RIDING THE TIDE
If there weren’t any weather and the land were perfectly stable, tides would be a pretty dry subject, with the timing and height totally predictable by a series of humongous equations. No one’s house would get washed away because we would know exactly how high the sea would rise each month and that it would rise no further.
Fortunately for climatologists who need work, weather has a profound influence on tidal levels, and so does the relative position of land. Weed like to concentrate on the former and only touch upon the other, more geological, process in passing.
Day-to-day weather, in the form of wind and barometric pressure, moves the relative sea level, upon which the day’s high and low tides are superimposed. Both high and low pressure systems are important.
Climate, Weather and Tides
The overall circulation of the atmosphere creates several transient phenomena that change tidal amplitude. These include tropical storms and hurricanes, mid-latitude cyclones (“northeasters”), and high pressure systems (“anticyclones”).
Day-to-day undulations in the jet stream create high and low pressure systems. The westerly jet stream, familiar to all as the generator of good and bad weather, is a rapidly-moving band of wind that separates tropical warmth from polar cold.
This stream is not stable, moving north and south with the seasons, and containing internal ripples (“troughs” and “ridges” in Weather Channel parlance) that propagate through it. The average altitude of the jet stream is around 30,000 feet, but it leaves traces down to 10,000 feet on most days.
These disturbances are generated by a number of processes, including the fact that the seasonal north-and-south movements of the jet take place over a highly varied earth which contains vast thermal differences between land and ocean. Further, the land surface is hardly uniform, being interrupted by mountains that can be as high as the lower bounds of the jet.
Along with other causes, these induce wave-like disturbances in the “normal” west-to-east flow. When the lower part of the wave moves through, there’s a trough aloft, and when the crest moves through, there’s a ridge.
Air piles up near and underneath jet stream ridges, so high pressure systems (and attendant fair weather) tend to form near the surface; the opposite is true for troughs, which give rise to low pressure and storminess.
Anticyclonic and Cyclonic Tidal Elevations
Our illustration shows this process schematically, with a high and a low placed in a typical position that results when a wave in the jet moves through eastern North America. The result is often two mechanisms that can elevate tides in our area.
Air flows clockwise around high pressure, diverging away from the barometric maximum. In doing so, a substantial “fetch,” or length of onshore flow can be generated. More and more persistent easterly and northeasterly winds pile increasing amounts of water near the shore, and each tide tends to be higher and higher.
Surfriders love these anticyclones because they usually combine good weather and big waves, but beachgoers with small kids—who tend to get swallowed up—aren’t as pleased. People who visit beachfront homes think they’re terrific, as the sun usually shines clearly over an ocean that is much closer than average.
People who own beachfront homes are usually less pleased, as the repeated tidal elevations can become severe enough to the erode the thin dune lines that keep their property from combining with the sea. Folks who own the high-rent properties where the brochure says “the ocean couldn’t be closer” get very antsy. People who own properties where the dune has already eroded (the brochure says: “No dune to cross for beach access!”) might pack up and leave.
Transient low pressure systems associated with jet stream troughs create larger tidal disturbances, even though they may be of shorter duration than those associated with high pressure. That’s because the change in pressure, or gradient, surrounding a low is usually greater than the one that surrounds a high. As a result, the wind flows faster, even fast enough to overcome a relatively small fetch compared to what accompanies a high pressure system.
In combination, the placement of a strong high pressure system to the north, along with a low pressure system to the immediate south, off the North Carolina coast, creates the strongest net movement of wind and water. It was just such an arrangement, which was in place for several days in March, 1962, that created the infamous “Ash Wednesday” inundation in Tidewater Virginia described below.
Another type of atmospheric circulation which elevates the tide is the tropical cyclone, the generic term for tropical depressions and storms and hurricanes. Unlike temperate storms, which result from disturbances in the westerly jet stream, tropical cyclones are usually disturbances in the easterly trade winds.
The trade winds themselves are ultimately generated by the great heat engine that rings the earth’s low latitudes, known as the Intertropical Convergence Zone (ITCZ). This is a region of upward motion, and relatively low barometric pressure, that tends to define the earth’s thermal equator. It is often evident on satellite images as an extensive band of thunderstorms.
Thunderstorms are merely columns of vertical motion in which moisture has condensed. The relatively constant upward movement through the ITCZ means that the air, once aloft, has to go somewhere. If it fell back in on itself there would be no ITCZ because the upward motion would stop. Instead, it diverges away from the ITCZ and descends, creating the giant subtropical high pressure systems that tend to center around latitudes 25-30. The low-level air returning from these highs to the low pressure of the ITCZ comprises the trade winds.
These are obviously very moisture-laden, as is evident from the tremendous thunderstorms that blow up in the ITCZ. Sometimes a zone of low pressure develops within the trades and finds a high-altitude environment that evacuates the air (so it doesn’t fall in on itself). This new area of thunderstorms then feeds on itself—more upward motion sucking more moist air inwards, to the point that it becomes warm with respect to its surroundings.
It’s a physical fact that heat is released to the environment whenever matter (in this case water) goes from a less ordered (water vapor) to a more ordered (rain) state. Circular low pressure systems that form in this fashion are called tropical cyclones, and their most flagrant expression is the hurricane.
As any Weather Channel addict knows (and we suspect that few avid Advisory readers can resist that habit), tropical cyclones come in a variety of sizes and flavors—from weak tropical depressions to Saffir-Simpson Category 5 hurricanes sporting maximum sustained winds over 155 mph and storm tides in excess of 18 feet above normal level.
During this century, the biggest hurricane storm surge to hit Virginia was on August 23, 1933. The tidal elevation at Sewell’s Point (see Sidebar) was 7.01 ft above mean sea level. (The more commonly cited figure is 9.8 ft., but this is above mean low tide, and the reference standard has recently been changed, as noted in the sidebar).
THE BIG ONES
As noted in our sidebar, the most accessible long-term tide record is from Sewell’s Point, extending back to 1928. The largest tidal elevation noted is, indeed, the 1933 hurricane.
The descriptor, “Hurricane” is a little problematical here. Official records from the National Hurricane Center designated the once-powerful (Category 3) system as only a strong tropical storm by the time its strongest winds were affecting Tidewater Virginia. However, the Weather Bureau publication, Climatological Data, Virginia Section, for August 1933, reports a peak wind gust of 88mph.
When it hit Virginia, the storm was in the process of decay and some of the winds in the outer feeder bands, which would move into Tidewater from the unimpeded ocean side, maintained some high gusts. Recent studies of hurricanes have noted that the strongest winds in storms that approach southeastern Virginia can be displaced far away from the center, if the center has been on land for several hours. The 1933 storm made landfall near Duck, North Carolina about three hours before the center moved slightly to the west of Norfolk.
Although we prefer to deal with elevation above mean sea level, the available numbers for the 1933 hurricane away from the tide gauge are given above mean low water, which was 9.8 ft. at Sewell’s Point. A number of factors conspired to raise sea level in the Chesapeake Bay and especially the Tidal Potomac even higher than at Sewell’s Point. We, therefore, see figures of 10.6 feet along the Potomac at Ft. Belvoir, 10.9 feet at Alexandria and a remarkable 12.2 feet at Anacostia in the southeastern District of Columbia.
The track, shown in our figure, was perfect for dragging water up the Potomac. The center was only a few miles west of the estuary and the resultant easterly winds sent a large slug of water up the Bay and into the River.
The water funnelled up the river, creating a tidal bore that can increase the elevation of the water beyond what it is in the open ocean. Strong southeasterly winds also impeded the normal flow of water from the river into the Bay, causing more water to remain upriver.
OTHER MAJOR TIDAL ELEVATIONS
Despite the (largely geological) rise in sea level, there is little evidence that major inundations are becoming more frequent in the Virginia tidewater. Our table shows all of them greater than 4.75 feet above mean sea level at Sewell’s point.
Ash Wednesday, 1962
The “Ash Wednesday,” 1962, storm created the second-highest tidal elevation, and the largest in the modern record for a Northeaster. More precisely, the 1962 surge was caused by a combination of a strong and virtually stationary low—whose center oscillated for days about 200 miles off the Virginia/North Carolina border—and an unusually large high pressure system immediately to the North.
Besides about ten inches of sea level, the major difference between Ash Wednesday and the 1933 hurricane was that the former was severe for three days or so, resulting in at least six inundating tidal cycles. As shown vividly by this fall’s category 1 hurricane/tropical storm Dennis over North Carolina’s Outer Banks, multiple flooding over the same beach eventually results in substantial damage, even from a relatively weak storm, as dunes ultimately get breached and inlets try to form.
The rest of the big elevations shown in our table are more “routine” disasters in which there is substantial beach erosion and overwash along Atlantic Avenue in Virginia Beach. Sandbridge, a bit to the south, usually sees enough sand movement to send in the road graders. Note that the big elevations in our table are equally distributed between hurricanes and northeasters, but also that the biggest hurricanes we can find (the 1933 storm, and 1960 Hurricane Donna) are “only” Category 2 storms.
Because it only goes back 1928, our Sewell’s Point doesn’t see some of the big hurricanes of the previous three centuries, which were detailed these Advisories in 1980 and 1988. We believe that the massive inundation of August, 1667, which appears to have produced a storm surge between 12 and 15 feet, may have been a Category 4 hurricane in or near Virginia, which would put it very close to theoretical maximum for tropical cyclones in the Mid-Atlantic region.