Lawrence J. Rouse

Cold-water stress in Florida Bay and northern Bahamas; a product of winter cold-air outbreaks

ABSTRACT During January 1977 three consecutive cold fronts crossed south Florida and the northern Bahamas which depressed shallow-water temperatures below the lethal limit for most reef corals. Digital thermal infrared data acquired by the NOAA-5 meteorological satellite, in situ water temperatures, and meteorological data were used to study the thermal evolution of Florida Bay and Bahama Bank waters. The third and most important frontal system depressed Florida Bay water below 16° C, a thermal stress threshold for most reef corals, for 8 days. The minimum water temperature recorded in situ was 12.6° C. Satellite data suggest that some Florida Bay coastal waters were at least 1° C cooler than water at this site. Cold-water plumes (detected on satellite imagery) suggest that offshelf or offbank movement of cold, dense water follows bathymetry-controlled routes. Absence of viable shelf reefs opposite tidal passes supports this contention. Coral mortality at Dry Tortugas was up to 91 percent during the 1977 event. Coral and fish kills were also reported from other parts of the Florida Reef Tract and northern Bahamas. Study results show that cold-water stress conditions can exist over vast shallow-water areas and have residence times of several days. These observations suggest that aperiodic chilling processes have a limiting influence on reef community development throughout the Florida Reef Tract and northern Bahamas.

The Great Flood of summer 1993: Mississippi River discharge studied

In the summer of 1993, the Mississippi River basin in the midwestern United States experienced anomalously high rainfall. Record flooding resulted from an abnormally persistent atmospheric weather pattern consisting of a quasi-stationary jet stream positioned over the central part of the nation, where moist, unstable air flowing north from the Gulf of Mexico converged with unseasonably cool, dry air moving south from Canada. In concert with the persistent weather pattern over the United States, highly anomalous circulation patterns were observed over much of the Northern Hemisphere [Richards, 1994]. The rainfall anomalies over the central United States produced abnormally high river discharges along the Louisiana coastline from the Mississippi and Atchafalaya Rivers during July and August, traditionally months of low river discharge. Some of the river water discharged into the northern Gulf of Mexico reached the Straits of Florida by September 1993 [Lee et al., 1994].

Thermal history of reef-associated environments during a record cold-air outbreak event

SummarySeveral polar continental air masses intruding into the south Florida/northern Bahama Bank region during January 1981 caused record low air temperatures and rapid chilling of extensive shallow-water carbonate systems. Numerous “coral kills” along the Florida reef tract and massive fish mortalities in Florida Bay were attributable to unusually cold waters generated at this time. Thermal evolution of Florida Bay/Florida reef tract and northern Bahama Bank waters from 8 to 21 January was assessed from thermal infrared data acquired by the NOAA-6 environmental satellite, in situ water temperatures, local meteorological data, and a computerized heat flux model. Field observations and laboratory experiments identify 16°C as a thermal stress threshold for most reef corals (Mayor 1915; Davis 1981). Temperaturecorrected digital satellite data indicated that water temperatures below 16°C were generated in Florida Bay and on Little and Great Bahama Banks during a 10-day period in January. Lowest temperatures on the Florida reef tract resulted from offshelf transport of Florida Bay water through major tidal channels. Offshelf movement of bay water is driven primarily by strong northerly winds, density gradients, and tidal pumping. Absence of reef development opposite major tidal passes along the Florida reef tract (Ginsburg and Shinn 1964) and aperiodic coral kills along bank margins can be attributed to this process, which has probably had a limiting influence on Holocene reef development in these areas.

Evolution and structure of a coastal squirt off the Mississippi River delta: Northern Gulf of Mexico

In early October 1992, satellite-derived sea surface temperature data revealed a 200 km long and 10- to 30-km-wide stream of cool water flowing toward the southwest from the Mississippi River delta region. Satellite imagery and in situ measurements have enabled a detailed study of the squirts kinematics and subsurface characteristics over a 2-week period. In its early stages, the squirt appeared as a narrow, high-speed (>75 cm/s) jet of water which flowed westward over the Mississippi Canyon, forcing a semi-submersible drilling rig to suspend operations from October 2 to 4. After crossing back onto the shelf, the squirt spread laterally, yielding a mushroom-shaped feature, 75 km wide, which consisted of counter-rotating vortices. Northeasterly wind forcing (averaging 10–15 m/s) and water level setup east of the delta appear to have been the primary mechanisms for evolution of the high-velocity currents. Satellite and in situ measurements demonstrate that the dipole eddy was comprised of a cool, low-salinity, low-density water mass at least 26 m deep in the center and 16 m deep along its margins. This event demonstrates that strong northeasterly winds over the northern Gulf of Mexico can initiate along-shelf and off-shelf flows of cooler coastal waters, contributing significantly to seasonal cooling and freshening of the continental shelf and to shelf/slope exchanges of water. During this event, approximately 100 km3 of inner shelf and river water was transported off the continental shelf, a volume equivalent to 17% of the average annual discharge of the Mississippi and Atchafalaya Rivers.

Satellite observation of the subaerial growth of the Atchafalaya Delta, Louisiana

Since the early 1970s a new subaerial delta has been forming along the central Louisiana coast, an area traditionally characterized by coastal retreat. This delta of the Lower Atchafalaya River has been monitored by using data from the Landsat satellite. With these data, the formation and year-to-year growth of a major new deltaic lobe of this Mississippi River distributary have been measured for the first time. The analyses show that the formation of new land coincides with flood pulses of the river and that, as would be expected, intertidal lands below mean sea level accrete at a faster rate than lands above mean sea level. Measurements indicate that by early 1976 approximately 32.5 km 2 of new land had been formed at an average growth rate of 6.5 km 2 /yr. Delta growth initiated bay filling and shoreline progradation; the result was creation of extensive new marshlands along the south-central portion of the Louisiana coast.

Warm‐core eddy discovered in Gulf of Mexico

Thermal infrared satellite data of June 6 and 8 (Figure 1) reveal that a new large warm-core anticyclonic eddy has recently separated from the Loop Current within the Gulf of Mexico. On June 8, the eddys dimensions were 546×300 km, with a center at 26° 20.9′ N latitude, 88° 05.8′W longitude. The eddy was justifiably named “Eddy Whopper.” The distance between the eddy and the Loop Current increased by about 70 km during the 56 hours that elapsed between the images shown. Separation of the eddy was preceded by southwestward advection of cool Florida shelf water observed between May 31 and June 6 (Figure la).

Satellite observations of the circulation east of the Mississippi delta: cold-air outbreak conditions

Abstract Examination of 12 years of Landsat multispectral scanner images shows a recurrent pattern of westward flow immediately south of the Mississippi-Alabama barrier islands under northerly winds. Such flow patterns are also seen under similar conditions in imagery from the Advanced Very High Resolution Radiometer (AVHRR) of the NOAA-series satellites. The flow enters Chandeleur Sound between Ship Island and the northern end of the Chandeleur Islands. It appears to be driven by northerly winds, which force water south through the ChandeleurBreton Sound, drawing water in from the shelf region south of the Mississippi-Alabama barrier islands. These observations on circulation can be simply explained assuming linear dynamics. These two operational satellite systems are accumulating valuable records of coastal circulation patterns under clear-sky conditions.

Response of subtropical shallow-water environments to cold-air outbreak events: Satellite radiometry and heat flux modeling

Cold-air outbreak induced chilling of shallow bay, bank, and shelf waters of southern Florida and the northern Bahamas was examined using satellite thermal infra-red measurements, in situ measurements, and a shallow-water heat flux model. Vast expanses of shallow waters are rapidly modified by the cold, dry continental air and high wind speeds characteristic of cold-front passages. Although water mass modifications are more rapid in shallow areas, prolonged effects are experienced in deeper shelf regions. Northerly winds accompanying the cold-air outbreak induce a net offshelf circulation, subjecting deeper regions to an inflow of chilled waters generated in shallower areas. Absence of coral reef development along preferential routes for offshelf water movement suggests that these winter processes are a limiting influence to southern Florida and northern Bahamas reef distribution.

Remote sensing of coastal environmental hazards

Remote sensing is playing an increasingly important role in management and development of coastal environments through the detection and monitoring of coastal processes. Coastal geomorphic changes, estuarine circulation, pollution and sediment transport, flood area extent, fires, and foliage die-back are readily detectable from aircraft and spaceborne sensors. These data have their greatest value when they are available in near real-time to decision makers such as civil officials (crisis response), corporate officials, and coastal/marine environment operators. Examples of flooding, fires, storm-driven and man-induced damage to coastal environments graphically illustrate the dynamics of these environments. This paper illustrates how a low-cost NOAA high resolution picture transmission (HRPT) earth station provides technological leverage to deal with some of them.