Abstract Numerical investigation of Hurricane Gilbert (1988) effect on the LoopCurrent warm core eddy (WCE) in the Gulf of Mexico is performed using theModular Ocean Model version 2 (MOM2). Results show that the storm-inducedmaximum sea surface temperature (SST) decrease in Gilbert’s wake is over 2.5�C, ascompared with the 3.5�C cooling in the absence of the WCE. The near-inertialoscillation in the wake reduces significantly in an along-track direction with thepresence of the WCE. This effect is also reflected between the mixed layer and thethermocline, where the current directions are reversed with the upper layer. Aftertwo inertial periods (IP), the current reversal is much less obvious. In addition, it isdemonstrated that Hurricane Gilbert wind stress increases the current speed of theWCE by approximate 133%. With the forcing of Gilbert, the simulated translationdirection and speed of the WCE towards the Mexican coast are closer to theobserved (42% more accurate in distance and 78% more accurate in direction)compared with the simulation without the Gilbert forcing. The simulated ocean re-sponse to Gilbert generally agrees with the recent observations in Hurricane Fabian.

Keywords Hurricane Gilbert Æ Warm core eddy Æ Near-inertial oscillation ÆOcean response

1 Introduction

Hurricane Gilbert attained a record minimum sea level pressure (MSLP) of 888 hPa at2152 UTC 13 Sept 1988 when traveling toward west-northwest (WNW) in the western

X. Hong (&) Æ S. W. ChangNaval Research Laboratory,7 Grace Hopper Ave., Monterey,CA 93943, USAe-mail: xd.hong@nrlmry.navy.mil

S. RamanNorth Carolina State University,Raleigh, NC, USA

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Nat HazardsDOI 10.1007/s11069-006-9057-2

ORI GI N A L P A PE R

Modification of the loop current warm core eddyby Hurricane Gilbert (1988)

Xiaodong Hong Æ Simon W. Chang Æ Sethu Raman

Received: 9 April 2005 / Accepted: 3 August 2006� Springer Science+Business Media B.V. 2006

Caribbean. Gilbert entered the Gulf of Mexico after landfall on the Yucatan Penin-sula, moving in a northwestward direction at about 5.6 m s–1 (Fig. 1). Although thestorm was much weaker when it entered the Gulf of Mexico, Gilbert maintainednearly constant intensity (MSLP of 940 hPa) before the final landfall in Mexicoapproximately 200 km south of Brownsville, Texas on 17 Sept 1988. Analyses of theAXBT and drifting buoy data and satellite images indicated that there was a LoopCurrent warm core eddy (WCE) F located within 200 km to the right of Gilbert’strack. Objectively analyzed temperature and estimated geostrophic flow fields fromAXCPs deployed in the western Gulf of Mexico one day after the passage of Gilbert(Wake I Experiment in Shay et al. 1998) indicated that there was interaction betweenhurricane-induced flows and the background eddy field. Maximum geostrophic cur-rents in the upper 30–50 m were approximately 0.9–1 m s–1 rotating anticyclonicallyaround the WCE F. In the core of the WCE F, observed temperature was 27–27.5�C,about 1�C cooler than the observed undisturbed Gulf water temperatures of 28–28.5�C. Upper ocean cooling reached 3–4�C with a right-side bias was observed alongthe storm track to about 200 km (Shay et al. 1992). However, cooling at 50 m depth inthe WCE F was not as evident. There is a large thermal gradient between the coldwake and the warm eddy in the central Gulf of Mexico as indicated by the AVHRRimages and the Airborne InfraRed Thermometer (AIRT).

The purpose of this study is to discuss the simulated ocean response to HurricaneGilbert using the GFDL Modular Ocean Model version 2 (MOM2). The simulatedmodel results will be compared with observation. The model results, as it will beshown, provide further detailed ocean response to Hurricane Gilbert as comparingto the relatively sparse observation. In addition to investigate the upper oceanthermal response, near-inertial and vertical response, the modification of WCE F byHurricane Gilbert surface forcing will also studied.

Yearday 257

16/06/

17/00

15/12

13/18

Fig. 1 Sea surface temperature (shaded), surface height (contour) and surface current (vector) atyearday 257 as initial condition in the simulations. The track of Hurricane Gilbert (dark solid line) ismarked with date and time in UTC

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The research described in this chapter is presented in four sections. Section 2briefly describes the ocean model and numerical experiments. Section 3 discussesand summarizes the numerical results. Section 4 compares the model-simulatedresults with recent observation. Section 5 gives conclusions.

2 The ocean model and experiment design

The ocean model used in this study is the Geophysical Fluid Dynamics Laboratory’smodular ocean model version 2 (MOM2), which is a three-dimensional primitiveequation model (Pacanowski 1996). The model has adapted the same ocean domainand resolutions as used in the study on the interaction between Hurricane Opal(1995) and a warm core ring in the Gulf of Mexico (Hong et al. 2000), since the twohurricanes had traversed in the Gulf of Mexico. The coefficients for the verticalmixing scheme such as maximum mixing coefficient, background diffusion coeffi-cient and the viscosity coefficient are also same as in Hong et al. (2000).

In lieu of an effective ocean data assimilation and prediction system, a 2-yearspin-up ended at yearday 257 with the limited-area domain for the Gulf of Mexicoprovides an ocean state resembling the observed state prior to Hurricane Gilbert(Fig. 1). This realization is used as the initial condition in this study. In thisrealization there is a WCE with location and strength similar to the observed,located 200 km to the right of Gilbert’s track. A prescribed surface wind stressfield (Chang and Anthes 1978) for Hurricane Gilbert is used as a surface forcing.The maximum wind stress specified is 42 dyn cm–2 at the radius of maximum windsof 60 km. The translation direction and the speed of the stress field follow the besttrack of Hurricane Gilbert, as depicted in Fig. 1 and listed in Table 1 of Shay et al.(1992).

Three numerical experiments are conducted for this study. Case C1 is a controlexperiment that simulates the response of the Loop Current and the WCE F toHurricane Gilbert wind field. Case C2 studies the Gilbert-induced response of aninitially motionless ocean without the WCE with uniformly distributed temperature.Case C3 is conducted without the hurricane forcing, in which the WCE is allowed todrift freely. All simulations start at 1200 UTC Sept 13. The simulation periods forCases C1 and C3 are 45 days and for Case C2 is 10 days. These simulationsencompass the periods prior to, during, one day after (Wake I), and three days after(Wake II Experiment in Shay et al. 1998) the passage of Gilbert. These results arecompared to the observation (Shay et al. 1992; 1998). The 45-day experiments forCases C1 and C3 extend to the yearday 302, which the analyses from observed WCEF are available for the comparison.

3 The ocean response in the Gulf of Mexico

Since the WCE F is located to the right of the storm track at approximately 200 km,or about 3 radius of maximum wind (Rmax), the Gilbert-induced ocean response is asexpected, complicated due to this mesoscale variability. The horizontal extent of thecold wake region induced by the hurricane is limited by the WCE because it has adeeper mixed layer depth, so is the SST cooling. The induced divergent and con-vergent currents are modulated by the anticylonic current associated with the WCE.

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The structure and the drifting speed of the WCE F are also modified as Gilbertpasses through the domain (Shay et al. 1992).

3.1 The upper ocean thermal response

The simulated sea surface temperature (SST) distributions and changes after the stormat 1500 UTC 19 Sept 1988 are shown in Fig. 2a for Case C1 and in Fig. 2b for Case C2.In Case C1, the maximum simulated SST decrease is over 2.5�C and the pool of coldwater (<26.0�C) extends to the western region of WCE F. The temperature gradientbetween the cool pool and WCE is about 2.0�C over a 100-km distance. This largehorizontal temperature gradient induces advection that may reduce the temperatureover the WCE. To the west, the decrease of SST is less than 1.0�C near the Mexicancoast due to the shallow shelf. The cooler SST is caused by the mixing in strong near-inertial current shears across the base of the mixed layer (Shay et al. 1992). This is incontrast to the central WCE where the maximum surface cooling is <1�C during thestorm as well as wake periods due to the deep warm layer.

The pattern of upper-ocean thermal response differs greatly between Cases C1and C2 as expected. The difference of temperature change between the two cases isdue to the existence of WCE adjacent the storm track. The deeper and warmerlayers associated with the WCE F reduce much of the storm-induced cooling. InCase C2, the temperature change is uniformly distributed between 1 and 2Rmax onthe right side of the storm track and in the wake of the storm (Fig. 2b), similar toidealized studies of ocean response to hurricane (Chang and Anthes 1979, Price1981). The uniform initial temperature field over the Gulf of Mexico is the mainreason for this distribution. The SST decreases in C2 from above 28.5�C to below25.0�C on the right side of the storm track, suggesting a maximum cooling of 3.5�C.

3.2 Near-inertial response

The time series of normalized surface current speed of u- and v-components have beenplotted for the cross-track distance at 2Rmax and for the along-track distance at 0.2k forCases C1 and C2 (Fig. 3a–d). The inertial wavelength k is ~600 km, given Gilbert’stranslation speed. The initial time is 0600 UTC 16 September to indicate the beginningof the storm period. The abscissa is scaled in inertial period (IP, about 30 h) relative tothe initial time. Therefore, the along-track distance at 0.2k is closer to the location ofthe WCE than at 2Rmax. At 2Rmax, the time evolution of the surface current patterns inthe two cases is very similar (Fig. 3a, b) and is only slightly larger magnitude in C2. Thesurface velocity u- and v-components are in quadrature and have an asymmetric re-sponse skewed to the right of the storm track, as the results of Shay et al. (1998).However, the responses at the distance of 0.2k (120 km) from the two cases are sig-nificantly different. Although the near-inertial oscillation exists as in Shay et al. (1998)with the WCE F (Fig. 3c), the simulated surface velocity components are dominated bythe WCE current speed. In Case C2 (Fig. 3d), the surface current response at 0.2k issimilar to the idealized case in previous studies.

3.3 Vertical structures

The depth-time cross-sections of the u-component for the cross-track distance at2Rmax and for the along-track distance at 0.2k for C1 are shown at 2Rmax (Fig. 4a)

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and at 0.2k (Fig. 4b). During the storm-forcing period (0–0.3 IP), the upper layercurrents are accelerated to a maximum speed of 130 cm s–1 in the east-west directionand 180 cm s–1 in the north-south direction (Figure not shown). The maximum

(a) With the WCE (1500 UTC 19 Sept)

(b) Without the WCE (1500 UTC 19 Sept)

(a)

Fig. 2 Sea surface temperature SST (shaded) and the SST change (contour) for (a) Case C1,(b) Case C2 at 1500 UTC 19 Sept 1988

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surface velocity in the C1 is larger than the observed maximum mixed layer veloc-ities of 110 cm s–1 (Shay et al. 1998). The maximum current components decrease to80 cm s–1 within the first 1.5 inertial periods. The current reversed direction belowthe thermocline, indicating a downward energy propagation of near-inertial waves(Shay et al. 1989). As shown in Fig. 3c, the near-inertial oscillation in the along-trackdirection weakens due to the presence of the anticyclonic circulation of the modu-lated WCE (Fig. 4b). The near-inertial oscillation is evident within the first 2 IP witha reversal of currents between the mixed layer and the thermocline. The currentreversal also exists after 2 IP, but with decreasing amplitude. In the case without theWCE F (C2), the near-inertial oscillation and reversal current between the mixed

(b)

(a)

(c)

(d)

Fig. 3 Normalized u- and v-components of surface current for Case C1 and Case C2 at 2Rmax and0.2 k respectively, where the vm = 160 cm s–1. The abscissa is scaled in inertial period relative to thepoint where Hurricane Gilbert approached at 0600 UTC 16 Sept 1988

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layer and the thermocline are comparatively more discernible after 2 IP (Fig. 4c),which is consistent with observations of Shay et al. 1998.

3.4 The modification of WCE F

Hurricane Gilbert moved in a WNW direction and passed within 200 km south ofWCE F (Fig. 1), it disturbs the WCE F more along the west side than along the eastside of eddy. The time change of surface current at the center of WCE F along24.3� N is shown in Fig. 5. The WCE F had a nearly asymmetric anticyclonic cir-culation before it was affected by Gilbert’s wind stress. The induced current re-sponse first takes place along the east side of WCE when Hurricane Gilbert passedthe Yucatan Peninsula and entered the western Gulf of Mexico, as indicated by thechange of southward currents along the east side of eddy to the southwest. Bothdirection and speed of the current show significantly changes along the west side ofeddy or west of 92� W after Sept 16. The simulated surface current speed increasesfrom 60 to 140 cm s–1, a 133% increase under Gilbert’s forcing. In the same period,

(a)

(b)

(c)

Fig. 4 Depth-time cross section of u-component at (a) 2Rmax, (b) 0.2k for Case C1, (c) 0.2k for theCase C2. The abscissa is scaled as in Fig. 3

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the current direction changes from northward to northwestward then westward.Subsequently, as Gilbert moved on, the induced current backs from eastward towestward. The isotach patterns in Fig. 5 delineated the inertial oscillation in thesurface current. The current speeds in general decrease with time after Sept 17 asGilbert continued to more away from the eddy. The current east of the eddy wasonly increased slightly over 60 cm s–1. The current speed in the center of WCE F hasa corresponding increase and decrease pattern due to the induced oscillation. Thewestward movement of WCE F over the 8-day period is evident in Fig. 5.

The shape and the position of WCE F were affected by Hurricane Gilbert too.Results from the two 45-day simulations with (C1) and without (C3) Gilbert’sforcing are compared in Fig. 6, where the 0 and 30 or 40 cm surface height contourshave been plotted for C1 (solid lines) and C3 (dotted lines) at various days from thesimulations. Both simulations start from Sept 14 with the same initial condition with

Fig. 5 Time change of surface current at the center of the WCE F along 24.3� N for Case C1

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the WCE as shown in Fig. 6a. After Hurricane Gilbert passes the WCE F (Fig. 6b),the eddy center location moves up more to the northwest in comparison to C3. Theseare caused by the strong hurricane-induced northward current along the south sideof eddy. The shape and the translation speed induced differ when the WCE Fpropagates towards the Mexican coast (Fig. 6c–f). The moving direction changesfrom southwest to west in C1 by the Hurricane Gilbert effect. By Oct 26, i.e.,

Fig. 6 The contour lines of 0 and 30/40 cm surface height for Cases C1 (solid lines) and C3 (dottedlines) at the various days into the simulations

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yearday 300 (Fig. 6f), the WCE F locates at 24.4� N and 93.6� W in C1, and at23.7� N and 93.3� W in C3. The central location of WCE F at the yearday 300 fromthe observation given by Shay et al. (1998) is at 24.5� N and 94.25� W. The distancesfrom the observed location to the model locations of C1 and C3 are 72 km and138 km. The location in C1 is about 8.6� south of the observed location, but thelocation in C3 is 40� south of the observed location. There is about 42% moreaccurate in distance and 78% more accurate in direction for C1. Hence, the drift of

Fig. 7 Time series of surface temperature from (a) buoys for Fabian (Niiler, personal communi-cation), (b) model simulated for Gilbert. Letters ‘‘left’’ and ‘‘right’’ indicate the temperature changesare collected at the left and right side of the storm

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the warm core eddy is more accurately reproduced in the simulation with Gilbert’sforcing (C1) than that without (C3).

4 Comparison with recent observation

Observational campaign in the Hurricane Component of the coupled boundarylayers air-sea transfer (CBLAST) has collected unprecedented rich data sets ofocean response, among them SST and temperature profiles during and afterHurricane Fabian (Sept 2–4, 2003) and Isabel (Sept 12–14, 2003) by drifting buoysand subsurface floats (Black, 2004). Time series of observed surface temperaturefor Fabian (Niiler, personal communication), and our Gilbert simulation (C1) areshown on Fig. 7. All the temperature variations feature a rapid decrease for1.5 day after the storm passage, and oscillated in the wake. The inertial period isabout 24 h for Fabian and 30 h for Gilbert. Since some buoys were located at theleft side of the storm and others at the right side of the storm, the decrease oftemperature and the inertial oscillations had different magnitude and amplitudeindicating a strong right bias. Larger temperature decrease and oscillation ampli-tude are shown for the right side. This right bias of ocean response to a storm isconsistent with previous observation (Price 1981; Sanford et al. 1987; Shay et al.1992). There are noticeable differences of surface and subsurface temperaturechanges between the observed in Fabian and simulated for Gilbert. These can beattributed to the differences in different mixed layer depth and structure in dif-ferent basins, and the model initial state and imperfections, such as a lack ofrecovering mechanism.

Temperature profiles from model simulation for Gilbert (Case C1) during thestorm and after the passage (Fig. 8a) are compared with those collected from CTDfloat in Fabian (Terrill, personal communication, Fig. 8b). There are generalsimilarities in the observed and simulated mixed layer structures after the stormpassage. Generally speaking, temperature decreases at the surface and uppermixed layer, and increases below the thermocline. The model is obviouslyincomparable to depict the sharp gradient at the bottom of the mixed layer due tolimited vertical resolution. In late time, the variation of deep layer temperature iscomplicated by upwelling and horizontal advection in both the observation andsimulation.

5 Conclusions

The ocean response to Hurricane Gilbert has been studied using the GFDL MOM2with an initial ocean state including the Loop Current and the WCE F similar to theobserved features and locations prior to Gilbert. The results from the numericalsimulation were able to conform to the observation about the modification of theLoop Current WCE F by Hurricane Gilbert and to provide us a more detailed oceanfeatures in three-dimensional and over a wider region than observation.

The maximum upper ocean temperature cooling due to the storm-induced mixingand upwelling was over 2.5�C. The existence of Warm Core Eddy adjacent to theright side of Gilbert’s path reduced the decrease of temperature caused by the stormdue to the fact of deep warm mixed layer inside the eddy. In a sensitivity test where

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Fig. 8 Temperature profiles from (a) CTD float (Terrill, personal communication) obtained every4 h for Fabian, (b) model simulation for Gilbert

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the initial condition in the Gulf of Mexico is quiescent, the maximum induced SSTdecreased 3.5�C.

Gilbert modified the WCE F more in the west side than in the east side because ofits track. The maximum surface current speed of the eddy under Gilbert’s forcingincreased by 133% of its pre-storm value with the current direction changed fromnorthward to northwestward then westward periodically with inertial period. In thevertical direction, the near-inertial oscillation propagated downward, however, inthe case with the WCE the vertical current reversal between the mixed layer and thethermocline is reduced after 2 IP due to the well-balanced eddy fields. The driftspeed and location of WCE F in its translation to the west of the Gulf of Mexico wascloser to the observed with Gilbert’s forcing.

The simulated ocean response to Hurricane Gilbert is compared with recentCBLAST observations in Hurricane Fabian. There are general agreements in termsof rapid cooling during the storm period, inertial oscillation during the wake period,and evolution of the mixed layer structure. However, due to the limited verticalresolution, the model is incomparable to depict the sharp gradient at the bottom ofthe mixed layer. Furthermore, the model imperfections, such as a lack of recoveringmechanism could be one of the reasons that results surface temperature decreaseduring the inertial oscillation (Fig. 7).

The Gulf of Mexico is characterized by the Loop Current, episodic shedding ofwarm core eddies and their westward propagation. These mesoscale activitiestransport the warm and salty Caribbean subtropical underwater (SUW) into theGulf. The life cycle of the eddy have a profound effect on the circulation in the Gulfof Mexico and greatly affect the local fishing, offshore drilling and other industriesand can impact the behavior of atmospheric disturbances such as Hurricane Opal(Hong et al. 2000; Shay et al. 2000; Bosart et al. 2000). Therefore, accurate analysisand forecast of eddy propagation will be important not only for hurricane forecastingbut also for many local industries. Since there is about 47% chance every year forintense hurricane cross the Gulf of Mexico, it will be necessary to consider theinfluence of hurricane in the studies of Loop Current WCE propagation.

Acknowledgements This research was supported by NRL Basic Research Program PE601153Nand the ONR 6.2 program PE602435N. The computations were performed at the Naval Oceano-graphic Office (NAVOCEANO) of the Department of Defense Major Shared Resource Center.

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