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

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— 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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