Role of Anomalous Warm Gulf Waters in the Intensification of Hurricane Katrina

Abstract

1] The year 2005 experienced several strong hurricanes intensifying in the Gulf of Mexico before making landfall that severely damaged the Gulf States, especially Hurricane Katrina. Remarkable similarities between sea surface temperature anomaly (SSTA) and major hurricane (categories 3 and higher) activity over the Gulf are identified. However, the intensification of individual hurricanes may not necessarily be temporally and spatially coincident with the distribution of warm waters or high sea surface temperature (SST). High SST values are found in advance of significant intensification of Hurricane Katrina. We emphasize that high SSTA which occurred at the right time and right place was conducive to the hurricane intensification. In particular, high SSTA in the northeastern quadrant of the storm track induced significant increases in surface latent heat fluxes (LHF) contributing to the rapid intensification of Katrina. We also compared and verified model simulations with buoy observations.

Full text

Role of anomalous warm gulf waters in the intensification of Hurricane
Katrina
Menas Kafatos,
1
Donglian Sun,
1
Ritesh Gautam,
1
Zafer Boybeyi,
1
Ruixin Yang,
1
and Guido Cervone
1
Received 18 April 2006; revised 7 June 2006; accepted 17 July 2006; published 1 September 2006.
[1] The year 2005 experienced several strong hurricanes
intensifying in the Gulf of Mexico before making landfall
that severely damaged the Gulf States, especially Hurricane
Katrina. Remarkable similarities between sea surface
temperature anomaly ( SSTA) and major hurricane
(categories 3 and higher) activity over the Gulf are
identified. However, the intensification of individual
hurricanes may not necessarily be temporally and spatially
coincident with the distribution of warm waters or high sea
surface temperature (SST). High SST values are found in
advance of significant intensification of Hurricane Katrina.
We emphasize that high SSTA which occurred at the right
time and right place was conducive to the hurricane
intensification. In particular, high SSTA in the
northeastern quadrant of the storm track induced
significant increases in surface latent heat fluxes (LHF)
contributing to the rapid intensification of Katrina. We also
compared and verified model simulations with buoy
observations.
Citation: Kafatos, M., D. Sun, R. Gautam, Z.
Boybeyi, R. Yang, and G. Cervone (2006), Role of anomalous
warm gulf waters in the intensification of Hurricane Katrina,
Geophys. Res. Lett., 33, L17802, doi:10.1029/2006GL026623.
1. Introduction
[2] Despite large reductions in track forecast errors over
the past three decades [McAdie and Lawrence, 2000], there
has been little improvement in forecasts of storm intensity.
Observational and modeling studies have shown the signif-
icant influence of vertical wind shear on hurricane inte nsity
changes. Stron g vertical wind shear (jV
z
j) between t he
upper and lower troposphere prevents the intensification
of tropical cyclones due to the so-called ‘‘ventilation’’ effect
of the hurricane warm core [Gray, 1968; Goldenberg and
Shapiro, 1996; Bracken and Bosart, 2000; Wong and Chan,
2004]. However, Zhu et al. [2004] found that Hurricane
Bonnie intensified simultaneously with the increase s of
vertical shear; suggesting that in addition to the magnitude
of wind shear, the direction of wind shear may as well be
significant for the intensification of hurricanes. The overall
dependence of tropical cyclone intensity on SST is well
documented [Fisher, 1958; Leipper, 1967; Emanuel, 1986,
1988; Holland, 1997], with an increase of 1K of SST
leading to a 12–14 mb deepening in hurricane minimum
central pressure [Hong et al., 1995; Zhu et al., 2004]. SST
plays a fundamental role in the inter-annual variability of
tropical storm frequency and intensity [Vitart et al., 1999],
and a direct role in providing moist enthalpy (i.e., latent and
sensible heat flux) to intensify tropical cyclones [Goldenberg
et al., 2001].
[
3] The recent active period of intense hurricanes has
triggered a hot debate in the scientific community whether
the increase in the frequency and intensity of hurricanes is
due to either the natural climate variability such as the El
Nin˜o/Southern Oscillation (ENSO), quasi-biennial oscilla-
tion (QBO), and Atlantic Multidecadal Oscillation (AMO)
[Bove et al., 1998; Elsner et al., 1998; Gray, 1984; Shapiro
and Goldenberg, 1998; Goldenberg et al., 2001; Virmani and
Weisberg, 2006], or the human-induced global warming
[Knutson and Tuleya, 2004; Emanuel, 2005; Webster et al.,
2005]. Several studies suggest that global warming would
likely result in SST increase, which may result in an increase
in the inte nsity of tropical cyclones [Tsutsui, 2002; Webster et
al., 2005]. Other studies indicate the effect of warm core ring
associated with the loop current in the intensification of
hurricanes is important [Hong et al., 2000; Scharroo et al. ,
2005]. Nevertheless, they are all associated with the effects of
warm SST. At the same time, we note that not all tropical
cyclones associated with warm waters attain peak intensity
(categories 4 and 5) during their life cycle.
[
4] Specifically, the current period after 1995 has sig-
naled an active period, especially for major hurricanes
(categories 3, 4, and 5) [ Goldenberg et al., 2001]. The last
several years set records for the most intense hurricanes ever in
any given year, except for the El Nin˜o year 1997 when
hurricane occurrences wer e low. The SSTA trends over the
North Atlantic coincide well with the Atlantic Multidecadal
(50 years) Oscillation (AMO) [cf. Virmani and Weisberg,
2006]. However, as we will report later, there is evidence of
increasing trends that may be associated with global warming.
[
5] On August 23, 2005, Katrina formed into a tropical
depression (TD) from a broad area of low pressure in the
central Bahamas. Over the next few days, Katrina rapidly
intensified into a category 5 after it crossed south Florida on
August 26, 2005 and entered the warm Gulf of Mexico with
SST values over 30C. Katrina made second landfall as a
powerful category 4 storm over the southeastern Louisiana
and southern Mississippi on August 29, 2005, causing
catastrophic flooding to the city of New Orleans and the
surrounding areas, resulting in thousands of deaths.
[
6] In this study, we focus our analysis on the variations
of SST for the Hurricane Katrina case. We note the recent
increase in Gulf hurricane activity, especially for intense
hurricanes (categories 3, 4, and 5), while studies in the past
predicted only minor differences in intense hurricane activ-
ity in the Gulf [Goldenberg et al., 2001]. Furthermore, the
significant role of SST and the resulting air-sea interactions
GEOPHYSICAL RESEARCH LETTERS, VOL. 33, L17802, doi:10.1029/2006GL026623, 2006
1
Center for Earth Observing and Space Research, School of Computa-
tional Sciences, George Mason University, Fairfax, Virginia, USA.
Copyright 2006 by the American Geophysical Union.
0094-8276/06/2006GL026623
L17802 1of5

associated with the intensification of Katrina are inves-
tigated, t hrough the combination of satellite and buoy
observations along with numerical model simulations.
2. Data and Methodology
[7] The following data and methodology apply in our
study:
2.1. Data
[
8] a. SST from the tropical rainfall measuring mission
(TRMM) microwave imager (TMI) at 25 km resolution
from the Remote Sensing Systems (http://www.ssmi.com).
The advantage of using SST from microwave observations,
like the TMI, is that it provides retrievals even under intense
cloudy conditions associated with hurricanes.
[
9] b. Buoy observations of winds, SST, surface air
temperature, and dew point from the National Data Buoy
Center (NDBC) (http://www.ndbc.noaa.gov/).
2.2. Methodology
[
10] In order to investigate the impact of warm SSTA on
Katrina’s intensity variations, using the latest PSU/UCAR
mesoscale model MM5 (version 3.7), two control experi-
ments were designed. In the first experiment, SST data from
the TMI observations on August 26, 2005 were used as the
model’s initial conditions (hereafter referred to as fixed SST
or FSST). In the second experiment, a uniform 1.5C
anomaly was added to FSST in order to capture the impact
of high SST anomaly on the hurricane’s characteristics
(referred to hereafter as FSST + 1.5). We performed a
96-h simulation initialized at 00Z August 26, 2005 using
a triply nested grid configuration with grid resolutions of
54, 18 and 6 km, covering the stages of Katrina’s rapid
intensification across the Gulf and the subsequent landfall in
the northern Gulf coast. The other model’s initial and lateral
boundary conditions were obtained from the NOAA NCEP
GFS (Global Forecasting System) 1  1 global analysis.
A bogus vortex representing the inner circulation of Katrina
was used in the model initial conditions [Goerss and
Jefferies, 1994; Zhu et al., 2004]. In order to test the effect
of pre-existing warm SST, SST was held unchanged during
the simulations.
3. Warm Gulf Waters and Hurricane Katrina
[11] As shown in Figure 1, the NDBC buo y 42040
observations at South of Dauphin Island, AL (29.18N,
88.21W), located northeast of Katrina’s track (Figure 2a
shows the location of the buoy with a red star), indicated
SST over 30C in advance of about two days before Katrina
reached its strongest intensity or the maximum wind speed
(gust). However, the increase of wind speed associated with
Katrina’s intensification resulted in the decrease of SST. As
a result of evaporative cooling in the near-surface environ-
ment under warm SST (at least 27C) conditions, the surface
air temperature (Ta) decreased significantly (Figure 1a)
[Cione et al., 1999] and the sea-air temperature difference
(SST-Ta) reached its largest value at the time of the peak
hurricane intensity (Figure 1b) [Shay et al., 2000]. The
increase in air-sea temperature contrast induced the strength-
ening of atmosphere-ocean heat flux exchange. We have
computed the surface heat fluxes from the buoy observations,
including sensible heat flux (SHF) and latent heat flux (LHF)
as follows:
SHF ¼ r
a
C
p
C
s
U
10
SST  T
a
ðÞ
LHF ¼ r
a
LC
E
U
10
q
s
 q
a
ðÞ
ð1Þ
where q
s
is the specific humidity (gkg
1
) of the water
surface, q
a
is the specific humidity of the air near the
surface, U
10
is the surface wind speed (m/s) at 10m height,
L is the latent heat of vaporization (M Jkg
1
), and r
a
is the
surface air density (kgm
3
), Cp is the specific heat of air,
Cs and C
E
are the exchange coefficients. The calculated
SHF and LHF, with the use of constant exchange
coefficients (Cs = 0.9e  3 and C
E
= 1.35e  3), are the
strongest at the time of maximum hurricane intensity
(Figure 1c) due to the atmosphere-ocean energy exchange,
which causes storms to receive energy from warm oceans
for further intensification.
[
12] From the buoy observations, it is found that the
storm intensity changes are well correlated with the sea-air
temperature contrast (SST-Ta) and surface heat fluxes over
the time. However, buoy stations are sparse and unevenly
distributed. From the numerical model (MM5) simulations,
the spatial distribution of the maximum LHF values at the
intense stages (category 3 and up) was found to be located
at the right side of the storm track (Figure 2a), where winds
were also usually stronger (Figure 2a) and most clouds and
precipitation develop [ Zhu et al., 2004]. LHF is believed to
Figure 1. Time series of (a) wind speed, SST, and Ta,
(b) sea level pressure (SLP) and sea-air temperature contrast
(SST-Ta), and (c) surface sensible heat flux (SHF) and latent
heat flux (LHF) from buoy 42040 observations.
L17802 KAFATOS ET AL.: WARM GULF WATERS AND HURRICANE KATRINA L17802
2of5

be one of the major drivers in the hurricane energy systems
and plays a vital role in development and intensification of
tropical cyclones (TC) [Guinn and Schubert, 1993; Bender
and Ginis, 2000; Hong et al., 2000; Shay et al., 2000;
Gautam et al., 2005].
[
13] Satellite observations show the SST was unusually
warm and around 30C over the entire Gulf before Katrina’s
perturbation [Scharroo et al., 2005]. At the same time, the
SSTA from the TMI observations was more than 1C along
the hurricane track and its right side (Figure 2b). Consid-
ering the large heat capacity of the ocean, this anomaly is
very significant. It has been shown that relatively modest
changes i n SST of order 1C can effectively alter the
maximum total enthalpy (latent plus sensible heat flux) by
40% or more [Cione and Uhlhorn, 2003]. Katrina under-
went rapid intensification into category 5 when it moved
across the Gulf with high SSTA to the right side of its track
(also at the location of the maximum LHF). This may imply
high SSTA at the right side of the storm track may be a very
important factor for hurricane intensification.
[
14] The model simulations indicated that the maximum
LHF was always located at the northeastern quadrant of the
storm track, at least during the intense stages (category 3
and up), primarily due to the ambient flow relative to the
storm motion and diabetic heating resulted from the spatial
distribution of SSTA. Also, time series of LHF always
shows the largest value at the time of the hurricane’s
maximum intensity.
[
15] From equation (1), we can see that LHF depends on
both wind speed and SST, as q
s
is calculated from SST.
Increase in wind speed also enhances the LHF. In order to
distinguish the effect of SST on the LHF from wind speed,
we performed two numerical experiments in which all other
conditions, including winds, were identical, but only the
SSTs were different. As consistent with the buoy observa-
tions (Figure 1c), numerical model simulations show that
when the LHF increased and peaked during August 28 and
29, 2005, Katrina received and accumulated energy through
the ocean-atmosphere energy exchange and the central sea
level pressure (SLP) reached its lowest value and Katrina
attained its strongest intensity (Figure 3). From these two
experiments with the same wind speed but different SSTs,
we can clearly identify the effects of warm SST inducing
significant increases in LHF, especially at the stages when
Katrina underwent rapid intensification (Figure 3). While
the difference in the minimum SLP between the two experi-
ments is not evident until the 48-h simulation, when the
storm began to receive more energy supply through the air-
sea interaction processes (Figure 3), this is consistent with
the buoy observations (Figure 1).
[
16] In order to validate the model simulations, we have
also calculated surface LHF from buoy 42001 observations
according to equation (1). The exchange coefficient C
E
in
equation (1) reflects the efficiency of the vertical exchange
Figure 2. (a) MM5 simulated track, surface winds
(vectors) and LHF (W/m
2
) at 72 h simulation valid at 00Z
29, August 2005. The locations of buoys 42040 and 42001
are indicated with red stars. (b) Weekly mean (ending on
August 27, 2005) SST anomaly in relative to 8-year (1998 –
2005) average and the circles of different colors indicate the
observed track and intensity of Hurricane Katrina.
Figure 3. (a) Time series of MM5 simulated area-averaged
(400 km  400 km over the inner region centered on the
eye) LHF with the MRF PBL scheme and minimum SLP
from the two numerical experiments, and the observed
minimum SLP. (b) Comparison of LHF values from buoy
42001 observations using exchange coefficients from Cione
et al. [1999] and Bentamy et al. [2003] methods and MM5
simulations with Blackadar and MRF PBL schemes.
L17802 KAFATOS ET AL.: WARM GULF WATERS AND HURRICANE KATRINA L17802
3of5

of water vapor and energy flux and is affected by the
stability of the surface air. Several studies show that C
E
depends on wind speed and different empirical relationships
[Liu et al., 1979; Large and Pond, 1982; Cione et al., 1999;
Bentamy et al., 2003], while Emanuel [2005] suggests the
dependence of C
E
on temperature. Here, we performed two
experiments: In one experiment, C
E
was determined under
relatively high wind c onditions >20 m/s and increase
linearly with wind speed [Cione et al., 1999], referred as
the Cione method here:
C
E
¼ 0:75 þ 0:067U
10
ðÞ*10
3
ð2Þ
[17] While in another method, C
E
decreases with wind
speed [Bentamy et al., 2003], referred as the Bentamy
method here.
C
E
¼ a exp bU
10
þ cðÞ½þ
d
U
10
þ 1

*10
3
ð2
0
Þ
Where a = 0.146, b = 0.292, c = 2.206648, and
d = 1.6112292.
[
18] The LHF values from the Bentamy method, during
the maximum hurricane intensity (category 5), are almost
half compared to that from the Cione method, while the
differences during other times are small. The LHF values
from numerical model MM5 simulations depend on the
PBL schemes used. The Blackadar an d Medium-Range
Forecast (MRF) schemes are selected for tests here; since
they have nearly identical representations of surface fluxes
except for the exchange coefficients or the definition of the
non-dimensional stability functions are different. The LHF
values produced by the Blackadar PBL scheme [Zhang and
Anthes, 1982] are almost twice as low as those from the
MRF PBL scheme at the time of the maximum intensity,
while the differences during other stages are not evident. As
a result, the simulated intensity from the Blackadar PBL
scheme is weaker than that from the MRF scheme (not
shown). It is found that MM5 simulated LHF from the
Blackadar PBL scheme is close to that calculated from the
buoy observations using the C
E
decreasing with the wind
speed (the Bentamy method), while the MRF PBL scheme
is close to the Cione method with the C
E
increasing with the
winds (Figure 3b). Previous study by Braun and Tao [2000]
also showed the remarkable sensitivity of Hurricane Bob
(1991) to several PBL schemes in the MM5 and suggested
the dependenc e of simulated intensity on surface exchange
coefficients for heat and momentum or the parameterization
of surface fluxes. A special PBL model may be needed
especially for hurricane case.
[
19] Nevertheless, what we want to emphasize here is that
no matter what method or PBL scheme is used, the spatial
and temporal distributions of LHF are found to be similar.
The maximum LHF is always located to the right side of the
storm track at its intense stages, coincident with the location
of high warm SSTA. Time series of LHF always shows the
largest value at the time of hurricane’s maximum intensity.
4. Summary and Discussions
[20] Remarkable resemblance is found between SSTA
and the major hurricane acti vities over the Gulf. High SSTA
played an important role in the recent increase of intense
hurricane activity over the Gulf, especially after 1995.
However, the intensification of individual hurricanes may
not be spatially and temporally coincident with the distri-
bution of warm waters or high SST. This may be due to the
fact that in addition to warm SST, the spatial location of
high SST anomaly over the open ocean with respect to the
storm track is another significant factor.
[
21] Observations show Katrina intensified when it
entered the warm Gulf with SST over 30 C. SST is
found to increase in advance of the intensification of
Hurricane Katrina, also confirmed by numerical simula-
tions. This is because it may need some time for a
tropical cyclone to accumulate energy for further intensi-
fication. High SSTA at the northeastern quadrant of the
storm track over the Gulf induced significant increases in
surface heat fluxes and appears to have played a vital role
in Katrina’s intensificatio n. Our present analysis combin-
ing numerical model simulations together with satellite
and buoy observations shows the significant impact of
anomalous warm Gulf waters on the rapid intensification
of Hurricane Katrina and th e r ole of high SSTA in
governing the air-sea interactions associated with the
intensification of Katrina into a category-5 hurricane.
[
22] Acknowledgments. This work was supported by the VAccess/
MAGIC project funded by the NASA’s Science Applications Program, and
the NSF from grant NSF0543330. We are grateful to the reviewers for their
thorough review and helpful comments.
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
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