Natural Science Forum / Physics / General Physics / January 2008

Rising ocean temperatures boost hurricanes, says study
  http://physicsworld.com/cws/article/news/32646

  Researchers in the UK claim to have the first firm evidence that
  rising sea-surface temperatures have boosted the intensity and
  incidence of hurricanes that form in the tropical North Atlantic,
  Caribbean and Gulf of Mexico. Mark Saunders and Adam Lea of
  University College London (UCL) say that a 0.5-degree increase in the
  temperature of the ocean surface results in a 40% increase in
  hurricane activity.

  Climate scientists have long suspected that warmer oceans mean more
  hurricanes. In the 1950s and 1960s, the sea-surface temperature (SST)
  in the tropical North Atlantic was relatively high and hurricanes
  were relatively common, while in the 1970s and 1980s, the temperature
  dipped by nearly one degree Celsius and the intensity and incidence
  of storms also dropped. Then in the 1990s, the temperature began to
  increase along with hurricane activity.

  Notoriously complex

  However, the Earth's climate is a notoriously complex system and
  researchers have struggled to establish a direct relationship between
  SST and hurricanes. An important complicating factor is that changing
  wind patterns over the Atlantic play an important role in hurricane
  formation \u2013 and it had been tricky to determine the relative
  impact of wind and temperature.

  Saunders and Lea have now got around this problem by doing a careful
  statistical analysis of storm, SST and wind data stretching back to
  1965 (Nature 451 557). First, they showed that about 75-80% of the
  variation in hurricane activity was a direct result of changes in the
  SST and wind patterns. Then, they assumed that the temperature and
  wind patterns had independent effects on hurricane intensity and used
  an analysis technique called multiple linear regression to remove the
  influence of wind.

  Accumulated cyclone energy

  The researchers described the hurricane frequency and intensity in
  terms of the accumulated cyclone energy (ACE) index for the North
  Atlantic, which measures the total energy dissipated by hurricanes.
  ACE is considered the most meaningful measure of hurricane activity
  because it combines intensity and duration of all tropical storms in
  a given season.

  They found that a 0.5 degree Celsius increase in SST boosted the ACE
  by about 40%. Furthermore, they say that much of the recent increase
  in the ACE since 1996 can be attributed to a rise in the SST.

  While Saunders and Lea were able to link SST to hurricane activity,
  their work says nothing about why the SST has increased over the past
  40 years. According to Saunders, this could be down to global warming
  or it could be part of the cyclical variation of SST called the
  Atlantic multidecadal oscillation, which seems to occur with a period
  of about 60-80 years.

  Improved climate models

  However, Saunders told physicsworld.com that the result should help
  those developing models that try to predict the outcomes of climate
  change, because in order to be correct, these models should reproduce
  the observed connection between increasing SST and hurricane
  activity.

  According to Stefan Rahmstorf, a physicist at the Institute for
  Climate Impact Research at Potsdam University in Germany, this latest
  work backs up previous studies that suggest that the connection
  between SST and ACE is very strong indeed. "What is remarkable
  in this study (and similar previous studies) is how strong the effect
  of a very modest warming of half a degree is", Rahmstorf told
  physicsworld.com "It makes you wonder what kind of hurricane
  activity you'd get after three degrees warming."

  Saunders, however, is more cautious about predicting what would
  happen if the SST rose by such an amount. It is possible, for
  example, that the SST is currently at a critical value for hurricane
  formation and small deviations cause large changes in hurricane
  activity. This might not be the case if climate change caused the SST
  to rise by several degrees.