Wednesday, July 11, 2012

Quiet In The Tropical Atlantic

In the wake of Debby drenching Florida two weeks ago, the Atlantic has turned significantly quieter as the axis of activity in the tropics in the Western Hemisphere has shifted westward into the Pacific basin, where two storms are churning in the open ocean.  Looking at the satellite imagery below, you can see the dearth of tropical activity across the tropical Atlantic and Caribbean...with a look that's much more typical of July.

There are a couple of reasons for this. One of these is that upper level winds aren't the most favorable for tropical development. The brighter yellows and oranges on the graphic below show the upper level wind shear in the Atlantic basin.  There's a pretty decent west to southwest upper level flow across the Atlantic, which doesn't help the cause for any tropical development.  Upper level winds are typically one factor in the mix when we look at potential for development if there are any tropical waves.  However, with no waves drifting across the Atlantic, this isn't the big reason for the lack of storms right now.  As El Nino develops down the line, this may loom as a larger factor for preventing Cape Verde storms from intensifying significantly east of the Caribbean.

A more significant reason is the presence of dry air in the atmosphere.  Low and mid level winds are pushing significant amounts of dry air off of the Sahara, westward across the Atlantic.  This satellite image below shows the amount of dry air in the atmosphere...the Saharan Air Layer is a common feature in the tropical Atlantic as it blows dust and haze westwards in the lower atmosphere, bringing some vivid sunsets to the Caribbean and also a general dry pattern in the tropical Atlantic since the dry and relatively warmer air prevents tropical waves and moisture from firing into showers since it caps the atmosphere above the humid ocean.

The result of this dry air layer in the atmosphere is a limit of the potential rainfall that could fall if a system were strong enough to fire precipitation.  The graphic above shows the potential for precipitation, in this case reds and oranges are where precipitation chances would be higher.  A strong layer of drier air exists across the middle of the Atlantic, reaching towards the Caribbean.   While a more favorable fetch of moisture exists near a tropical wave that is exiting Africa, the presence of dry air a little further west will likely prevent this wave from strengthening much.

This won't be the trend through the summer...we'll probably see some shifts towards a more favorable pattern in August if one were to peg out the MJO (Madden-Julian Oscillation, which is an indicator that can be used to time potential favorable cycles for tropical development) and likely see more tropical systems fire as we approach the more traditional peak of hurricane season in eight weeks.  However, with the increasing potential of unfavorable upper winds in the Atlantic due to El Nino's development, the most likely places for tropical development this year will be closer to home or in the Central Atlantic, places where we've seen storm development take place already this year.