Satellite thermal infrared data on the Gulf of Mexico show that a seasonal cycle exists in the horizontal surface temperature structure. In the fall, the surface temperatures of both coastal and deep waters are nearly uniform. With the onset of winter, atmospheric cold fronts, which are accompanied by dry, low temperature air and strong winds, draw heat from the sea. A band of cooler water forming on the inner shelf expands, until a thermal front develops seaward along the shelf break between the cold shelf waters and the warmer deep waters of the Gulf. Digital analysis of the satellite data was carried out in an interactive mode using a minicomputer and software. A time series of temperature profiles illustrates the temporal and spatial changes in the sea-surface temperature field

Huh, O. K.

Rouse, L. J., Jr.

Smith, G. W.

English

NASA Technical Reports Server

N78-146 06
SURFACE TEMPERATURES AND TEMPERATURE GRADIENT FEATURES
OF THE U.S. GULF COAST WATERS
Oscar K. Huh, Lawrence J. Rouse, Jr., and Glenn W. Smith
Coastal Studies Institute, Louisiana State University
Baton Rouge, Louisiana
ABSTRACT
Satellite thermal infrared data on the Gulf of Mexico
show that a seasonal cycle exists in the horizontal surface
temperature structure. In the fall, the surface tempera-
tures of both coastal and deep waters are nearly uniform.
With the onset of winter, atmospheric cold fronts, which are
accompanied by dry, low-temperature air and strong winds,
draw heat from the sea. Penetrative convection and wind-
driven mixing lower temperatures, first in the shallowest
waters and then, as the winter season progresses, in deeper
and deeper portions of the Gulf. A band of cooler water
forming on the inner shelf expands, until a thermal front
develops seaward along the shelf break between the cold
shelf waters and the warmer deep waters of the Gulf.
Digital analysis of the satellite data has been carried
out in an interactive mode using a minicomputer and software
developed at the Coastal Studies Institute. A time series
of temperature profiles illustrates the temporal and spatial
changes in the sea-surface temperature field.
1. INTRODUCTION
The most fundamental property of coastal and shelf waters routinely avail-
able from remote-sensing systems is temperature. The data are limited to sur-
face temperatures only, but the prominent surface expression of important
processes (coastal oceanic fronts, river and estuarine discharge plumes, and
conditions of strong vertical mixing) make these data fields particularly useful.
A series of images acquired by the NOAA satellites has been obtained (in
image and tape format). The images span the time from early fall to mid-spring.
These data are products of the thermal sensors in the Very High Resolution
Radiometers (VHRR). The spatial resolution of these scanners is 0.9 km at the
satellite subpoint and degrades to 1.5 km at the image margin. Maximum thermal
resolution is 0.5K. Thermal infrared images of any particular area are acquired
every 12 hours, and a visual image is acquired once every 24 hours. In all the
images the warmer-scene temperatures are represented by the darker gray shades.
The data reveal the response of the coastal waters to the passage of winter-
season atmospheric cold fronts.
The satellite data show highly organized patterns of surface temperatures,
strong thermal fronts, and many complex gradient features. River and estuarine
discharges, coastal and shelf waters, offshore gulf water, and the Loop Current
show up as thermally differentiated water masses. The Loop Current is the
stream of warm equatorial water that enters the Gulf of Mexico through the
Yucatan Channel. After entering the Gulf it makes a broad curved 180° turn,
exits through the Straits of Florida, and becomes a source of the Gulf Stream.
With the exception of the thermally conservative Loop Current and the already
chilled riverine discharges, the coastal and continental shelf waters undergo
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important changes. These are primarily alterations in horizontal and vertical
temperature structure initiated by the cold, dry continental air behind the
atmospheric polar fronts. The coldest waters occur in the estuaries and along
the inner shelf; the waters become progressively warmer offshore in the deeper
portions of the Gulf. The shallow waters of the shelf are generally well mixed
and the sea surface temperature is the same as that of the vertically uniform
water column (Etter and Cochrane, 1975). The fall and spring months are times
of transition between these two extremes of heat content and surface temperature
gradients.
Figure 1 is an outline map of the Gulf of Mexico. The location of the
continental shelf break is indicated along the northern coast of the Gulf by the
100-meter depth contour.
2. DISCUSSION
The NOAA-5 image for 1 October 1976 (Fig. 2) shows the conditions that are
typical of the summer season. The surface temperatures of the coastal bays and
shelf waters are the same as the temperature of the deep water of the Gulf of
Mexico. Historical oceanographic data show that subsurface temperatures during
the warm summer months are also uniform horizontally (Etter and Cochrane, 1975).
By 11 October 1976 (Fig. 3) the first major cold front of the season had moved
across the Gulf Coast. The air behind this front was much colder, and nighttime
temperatures up to 7.5°C lower than those prevailing earlier in the season were
measured at several coastal weather stations. The result of this initial out-
break of cold air was the lowering of the surface temperature of the coastal
lakes and bays. The cool estuarine discharge of plumes overrunning the still-
warm inner shelf waters shows up on the imagery; the maximum thermal contrast
occurs at this point in the winter season. Offshore, horizontal temperature
gradients still remained very weak. Along the shelf east of the Mississippi
River the warm offshore water extended to the coast. West of the Mississippi
River, cooler water was present tens of kilometers offshore. This difference
between the regions east and west of the river is a consequence of the much
shallower water on the shelf along the Louisiana coast. As a cold, dry air mass
passes over the Gulf waters, energy is transferred from the ocean. Long-wave
radiation is readily transmitted through the clear skies into space. Sensible
and latent heat transferred to the atmosphere by conduction and evaporation at
the water surface are carried upward by turbulent air motions. Persistent
offshore winds sweep this heat and water vapor seaward, and dry air is replen-
ished from the land. Aided by the wind stress, the increase in density of
surface waters resulting from heat loss and evaporative salinity increase ini-
tiates penetrative convective mixing, which dissipates stratification, and the
surface temperatures become more representative of the resulting isothermal
water column. Because the amount of temperature decrease depends upon depth,
the shallowest regions experience the largest temperature drop. Surface tem-
perature profiles extracted from the digital tapes for the NOAA-5 image of 11
October 1976 are shown in Figure 4. Figure 4a is the profile of line A-A' in
Figure 3. Three temperature regions are. evident: the cold bay waters, the
slightly warmer nearshore waters, and the as yet uncooled offshore waters. The
profile (B-B1) from the Florida coast (Fig. 4b) shows the relatively uniform
surface temperatures. Note that no attempt has been made to correct the satel-
lite-observed temperatures for atmospheric attenuation.
The next phase, the winter cycle, is represented by an image taken on 18
January 1977 (Fig. 5). At the time this image was acquired the coastal waters
of the Florida shelf had cooled and a temperature front was situated approxi-
mately 30 km offshore. The cooling of the water column that had occurred much
earlier west of the Mississippi was beginning along the Florida coast. Figure
6 contains the surface temperature profiles extracted from the NOAA-5 image of
18 January 1977. Two temperature regions are evident in this image along the
Florida coast (profile B-B'): cooled nearshore waters and the still-warm off-
shore waters.
With the passage of a succession of cold fronts, the band of cooler inner
shelf waters forming along the coast expands until an oceanic thermal front
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develops between the cold shelf waters and the warmer deep Gulf waters approxi-
mately along the 100-meter isobath (Fig. 1}. This condition is illustrated by
the NOAA-3 image of 13 February 1976 (Fig. 7). The synoptic sea surface tem-
perature pattern at this time was divided into three distinct zones: the cold
shelf waters, the warmer open Gulf waters, and, warmest of all, the Loop Cur-
rent. Mesoscale irregularities had developed along the thermal front between
the shelf waters and those of the open Gulf. The origin of these features is as
yet unclear. Possible mechanisms include horizontal eddy motion, intrusions of
surface water driven by strong winds, and overrunning by strong offshore cur-
rents. Another possible cause of excursions of the warmer water toward the
coast could have been internal density cascading. This process involves down-
slope flow of the colder and hence denser water that had formed on the shelf and
a compensating landward flow of warmer offshore surface water. The shallow
water of the Bahama Banks had also been cooled and was distinctly outlined in
this image. The profiles in Figure 8 are taken from the NOAA-3 image of 13
February 1976. Profile A-A', from off the Louisiana coast, shows evidence of
three temperature regions. The profile from off the Florida coast (B-B1) is
more complex owing to the presence of the mesoscale irregularities, as seen in
Figure 7.
The continued outbreaks of polar continental air masses over the Gulf
during the winter of 1976-77 caused the temperature patterns of the Gulf waters
to evolve an additional step. By 1 March 1977 (Fig. 9) the gradient between the
shelf and the deep Gulf waters had broken down. Strong cold fronts had sig-
nificantly chilled the entire northern part of the Gulf, and only the equatorial
waters of the Loop Current remained warm.
As the strength of the polar air masses decreases, with the advent of
spring, the Gulf water begins to warm. The first waters that respond
to the warming trend are the shallow waters in the coastal lakes and bays and
along the shoreline. The NOAA-3 image of 24 April 1976 (Fig. 11) shows this
warming along the Florida coast. The surface temperature of the bays along the
northwest coast was warmer than that of the nearby Gulf water. The shallow
banks in the Bahamas also had higher sea-surface temperatures than the adjacent
Atlantic Ocean water. As the length of the days increases and the air tempera-
tures rise, the waters of the Gulf will return to the warm-season conditions of
Figure 1.
3. CONCLUSIONS
1. Surface temperature patterns of the Gulf of Mexico display a highly
organized character. During the winter months three distinct regions are
present: the cold shelf waters, the warmer deep Gulf waters, and the equatorial
waters of the Loop Current.
2. A seasonal cycle of progressive cooling changes the uniform surface
temperature distribution of the summer months into the thermally structured
surface of the winter season.
3. The locations of the thermal oceanic fronts are controlled by the
bathymetry; the shallowest areas cool quickest.
4. The shallow coastal lakes and bays are the first areas to show a
response to both heating and cooling and therefore serve as indicators of the
direction of the heat fluxes between ocean and atmosphere.
5. Poorly understood mesoscale irregularities occur along the oceanic
thermal fronts. Sources of these features are uncertain because of the coarse
spatial and temporal resolution of previous oceanographic measurements.
6. Surface measurement programs, guided by real-time satellite infrared
1611
observations, are suggested to aid in resolving the sources of frontal irregu-
larities.
7. During the winter season the Loop Current is readily discernible
because of the maximum thermal contrast that exists between it and the adjacent
Gulf water.
ACKNOWLEDGMENTS
This research was performed under a contract between the Geography Pro-
grams, Office of Naval Research, Arlington, Virginia 22217, and the Coastal
Studies Institute, Louisiana State University. Appreciation is extended to Mr.
Jack Klein and Mr. Larry Breaker, of the NOAA/NESS Satellite Field Services
Station, Redwood City, California, to Mr. Frank Kniskern, of NOAA/NESS, Washing-
ton, D.C., and to Dr. Stephen Baig, of the NOAA/NESS SPSS, Coral Gables, Florida,
for the satellite data.
REFERENCE
Etter, P. C., and J. C. Cochrane, 1975, Water temperature on the Texas-
Louisiana shelf. Texas A & M Marine Advisory Bulletin TAMU-SG-75-604.
) .'.-^.L*5- AIA- ! GA.
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GULF OF
MEXICO
FIGURE 1. THE GULF OF MEXICO. The location of the continental shelf break is
indicated by the 100-meter depth contour.
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!
. .
FIGURE 2. NOAA-5 INFRARED IMAGE, ORBIT 785, 1 OCTOBER 1976, 0225Z. Acquired
shortly after the passage of an early-season cool front. Surface temperatures
of coastal and deep waters are uniform; no strong thermal gradients exist. This
image is representative of summer conditions.
FIGURE 3. NOAA-5 INFRARED IMAGE, ORBIT 915, 11 OCTOBER 1976, 1500Z. Acquired
after the first strong cold front of the season. Coastal lakes and bays chilled.
Estuarine discharge plumes thermally distinct. Offshore cooling along Louisiana
coast and weak horizontal temperature gradients.
1613
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FIGURE 4. TEMPERATURE PROFILES. Extracted from the digital
data for the NOAA-5 image of 11 October 1976 (lines A-A1 and
B-B' in Fig. 3). The arrows indicate the approximate
location of the coastline. No attempt has been made to
correct the observed temperatures for atmospheric attenuation.
Each dot represents the temperature from one picture element
along the lines.
FIGURE 5. NOAA-5 INFRARED IMAGE, ORBIT 2134, 18 JANUARY 1977, 0204Z. Sharp
thermal gradien formed off Florida coast as a result of heat loss by the shallow
water column. Continuation of offshore cooling.
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ORIGINAL PAGE IS
OF POOR QUALITY
— I
T ..
\f.'^ *Sj\\'?~ —
FIGURE 6. TEMPERATURE PROFILES. Lines A-A' and B-B' in Figure 5. Extracted
from the digital data for the NOAA-5 image of 18 January 1977. The temperatures
off the Florida coast (Fig. 6b) show the effect of the negative heat fluxes.
Waters near the coast are cooler than those offshore, where depth is greater.
Temperatures off the Louisiana coast are also cooler nearshore, but there is no
sharp gradient offshore. Location of the coastline is indicated by the arrows.
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FIGURE 7. NOAA-3 INFRARED IMAGE, ORBIT 10267, 13 FEBRUARY 1976, 0157Z. Strong
negative heat fluxes affect the entire Gulf region, including the Bahamian and
Cuban banks. Water in all shallow regions becomes cold. Three water types
defined: cold shelf and coastal waters, cool Gulf waters, and warm equatorial
waters in the Loop Current and Gulf Stream.
ORIGINAL PAGE IS
OF POOR
25
. 1 3 —(— i • • i
. i-^ —«.. •.. " . ..^ ..^ jtfV»^ yvJijrVnr
°_
IK
3
25
20
FIGURE 8. TEMPERATURE PROFILES, Lines A-A' and B-B1 in Figure 7. Extracted
from the digital data for the NOAA-3 image of 13 February 1976. The two ver-
tical lines in profile A-A1 approximate the locations of the gradients between
the three temperature regions that are evident off the Louisiana coast (Fig. 7)
Off the Florida coast (profile B-B1) there does not seem to be such a sepa-
ration. Location of the coastline is indicated by the arrows.
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PAGE IS
POOR QUALITY
FIGURE 9. NOAA-5 INFRARED IMAGE, ORBIT 2654, 1 MARCH 1977, 0218Z. Strong cold
fronts of the severe winter continue to cool shelf and Gulf waters, and thermal
contrast at the shelf decreases. Eddies from the Loop Current extend far to the
north.
FIGURE 10. NOAA-3 INFRARED IMAGE, ORBIT 11085, 19 APRIL 1976, 0158Z. Warming
of shallow regions along the Florida coast; evidence of positive heat flux along
the inner shelf.
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Surface temperatures and temperature gradient features of the US Gulf Coast waters

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