Sanchez-Rubio et ai.: Occurrence of pelagic Sargassum in waters of the U S. Gulf of Mexico 
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Figure 1 
Occurrence of pelagic Sargassum macroalgae in (A) summer (May-July) and (B) fall (September-November) across the 
U.S. Gulf of Mexico during the coupling of phases of the Atlantic Multidecadal Oscillation (AMO) and North Atlantic 
Oscillation (NAO) from 1982 through 2012. Values within each square degree area (1 degree of latitude by 1 degree 
of longitude) represent the percentages of samples associated with species of Sargassum during the coupling of AMO 
warm and NAO negative phases (top values) and AMO cold and NAO positive phases (bottom values). 
42003: 1979-2011) GOM regions. The wind data were 
limited to those months in which more than 400 hours 
were recorded. The hourly data for each of the buoys 
were grouped according to the direction of the wind 
(northern wind: 337.6-22.5°, northeastern wind: 22.6- 
67.5°, eastern wind: 67.6-112.5°, southeastern wind: 
112.6-157.5°, southern wind: 157.6-202.5°, southwest¬ 
ern wind: 202.6-247.5°, western wind: 247.6-292.5°, 
and northwestern wind: 292.6-337.5°). These data 
were then divided by season (fall: September-Novem¬ 
ber, winter: December-February, spring: March-May, 
and summer: June-August). The seasonal average 
wind speed (meters per second) was calculated for each 
direction of the wind for each buoy. Using these data, 
we calculated seasonal wind stress (T) in newtons per 
square meter: 
T - p x CD x f/ 10 2 , (1) 
where p = the density of air at 1.225 kg/m 3 ; 
U w , - wind speed in meters per second at 10 m 
above the water surface; and 
CD - the drag coefficient (Hellerman, 1965). 
Smith (1980) proposed a formula to calculate CD: 
1000 CD = 0.44 + (0.063 x U 1Q ). (2) 
Seasonal data of wind stress were then transformed 
to wind momentum (newton-second [Ns] per square 
meter) by multiplying the wind stress by the average 
number of seconds per hour that the wind blows for 
each of the directions. The index of wind momentum 
represents the direction and flux of water driven by 
the seasonal winds in the 3 GOM regions. Monthly 
