2003 Coastal Upwelling in
East Central Florida
Description:
During the month of July, 2003 there was a prolonged period of upwelling
along the east central Florida coast. Surf temperatures at Daytona
Beach were below 70 degrees on numerous days, with some readings in the
mid 60s (coolest was 62 degrees). There was not one day when
the surf temperature reached 80 degrees! Further south, the cooling was
noticed at Port Canaveral/Trident Pier and Fort Pierce, with readings
occasionally in the low/mid 70s.
Past Events:
A cursory review of surf temperature data back through 1992 shows that
this event has been much more prolonged than in other years. The year
with the next most prolonged and intense event was 1994. In 2000, there
was also an extended period of cool surf temperatures at Daytona
Beach and Jetty Park from late June through early August, but the event
was not nearly as extreme. All years had a period with cooler than
normal surf temperatures (i.e. below 80 degrees) at Daytona Beach with
the exception of 1997. Cocoa Beach/Jetty Park had several years with
surf temperatures remaining above 80 for the whole summer (1993, 1995,
1997, 2001)
Upwelling Theory:
The reason upwelling occurs is because of Ekman transport. The flow of
most surface currents in the oceans is driven by wind. When wind blows
over water, the surface of that water is not pushed directly in front of
the wind, but moves at about 45 degrees to the right of the wind's
motion in the Northern Hemisphere because of the Coriolis force (which is caused
by the rotation of the earth). As one descends in the water, the
direction of flow continues to be deflected to the right, until
ultimately a three-dimensional spiral is formed vertically in the water.
The net transport of water, as explained by Ekman transport, is at an angle of roughly 90
degrees to the direction of the wind. In short, a layer of water near
the surface is pulled directly away from the coast - causing cooler deep
water to rise and replace it near the shore.
Reason for 2003 Event:
This year has been so extreme because of the strength and persistence of
the subtropical high pressure ridge. Surface pressures in the Atlantic
have been several millibars greater than average. This has produced a
long period where surface winds were predominantly southeast to
south. This wind flow, when combined with Ekman transport, will produce
a net transport of water away from the coast (which is oriented
340-160 degrees). I have seen some web sites/news articles that show an
offshore wind as the cause for upwelling. Based on the Ekman
transport principle, this explanation is incorrect. The longtime
Official in Charge at the Daytona Beach National Weather Service Office
observed upwelling events for many years when the wind flow was
southeast.
Ramifications/Proposed Theory:
With cooler sea surface temperatures near shore, the east coast sea
breeze should be be stronger than normal at times. This most likely will
lead to a more vigorous Atlantic/Gulf sea breeze boundary collision and
enhance the strength of storms. I think that the wet season in 1994 also
had a lot of severe weather.
Randy Lascody
National Weather Service
Melbourne, FL
Aug. 1, 2003
posted August 7, 2003 - dlj
|