Eos, Vol. 87, No. 8, 21 February 2006 landfall at the northern Gulf coast, initialized at 0000 UT on 26 August 2005.This model sim ulation helps to measure SST as a function of the heat flux or energy exchange through the air-sea interactions. The maximum latent heat flux (LHF), which is associated with intensity variations of hurricanes [Gautam et al., 2 0 0 5 ] , shows significant increases (when additional SST was fed into the model's initial c o n d i tions) during rapid intensification of Katrina (Figure l c ) . S S T was found to i n c r e a s e prior to the significant d e e p e n i n g of the h u r r i c a n e central minimum SLR w h i c h o c c u r r e d after the 4 8 hours w h e n the simulated storm began to receive m o r e energy supply through the air-sea interac-
FORUM COMMENT & REPLY
Comment on "Satellite Altimetry and the Intensification of Hurricane Katrina" PAGE 8 9 In a recent Eos article, Scharroo et al. [2005] reported that the dynamic sea topography anomalies along the track of Hurricane Katrina were the most prominent factors caus ing the intensification of Katrina as it passed over these anomalous regions in the Gulf of Mexico. They show that the sea surface temper ature (SST) in the entire Gulf of Mexico was uniformly ~30°C and was not associated with the rapid intensification of Katrina. We partly agree with their findings b a s e d on the results of dynamic topography associ ated with Katrina's intensification; however, we do not c o n c u r with their idea that SST was not linked with the rapid intensification of Katrina. Here, we show the significant impact of high SST a n o m a l y in the Gulf on Katrina's rapid intensification and the role of a n o m a l o u s SST in governing the air-sea inter actions during its intensification. The SST distribution over the Gulf of Mex ico during Katrina's intensification shows a discernible warm patch of ~32°C associated with the upper shelf in the northern Gulf [see also Sharroo et al, 2005, Figure 2 a ] . According to Sharroo et al,2005,the warm SST along the Gulf c o a s t may b e shallow. However, a more than 1°C SST anomaly (SSTA) is found at the northeastern quad rant or to the right of the storm track (Figure la, outlined in red), where winds are usually stronger and most clouds and intense pre cipitation develop [Zhu et al, 2 0 0 4 ] . The SST over the Gulf and along the track of Katrina shows a significant increase prior to the drop of s e a level pressure (SLP) to its minimum value of 9 0 2 m b a r (Figure l b ) . T o investigate the impact of SST on Katrina's intensity variations, we used the latest Penn-
Fig. 1. (a) Sea surface temperature (SST) (shad ed) and SST anomaly (contours overlaid) dur ing 21-27August 2005 (the 7 s indicate areas where SST anomaly is above 1 °C) (b) SST averaged over the Gulf of Mexico (22~30°N, -98 ~ -81 °W) and along the track of Katrina (1000 kmxlOOO km area-average centered on the eye) with the observed minimum sea level pressure (c) Minimum sea level pressure and maximum latent heat flux from two numerical simulations: (1) CSST, where monthly mean SST during August 2005 was input as the models initial conditions, and (2) CSST+ 2, where an addition of2°C was inputted in order to cap ture the impact of the observed SST anomaly in relation to 8-year average from 1998 to 2005 on the hurricane's characteristics. Original color image appears at the back of this volume.
sylvania State University/University Corpora tion for Atmospheric Research (PSU/UCAR) m e s o s c a l e model MM5 (version 3.7) to perform 96-hour simulations covering the period of rapid development across the Gulf and 1
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Eos, Vol. 87, No. 8, 21 February 2006 tion p r o c e s s e s (Figure l c ) . W e found a phase lag of about two days b e t w e e n SST increase a n d the significant drop of the minimum SLR which is consistent with the observations (Figure l b ) . These model simulations, c o m p l e m e n t e d by remote sensing observations, show that high SST anomaly affected Katrina's rapid intensification by inducing significant increases in latent heat releases associated with the hurricane through dominant sea-air interactions. However, the intensification may not b e spatially a n d temporally consis tent with the distribution of warm SST. Since sea topography reflects the inte grated influence of S S T and salinity through
the water column, and a c a n o n i c a l correla tion analysis shows that sea topography and SST anomalies vary coherently at large spatial scales [Cummins et al, 2 0 0 5 ] , our present results therefore confirm the impact of SST on the rapid intensification of Katrina a n d the role of anomalous SST in governing the airsea interactions associated with the intensifi cation of Katrina into a Category 5 hurricane.
Gautam, R.,G. Cervone, R. PSingh,and M. Kafatos (2005), Characteristics of meteorological param eters associated with Hurricane Isabel, Geophys. Res. Lett., 32, L04801, doi: 10.1029/2004GL021559. Scharroo, R.,W H. FSmith, and J. L. Lillibridge (2005), Satellite altimetry and the intensification of Hur ricane Katrina, Eos Trans. AGU, 86(40), 366. Zhu,T.,D.L. Zhang,and FZ.Weng (2004), Numerical simulation of Hurricane Bonnie (1998): Part I. Eyewall evolution and intensity changes, Mon. Weather Rev., 732,225-241. —DONGLIAN SUN, RITESH GAUTAM, GUIDO CER
References
VONE, ZAFER BOYBEYI, AND MENAS KAFATOS;
Cummins, P F, G. S. E. Lagerloef, and G. Mitchum (2005), A regional index of northeast Pacific variability based on satellite altimeter data, Geophys. Res. Lett., 32, L17607, doi:10.1029/ 2005GL023642.
E-mail:
[email protected], Center for Earth Observing and Space Research, School of Computational Sci ences, George Mason University, Fairfax,Va.
Reply to Comment on "Satellite Altimetry and the Intensification of Hurricane Katrina"
face layer. As Figure 1 shows, the minimum sea level pressure near the eye of the storm c o i n c i d e s with the maximum in dynamic topography while crossing the W C R , not with the maximum SST near the shore. Shay et al [ 2 0 0 0 ] have reported a similar c a s e for the intensification of Hurricane Opal ( S e p t e m b e r a n d O c t o b e r 1 9 9 5 ) while crossing a W C R in the Gulf of Mexico. They, too, c a m e to the c o n c l u s i o n that high SST is a necessary, but insufficient, condition for hurricane intensification. However, more relevant is t h e upper o c e a n heat content, as determined b y t h e upper o c e a n temperature a n d t h e depth of the thermocline, which is reflected in t h e dynamic topography. In the c a s e of Typhoon Imbudo (July 2 0 0 3 ) , Goni and Trinanes [ 2 0 0 3 ] showed that the depth of the 2 6 ° C isotherm d e c r e a s e d as a result of the release of thermal energy to the atmosphere. T h e s e c a s e s e m p h a s i z e t h e important contribution of t h e s e a surface height to the estimation of t h e tropical c y c l o n e heat potential, which more efficiently predicts h u r r i c a n e intensification than s e a surface temperature a l o n e .
PAGE 8 9 In their c o m m e n t o n our Eos article [Scharroo et al, 2 0 0 5 ] , S u n et al [this issue] c o n c l u d e that s e a surface temperature (SST) had a significant impact on the rapid inten sification of Hurricane Katrina. Although SST may have played s o m e role, we want to stress that dynamic topography is a more reliable proxy than S S T for upper o c e a n heat content, which is ultimately responsible for the cyclone's intensification. As e v i d e n c e of the impact of SST on s e a level pressure ( S L P ) , S u n et al [this issue, Figure l c ] present t h e o u t c o m e of model simulations in which the SST over the Gulf of Mexico was raised by as much as 2°C. After a two-day lag, this comparatively large increase in SST c a u s e d SLP to d e c r e a s e by a mere 1 0 millibar. However, s i n c e the SST
along Katrina's track across the Gulf of Mex ico varied by n o more than 1°C, whereas the SLP dropped by about 9 0 millibar (Figure 1 ) , their results actually indicate that the impact of SST on t h e hurricane intensification was neither rapid nor significant. Moreover, the perceived correlation between SST a n d SLP was very weak. Although the shelf water in the northern part of the Gulf of Mexico is about 1°C warmer than the central Gulf, the hurricane in fact lost strength as soon as it entered this region ( s e e Figure 1 ) . Katrina reached maxi mum intensity somewhat earlier as it crossed a warm c o r e ring (WCR) and experienced the most rapid drop in sea level pressure when crossing the Loop Current. Both o c e a n ographic features are prominently displayed as maxima in the dynamic topography but are obscured in SST due to a thin, warm sur
References Goni,G. J.,and J. A.Trinanes (2003),Ocean thermal structure monitoring could aid in the intensity forecast of tropical cyclones, Eos Trans. AGU, 84(51), 573,577-578. Scharroo, R.,W H. FSmith, and J. L. Lillibridge (2005), Satellite altimetry and the intensification of Hur ricane Katrina,£bs Trans. AGU, 86(40), 366. Shay, L. K., G. J. Goni, and PG. Black (2000), Effect of a warm oceanic feature on Hurricane Opal, Mon. Weather Rev., 126, 1366-1383. Sun, D., R. Gautam, G. Cervone, and M. Kafatos (2005), Comment on "Satellite altimetry and the intensifi cation of Hurricane Katrina," Eos Trans. AGU, this issue. 26
27
28
29
Day in August 2005
— R E M K O SCHARROO, Altimetrics LLC, Cornish,
N.H.; E-mail:
[email protected]; WALTER H. E Fig. 1. Sea surface temperature (SST, triangles), dynamic topography (DT,open circles), and sea level pressure (SLPsolid circles) along the track of Hurricane Katrina during the period 26-29 August 2005. Shaded bars indicate the crossing of the Gulf Loop Current, a warm core ring (WCR), and the northern Gulf shelf water. Intensification of Katrina, indicated by decreasing SLP correlates well with higher dynamic topography.
SMITH and JOHN L . LILLIBRIDGE, National Oceanic
and Atmospheric Administration, Laboratory for Satellite Altimetry, Silver Spring, Md.
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