Unusual response of sea surface reflectance and temperature 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
Response of Sea Surface Reflectance (SSR) and temperature (SST) to a category 4 hurricane,
Dennis, in early July 2005 was assessed using data from MODIS, SeaWiFS, AVHRR, historical
hydrographic and nutrient surveys, and a simple mixing model. The near-shore shallow
bathymetry, the offshore deep nutricline, and the Loop Current eddies led to hurricane-induced
SSR and SST changes that have not been observed in other regions. Significant SSR increases
were found for < 50 m waters due to sediment resuspension, accompanied by increases in
MODIS fluorescence line height (FLH). The short response time (< 2 days) implied that such
fluorescence increases were likely due to resuspended benthic algae and/or bottom chlorophyll
maxima, as opposed to new primary production. 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, almost entirely biased to the right-side of the hurricane track (–1.960.66oC
for an area of 158,600 km2), and SSR changes in < 50-m waters recovered to normal conditions
after about 10 days. This study shows that a natural disturbance over a shallow shelf may be used
to infer water column and benthic properties when field measurements are not available.
Introduction
Dramatic surface cooling and color changes can occur in the ocean after a hurricane’s passage. In
the coastal ocean, hurricanes can also cause sediment resuspension/transport as well as coastal
erosion and flooding. Although far from comprehensive, all these phenomena have been
documented in the literature (Stramma et al., 1986; Shay et al., 1992; Hoge and Lyon, 2002;
Babin et al., 2003; Lin et al., 2003; David and Yang, 2004; Walker et al., 2005; others), where
changes in sea surface temperature (SST) are well-known and there is general consensus on the
driving mechanism, but other changes are still under research. For example, some proposed that
the color (or sea surface reflectance, SSR) change is due to increased primary productivity from
deeper-mixing or upwelling induced nutrient enhancement (Babin et al., 2003) and/or
entrainment of the deep chlorophyll maxima (Walker et al., 2005), while others argued that from
the perspective of ocean color algorithms these SSR changes are more likely due to enhanced
CDOM (colored dissolved organic matter, also called yellow substance, a material from plant
degradation from either marine or terrestrial origin) rather than phytoplankton (Hoge and Lyon,
2002). In addition, study of shallow water response is often limited to sediment resuspension and
river plume injection (e.g., Yuan et al., 2004), but documentation of changes in physical and biooptical properties is scarce, partly due to lack of resource for timely cruise survey or lack of
reliable algorithms to estimate these properties accurately from satellite measurements.
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In early July 2005, Hurricane Dennis (category 4) swept the eastern Gulf of Mexico (EGOM),
including the west Florida shelf (WFS). In addition to sediment resuspension and surface cooling,
there are some unique features observed from SSR and SST satellite imagery that have never
been reported elsewhere, including 1) sharp contrast in SST response along the 30-m isobath
where hurricane-induced heat loss and deeper mixing is expected to be more or less
homogeneous across the isobath; 2) significant increase in the solar stimulated fluorescence on
the shallow shelf; and 3) negligible changes in SSR in the deeper shelf where surface cooling is
maximum.
Although the upper ocean heat and temperature response to Dennis has been modeled by Morey
et al. (2006), these unusual physical and biological features have not been documented or
explained. Using concurrent satellite data, historical field surveys, and a simple mixing model,
we report these unique changes and explain the underline driving mechanisms. We further
hypothesize that hurricane perturbation over shallow waters can serve as a natural experiment to
infer summertime thermocline and nutricline depths, and the perturbation may also yield
information on water depth.
Data Sources
SST data were obtained from AVHRR (n12, n15, n17) and MODIS (Terra and Aqua) satellite
sensors. SSR and related products (chlorophyll concentration or Chl, fluorescence line height or
FLH) were obtained from 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 same area were obtained from the NEGOM program
(1998-2000, Hu et al., 2003), the EcoHAB program (1998-2001), and a field survey to the
Florida Current in August 2004 (Hu et al., 2005a), respectively. Data for Hurricane Dennis,
including the category, eye location, sustained and maximum wind speeds, were obtained from
the National Hurricane Center of the US.
Results
SST and SSR Response to Hurricane Dennis
Fig. 1 shows that the surface cooling is nearly entirely biased to the right side of the hurricane
track, with average cooling of –1.960.66oC (maximum reached < – 4oC) in the outlined area
(~158,600 km2), defined by the difference between SST on July 11 (26.470.58oC) and 1-week
average SST before Dennis (28.430.37oC). However, for the < 30-m waters between Cape San
Blas and Dry Tortugas (area ~ 36,000 km2), SST response is smaller (-1.660.60oC, from
29.230.44 to 27.570.59). Note that before Dennis the <30-m waters are ~0.8oC warmer than
the deeper area, partially leading to the sharp SST contrast near the 30-m isobath after Dennis
(Fig. 1b). The significant sediment resuspension over the entire shallow (< 50 m) shelf (Fig. 2b)
suggests that the smaller SST changes over < the 30-m waters are not because of the limited
fetching distance of the hurricane wind.
Accompanying the sediment resuspension is the significant enhancement of the MODIS FLH
signal (Fig. 2d), also restricted to the 50-m isobath. Except for a small filament along the eddy
2
edge (Fig. 2d), there is no detectable change in FLH for waters > 50-m isobath, even in the area
of significant surface cooling. The MODIS FLH data are not sensitive to detect small changes (<
0.1 mg m-3) in chlorophyll concentrations (Chl) (Hu et al., 2005b). SeaWiFS Chl time-series data
did not reveal any significant changes after Dennis for waters > 50-m isobath except the small
filament. Further, there was no significant change in either Rrs(443) or Rrs(555) (Fig. 3). The
slight decrease in Rrs(443) on July 13 (about –0.001 sr-1) is perhaps mainly due to the imperfect
atmospheric correction, because of the inherent atmospheric correction uncertainty of 0.00064
sr-1 at this wavelength (Gordon, 1997). Clearly, for waters > 50-m isobath, bio-optical and
biological responses are minimal. This result and the SST contrast across the 30-m isobath are
different from all those reported in the literature.
Forcing
What could have caused the unusual SST and 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 track. However, if the water is already well mixed down to the bottom, 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 to 1000-m isobath. Then, if the bottom depth is less than 30 m, hurricane perturbation
should have less impact than over deeper waters. For the same reason, significant cooling was
observed in the outlined area in Fig. 1b because of the larger isopycnal displacement of the
seasonal thermocline.
The isopycnal displacement can be approximated as (Price et al., 1994; Babin et al., 2004):  =
/(wfUh), where  is the wind stress that can be derived by 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 transit speed. Using data from the
NHC, we estimated that for the average wind speed of 56.17.9 m s-1 during Dennis’ passage,
the isopycnal displacement in the eastern GOM is 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. Consider the size of the
cooling area (158,600 km2) and other factors (heat loss, inhomogenous vertical mixing), 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 eastern GOM is typically below 5060 m, above which nitrate concentration is 0.5 M or not detectable (Belabbassi et al., in press).
Therefore, the deep mixing did not bring much nutrient to the surface to stimulate phytoplankton
growth. For the same reason, one can also infer that CDOM above the nutricline was small and
homogeneous, for otherwise one would expect a significant drop in Rrs(443).
Discussion
Walker et al. (2005) observed average surface cooling of about –1.9oC on both side of the
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 its maximum at 0.81 mg m-3. Similarly, based on SeaWiFS data Davis and
3
Yan (2004) reported Chl-rich coastal filaments after hurricane’s passage. More generally, Chl
enhancement for the open ocean was investigated by Babin et al. (2004). However, using a biooptical model that explicitly differentiates CDOM from Chl, Hoge et al. (2002) concluded that
the color change in the open ocean was due to increased CDOM rather than Chl. Here, only a
small filament of enhanced MODIS FLH was observed along an eddy edge (Fig. 2d), where
SeaWiFS Chl showed an increase from ~0.12 to 0.25 mg m-3. The MODIS fluorescence data
unambiguously revealed the phytoplankton increase, a result of the already uplifted thermocline
and nutricline within the eddy. Elsewhere in the deep water, we observed minimal ocean color
(SSR) change, even in the waters with maximum SST response, a result of deep nutricline and
unique bio-optical profiles in the eastern GOM. Clearly, hurricanes do not necessarily induce
color changes, and further research is needed to verify the bio-optical responses, particularly in
terms of reliable bio-optical algorithms and timely field survey.
Similar to the CDOM interference to the Chl bio-optical algorithm for the open ocean, sediment
resuspension in the shallow shelf (0-50 m, Fig. 2b) can also cause overestimates of Chl if a bandratio algorithm is used. However, MODIS FLH is a reliable index for solar stimulated
fluorescence under most circumstances. In highly turbid waters the presence of significant
amount of sediments may cause overestimates in the FLH signal, but such overestimates for
sediments concentrations of 40-50 mg L-1 (based on Fig. 3b) are expected to be much smaller
and therefore could not account for the observed significant increase in FLH (Fig. 2d). Then,
does the increased FLH over the <50 m shelf (Fig. 2d) suggest increased phytoplankton biomass
and/or enhanced primary productivity?
The EcoHAB data showed that the waters of >30 m isobath are relatively nutrient depleted (N, P,
Si) in the surface layer, with slight increase in the bottom layer. For waters between the 0-20 m
isobaths both P and Si are larger (P ~ 0.05 – 0.3 M; Si ~ 1 – 8 M) but they are vertically
homogenous. However, there may be significant amount of nutrient in the bottom sediment (pore
water) that was brought to the surface by the hurricane perturbation. The EcoHAB data also
showed high Chl in the bottom layer (~ 0.3 – 1 mg m-3) when the surface layer is oligotrophic
(Chl ~ 0.1 mg m-3). Because high FLH was observed on July 11 (image not shown due to partial
sun glint), only after one day of the hurricane passage, the bottom Chl layer and the significant
amount of benthic algae throughout the shallow shelf (Gabe Vargo, USF, personal comm.)
should be the dominant reason of the increased FLH after Dennis.
The sediment resuspension is restricted to < 50-m isobath for the average wind speed of 56.17.9
m s-1 (category 4 hurricane) and average transition speed of 7.73.0 m-1. This is perhaps the
maximum water depth where hurricane perturbation can bring the bottom sediment to the surface
for this region. Indeed, three hurricanes landed on Florida during summer 2004 (Charley,
Frances, and Jeanne), where sediment resuspension was always restricted to < 50-m or shallower.
Hence, combined with physical models, hurricane perturbation may be used to infer water depth.
Why did the surface cooling occur almost entirely to the right side of Dennis’s track (Fig. 1b)? In
addition to the asymmetric wind and physical forcing, the heterogeneity of the 3D hydrographic
properties in the eastern GOM should be the dominant reason. Fig. 1a shows the dominant eddies
and associated current direction, confirmed by concurrent satellite altimetry data. Clearly, most
of the waters to the left side of the hurricane track are from the Loop Current (Caribbean origin).
4
Historical data (e.g., Fig. 4b) showed that the surface mixed layer of this water in the Florida
Current is at least 60 m, leading to less surface cooling. The shape of the cooled area (outlined at
black line on both Figs 1a and 1b) also suggests that differences in the hydrographic properties
inside and outside the LC eddies should be another important reason to result in the observed
cooling patterns.
Conclusion
Unusual patterns in hurricane-induced surface cooling and SSR changes occurred after Hurricane
Dennis’ passage in the EGOM in early July 2005. These include the significant surface cooling
that is almost entirely biased to the right-side of the hurricane track, less cooling on the < 30-m
shelf, significant sediment resuspension on the <50-m shelf accompanied with increases in the
solar stimulated fluorescence, and more importantly, the negligible changes in SSR in the area of
maximum cooling. These unusual responses contrast to all those reported in the literature, and
can be well explained by the historical 3D hydrographic and nutrient data. Based on these
observations, we hypothesize that hurricane perturbations over shallow shelves may be used as
natural experiments to infer the hydrographic and bio-optical properties of the water column that
could otherwise not be observed from satellites.
Acknowledgement
Funding was provided by NASA through Grants NNS04AB59G and NAG5-10557 to MullerKarger and 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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(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
88oW
86oW
(b) July 11, 2005
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 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) after Dennis,
where the entire <50-m shallow shelf experienced significant sediment resuspension and increase
in FLH. Elevated FLH values were also found along 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
20.0
Rrs(443)
Rrs(555)
SST
Chl
-3
0.5
o
7.0
30.0
Chl (mg m )
9.0
(a)
SST ( C)
Rrs (x1000 sr-1)
Rrs (x1000 sr-1)
Dennis
(b)
10.0
0.06/30
7/5
7/10
7/15
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 < 50-m shallow shelf from Cape San Blas to Dry Tortugas. The dashed lines
denote the one-week mean values before Dennis. The 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 at about 27oN
from the July 2000 and July 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 (FC).
10