The Interface Between Sky and Sea
The flora, fauna, terrain, and geography of the Florida Keys and adjoining coastal waters are associated with an extraordinary contrast of things animal, vegetable and mineral. This natural variation extends to the realms of sky and sea. The Florida Keys mariner, in particular, lives, works, and plays within the membrane which connects creatures and currents of the sea with the critters and forces of the sky above. He or she therefore is intimately familiar with the ever-changing, but primeval character of the weather and ocean. The ocean and air above each are in constant motion and exchange, as are its inhabitants and particulates. Think of a seaweed line as an example of a focal point at the air-sea interface, attracting birds, fish, and people. A coral reef ecosystem thrives at the healthy nexus of ocean and atmosphere. The forces of the moon and sun drive the daily rhythm of the tides. The Gulf Stream, a product of large-scale prevailing winds in the North Atlantic Gyre, constantly transports warm water, nutrients, marine life, and much more to these “islands in the stream” known as the Florida Keys. It should be no surprise, then, that the weather experienced each night and day in and around the Florida Keys is strongly influenced by the connections binding ocean and atmosphere. Take the wind, for example. Wind is air in motion relative to the surface of the earth. Prevailing winds are those most common for a given season or time of year. Such prevailing winds are forced by the large-scale pressure systems which dominate particular regions of the planet. However, the actual wind observed at a place in time may be highly influenced by terrain, the ocean, and other small-scale aspects of the local atmosphere, such as the presence of nearby rain showers. Wind records at Florida Reef lighthouse weather stations indicate November as the windiest month. One of the reasons for this is the significant difference in air temperature which arises between the increasingly cold November post-frontal breezes, and the warm sea surface over which these breezes blow. The first cool, dry air masses which penetrate into the subtropics during the late autumn replace the warm, moist, tropical air which has been resident for months. However, the portion of these air masses in contact with the ocean surface warms and moistens quickly. This leads to a more “unstable” stratification of air temperature, as air near the ocean warms relative to the cool air aloft. An unstable air mass will become increasingly turbulent, as horizontal air streams are diverted both upward and downward due to constantly rising warm air and sinking cool air. The result in this situation usually is an increase in both the surface wind speed and gustiness. Such a mechanism has been at work during many a November gale in the Keys. Success at marine weather forecasting in the Florida Keys depends greatly upon the successful diagnosis and prediction of the “marine boundary layer”, the portion of the lower atmosphere which borders, influences, and is influenced by the ocean. Two challenges are of special concern for the marine meteorologist when forecasting the marine boundary layer: 1) scarcity of weather data over the ocean, and 2) limitations of numerical weather prediction computer models. Weather data are in short supply over the ocean because buoys and instruments are expensive to purchase and maintain in a salt-water environment. Therefore, Florida Keys National Weather Service marine duty forecasters rely upon and appreciate greatly any and all reports of wind, weather, and sea from local captains, guides, and recreational boaters (find us at www.weather.gov/keywest). The second challenge involves limitations of the computer models which simulate atmospheric motions and thermodynamics. Most weather prediction models do not yet possess the capacity to explicitly predict many of the physical processes present at the interface between sky and sea because these processes are either too small, or too complex. Fortunately, your Florida Keys National Weather Service meteorologists have developed some skill and local knowledge over the years, allowing them to translate the mix of observational data, computer model output, and meteorological knowledge into meaningful forecasts of wind, waves, and weather. Weather forecasts are not perfect, but they are improving little by little each year. If you live, work, or play on the water in the fabulous Florida Keys, you can help your local NOAA meteorologists improve marine weather forecasts by assisting us in establishing the “sea truth” in the beautiful, yet mysterious interface between sky and sea. How? Tell us about the wind, waves, and weather at your location! Post on our Facebook wall (https://www.facebook.com/US.NationalWeatherService.KeyWest.gov). Tweet us @NWSKeyWest. E-mail us at sr-key.marine@noaa.gov. Or, call us at 305-295-1316 ext. 3. As always, be weather-ready, and stay safe!
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