracking the sea surface temperatures across the central and eastern Pacific, in an area known as the El Niņo region, have seen a dramatic decrease in temperatures indicating a shift from El Niņo into La Niņa conditions. A shift between El Niņo and La Niņa is normal although typically there is a period of neutral conditions between the two phenomena. This time, the neutral period has been fairly short as last winter we experienced moderate El Niņo conditions.

Although the cooler than average temperatures indicate La Niņa conditions, it is not officially considered to be a La Niņa 'episode' until there are five consecutive months of such cooler sea surface temperatures. The cooling trend has only been indentified for the past three to four months. This trend may change but meteorologists are fairly confident that La Niņa will continue into early 2011 with increasing impacts on North American weather patterns as 2010 winds down.

La Niņa and the 2010 Atlantic Hurricane Season
Early indications of La Niņa include the recent increase in tropical storm activity. Last summer, when we experienced El Niņo conditions, the hurricane season in the Atlantic was fairly quiet with only nine named storms and only three storms reaching hurricane strength. It should be noted that two of those storms made landfall in Atlantic Canada including Hurricane Bill. While the Atlantic storm season was quiet, the Pacific storm season was anything but with 17 named storms and seven reaching hurricane status.

In 2010, after a slightly above average start to the Atlantic hurricane season in June and July with one hurricane (Alex), one tropical storm (Bonnie) and one tropical depression (Two), there have been four systems in the month of August alone. Though such an increase in August is not unusual in itself, the hurricane season is heating up and La Niņa is partially responsible. Cooler sea surface temperatures in the Pacific El Niņo region result in lower wind shear high up in the atmosphere over the Atlantic basin. This allows tropical cyclones to organize and grow in strength rather than inhibiting their development.

The storm track of Hurricane Earl show it curving away from the Gulf of Mexico and heading north up the Eastern Seaboard towards Atlantic Canada
The storm track of Hurricane Earl show it curving away from the Gulf of Mexico and heading north up the Eastern Seaboard towards Atlantic Canada

Atlantic Canada
La Niņa is just one weather pattern influencing the hurricane season risk for Atlantic Canada this year. Another weather pattern is increasing the potential for tropical cyclones in the Atlantic Ocean to curve north off the east coast of North America instead of moving into the Gulf of Mexico.

This year, the Bermuda High has been decidedly weak across the subtropical North Atlantic. This semi-permanent high pressure area has the capacity when strong to block hurricanes from curving north as they move west across the tropics until they get to Florida or the Gulf of Mexico. This blocking action can spare the Eastern Seaboard from hurricane activity by shifting their course into the Gulf Coast states if and when the usual northward curve in their tracks occurs. This weakness in the Bermuda High is related to the North Atlantic Oscillation (NAO) which is a measure of the strength of pressure systems in the North Atlantic. Specifically, the NAO represents the pressure differences between the Azores High well off the Portuguese coast, an inseparable cousin of the Bermuda High, and the Icelandic Low. Currently the NAO is negative meaning weaker than average high pressure over the Atlantic subtropics.

With this current configuration and given their location, Atlantic Canada runs a higher risk of feeling the impact of storms whose tracks may take hem up the Eastern Seaboard with greater frequency unless the Bermuda High strengthens.

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