Bay St. Louis Hurricane

Southern United States, July 27–28, 1819 

Touted as one of the most destructive tropical cyclones to have affected the fledgling United States during the first half of the 19th century, the Bay St. Louis Hurricane unleashed pulverizing winds and a deadly five to six foot storm surge on the coasts of Alabama, Louisiana, and Mississippi between July 27 and 28, 1819. In Mississippi, where the tightly coiled eye of the 24-hour hurricane made landfall just before midnight on September 28, nearly every house, warehouse, and wharf along the banks of Bay St. Louis was destroyed.

Eyewitness accounts state that the entire coastline from Pass Christian, Mississippi, to Mobile, Alabama, was littered with the remains of shattered buildings, uprooted fences, and snapped trees. Dozens of human bodies, along with the carcasses of several hundred head of cattle, festered on the beaches, in the swamped bayous, in the piles of wreckage that in some places formed an almost impenetrable wall. In Alabama, where the hurricane’s broad surge was funneled inland across Mobile Bay’s narrowing shores, deadly alligators, snakes, and snapping turtles were washed into the city’s clogged streets, causing a number of gruesome fatalities. Several vessels—from small boats to 60-ton brigs—were driven ashore by the enormous six to ten foot (2–3 m) surge, one of them coming to rest with its bowsprit piercing the side of a quayside warehouse on Dauphin Street. At Cat Island, located 11 miles (18 km) to the southeast of Bay St. Louis, 39 sailors from United States sloop-of-war Firebrand were killed when the burgeoning storm first capsized their swift, 12-gun craft and then deposited it, overturned, on the shore. Farther inland, in the midst of the pine forests that blanketed Mississippi’s midlands, the Bay St. Louis Hurricane overtook a contingent of U.S. soldiers who had encamped for the night in a shallow valley. 

First surprised, then terrified by the sudden tempest of wind and flooding rains that fell upon them during the evening hours of July 27, the soldiers fled to higher ground, into woods where trees were falling like nine-pins. At least one man was killed and twenty others seriously wounded as the hurricane shredded tents, scattered provisions, and showered the stricken party with arrowlike splinters. In Louisiana, where the hurricane’s track first took it ashore just west of the lower Mississippi Delta, minimal but sustained winds buffeted New Orleans, running three ships aground at the mouth of the river and flooding many farms along the rims of lakes Borgne and Pontchartrain. Before dissipating over the cooling reaches of the continental United States on July 29, the Bay St. Louis Hurricane had claimed between 100 and 175 lives and caused untold thousands of dollars in property damage to the Mississippi Sound’s budding trade and industry.

Bay of Bengal

Roughly situated between the equator and the tropic of Cancer (23.5 degrees North), the expansive, V-shaped Bay of Bengal stands as the northeasternmost extension of the Indian Ocean. Bordered to the southwest by the island of Sri Lanka, to the west and northwest by India, to the north by Bangladesh, and to the north-northeast by Myanmar (Burma), the Bay of Bengal’s position within the intertropical convergence zone (ITCZ) has long made it one of the most active spots on Earth for cyclone generation. On average, eight percent of the world’s tropical cyclones originate over the warm waters of the bay in any given year. Between 1737 and 2006, some 742 recorded cyclones were formed; this represents an average of three storms annually. While cyclones have occurred in the Bay of Bengal during every month of the year, statistics indicate that peak periods for cyclone generation come in July and September, or during those periods when the Asiatic monsoon begins its advance and retreat. Furthermore, a number of these storms have been of significant intensity and duration, posing a grave hazard both to the shipping that regularly plies the bay and to those nations—in particular India and Bangladesh—that ring it.

Nearly three centuries of comprehensive study into the character of the Bay of Bengal’s frequent cyclones have increasingly revealed a cyclical connection between such storms and the ever-changing seasons of the monsoon. Prompted by the sharp temperature and pressure differences between the landmass of the Indian subcontinent and the large bodies of water that surround it, the early summer arrival of the monsoon is characterized by both an abrupt reversal in the direction of the prevailing winds and the rapid formation of tropical cyclones. For this reason, cyclone generation in the Bay of Bengal tends to be most common during the change of seasons, or when the prevailing winds of May and June first shift to the north and flow up from the Indian Ocean and into the hot, humid regions of Bengal and northwest India.

Similarly, the retreat of the monsoon, which generally occurs between September and November, brings with it another peak in cyclone origination, one noted for its predilection toward violent cyclone strikes. Indeed, some of the most destructive cyclones on record have come ashore from the Bay of Bengal during September, October, and November, including the False Point Cyclone of September 21, 1885, the Hooghly River Cyclone of October 7, 1737, the Great Calcutta Cyclone of October 5, 1864, the Backergunge Cyclone of October 31, 1876, and the Great Cyclone of 1970.

Although death-toll estimates from these storms vary from source to source, their cumulative losses can be accurately measured in terms of hundreds of thousands of people either left dead or missing. A majority of early season Bay of Bengal cyclones tend to originate in the northern half of the bay during the months of June and July. These storms, which are usually of smaller size and intensity than those late-season cyclones that develop south of the 16th parallel, almost always move in a northwesterly direction, a course that takes them ashore on India’s northeast coast or in the vicinity of the major port cities of Calcutta and Visakhapatnam.

Although early season Bay of Bengal cyclones do not as a rule prove as destructive as their late-season cousins, they do remain better organized for a longer period of time, bringing huge precipitation counts and gusty winds to the inland regions of the Ganges River Valley. On the other hand, late-season cyclones in the Bay of Bengal, those storms that form between September and November, are generally of greater size and intensity at landfall because of the long distances over which they must travel. 

Such cyclones, which originate in either the south or southeast quadrants of the bay, are steadily carried to the north or northwest by the vast influx of moist air that sweeps up from the west-southwest as part of the retreating summer monsoon. Powered by both their energy-laden steering currents and the warm 86°F (30°C) waters over which they pass, these Bay of Bengal cyclones often grow into storms of enormous destructive potential. With central barometric pressures below 27.91 inches (945 mb), winds in excess of 155 MPH (250 km/h), and record-breaking storm surges, these cyclones frequently slam into India’s southeast beaches or inundate the low-lying south coast of Bangladesh, with catastrophic results for the indigenous populations who make their lives along the shores of the Bay of Bengal.

Tropical Storm Barry

Southern United States, June 1–2, 2007 

As though inspired by its 2001 predecessor, the fourth North Atlantic tropical system dubbed Barry originated in the eastern Gulf of Mexico (near latitude 24 degrees North and longitude 85 degrees west) on June 1, 2007. A poorly organized early season system with maximum sustained winds of 52 MPH (83 km/h), Barry tracked to the northeast and made landfall near Florida’s Tampa–St. Petersburg during the daylight hours of June 2. Its minimum central barometric pressure of 29.44 inches (997 mb) at landfall produced precipitation counts in excess of six inches (152 mm) across much of northern Florida. After being downgraded to a tropical depression immediately upon landfall,

Barry’s low-pressure center moved across northern Florida and trailed northward along the eastern U.S. seaboard, dropping tremendous quantities of moisture in South and North Carolina, Virginia, and Pennsylvania. In New York City and across southern New England, a now-extratropical Barry delivered gale-force gusts and some three to four inches (76–102 mm) of precipitation, causing some localized flooding. Although more than seven inches (179 mm) of rainfall was recorded in Cuba, and another eight inches (203 mm) over Georgia, no direct deaths were reported in Barry’s wake. Indeed, Barry’s early season rains fortuitously broke a drought condition over northern Florida and helped firefighters battle an extensive wildfire in central Georgia. The name Barry has been retained on the North Atlantic lists and is scheduled to reappear during the 2013 hurricane season.

Hurricane Barry

Southern United States–Gulf of Mexico–Mexico, August 23–29, 1983 

Between August 23 and 29, 1983, Hurricane Barry tracked nearly due westward from its origination point off the eastern coast of the Bahamas, striking the central eastern coast of Florida as a tropical storm, then passing into the Gulf of Mexico, where it was downgraded to a tropical depression. As it moved westward across the warming waters of the Gulf, Tropical Depression Barry slowly reintensified, becoming a tropical storm again on August 28, and a hurricane with sustained winds of 81 MPH (130 km/h) just prior to making landfall on the Mexico-Texas border. A strengthening tropical system at landfall, Hurricane Barry produced a central pressure reading of 29.11 inches (986 mb) making it a relatively powerful Category 1 system. Tracking due west into northern Mexico, Barry dissipated by August 30.

Wind-Driven Circulation

Wind-driven circulation (WDC) refers to ocean currents initiated and propelled by winds blowing across the surface of the ocean. Wind-driven circulation is part of the complex system of energy and heat redistribution that helps to draw tropical heat away from equatorial regions to moderate the Earth’s climate.

• Wind Generation and Oceanic Circulation

Wind is caused by the uneven solar heating of the Earth’s surface, which creates areas of high and low atmospheric pressure. Pressure differentials cause air to circulate from areas of high to low pressure. Solar heating is greatest near the equator, causing a region of low pressure. Rising air moves northward as cooler air from high pressure, mid-latitude regions moves toward the equator. The Coriolis effect, the rotational action of the Earth spinning on its access, deflects the winds toward the east, north of the equator, and to the west, south of the equator. This explains why prevailing winds in the mid-latitude Northern Hemisphere come from the west.

An understanding of wind generation helps to explain the operation of ocean currents. There are two general types of ocean circulation and both result from solar heating: WDC and thermohaline circulation (THC). Differential solar heating generates wind. Wind blowing across the surface of the ocean causes the water to move. The direction of the ocean current is determined in part by the direction of the prevailing winds at a given latitude. WDC is responsible for currents near the ocean’s surface.

THC drives deep-water ocean currents and vertical movement of water. THCs occur as water density increases, either through cooling as water moves toward the poles, or because of increased salinity. In both cases, the THC is responsible for carrying warmer surface water to the ocean depths. There is an interplay, then, between the WDCs and THCs that is exemplified in the Gulf Stream that moves warm water from the Caribbean in a northeasterly direction, along the eastern seaboard of North America, toward northern Europe. As WDCs carry water away from the tropics, the water cools and becomes more saline. It consequently becomes more dense, which causes it to sink along a THC. 

This mechanism, whereby warm surface water is transported toward the poles, descends as it becomes cooler, to be re-circulated southward as cool deepwater currents, is sometimes referred to as

the North Atlantic conveyor. Interplay of WDCs and THCs The Gulf Stream starts as a WDC because it gets its initial energy from the trade winds. Global warming could affect the Gulf Stream in two ways. The first, and more widely reported mechanism occurs because global warming could melt Arctic ice, decreasing sea salinity. This will in turn hinder the ability of the water to descend as part of the THC.

The end result is the cooling of northern latitudes. Global warming could also affect the initial forcing of the Gulf Stream. Some researchers suggest that global warming will disrupt and weaken the trade winds. Diminished trade winds means less energy driving the Gulf Stream northward, also contributing to a cooler climate at high latitudes. This could cause dramatic cooling in northern Europe, which needs the North Atlantic Conveyor to moderate its climate. The paradox is that global warming could ultimately cause regional cooling in some parts of the world by affecting both the THCs and WDCs.

Western Boundary Currents

Western boundary currents are intense jet currents at the western periphery of large-scale oceanic gyres in the World Ocean. As was shown in the pioneer paper of Henry Stommel in 1948, they are the result of two following causes:

1. The β-effect, a term that has arisen from the traditional representation of the Coriolis force in the following formula: f = f0 + βy, where f0 is a Coriolis parameter at a definite latitude; in other words, the β-effect is due to spherical form of the Earth turning on its axis.

2. Low conservation of absolute vortex for oceanic motions.

Oceanic gyres are forced by horizontally inhomogeneous, large-scale wind fields (or wind vorticity). For instance, in the North Atlantic Ocean, the anticyclonic subtropical gyre is situated under the northeastern trade wind and midlatitude westerly wind as a result of clockwise wind vorticity, whereas the north tropical cyclonic gyre is a result of anticlockwise wind vorticity between the Intertropical Convergence Zone and the northeastern trade wind. Currents in the western part of each gyre are more intense than in the eastern part because they are dictated by the law of conservation of absolute (relative plus planetary) vortex.

Each particle moving northward (southward) gets an additional (loses) planetary vorticity as a result of spherical form of the rotating Earth. In the clockwise gyre, this should be compensated by the increase of relative negative vorticity; that is, by the intensification of clockwise rotatation. In the counterclockwise gyre, this should be compensated by the increase of relative positive vorticity; that is, by the intensification of counterclockwise rotatation. In both cases, this leads to intensification of currents in the western periphery of the basin. In the eastern part of a gyre, all particles move in the opposite direction in comparison with the western part. It leads to the weakening of circulation in the eastern gyre’s end.

The β-effect may be also understood in terms of Rossby waves. Long, nondispersive Rossby waves carry (kinetic) energy from the east to the west within each gyre. After their reflection from the western boundary of the basin, the short, dispersive Rossby waves are generated and move to the east. However, the short Rossby waves are dissipated in the relatively narrow vicinity of the near coastal zone just as a result of their shortness and dispersive properties, which leads to more affective realization of dissipative processes. Thus, the kinetic energy of the planetary Rossby waves is accumulated in the vicinity of the western periphery of the gyres.

In fact, the western boundary currents (especially in the Atlantic Ocean) are also controlled by thermohaline factors. The β-effect impacts the thermohaline circulation and causes the intensification of the thermohaline currents in the western part of the basin. Deep thermohaline currents in the North Atlantic Ocean (generating in the region of the sinking of deep Atlantic Ocean water and spreading at depths between 1.5 and 2.5 mi., or 2.5 and 4 km) are southward, while compensative thermohaline currents in the upper baroclinic layer (between the surface and 0.6 to 1.2 mi., or 1 to 2 km) are northward. As a result of superposition of the meridional thermohaline circulation, the wind-driven, northward western boundary currents in the clockwise gyres of the North Atlantic Ocean intensify, while southward currents in the counterclockwise gyres weaken.

The most intense western boundary currents in the Northern Hemisphere are the Gulf Stream, Labrador current, North Brazilian current (Atlantic Ocean), Kuroshio current (Pacific Ocean), and Somali current (Indian Ocean). The velocity in these currents’ axes reaches or even exceeds 6.5 ft. (2 m) per second. Detailed analysis of the structure and origins of western boundary currents (such as the Gulf Stream) was conducted by Henry Stommel in 1958 and 1966.

Western boundary currents in the North Atlantic Ocean carry ~100 Sv (1 Sverdrup = 106 cu. m per second) of water in the upper baroclinic layer. The wind vorticity accounts for about 30 to 60 Sv (30-60 multiplied by 106 cu. m per second). The average power of the source of deep Atlantic Ocean water is about 20 Sv (20 multiplied by 106 cu. m per second). Therefore, the joint effect of wind vorticity and meridional thermohaline circulation can explain up to 80 percent of observed transport of the western boundary currents in the North Atlantic Ocean.

The remaining (at least) 20 percent of total transport is a result of the mesoscale eddies. In fact, the western boundary currents, which look like meandered jets, generate the intense mesoscale eddies, the “rings.” The typical horizontal size of rings is about 60 mi. (~100 km), and orbital velocity is 3.3 to 6.5 ft. (1 to 2 m) per second. Rings trap the water in their central part and carry it with a typical speed of about a few centimeters per second.

The lifetime of the rings may reach four years, after which time most of them are recirculated and feed the western boundary currents. Thus, the mesoscale eddies account for a significant portion of volume transport of the western boundary currents. Recirculation of the Gulf Stream is one of the integral manifestations of mesoscale effects.

Rossby Waves

Space Studies describe Rossby waves as “slowmoving waves in the ocean or atmosphere, driven from west to east by the force of Earth spinning.” These are naturally occurring phenomena first recognized in 1939 by Swedish American meteorologist Carl-Gustav Rossby. These waves, which are found in both the atmosphere and the oceans, are important mechanisms for the redistribution of energy around the globe.

Atmospheric and Oceanic Waves This phenomenon was first identified as atmospheric oscillations that occurred in the mid-latitudes in the Northern and Southern Hemispheres. In Europe and North America, people typically experience a Rossby wave as a large cold front plunging southward.

The jet stream, guised as a tongue of cold air, dips southward as a large tropical air mass moves northward. The interaction between these air masses, affected by the Coriolis affect that intensifies at lower latitudes, generates changing weather on a day-to-day and week-to-week basis. During televised weather reports, North American Rossby waves appear as large-scale oscillations of clouds moving from west to east across a continent.

Scientists later identified a similar phenomenon at work in the water of all ocean basins. Researchers discovered that oceanic Rossby waves represent a mechanism by which the ocean responds to significant atmospheric “forcing” or windrelated disruption. Rossby waves disperse the atmospheric energy across ocean basins and can be measured through satellite imagery. Because of the impact of the Earth’s axial rotation and the Coriolis effect, the oceanic Rossby waves tend to spiral away from the equator in both the Northern and Southern Hemispheres.

Scientists are increasingly interested in Rossby waves because of the possible connection between these atmospheric and oceanic waves and global warming. An understanding of atmospheric Rossby waves enabled researchers to effectively study long term temperature fluctuations and provide concrete evidence that global warming was occurring. Rossby waves can affect entire ocean basins.

They also tend to move from the eastern part of the Pacific and Atlantic oceans toward the west on either side of the equator. A complementary Kelvin wave moves in the opposite direction from west to east along the equator. The multiple axes along which the ocean moves has the ability to disrupt oceanic circulation. Researchers propose that global warming will generate stronger weather events with greater frequency. Because oceanic Rossby waves transmit atmospheric disruptions, the theory is that as storms occur more frequently, the wavelength and frequency of Rossby waves will also change—with a potentially disruptive impact on ocean currents such as the Gulf Stream.

A disrupted Gulf Stream could cause cooling at higher latitudes in the North Atlantic. Researchers also see a connection between changes in Rossby waves and the intensity of El Niño and La Niña ocean surface water temperature fluctuations in the Pacific Ocean. Also known as the El Niño-Southern Oscillation (ENSO), sea surface temperature increases during El Niño events, and decreases in La Niña events can alter or intensify the monsoonal rainfall and hurricane patterns in North and South America. The challenge for climatologists and oceanographers is to understand how disrupted Rossby waves are a cause and consequence of global warming.

The broader point is that global climate change is a complex process that involves the interplay between large scale atmospheric and oceanic processes operating at multiple scales.