"HA! Look at 2006! Where are the hurricanes?"
Part of the Common Arguments from Climate Change/Global Warming Skeptics and Deniers Series
From Logical Science

The Saharan Air Layer (SAL)

  Weather is complex and unless you are an expert on climate you can't cherry pick any one day, week, or even year.  To avoid confusion that can be caused by short term noise you should only look at multi-year global averages.  When you take multi-year global averages you switch from a weather observation mode to a climate observation mode.  Hurricanes are powered by warm water.  The quantity and location of wam water is controlled by a natural cycle called the Atlantic multidecadal oscillation (AMO) as well as global warming.  The lack of hurricanes in the 2006 Atlantic ocean is not due to a lack of warm water but something called the Saharan Air Layer.  This very dry and very dusty layer of air originates from the African Sahara Dessert.  This air pocket keeps the mid-troposphere warm and dry which limits development of the storms.  Normally it goes away rather quickly but this year it has decided to stick around for a little longer than usual.  Once this air layer goes away then storms will start to develop.  The very dusty Saharan Air Layer (SAL) can actually be seen from space.

Satellite photo of dust over the Atlantic SAL traversing the Atlantic
source Laboratory for Atmospheric and Space Physics source: NOAA, FAQ

        In 2006 a particularly strong and persistent Saharan Air Layer blanketed the Atlantic.  Because of this we saw very few hurricanes in the 2006 Atlantic season. Since there was no Saharan Air Layer in the opposite side of the planet, record numbers of typhoons (an Asian hurricane) occurred in the 2006 Pacific season.  As you can see below the western Pacific had 30 tropical storms (60 if you count eastern & south) while the Atlantic had only 9 in 2006.

 30 Pacific Tropical Storm Tracks 9 Atlantic Tropical Storm Tracks
Source: Unisys: 2006 Western Pacific Source: Unisys: 2006 Atlantic

So what will happen this year?  Well real-time satellite imagery for tracking the SAL can be found here.  Feel free to make your own predictions.




Hurricanes, Global Warming and the Natural Cycle


 Source: SEED: Cribsheet #6 via NOAA, MIT, et. al.

  The above graph shows storm intensity plotted against sea surface temperatures.  As you can see there is a strong correlation.  Dr. Judith Curry has a peer review article published by the American Meteorological Society (AMS) that covers past hurricanes and climate change.  Dr. Curry presents the following data which supports the consensus among climatologists (which is different than a meteorologist) that the hurricane cycle has had a boost from global warming:

Source: Curry et. al   Figure 2
Sea surface temperatures over time.  

which goes along with the following quote:

"The strength of the tropical storm activity during the period of 1995–2005 (which is at least a decade away from the expected peak of the current AMO cycle), relative to the previous maximum 11-year period of 1945–55 (Table 1), shows a 50% increase in the total number of tropical storms, number of hurricanes, and number of category-4 and -5 storms."1

The part in bold is very important.  Even though we are still 10 years away from the next peak, we've surpassed the previous peak by 50% in tropical storms, number of hurricanes, and number of category 4 & 5 storms.  It is also important to note that tropical warming of sea surfaces (Figure 2) "is consistent with a similar increase in global surface temperatures"1


The SAL and Hurricanes: A more detailed look

The following is a description as to what occurs when the SAL interacts with hurricanes:

"Tropical cyclones interact with the Saharan Air Layer (SAL) in several ways. Some tropical cyclones are trapped in the Saharan Air Layer for their entire life cycle and often struggle to intensify beyond strong tropical storm strength (>=34 kt to <64 kt). Tropical Storm Chantal of 2001 is an example of this type of interaction (Animation 1). Other tropical cyclones like Joyce of 2000 can be overrun by the faster moving Saharan Air Layer and are quickly weakened (Animation 2).  Tropical cyclones that are trapped in the Saharan Air Layer early in their life cycle often rapidly increase in strength when they break free of the Saharan Air Layer. Hurricane Erin of 2001 is an example of such an interaction Animation 3."
source: University of Wisconsin-Madison Space Science and Engineering Center

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