Hurricane season 2011 begins

  • The tracks of last year's Atlantic hurricanes and tropical storms. Image: NOAA

    A schematic diagram showing the favourable conditions across the Atlantic during the 2010 hurricane season. Image: NOAA

    For comparison, these were the hurricane and tropical storm tracks during 2004. Image: NOAA

    Hurricane Ivan just before it slammed into the Gulf Coast of the USA in September 2004. Image: NOAA

    La Niña in August 2010 as revealed by sea surface temperature (SST) anomalies: note the anomalously cool strip of water across the equatorial Pacific and the warm anomalies in the Atlantic. Image: NOAA Climate Prediction Center

  • Hurricane season 2011 begins
    02.06.2011 15:29


    The first day of June marked the customary start of the hurricane season in the Atlantic, a period which finishes at the end of November.

    That is not to say that hurricanes cannot form outside of those dates but favourable atmospheric and oceanic circumstances are less likely.

    Perhaps the most important of these is the temperature of the sea. Hurricanes and typhoons are fuelled by moisture and warmth, and the sea surface below the needs to be at a temperature of at least 26.5 degrees Celsius to sustain development.

    Before June and after the end of November the sea more often than not is just not warm enough. There are exceptions, however, and parts of the Atlantic were particularly warm in 2010, including the region near the Cape Verde Islands off the coast of northern Africa where many tropical storms spawn.

    This is one reason why 2010 was such an active year for tropical storms in the Atlantic, with far more than average and the greatest number of named storms since 2005: with a total of 19, it tied for third place alongside 1995 and 1887.

    Twelve of these cyclones developed into hurricanes, the joint second highest number on record, equalling 1969.

    Another factor was the existence of a strong La Niña episode, the anomalous cooling of water in the equatorial Pacific. This might not, on the face of it, make a lot of sense when the talk is of Atlantic storms.

    However, La Niña tends to reduce the speed of winds aloft around the northern hemisphere. Nascent tropical storms are apt to get ripped apart as the grow upwards if these winds are strong; for vigorous development they need winds to be reasonably uniform upwards through the atmosphere – what is known as low wind shear.

    El Niño events – the warming of the equatorial Pacific - have the opposite effect, and hurricane formation tends to be suppressed during those years.

    La Niña all but disappeared by the start of June this year to leave neutral conditions but its effects are likely to linger for a while. Because of this and other factors, including continuing anomalous warmth of Atlantic waters, there is a consensus among tropical forecasting experts that 2011 will be another busy year in the Atlantic, although not quite as much as 2010.

    NOAA’s Climate Prediction Centre, for example, is predicting between 12 and 18 named storms, including 6 to 10 hurricanes, of which between 3 and 6 may become major hurricanes. The average is 11 named storms, six hurricanes and two major hurricanes.

    Fortunately, and rather unusually, high pressure systems steered many of last year’s hurricanes on a track farther east than usual and so far fewer than might be expected approached the USA, although more than usual formed farther south across the Caribbean Sea.

    Nothing is certain even a few days ahead when it comes to forecasting hurricanes but with a different atmospheric set up this year it is possible that America’s coasts may under greater threat.

    By: Stephen Davenport