Florida Keys Marine Weather Regions: Part I - The Gulf of Mexico

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march2014 Wind, waves, and weather change on all time scales:  annually; seasonally; monthly; weekly; daily; hourly; even minute by minute.  This is the nature of the mutable membrane between sky and sea within which we live, recreate, or ply a trade in the Florida Keys.  Moreover, the weather, wind, and sea often differ from one location to the next, depending on many factors, including small-scale variations in the atmosphere, water temperature, water depth, wind fetch over open water, and proximity to land, among others.  This article is the first in a series to explore the spatial variations in marine weather covering various “regions” around the Florida Keys, including the southeastern Gulf of Mexico, Florida Bay and the shallow Gulf waters inside five fathoms, Hawk Channel, and the Straits of Florida.  Typical wind, wave, and weather conditions and common marine weather hazards will be presented in the context of atmospheric, oceanographic, and geologic factors unique to each region. The first region to be explored is the southeastern Gulf of Mexico beyond five fathoms (one fathom is equal to six feet), including Dry Tortugas and Rebecca Shoal Channel.  This region is located at the extreme southeastern corner of the Gulf of Mexico basin atop the southern portion of the West Florida Continental Shelf.  Water depths average 30–100 feet, with the shallowest areas to the east, near the Florida Peninsula and Keys.  Progressively deeper waters are encountered toward the west, with the deepest waters just west and south of Dry Tortugas. The extreme southeastern Gulf of Mexico is interesting for several reasons.  First, its shallowness results in sea surface temperatures that are both higher in the summer and lower in the winter than deeper adjoining regions of the Gulf of Mexico and Straits of Florida (a shorter water column usually is heated and cooled more quickly than a deeper one).  Monthly average sea surface temperatures in the Dry Tortugas range from about 71°F in February to about 86°F during the July–September period.  However, sea surface temperatures during winter commonly fall into the 60s across a significant portion of the southeastern Gulf north of Dry Tortugas.  The cooler water and adjoining air mass near the surface of the water may lead to a decoupling of momentum in the lower atmosphere if the air mass being transported into the region is warmer than the one in residence.  Consequently, wind speeds over the cooler waters may be significantly lower and less gusty than those winds observed over adjacent warmer water bodies.  Also, cooler winter water temperatures lead to increased fog and low cloud formation in this region.  This happens when warm, moist air of tropical origin meets the chilly air in place over the winter-cooled shallow Gulf waters, thereby saturating the cooler air, with subsequent fog formation a common occurrence.  During some years, the mid-summer sea surface temperatures in this region may soar to near 90°F.  Unlike winter, summertime sea surface temperatures are warmer in the southeastern Gulf of Mexico than those observed in the nearby deeper waters of the open Gulf of Straits of Florida.  The added warmth is transferred to the lower atmosphere, resulting in a more thermodynamically “unstable” situation conducive to thunderstorm formation, especially at night. The sea state is another fascinating aspect of the southeastern Gulf of Mexico.  A large spectrum of wave types, heights, and periods seems to be characteristic of this region, depending on a particular arrangement of wind velocity, fetch, and duration for a given weather event.  Unfortunately, no wave-measuring buoys have ever been deployed to this region, so a comprehensive data record of wave heights is not possible.  However, numerical wave hindcast models have indicated that the “significant wave height” (average of highest one third of waves in a given sea state) has averaged two feet or less in nearly half of all hours during the last 30 years, while significant wave heights of three feet or less were probable 75% of the time.  Rarely (about 3–4% of the time), seas are likely to have grown to exceed 6.5 feet, with extreme values exceeding 15 feet associated with passages of hurricanes or severe winter storms.  The models suggest that peak significant wave heights may have exceeded 30 feet in this region for a storm like Hurricane Wilma in 2005. Of course, waves are generated by wind, and prevailing breezes blow from the northeast, east, or southeast across the southeastern Gulf of Mexico.  A limited “fetch” of wind (distance traveled by wind over open water) results from these directions thanks to the presence of the Florida Peninsula and Keys just upstream.  As a consequence, most sea states are characterized by young, wind-generated waves with wave periods of 4–6 seconds.  A wave “period” is the time between successive wave troughs or crests.  Occasionally, longer-period Gulf swells may move in from the northwest, especially during the winter in the wake of cold fronts with long trajectories of post-frontal northwest breezes.  In these cases, swell periods may exceed 10 seconds.  Also during these events, the incoming waves and swell may shoal as they travel from deep to shallow water, resulting in a “confused” sea state. Next month, we will explore the marine weather across Florida Bay and the Gulf waters inside five fathoms.  Until then, remember to be weather-ready, and stay safe!


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