Higher in the Gulf Stream

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The Gulf Stream influences Florida Keys marine weather in a variety of ways, and therefore is of great interest to local weather forecasters. The Gulf Stream also interests oceanographers, anglers, marine biologists, fishing guides, dive boat captains, and the masters of the numerous freighters and tankers that routinely ply the Straits of Florida. For the National Weather Service meteorologist, the Gulf Stream provides a myriad of ways in which to complicate a simple marine forecast, if there were such a thing. The most obvious effects are related to the jacking up of wave heights when wind opposes current. However, the Gulf Stream also influences patterns of both air and water temperature around the Keys, wind velocity, humidity, the development of clouds and local showers, and on occasion, the intensity of nearby tropical storms or hurricanes. The proximity of the Gulf Stream to the Florida Keys is an accident of geology and oceanography. The swift current which meanders through the Straits of Florida is actually the southern leg of the “Florida Current”, a single albeit large component of the “Gulf Stream System”. This “system” is part of a class of large-scale ocean currents which transport warm water from low to high latitudes in deep, narrow ribbons of fast-moving sea. You will find similar ocean current systems around the globe, including those featuring the Kuroshio Current (off Japan), the East Australian Current, the Brazil Current, and the Agulhas Current (off South Africa). All of these currents are located in the western portions of the world’s great ocean basins because here the rotation of the earth causes an accumulation of energy. This energy must subsequently be dissipated, and the result is the highly unstable nature of these currents. They are constantly moving, shifting, changing, and shedding eddies. The Gulf Stream off the Florida Keys is most accurately identified by the appearance of a swift, steady current setting west to east, or southwest to northeast. Current speed averages 2–3 knots, and occasionally exceeds 4 knots. During the cool season, the Gulf Stream can be identified by a sharp horizontal gradient in sea surface temperature. The shallower waters nearshore are much more at the mercy of cold-air incursions owing to their reduced water volume and heat capacity. Gulf of Mexico and Florida Bay sea surface temperatures dip below 65° at least once or twice during a typical cool season, and more frequently in some years. On occasion, the shallow waters turn downright cold, as in January of 2010 when sea surface temperatures in Florida Bay were in the 40s! However, the waters in the core of the Gulf Stream rarely drop much below 80°, regardless of the year. So, as the nearshore waters cool, a sharp temperature gradient develops near the edge of the Gulf Stream. This gradient can be seen clearly in pictures obtained from special instruments aboard weather satellites which measure sea surface temperature patterns from space! Of course, the Gulf Stream or an associated eddy often can be identified visually at sea by anglers and other mariners. This is probably due to the fact that Gulf Stream water typically is clearer than shallower waters nearshore which are infused with muddier freshwater from rivers and runoff. The nearshore waters also tend to contain higher concentrations of nitrogen and algae. In addition, the Gulf Stream flows through deep water which scatters light differently than shallower water where the varying bottom types are visible (think “brown, brown run aground”). In fact, Gulf Stream water tends to have a distinct blue color; hence, the popularity of the term “blue water”. Ocean waves generated by strong northeast breezes will grow higher and steeper in the Gulf Stream because the current, running toward the east or northeast, lies in opposition to the wind-generated waves. The effect is a magnification of the wind force combined with a shortening of wave period which creates taller and steeper waves. Calculations derived from basic wave physics indicate that wind-current interactions in the Gulf Stream may account for seas up to 40% higher than those which would be observed in the absence of current. Florida Keys National Weather Service marine forecasters now are experimenting with ocean wave models which explicitly account for the effects of the Gulf Stream. For years, the simple, monotonous phrase, “…higher in the Gulf Stream” has been used in marine forecasts as an acknowledgment of sorts that certain wind velocities would lead to higher Gulf Stream waves. However, forecasters now are producing graphics of wave heights in the Gulf Stream routinely, and making them available to the marine public (see http://www.srh.noaa.gov/key/?n=marine). A future article will explore Gulf Stream eddies and meanders. Until then, remember to be weather-ready, and stay safe!