By Paul Roat
Winter cold fronts often bring a surprising eruption of land from the watery depths of Sarasota Bay.
A confluence of wind, air pressure, lunar cycles, and other factors can drain the bay this time of year. Water gives way to acres and acres of seagrasses and mud flats.
What’s up with that?
Dr. Ernest Estevez is the keeper of all lore nautical in our part of the world. For more than three decades he practiced his craft at Mote Marine Laboratory, publishing scores of technical papers on riverine, estuarine, and all other coastal ecosystems. He is truly a font of marine knowledge.
Now retired, he offered some thoughts years ago about Sarasota tides in the following essay.
“I moved here from New England where you could set your clock by the tides, but down here the tides are different every day. And the bay is a whole lot shallower than it was this summer! What’s going on?” … is a question I’ve come to expect this time of year. The answer is not complicated, but it has several parts.
For starters, the Florida west coast experiences “mixed” tides, or a combination of diurnal (one high and low tide per lunar day) and semi-diurnal (two high and two low tides) patterns. Each by itself is regular, but when both patterns occur together we can have days with only one or, rarely, even five tides. Two to four are the most common, but even these will have unequal heights, causing us to speak of higher and lower high tides or higher and lower low tides.
Mean tide is the average of all high and low tides. To make boating as safe as possible, all navigation charts show depths relative to mean lower low tide. But there is more at work than that. The average level of the near-shore Gulf changes from month to month, being lowest around February and highest in late September to mid-October. The main reasons for this pattern are thermal expansion of water on the west-Florida Shelf (warm water occupies a larger volume) and runoff of river flow in summer months. The barnacle or oyster that settles on a seawall near high tide in summer will die from exposure to air when winter comes, just because of these differences.
Seasonal winds accentuate matters, too. Winds from the north and especially east tend to drive water out of rivers and bays along our coast, whereas west or south winds push water inland. Our prevailing winter winds are of the first kind, especially after cold fronts, causing the already-lowered tides to reach even lower levels. Folks are hard-put to float their boats in the middle of a winter day following a cold front, but it’s a great time to walk out on mudflats and sand bars looking for clams!
The savvy Floridian or visitor knows some other main points about coastal water levels. For example, the tide range is not the same everywhere along the Gulf coast. The difference between highest and lowest tides varies from 1 or 2 feet to 4 or almost 5 feet, and there is no regular north-south trend from the Panhandle to the Florida Keys. A main reason for tidal changes geographically is the distance across the continental shelf each oceanic tidal wave must travel every day; where the shelf is very wide, for example off the Ten Thousand Islands or Big Bend coastline where the tide range is greater, versus where it is relatively narrow, off the Panhandle where the tide range is small.
Also, the overall level of the sea is gradually rising as a remnant of global warming since the last ice age. For west-central Florida, the rate of sea-level rise has been about 9 inches per century, a seemingly slow rate but one with enormous ecological importance in so flat a state as ours. The rate of sea-level rise is expected to increase as a result of climate change.
The rarest and most extreme sea level is probably the one we most anticipate —hurricane storm surge. In a stairwell at Mote Marine Laboratory, I once marked the wall to show where computer models predict the water will reach with hurricanes of increasing intensity. A Category 5 surge would put 3 feet of water into the second floor of our barrier-island facility, and inundate all low-lying mainland areas from Tampa Bay to Charlotte Harbor.