Response of sea surface properties to Hurricane Dennis in the eastern Gulf of Mexico
Chuanmin Hu and Frank E. Muller-Karger
Institute for Marine Remote Sensing, College of Marine Science, University of South Florida
140 Seventh Avenue, South, St. Petersburg, FL 33701
(727)5533987, hu@marine.usf.edu
Abstract
Sea Surface Reflectance (SSR) and Sea Surface temperature (SST) were assessed in the eastern
Gulf of Mexico in early July 2005 after passage of hurricane Dennis, a category 4 storm, using
data from the MODIS, SeaWiFS, and AVHRR satellite sensors, historical hydrographic and
nutrient surveys, and a simple vertical water column mixing model. The near-shore shallow
bathymetry, the offshore deep nutricline, and the Loop Current eddies led to unique SSR and
SST changes in this region. For waters shallower than 50 m, a strong sediment resuspension
event was evidenced by SSR increases in the visible and in MODIS fluorescence line height
(FLH), which recovered to normal conditions after about 10 days. The FLH change indicates
increasing biomass in the short response time (< 2 days), which might have been due to
resuspended benthic and/or near-bottom algae, as opposed to simply new primary production in
the overlying water column. For waters > 50 m deep, where significant surface cooling was
observed, there was no apparent change in SSR due to the deep nutricline. The SST changes
occurred almost entirely along the right side of the hurricane track (–1.960.66oC for an area of
158,600 km2). These observations can be explained by the three-dimensional hydrographic and
nutrient structure in waters near the west Florida shelf.
Introduction
Dramatic surface cooling and color changes in the ocean have been documented after a
hurricane’s passage (Stramma et al., 1986; Shay et al., 1992; Hoge and Lyon, 2002; Babin et al.,
2003; Lin et al., 2003; Davis and Yan, 2004; Walker et al., 2005; others). In the coastal ocean,
hurricanes can also cause sediment resuspension/transport as well as coastal erosion and flooding.
Changes in sea surface reflectance (SSR) may be due to increased surface primary productivity
resulting from nutrients supplied by deep mixing or upwelling (Babin et al., 2003) and/or
entrainment of the deep chlorophyll maxima (Walker et al., 2005). Some investigators have
proposed that SSR changes are more likely due to enhanced colored dissolved organic matter
(CDOM) (Hoge and Lyon, 2002). Over shallow waters, sediment resuspension and river plume
injections have been observed (Yuan et al., 2004). Yet, to date there are no studies of concurrent
in situ and satellite observations of physical and bio-optical properties in the immediate
aftermath of a hurricane over a shelf environment.
In early July 2005, Hurricane Dennis (category 4) swept across the eastern Gulf of Mexico
(EGOM) and the west Florida shelf (WFS). In addition to sediment resuspension and surface
cooling, other unique features were observed in satellite-derived SSR and SST. Specifically, 1) a
sharp contrast in SST response on either side of the 30-m isobath on the shelf; 2) a large increase
in the solar stimulated fluorescence over the shallow portions (<50 m) of the shelf; and 3)
negligible changes in SSR over deeper shelf waters, where surface cooling was maximum.
1
The upper ocean heat and temperature response to Hurricane Dennis has been modeled by Morey
et al. (2006), yet the unusual physical and biological features have not been documented or
explained. Here we report on these unique changes and use concurrent satellite data, historical
field surveys, and a simple mixing model to explain the underlying driving mechanisms. We
hypothesize that hurricane perturbation over shallow waters can yield information on
summertime thermocline and nutricline depths as well as on bottom-dwelling algae.
Data Sources
SST data were obtained from the Advanced Very High Resolution Radiometer (AVHRR on
NOAA Polar Orbiters 12, 15, and 17) and the Moderate-resolution Imaging Spectroradiometer
(MODIS on the Terra and Aqua satellites). SSR and related products (chlorophyll-a
concentration or Chl, fluorescence line height or FLH) were obtained from the Sea-viewing
Wide-Field-of-view Sensor (SeaWiFS) and MODIS. All satellite data were captured and
processed using the most updated software and algorithms at University of South Florida.
Historical CTD and nutrient data in the EGOM were obtained from the Northeastern Gulf of
Mexico (NEGOM) program (1998-2000, Hu et al., 2003), the EcoHAB program (1998-2001),
and a field survey conducted to the Florida Current in August 2004 (Hu et al., 2005a). Data for
Hurricane Dennis, including the storm category, eye location, and sustained and maximum wind
speeds were obtained from the National Hurricane Center, U.S.A.
Results
SST and SSR Response to Hurricane Dennis
Fig. 1 shows that the surface cooling is almost entirely located to the right of the hurricane track.
The average cooling in the core of the cold plume (area outlined in Fig. 1b) was –1.960.66oC
(minimum was < – 4oC). In total, an area of ~158,600 km2 was defined by the difference
between SST on July 11 (26.470.58oC) and the 1-week average SST derived for the week
before Dennis (28.430.37oC). However, for the < 30 m shelf waters between Cape San Blas and
Dry Tortugas (an area ~ 36,000 km2), the SST response was smaller (-1.660.60oC, from
29.230.44 to 27.570.59). Before Dennis the < 30 m waters were ~0.8oC warmer than the
deeper area. This likely led partially to the sharp SST gradient across the 30-m isobath after
Dennis (Fig. 1b). The significant sediment resuspension event over the entire < 50 m shelf (Fig.
2b) suggests that the smaller SST changes over the < 30 m shelf are not due to the limited
fetching distance of the hurricane wind, but rather due to the 3-dimentional hydrography before
Dennis.
The sediment resuspension event over the entire shallow (< 50 m) shelf (Fig. 2b) illustrates the
intensity with which hurricane Dennis mixed shelf waters. Accompanying the resuspended
sediment we observed a significant enhancement of the MODIS FLH signal (Fig. 2d). The
MODIS FLH data are most sensitive to detect larger changes in Chl (> 0.1 mg m-3; Hu et al.,
2005b). Except for a small filament near the edge of a Loop Current eddy around 26°N, 84-86W
(Fig. 2d), there was no detectable change in FLH for waters > 50 m depth, even in the area of
2
significant surface cooling. SeaWiFS Chl time-series in deep waters, not affected by sediment
resuspension, did not reveal any changes after Dennis except for the filament as shown in Fig. 2d.
Further, there was no significant change in either Rrs(443) or Rrs(555) (Fig. 3a). The slight
decrease in Rrs(443) on July 13 (about –0.001 sr-1) is perhaps primarily due to an imperfect
atmospheric correction, because of the inherent atmospheric correction uncertainty of 0.00064
sr-1 at this wavelength (Gordon, 1997). Clearly, for deep waters (> 50 m depth), bio-optical and
biological responses are minimal. This is in contrast to previous reports suggesting that
hurricanes at these latitudes stimulate phytoplankton blooms in their wake.
Forcing
How much force is needed to cause the unusual SST and ocean color response in the wake of
Dennis? The cyclonic wind forcing will result in deep mixing and upwelling, particularly to the
right-side of the hurricane’s track. If the water is already well mixed down to the bottom prior to
the passage of the storm, SST changes should be small. Data collected in the past around the
same time of the year (Fig. 4) suggest that the surface layer due to summer stratification is about
20-30 m for the EGOM, at least over most of the shelf and out to the 1000 m isobath. If mixing
were to occur to greater depths, a change should be expected in SST over these regions. Indeed,
significant cooling was observed in the outlined area in Fig. 1b. Where the bottom is shallower
than 30 m, hurricane perturbation of the SST should be relatively small.
The downward isopycnal displacement of the seasonal thermocline in waters deeper than 30 m
can be approximated as (Price et al., 1994; Babin et al., 2004):  = /(wfUh), where  is the wind
stress derived as a function of wind speed (U10, m s-1) and air density (a = 1.26 kg m-3) as
=a(0.49+0.065 U10)10-3 U102, w is the water density (1020 kg m-3), f is the Coriolis parameter,
and Uh (m s-1) is the hurricane transition speed.
Using data from the National Hurricane Center, we estimated that for the average wind speed of
56.17.9 m s-1 during Dennis’ passage, the isopycnal displacement in the EGOM was 38.319.3
m. The associated average surface cooling, estimated as the difference between the average
temperature for the water depth of 30+38.3 m and the pre-hurricane 30 m (Fig. 4), is 26.9 – 29.7
= 2.8oC. Considering the size of the cooling area (158,600 km2) and factors such as heat loss,
inhomogenous vertical mixing, which we do not estimate but which also affect SST, this result
agrees reasonably well with that observed from the satellites.
Why was there no apparent change in SSR in the area of maximum cooling? Nutrient profiles
collected by the NEGOM program along a west-east transect from Tampa Bay to the 1000-m
isobath suggest that the nutricline for this time of year in the EGOM is typically deeper than 5060 m. Above this depth, nitrate concentration is below 0.5 M and frequently undetectable
(Belabbassi et al., in press). The mixing did not penetrate the nutricline and therefore little
nutrients were available to stimulate phytoplankton growth near the surface. One may similarly
infer that CDOM concentration above the nutricline was low and homogeneously mixed, since if
there had been an upward injection of CDOM we would have observed a drop in Rrs(443).
Discussion
3
Walker et al. (2005) observed average surface cooling of about –1.9oC on both sides of a
hurricane track in the NGOM near the Mississippi Delta, and a filament of enhanced SeaWiFS
Chl only 1.5 days after the hurricane passage (from 0.36 to 0.50 mg m-3). Two days later the Chl
in the patch reached a maximum concentration of Chl=0.81 mg m-3. Similarly, Davis and Yan
(2004) observed Chl-rich coastal filaments after a hurricane’s passage using SeaWiFS data.
More generally, Chl enhancement in the wake of a hurricane in the open ocean was reported by
Babin et al. (2004). However, Hoge et al. (2002) concluded that the color change in the open
ocean was due to increased CDOM rather than Chl. Here, in the deep Gulf of Mexico we
observed a change in Chl as enhanced MODIS FLH and SeaWiFS Chl only in a filament within
an eddy (Fig. 2d). SeaWiFS Chl showed an increase from ~0.12 to 0.25 mg m-3 in this filament.
We believe that the hurricane-induced mixing reached the uplifted thermocline and nutricline
within the cyclonic eddy near 26oN. We did not observe other changes in the SSR or Chl in deep
water, even in the area of maximum SST response. This, again, is likely the result of mixing to
depths shallower than about 60 m. Clearly, hurricanes do not necessarily induce color or
chlorophyll changes every time they pass over deep waters as implied in previous studies.
Further research is needed to verify the bio-optical responses, particularly in terms of reliable
bio-optical algorithms and timely field surveys to examine biogeochemical changes in the wake
of a hurricane.
Similar to the CDOM interference to the Chl bio-optical algorithm for the open ocean, sediment
resuspension over the shallow shelf (0-50 m, Fig. 2b) can also cause overestimates of Chl if a
band-ratio algorithm is used. The MODIS FLH provides an alternative in the form of a reliable
index of chlorophyll (e.g., Hu et al., 2005b). In highly turbid waters, significant amount of
sediments may cause overestimates in the FLH signal. However, FLH overestimates for
sediments concentrations of 40-50 mg L-1 (based on Rrs(555) values in Fig. 3b and empirical
formulas to convert Rrs(555) to sediment concentration) are expected to be much smaller and
therefore could not account for the observed significant increase in FLH over the <50 m shelf
(Fig. 2d). Then, does the increased FLH over the <50 m shelf (Fig. 2d) suggest increased
phytoplankton biomass?
The EcoHAB data showed that surface waters where the shelf is deeper than 30 m are relatively
nutrient depleted (N, P, Si). Slightly higher nutrient levels are found in the bottom layer. In
waters between the 0-20 m isobaths both P and Si are typically higher (P ~ 0.05 – 0.3 M; Si ~ 1
– 8 M) and vertically homogenous. However, there may be significant amount of nutrients in
bottom sediment pore waters. These may be released into the overlying water column by
hurricane perturbation. The EcoHAB data also showed high Chl (~ 0.3 – 1 mg m-3) in the bottom
layer where the surface layer had low Chl (~ 0.1 mg m-3). The day after the passage of hurricane
Dennis (July 11), high FLH was observed over the <50 m shelf (image not shown due to partial
sun glint). We believe that the bottom Chl layer and possibly benthic algae that is prevalent
throughout the shallow shelf (Gabe Vargo, USF, personal comm.) were resuspended and
stimulated, leading to the increased FLH signal after Dennis.
Sediment resuspension to the surface was largely restricted to the < 50 m shelf for the average
wind speed of 56.17.9 m s-1 (category 4 hurricane) and average storm transition speed of
7.73.0 m-1. Similar observations were made during three hurricanes that affected the WFS
4
during summer 2004 (Charley, Frances, and Jeanne), when sediment resuspension events were
always restricted to < 50 m.
Why did the surface cooling occur almost entirely to the right side of Dennis’ track (Fig. 1b)? In
the past, much emphasis has been placed on the asymmetric distribution of wind and physical
forcing around the periphery of these storms. However, there is also spatial heterogeneity in the
hydrographic properties in the eastern EGOM over the shelf. Over deeper waters, most of the
waters to the left of the hurricane track are in the Loop Current and other deep Gulf waters (Fig.
1a). Historical data (e.g., Fig. 4b, profile labeled FC) shows that the water in the Florida Current
(an extension of Loop Current) is warmer to much deeper depths than outside the current, and
that the thermocline is at least 60 m deep. Therefore it is more difficult to mix to depths that will
lead to surface cooling. The shape of the cooled area (outlined as black line on both Figs. 1a and
1b) show that differences in the hydrographic properties inside and outside the LC and its eddies
are an important factor leading to spatial changes in the observed cooling patterns. These
patterns are clearly of interest in defining the heat content of surface waters that can provide heat
to sustain storm strength.
Conclusion
Unusual patterns in hurricane-induced surface cooling and SSR changes occurred after Hurricane
Dennis’ passage in the eastern Gulf of Mexico in early July 2005. These include significant
surface cooling to the right of the hurricane track over the >30 m shelf, less cooling over the < 30
m shelf, and significant sediment resuspension over the <50 m shelf accompanied with a strong
increase in the solar stimulated fluorescence. There was negligible changes in SSR and Chl in the
area of maximum cooling over deep shelf and Gulf waters. These responses are different from
those reported in the recent literature examining SST and chlorophyll changes in the wake of
hurricanes. The differences can be explained by the three-dimensional hydrographic and nutrient
structure in waters near the west Florida shelf.
Acknowledgement
Funding was provided by NASA through Grants NNS04AB59G and NAG5-10557 to F. MullerKarger and C. Hu. Dr. Douglas Biggs and Ms. Leila Belabbassi (Texas A&M Univ.) provided
historical hydrographic and nutrient data from the MMS-sponsored NEGOM project. We thank
Dr. Robert Masserini (USF) for providing historical EcoHAB data on the WFS.
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6
(a)
LC Eddy
Loop
Current
(a) July 2-8, 2005
Cape
San Blas
(b)
28oN
July 10, 2005
05:00 GMT
26oN
24oN
Dry
Tortugas
(b) July 11, 2005
88oW
86oW
Figure 1. Sea surface temperature (SST, oC) before (a) and after (b) Hurricane Dennis. The
weekly mean SST in (a) shows dominant current directions associated with Loop Current and LC
eddies (outlined in dashed red lines with arrows). The overlaid numbers indicate the location of
the hurricane eye and the category, at approximately 2-hour steps, starting from the south around
9 July 2005, 05:00 GMT. Overlaid green lines show bathymetry contours for 30, 40, 50, 100,
200, 500, and 1000 m. In (b), the maximum surface cooling is outlined by the black dashed line,
and the outline is overlaid in (a) to show its relationship with the pre-hurricane LC and eddies.
An area of about 20,000 km2 between 100- and 500-m in the center of the cooling (blue outline
in (b)) was chosen to study the changes in several bio-optical properties. The color scales used to
code the SST in the images were adopted from a fixed omni-scale to cover the entire SST range
(0 – 31oC) to assist time-series analysis for the global ocean.
7
(a)
(b)
July 7, 2005
18:55 GMT
July 11, 2005
18:30 GMT
(c)
(d)
86oW
84oW
28oN
26oN
July 7, 2005
18:55 GMT
July 12, 2005
19:11 GMT
Figure 2. MODIS imagery showing changes in SSR (a and b) and FLH (c and d) from before to
after Dennis, where the entire <50-m shallow shelf experienced significant sediment
resuspension and increase in FLH. Elevated FLH values were also found near an LC eddy edge,
as annotated with the black arrows in (d).
8
29.0
0.4
28.0
5.0
27.0
0.3
0.2
3.0
26.0
0.1
1.0
30.0
25.0
0.0
(b)
For <50 m WFS
-3
0.5
o
7.0
30.0
20.0
SST
Chl
10.0
Rrs(443)
Rrs(555)
0.06/30
7/5
7/10
7/15
Chl (mg m )
9.0
(a)
SST ( C)
Rrs (x1000 sr-1)
-1
Rrs (x1000 sr )
Dennis
7/20
Date (2005)
Figure 3.SST and SSR parameters before and after Hurricane Dennis in the EGOM: (a) for the
area between 100- and 500-m isobath and between 26.5 and 28.5oN (outlined in blue in Fig. 1b);
(b) for the entire shallow (<50 m) shelf region from Cape San Blas to Dry Tortugas. The dashed
lines denote the one-week mean values before hurricane Dennis. SSR parameters were derived
from SeaWiFS from the dates when at least half of all pixels in the area were valid.
9
0
0
10
10
20
30
28
30
Depth(m)
Depth(m)
20
28
30
30
26
26
24
24
40
Temperature
2000
40
Temperature
2001
22
20
20
50
50
-83.4
-83.2
-83
-82.8
-83.4
-83.2
Longitude
0
Depth (m)
22
20
-83
-82.8
Longitude
NEGOM station L11S16
7/27/1998; 8/21/1999; 8/7/2000
o
o
27.50 N, 85.23 W
40
60
FC station MR5
8/22/2004
o
o
23.90 N, 82.76 W
80
100
20.0
25.0
30.0
o
Temperature ( C)
Figure 4. (a) Temperature profiles along a west-east transect south of Tampa Bay near 27oN in
July 2000 and 2001 EcoHAB cruises; (b) Temperature profile (meanstandard deviation) for the
NEGOM station L11S16 (near the center of the maximum cooling area outlined in Fig. 1b), and
temperature profile from a station in the Florida Current (labeled FC).
10