3248 MONTHLY WEATHER REVIEW VOLUME 136

Loop Current Response to Hurricanes Isidore and Lili

LYNN K. SHAY Division of Meteorology and Physical Oceanography, Rosenstiel School of Marine and Atmospheric Science, University of Miami, Miami, Florida

ERIC W. UHLHORN Hurricane Research Division, NOAA/Atlantic Oceanographic and Meteorological Laboratory, Miami, Florida

(Manuscript received 8 February 2007, in final form 17 December 2007)

ABSTRACT

Recent hurricane activity over the Gulf of Mexico basin has underscored the importance of the Loop Current (LC) and its deep, warm thermal structure on hurricane intensity. During Hurricanes Isidore and Lili in 2002, research flights were conducted from both National Oceanic and Atmospheric Administration (NOAA) WP-3D aircraft to observe pre-, in- and poststorm ocean conditions using airborne expendable ocean profilers to measure temperature, salinity, and current structure. Atmospheric thermodynamic and wind profiles and remotely sensed surface winds were concurrently acquired as each storm moved over the LC. Observed upper-ocean cooling was about 1°C as Isidore moved across the Yucatan Straits at a speed of 4msϪ1. Given prestorm ocean heat content (OHC) levels exceeding 100 kJ cmϪ2 in the LC (current velocities Ͼ1msϪ1), significant cooling and deepening of the ocean mixed layer (OML) did not occur in the straits. Estimated surface enthalpy flux at Isidore’s eyewall was 1.8 kW mϪ2, where the maximum observed wind was 49 m sϪ1. Spatially integrating these surface enthalpy fluxes suggested a maximum surface heat loss of 9.5 kJ cmϪ2 at the eyewall. Over the Yucatan Shelf, observed ocean cooling of 4.5°C was caused by upwelling processes induced by wind stress and an offshore wind-driven transport. During Hurricane Lili, ocean cooling in the LC was ϳ1°C but more than 2°C in the Gulf Common Water, where the maximum estimated surface enthalpy flux was 1.4 kW mϪ2, associated with peak surface winds of 51 m sϪ1. Because of Lili’s asymmetric structure and rapid translational speed of 7 m sϪ1, the maximum surface heat loss resulting from the surface enthalpy flux was less than 5 kJ cmϪ2. In both hurricanes, the weak ocean thermal response in the LC was primarily due to the lack of energetic near-inertial current shears that develop across the thin OML observed in quiescent regimes. Bulk Rich- ardson numbers remained well above criticality because of the strength of the upper-ocean horizontal pressure gradient that forces northward current and thermal advection of warm water distributed over deep layers. As these oceanic regimes are resistive to shear-induced mixing, hurricanes experience a more sustained surface enthalpy flux compared to storms moving over shallow quiescent mixed layers. Because ocean cooling levels induced by hurricane force winds depend on the underlying oceanic regimes, features must be accurately initialized in coupled forecast models.

1. Introduction weather forecasting systems to prepare for landfalling systems (Marks and Shay 1998; Bender and Ginis Coupled models that predict hurricane intensity and 2000). For such models, it has become increasingly clear structure change are being used to issue forecasts to the over the past decade that the oceanic component will public, who will increasingly rely on the most advanced have to include realistic initial conditions to simulate not only the oceanic response to hurricane forcing (Price 1981; Sanford et al. 1987; Shay et al. 1992; Price Corresponding author address: Lynn K. Shay, Division of Me- et al. 1994; D’Asaro 2003) but also the atmospheric teorology and Physical Oceanography, Rosenstiel School of Ma- rine and Atmospheric Science, University of Miami, 4600 Rick- response to oceanic forcing (Shay et al. 2000; Hong et enbacker Causeway, Miami, FL 33149. al. 2000; Lin et al. 2005; Walker et al. 2005; Wu et al. E-mail: [email protected] 2007; Shay 2008).

DOI: 10.1175/2007MWR2169.1

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An important example of this latter effect was ob- embedded within its circulation pattern, with smaller- served during Hurricane Opal’s passage in 1995, when scale cold core rings located along their periphery. The atmospheric conditions were conducive for Opal’s anticyclonic circulation around the LC exits the GOM rapid deepening over 14 h over the Gulf of Mexico through the Florida Straits between the United States (GOM; Bosart et al. 2000). During this deepening pro- and Cuba to form the Florida Current and, eventually, cess, Opal passed over a warm core ring (WCR) shed the Gulf Stream. These ribbons of deeper and warmer earlier by the Loop Current (LC) as detected by radar ocean current features transport heat poleward, repre- altimeter measurements of the surface height anomaly senting an integral part of the gyre circulation (Gill (SHA) fields from the National Aeronautics and Space 1982). Administration’s (NASA’s) Ocean Topography Ex- Investigating a central question about upper-ocean periment (TOPEX)/Poseidon mission (Shay et al. heat, Leipper and Volgenau (1972) developed a rela- 2000). Although satellite-derived images revealed that tionship to estimate the hurricane heat potential or sea surface temperatures (SSTs) were 29.5° to 30°C, ocean heat content (OHC), namely, there was little evidence of this warm ocean feature’s ␩ signature compared to the surrounding Gulf Common ϭ ͵ ␳͑ ͓͒ ͑ ͒ Ϫ ͔ ͑ ͒ Q cp z T z 26 dz, 1 Water (GCW). Using a coupled model, Hong et al. h26 (2000) performed a series of sensitivity tests with and without this observed WCR. They found that Opal where cp is specific heat at constant pressure (4.2 kJ kgϪ1 KϪ1), ␳(z) is the density structure, the observed deepened an additional 14 mb over the WCR compared temperature is T(z), and integration limits stretch from to numerical experiments without it. Walker et al. the depth of the 26°C isotherm (h ) to the surface (␩). (2005) found that cold core rings located on the periph- 26 In subtropical regimes such as the LC, OHC values ery of the larger WCR helped to weaken Hurricane exceed 100 kJ cmϪ2 (Leipper and Volgenau 1972). That Ivan (2004) just prior to landfall. More recently, Shay is, the 20° and 26°C isotherm depths are located at (2008) showed that the LC and WCR did not signifi- ϳ300- and 150-m depths in this subtropical water mass, cantly cool during the passage of Hurricanes Katrina compared with ϳ100- and 50-m depths, respectively, in and Rita when these hurricanes rapidly deepened to the GCW. Category 5 status. These studies emphasize the impor- To improve our understanding of the LC response to tance of initializing models with realistic ocean features the passage of a mature hurricane, a series of experi- to couple to hurricane forecasting models (Jacob and ments was conducted from National Oceanic and At- Shay 2003; Falkovich et al. 2005; Halliwell et al. 2008). mospheric Administration (NOAA) WP-3D research The upper ocean’s transport from the northwest Ca- flights (N42RF, N43RF) and the NOAA Gulfstream- ribbean Sea and through the Yucatan Straits signifi- IV (N49RF) aircraft during the passage of Hurricanes cantly influences the GOM circulation patterns. These Isidore and Lili. The experimental sampling strategy ϳ ϵ 6 3 Ϫ1 transports, 24 Sv (1 Sv 10 m s ) through the was designed to deploy global positioning system straits, force LC variability and modulate WCR shed- (GPS) sondes (Hock and Franklin 1999), airborne ex- ding events (Maul 1977; Sturges and Leben 2000; Leben pendable current profilers (AXCPs), airborne expend- 2005). The LC transports warm subtropical water with able conductivity, temperature, and depth profilers a markedly different temperature and salinity structure (AXCTDs), and airborne expendable bathythermo- into the GOM compared to the GCW (Shay et al. graphs (AXBTs) prior to, during, and subsequent to 1998). As the LC intrudes north of 25°N, WCRs with hurricane passage. This experimental effort in Hurri- diameters of 100–200 km separate from the LC at an canes Isidore and Lili improved upon a previous Hur- average interval of 6–11 months, based on radar altim- ricane Gilbert experiment (Shay et al. 1992) by mea- eter-derived SHA fields (Sturges and Leben 2000). In suring prestorm, in-storm, and poststorm currents, contrast, when the LC retracts south of 25°N, the time temperatures, and salinities along with detailed atmo- envelope for WCR shedding events increases to an av- spheric temperature, humidity, and wind soundings erage of more than 17 months (Leben 2005). Regard- (Table 1). The objective of this paper is to document less of the northward LC penetration, these anticycloni- the evolving thermal and momentum ocean response to cally rotating WCRs propagate westward at speeds of the heat, moisture, and momentum fluxes across the 3–5kmdayϪ1 (Elliot 1982). Note that both the LC and air–sea interface based on the combination of GPS WCR features contain upper-ocean currents of up to sonde profiler data (Hock and Franklin 1999) and re- 1.7 m sϪ1 (Forristall et al. 1992; Oey et al. 2005). At any motely sensed surface winds from the Stepped Fre- given time, the GOM may have two or three WCRs quency Microwave Radiometer (SFMR; Uhlhorn et al.

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TABLE 1. Deployed probes (RF failures in parentheses) for Isidore and Lili research flights from NOAA/NESDIS ocean winds (OW), NSF, and landfall (LF) flights. Note that the 19 and 23 Sep flights represent pre-Lili conditions in the Gulf of Mexico (based on RF signals, success rates Ͼ85%). All dates are in 2002.

Isidore Lili Date Flight GPS BTs CPs CTDs Date Flight GPS BTs CPs CTDs 18 Sep NSF 11 20(4) 16(1) 23(2) 25 Sep OW 17 10 0 0 19 Sep OW 25 20 0 0 29 Sep NSF 23 35(2) 0 0 19 Sep NSF 11 19(2) 18(2) 21(1) 30 Sep NSF 18 31(3) 6(2) 0 21 Sep OW 53 18 0 0 30 Sep OW 14 10 0 0 21 Sep NSF 22 19(1) 30(7) 14(0) 2 Oct OW 47 20 0 0 22 Sep OW 15 10 0 0 2 Oct NSF 43 19(4) 26(4) 18(4) 22 Sep LF 10 19 0 0 3 Oct LF 20 1 0 0 23 Sep NSF 12 21(2) 27(4) 16(1) 4 Oct NSF 16 10(1) 35(4) 18(0) Total 16 347 282(19) 158(24) 110(8)

2007). To accomplish this objective, this paper is orga- GOM (Fig. 1b), making landfall on 26 September just nized as follows: a description of the profiler data, in- west of Grande Isle, Louisiana. cluding chronologies of Hurricanes Isidore and Lili, is in section 2; the experimental approach, including the b. Hurricane Lili surface wind forcing and air–sea parameters, is de- Lili was also a tropical wave of Cape Verdean origin, scribed in section 3; the observational analysis and air– starting on 16 September 2002 (Pasch et al. 2004). This sea fluxes are discussed in section 4; section 5 describes wave became a tropical depression on 21 September, the forced LC response with respect to ocean cooling and as the system moved just west of north at Ϸ10 and mixing; and the results are summarized, together msϪ1, initial intensification to TS status occurred on 23 with concluding remarks, in section 6. September. The TS subsequently weakened to an open tropical wave on 26 September, but as the wave slowed 2. Hurricane chronology it redeveloped into a TS late on 27 September, with a minimum central pressure of 994 mb. Lili intensified to a. Hurricane Isidore hurricane status at 1200 UTC 30 September while pass- ing over the Cayman Islands. As Lili tracked along a The tropical wave from which Isidore developed north-northwest trajectory after emerging off the Cu- originated over Cape Verde, where several periods of ban coast (Fig. 2), the hurricane intensified to Category fluctuating storm intensity occurred until this wave 3 status (ϳ51 m sϪ1) over the LC and to a Category 4 crossed the 50°W meridian (Pasch et al. 2004). How- Ϫ1 ever, only when the tropical depression moved into the storm (61 m s ) in the south-central GOM just north western Caribbean Sea was it named Tropical Storm of the boundary between the LC and the GCW. During R Isidore on 17 September 2002. Isidore then moved this period, Lili’s radius of maximum winds ( max) de- slowly along a northwest track, and at 1800 UTC 19 creased from 25 km over the LC to 18 km along the northern boundary between the LC and the GCW September, Isidore was upgraded to a hurricane with a Ϫ1 central pressure of 983 mb. Isidore’s winds exceeded 40 when the storm system was moving at 7 m s . Lili Ϫ1 rapidly weakened to Category 1 status owing to a com- ms on 20 September as it approached the Isle of bination of enhanced atmospheric shear, the intrusion Youth and made landfall along the tip of western Cuba. of dry air along the western edge (Pasch et al. 2004), The storm remained over Cuba for about 12 h, re- and interactions with the shelf water cooled by Isidore. emerging over the Yucatan Straits midday on 21 Sep- Ϫ1 Hurricane Lili made landfall at 1300 UTC 3 October tember. This slow-moving hurricane (ϳ4ms ) trav- near Intracoastal City, Louisiana. eled westward (Fig. 2), intensifying to a Category 3 storm just north of the Yucatan Straits over the LC (see Fig. 1). Isidore moved over the Yucatan Shelf on 22 3. Experimental approach September and made landfall on the Yucatan Peninsula During the 2002 NOAA Hurricane Research Divi- for the next 36 h, weakening to a minimal tropical storm sion’s (HRD’s) hurricane field program, a joint Na- (TS) embedded within a broad atmospheric circulation tional Science Foundation (NSF) and NOAA experi- pattern. Isidore subsequently moved northward and ment measured both the kinematic and thermodynamic created a cool wake of 28.5°C SSTs across the central upper-ocean response to a propagating mature tropical

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FIG. 1. (a) LC (red contour) position relative to the best tracks of Isidore (black track, originating in bottom center) and Lili (black track, originating in bottom-right corner) in September–October 2002 with intensities (see legend) and the location of NDBC 42001 (blue dot). (b) Observed SSTs (°C) at NDBC 42001 with gray shading indicating lifetimes of Isidore and Lili. cyclone over the LC. Motivated by the Hurricane Opal ber off the Yucatan Peninsula and along the Louisiana case, the experimental objective was to measure the Coast for Isidore and Lili, respectively. Success rates levels of upper-ocean cooling and shear-induced mixing for the oceanic profilers, defined here as receiving radio in the LC circulation system. To achieve this objective, frequency (RF) signals on the aircraft, were greater the experiment used 16 research flights, each deploying than 85%. GPS sondes were also concurrently de- oceanic and atmospheric expendable probes in the ployed from the aircraft, including the G-IV used to same location before, during, and after the passage of map the regional-scale atmospheric structure over the Hurricanes Isidore and Lili (Table 1). A set of prestorm GOM from flight level to the surface in the storm and flights was conducted during 18–23 September 2002; the surrounding environment. in-storm flights occurred on 21 September and 2 Octo- The comprehensive set of measurements included ber; and poststorm surveys were acquired on 22 and 23 both in situ and remotely sensed data. The data ac- September and 4 October. In addition, there were also quired included current and temperature profiles two landfall experiments on 22 September and 3 Octo- from AXCPs, temperature and salinity profiles from

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FIG. 2. HRD H*Wind surface wind analysis of Hurricanes (a) Isidore at 2130 UTC 21 Sep 2002 and (b) Lili at 0700 UTC 02 Oct 2002. Isotachs are contoured every 5 m sϪ1. Data used to generate this analysis include observations from the SFMR, GPS dropwindsondes, Quik- SCAT scatterometer, and available hourly buoy reports. The storm track is indicated by the solid line; the dark box shows the ocean data analysis region considered for this research.

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AXCTDs, temperature profiles from AXBTs, and sur- ployed profilers, including over the Yucatan Shelf. Dur- face directional wave spectra from the NASA scanning ing the poststorm experiment on 23 September (ϳ2 radar altimeter (Wright et al. 2001). Note that although days later), 64 probes were deployed along transects the AXCPs, AXCTDs, and AXBTs all measure tem- similar to those of the prestorm and in-storm flights. perature, the temperature measurements only pen- Seven of these profilers did not transmit data back to etrate to 350 m for the AXBTs compared to 1000 and the aircraft. 1500 m for AXCTDs and AXCPs, respectively. A sec- ond difference is that temperatures from AXBTs and b. Lili AXCPs are measured with a thermistor with an accu- racy of 0.2°C; an AXCTD’s thermistor accuracy is One week later, on 28 September 2002, TS Lili 0.12°C. In this study, SSTs from all expendable profilers moved to the northwest toward the Yucatan Straits fol- are defined to be near-surface temperatures within the lowing a track similar to Isidore. During this time, a first few meters of the sea surface. Also, deeper profiles prestorm flight was conducted in front of the projected allow estimates of geostrophically balanced currents TS Lili (and potential hurricane) track on 29 September relative to 750 m for assessing initialization schemes (not shown) by deploying AXBTs. On 30 September, used in ocean models (Halliwell et al. 2008). Finally, the additional profilers were deployed from an in-storm surface wind field was measured from observations by flight centered on Lili at 20°N, 81°W, including six GPS sondes (Hock and Franklin 1999) and SFMR AXCPs with four probes providing profiler data (not (Uhlhorn et al. 2007). These in situ data are cast within shown). As Lili continued along this northwest trajec- a regional-scale context using SHA fields measured tory, moving over the western tip of Cuba as a Category from radar altimetry (Cheney et al. 1994; Sturges and 1 storm, the in-storm flight on 2 October (Fig. 3f) was Leben 2000). TOPEX and Geosat Follow-On Mission centered fortuitously on the pre-Isidore grid of 19 Sep- SHA data (in Fig. 3) were objectively analyzed at tember (Fig. 3b). On this research flight, 63 profilers 0.5° using the method of Mariano and Brown (1992) were deployed, with 12 of them not providing any RF based on parameters used with the Hurricane Gilbert signals to the aircraft. It was during this early flight on dataset. N43RF and the later N42RF flight on 2 October [in support of a NOAA/National Environmental Satellite, Data, and Information Service (NESDIS) ocean winds a. Isidore experiment deploying AXBTs] that Lili deepened to a Given uncertainties in Isidore’s track, two grids of Category 4 storm just northwest of the boundary be- prestorm ocean profilers were deployed from research tween the LC and the GCW in the central GOM basin. aircraft in the northwest Caribbean Sea and in the A post-Lili experiment was then conducted on 4 Octo- south-central part of the GOM on 18 and 19 September ber by deploying the same number of profilers, with 2002 (Figs. 3a,b). Success rates in acquiring profiles only five RF failures. Thus, these oceanic and atmo- were Ϸ75% from oceanic probes. In some cases, the spheric measurements were acquired when two hurri- compound surrounding the thermistor was compro- canes, separated by 10 days, were intensifying over the mised below 100 m because of pressure effects that same oceanographic regime. caused bad temperature profiles from AXCPs. A few AXCTDs also had incorrect calibration coefficients and c. Air–sea parameters were not used in the analyses below. On each of the prestorm flights, there were eight probes with no RF Air–sea parameters and scaling arguments, defined signals. During the dual-aircraft Isidore flight on 21 in Table 2, are used to place the observations into a September (Fig. 3c), there were eight failures out of 63 nondimensional framework based on Price (1983). The deployed profilers, and 19 additional AXBTs deployed wavelength of the oceanic response induced by a mov- from N42RF with 5 RF failures. Several AXCPs mal- ing tropical cyclone is proportional to the product of functioned along the western boundary of the Yucatan the storm translation speed Uh and the local inertial Straits, where large currents and current shears caused period (IP; Geisler 1970). Based on a 4 m sϪ1 transla- the thin wire connecting the probe to the surface unit to tion speed and an IP of 1.3 days (Ϸ31 h), the predicted break. Given the dual-aircraft mission, this still pro- wavelength ⌳ for Isidore is 450 km (Table 2). For Lili, duced unprecedented coverage of the upper ocean as a this wavelength is ϳ770 km because of faster transla- hurricane intensified to Category 3 status. On 22 Sep- tion speeds of ϳ7msϪ1 and IPs ranging from 1.3 days tember, there were 29 additional AXBTs deployed (31 h) to 1.16 days (28 h) over the LC and GCW, re- over the same regime where the in-storm flights de- spectively. The Rmax for these storms over the LC

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FIG. 3. SHA field (colors indicate heights in cm) from radar altimetry and oceanic profile deployments for (a) pre-Isidore in the northwest Caribbean Sea, (b) pre-Isidore in the south central GOM, (c) Isidore in-storm across the Yucatan Straits, (d) post-Isidore (1 day), (e) post-Isidore (2 days), (f) Lili in-storm over the grid in (b), and (g) post-Lili on 4 October relative to the Isidore and Lili tracks with hurricane category levels as in Fig. 1. AXBT data are indicated by triangles; AXCP, by boxes; AXCTD, by circles. Black symbols indicate good data; white symbols indicate probe failure.

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TABLE 2. Air–sea parameters, nondimensional numbers and scales in Isidore and Lili (LC and GCW) based on Price’s (1983) scaling arguments. Maximum wind stress is estimated from SFMR data.

Isidore Lili Parameter LC LC GCW

Radius of maximum winds (km) Rmax 23 25 18 Ϫl Maximum wind (m s ) Wmax 50 50 61 Ϫ2 ␶ Maximum wind stress (N m ) max 7.1 7.1 10. Ϫ1 Speed of the hurricane (m s ) Uh 4 6.9 7.7 Wavelength (km) ⌳ 449 771 775 Ϫ1 First mode phase speed (m s ) c1 1.5 1.5 2.8 ␣Ϫ1 First mode deformation radius (km) n 28 26 46

Time scale (IP) t1 1.2 1.7 0.3 Inertial period (days) IP 1.3 1.3 1.16 Reduced gravity (m sϪ2 ϫ 10Ϫ2) gЈ 1 1 2.5 Mixed layer depth (m) h 110 110 35 Non-dimensional numbers

Froude number Fr Uh /c1 2.6 2.5 2.8 Nondimensional storm speed SUh /(2Rmax f ) 0.54 0.8 1.15 ϩ Ϫ2 Ј 2 Mixed-layer Burger number M (1 S )g h/(2Rmax f ) 0.08 0.04 0.035 Thermocline Burger number TbhϪ1M 0.072 0.036 0.2 ␣ Nondimensional forcing Fo 2Rmax/ 1 1.64 1.92 0.78 Scales Ϫ1 ␶ ␳ Wind-driven velocity Vml (m s ) maxRmax/( ␱hUh) 0.38 0.24 0.68 Ϫ1 Ϫ1 Thermocline velocity Vth (m s ) hb Vml 0.42 0.26 0.12 ␩ ␶ Isopycnal displacements s (m) max/(fUh) 10.9 6.1 7.3 Ϫ1 Ј␩ Geostrophic velocity Vgs (cm s )gs/(fRmax) 2.9 1.4 5.3 Frequency shift ⑀ M/2 0.04 0.02 0.018

ranged from 23 to 25 km; however, as Lili moved north- associated with the first baroclinic mode (Gill 1984),

westward over the GCW, Rmax decreased to 18 km dur- the baroclinic time scale required for a phase difference ing a deepening cycle to a Category 4 storm. In this of ␲/2 to develop between the first baroclinic mode and context, along- and cross-track directions are nondi- the other baroclinic modes is given by ⌳ mensionalized in terms of and Rmax to examine the forced ocean response relative to the observed storm ␲f t ϭ , ͑2͒ structure (Shay et al. 1998). 1 2 2 k c1 The first baroclinic mode wave phase speed in the LC Ϫ is approximately 1.5 m s 1; in the GCW, this phase where k represents the horizontal wavenumber of the Ϫ1 speed increases to 2.8 m s because of stronger strati- wind stress, f is the local Coriolis parameter, and c1 is fication at the base of the mixed layer. Reduced gravi- the first baroclinic mode phase speed. The relevant Ϫ Ϫ ties (gЈ) ranged from ϳ1 ϫ 10 2 ms 2 in the LC to time scales are 1.2–1.7 IPs in the LC regime compared, Ϫ Ϫ 2.5 ϫ 10 2 ms 2 within the GCW (e.g., Shay et al. with 0.3 IPs in the GCW. This more rapid time scale is

2000). The Caribbean subtropical water (STW) strati- due to a decrease in Rmax coupled with a faster phase fication is weaker, with large ocean mixed layers speed when Lili moved over the GCW. Note that this

(OMLs) of O(100 m) compared to the GCW stratifica- time scale was O(2 IPs) during Gilbert due to an Rmax tion, where the initial OML depth lies between 35 that was 3 times larger than that observed in either and 40 m. The Froude number, defined as the ratio Isidore or Lili (Shay et al. 1992). ␩ ␶ ␳ of the translation speed to the first baroclinic mode Isotherm displacements ( ) scale as max/( ofUh), Ϫ1 wave phase speed Uhc1 , exceeds 2.5 in both cases, about 11 m (6 m) in the expected Isidore (Lili) ocean implying a predominant baroclinic ocean response response. The geostrophic velocity response Vgs, pro- Ј␩ (Geisler 1970). The baroclinic radius of deformation portional to g /(fRmax), is predicted to be weak, ␣Ϫ1 Ϫ1 ( 1 ) was 28 km in the LC, compared with 46 km in the with values of 1 to 3 cm s in the LC compared with GCW. When the length scale of the wind stress forcing 5cmsϪ1 in the GCW, consistent with the Gilbert Ϸ (2Rmax 50 km) is greater than the deformation radius dataset (Shay et al. 1992). Wind-driven ocean veloc-

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␶ ␳ ␻ ␻ ity Vml scales as maxRmax/( ohUh) representing a veloc- z z kz Ϫ ͑ ͒ ϭ ͫ ͩ ͪ ϩ ͩ ͪͬ ϩ 1 u z C cos S sin e U ity of 24–38 cm s , with similar values in the ther- W W i mocline based on the expression Vmlh/b (where h is the Z Ϫ Z initial OML depth and b is the thermocline thickness). ϩ ͫ Ϫ ͩ iϪ1 iͪͬ ͑ ϭ ͒ Sz z , for i 1, 3 , Weak upper-ocean stratification distributed over i 2 deeper layers will cause the LC response to be weaker ͑4͒ than in the more stratified GCW (Price 1981; Shay and Elsberry 1987; Shay et al. 1992). In the GCW, however, where u(z) is the modeled east–west current profile [a the predicted wind-forced thermocline current is 12 Ϫ similar expression holds for the north–south compo- cm s 1 because of stronger stratification and shallower nent ␷(z)], C and S are the amplitudes of the orbital OML depths. The magnitude of the ocean response velocities associated with surface waves, ␻ ϭ ͌kg is depends not only on the characteristics of the hurri- cane forcing but also, crucially, on the background the wave frequency for wavenumber k following from ϭϪ Ϫ1 initial ocean conditions based on these scaling argu- linear theory, W 4.5 m s is the AXCP fall rate, and U and S are the mean current and shear in layer ments. Price (1983) defined a mixed-layer Burger num- i zi ber as i, respectively. Observed current profiles are fit to (4) using a standard Levenberg–Marquardt nonlinear least

Ϫ2 squares regression (Marquardt 1963) by minimizing the ͑1 ϩ S ͒gЈh ϭ M ϭ , ͑3͒ error over a range of trial wave periods T 7–14 s ͑ ͒2 2Rmax f (␻ ϭ 2␲/T). An additional constraint is that the current profiles are continuous across layer interfaces, thereby where S is the nondimensional storm speed reducing the number of free parameters from eight to

Uh/(2Rmax f ) and h is the OML depth. For the LC, M six. ranged between 0.04 and 0.08 because of large values of Model fits to the Isidore (21, 23 September) and Lili S for Lili (0.8) and for Isidore (0.54). Over the GCW, (2 October) current profiles are listed in Tables 3, 4, Lili’s acceleration toward the northwest coupled with a and 5, respectively. In-storm orbital velocity amplitudes Ϫ1 smaller Rmax caused S to increase to 1.15. The ther- were typically 1 m s , in accord with previous experi- mocline Burger numbers T ϭ b/hM are nearly equal to mental efforts (Sanford et al. 1987). As shown in Fig. 4, M in the LC because the ratio of thermocline and OML the RMS residual currents, estimated by calculating dif- thicknesses is O(1), as noted above. In the GCW, how- ferences between the model (4) and the observed pro- ever, T is 0.2 (i.e., a magnitude larger than in the LC), file to a depth where kz ϭϪ2 (i.e., where wave ampli- representing more dynamical coupling between the tudes are reduced to about 13% of their surface veloc- OML and thermocline layer. The resultant blue shift in ity amplitudes based on the fits), were generally less the mixed-layer inertial frequency is proportional to than 10 cm sϪ1, indicative of a reasonably good fit for M/2 (Table 2), equating to frequencies shifted above f strongly forced mixed layer conditions. However, there by 2% to 4%, consistent with previous results (Shay et were a few exceptions with relatively large residuals. al. 1998). Vertical shears of the horizontal velocity components in Ϫ2 Ϫ1 the upper two layers (Z1, Z2) were O(10 s ), whereas the shears in the lower layer (Z3) decreased by 4. Analysis approach an order of magnitude. The orbital velocity amplitudes have been removed from the observed current profiles a. Orbital velocities to examine the observed upper-ocean response (Shay To examine the current and shear measurements et al. 1992; Price et al. 1994). from the AXCPs, the surface wave–induced orbital ve- locity signals must be removed from the current profiles b. Objective analyses observed from the in-storm flights. When averaged over a cycle, the resolved low-frequency surface waves Objectively analyzed fields of observed variables are do not contribute significantly to the mean current produced using a statistical interpolation methodology. (with the exception of a small Stokes drift due to non- The analysis procedure used here is the OAX5 package linear wave structure). Observed current profiles are fit (developed at Canada Bedford Institute of Oceanogra- to the Sanford et al. (1987) three-layer model with an phy), which is based on the algorithm presented in assumed monochromatic, linear, deep-water surface Bretherton et al. (1976). This approach uses a linear gravity wave superimposed; thus, optimal interpolation technique to estimate values at

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TABLE 3. Coefficients from fits with the Sanford et al. (1987) model and the Isidore storm AXCP profiles in the upper 200 m where

Z0 is the start depth of the good data used in the fit; T is the period of the surface wave with coefficients of C and S; Z1,2,3, V1,2,3 and ϫ Ϫ2 S1,2,3 10 represent layer depth, layer–averaged currents, and current gradients in each layer, respectively; and R is the residual current not explained by the model to a depth of eϪ2.

Time TC SZ0 V1 S1 Z1 V2 S2 Z2 V3 S3 Z3 R UTC Variable s cm sϪ1 cm sϪ1 mcmsϪ1 sϪ1 mcmsϪ1 sϪ1 mcmsϪ1 sϪ1 mcmsϪ1 1824 u 10.5 Ϫ713Ϫ10 Ϫ18 0.11 Ϫ60 Ϫ33 0.58 Ϫ100 Ϫ42 Ϫ0.05 Ϫ200 4.4 ␷ 9.5 13 Ϫ21 Ϫ10 32 0.14 Ϫ60 Ϫ5 1.66 Ϫ100 Ϫ14 Ϫ0.49 Ϫ200 5.9 1835 u 7.5 Ϫ77 9 Ϫ18 Ϫ15 Ϫ0.48 Ϫ50 Ϫ31 1.19 Ϫ90 Ϫ40 Ϫ0.26 Ϫ200 7.7 ␷ 10.0 92 132 Ϫ18 70 3.29 Ϫ50 10 0.32 Ϫ90 8 Ϫ0.06 Ϫ200 7.8 1850 u 8.0 8 Ϫ2 Ϫ10 Ϫ39 0.10 Ϫ50 Ϫ5 Ϫ3.60 Ϫ70 17 0.21 Ϫ200 3.4 ␷ 7.0 23 9 Ϫ10 15 Ϫ0.78 Ϫ50 57 Ϫ2.64 Ϫ70 59 0.37 Ϫ200 12.0 1926 u 7.0 Ϫ28 Ϫ19 Ϫ811Ϫ0.02 Ϫ30 4 0.36 Ϫ70 Ϫ2 Ϫ0.02 Ϫ200 5.2 ␷ 6.0 45 Ϫ79 Ϫ8 23 1.80 Ϫ30 28 Ϫ1.23 Ϫ70 22 0.47 Ϫ200 11.0 1936 u 10.5 6 Ϫ16 Ϫ10 49 0.72 Ϫ50 30 0.11 Ϫ140 17 0.39 Ϫ180 4.1 ␷ 9.5 Ϫ8 Ϫ18 Ϫ10 40 Ϫ1.22 Ϫ50 58 0.15 Ϫ140 25 1.29 Ϫ180 8.5 1957 u 10.0 Ϫ11 18 Ϫ15 Ϫ13 Ϫ0.42 Ϫ50 7 Ϫ0.65 Ϫ90 21 Ϫ0.02 Ϫ200 8.7 ␷ 7.0 23 Ϫ57 Ϫ15 53 Ϫ1.07 Ϫ50 79 Ϫ0.37 Ϫ90 53 0.61 Ϫ200 16.3 2007 u 11.5 5 Ϫ0 Ϫ510Ϫ0.05 Ϫ50 11 0.01 Ϫ100 2 0.16 Ϫ200 2.9 ␷ 12.0 Ϫ310Ϫ5 24 0.04 Ϫ50 27 Ϫ0.14 Ϫ100 25 0.10 Ϫ200 2.9 2030 u 12.0 41 Ϫ74 Ϫ8 31 1.52 Ϫ50 20 Ϫ2.09 Ϫ70 23 1.82 Ϫ90 5.0 ␷ 8.0 Ϫ12 Ϫ76 Ϫ8 126 Ϫ0.13 Ϫ50 124 0.45 Ϫ70 103 1.61 Ϫ90 3.4 2036 u 8.5 Ϫ288 Ϫ85 Ϫ15 Ϫ30 Ϫ2.14 Ϫ60 7 0.51 Ϫ100 Ϫ1 Ϫ0.03 Ϫ200 6.1 ␷ 7.0 174 372 Ϫ15 166 2.36 Ϫ60 65 2.39 Ϫ100 19 Ϫ0.03 Ϫ200 9.7 2100 u 10.5 Ϫ36 Ϫ48 Ϫ8 Ϫ4 1.16 Ϫ30 Ϫ2 Ϫ2.96 Ϫ40 9 0.82 Ϫ50 5.4 ␷ 8.0 7 Ϫ63 Ϫ8 27 1.35 Ϫ30 10 0.55 Ϫ40 Ϫ24 6.28 Ϫ50 7.0 2110 u 7.0 291 Ϫ1155 Ϫ20 Ϫ45 Ϫ8.63 Ϫ40 40 0.28 Ϫ50 34 0.89 Ϫ60 2.6 ␷ 7.0 Ϫ491 Ϫ268 Ϫ20 12 Ϫ1.64 Ϫ40 0 5.70 Ϫ50 Ϫ30 0.25 Ϫ60 15.5 2120 u 8.0 37 72 Ϫ10 Ϫ24 Ϫ3.78 Ϫ40 13 1.29 Ϫ70 1 Ϫ0.19 Ϫ140 4.2 ␷ 7.0 4 13 Ϫ10 Ϫ28 Ϫ0.05 Ϫ40 Ϫ10 Ϫ1.17 Ϫ70 14 Ϫ0.20 Ϫ140 4.9 2145 u 12.0 Ϫ64 Ϫ48 Ϫ10 90 Ϫ0.17 Ϫ70 69 1.30 Ϫ110 20 0.52 Ϫ200 9.8 ␷ 8.0 Ϫ83 Ϫ132 Ϫ10 87 0.05 Ϫ70 77 0.45 Ϫ110 44 0.53 Ϫ200 6.0 2148 u 9.5 Ϫ44 0 Ϫ10 57 Ϫ2.93 Ϫ40 76 1.24 Ϫ80 18 1.67 Ϫ120 6.8 ␷ 9.5 Ϫ50 Ϫ68 Ϫ10 33 Ϫ0.87 Ϫ40 43 0.13 Ϫ80 39 0.07 Ϫ120 5.7 2159 u 12.0 26 Ϫ9 Ϫ10 6 0.56 Ϫ120 Ϫ23 Ϫ0.10 Ϫ150 Ϫ6 Ϫ0.62 Ϫ200 4.3 ␷ 10.0 Ϫ102 9 Ϫ10 Ϫ71 0.27 Ϫ120 Ϫ60 Ϫ1.64 Ϫ150 Ϫ10 Ϫ1.05 Ϫ200 7.1 2210 u 8.5 10 Ϫ43 Ϫ10 32 0.00 Ϫ60 14 1.83 Ϫ80 Ϫ6 0.03 Ϫ200 4.2 ␷ 11.0 62 23 Ϫ10 Ϫ71 0.65 Ϫ60 Ϫ52 Ϫ3.52 Ϫ80 Ϫ10 Ϫ0.12 Ϫ200 6.7 2320 u 9.0 Ϫ19 22 Ϫ8 30 0.65 Ϫ30 21 0.14 Ϫ60 11 0.26 Ϫ120 3.0 ␷ 10.0 2 Ϫ72 Ϫ8 Ϫ19 1.18 Ϫ30 Ϫ9 Ϫ1.54 Ϫ60 11 0.10 Ϫ120 5.8 2331 u 10.5 0 Ϫ7 Ϫ775Ϫ0.11 Ϫ60 70 0.81 Ϫ80 56 0.11 Ϫ200 5.1 ␷ 9.0 Ϫ35 Ϫ3 Ϫ7 Ϫ13 1.02 Ϫ60 Ϫ67 2.64 Ϫ80 Ϫ48 Ϫ0.76 Ϫ200 11.5 0007 u 8.5 Ϫ35 241 Ϫ8 Ϫ63 Ϫ6.94 Ϫ30 9 0.41 Ϫ50 6 Ϫ0.02 Ϫ200 2.8 ␷ 9.0 19 Ϫ396 Ϫ8 119 16.39 Ϫ30 Ϫ38 Ϫ2.36 Ϫ50 Ϫ4 Ϫ0.14 Ϫ200 9.5 0016 u 9.5 Ϫ108 28 Ϫ8 Ϫ11 0.11 Ϫ30 Ϫ18 1.18 Ϫ40 Ϫ32 1.71 Ϫ50 5.9 ␷ 8.0 Ϫ46 Ϫ202 Ϫ8 Ϫ24 3.15 Ϫ30 Ϫ61 0.57 Ϫ40 Ϫ44 Ϫ4.00 Ϫ50 15.2

grid points based on observations at the nearest neigh- able about LC structural and temporal variability over bors. A covariance model of the form (Freeland and the scales of interest exposed to severe forcing, a quan- Gould 1976) titative development of the optimal parameter scales is difficult (e.g., Baker et al. 1987). Therefore, scales are 2 Ϫ r chosen through trial and error to adequately resolve the ␳͑r͒ ϭ e rͩ1 ϩ r ϩ ͪ ͑5͒ 3 mesoscale ocean structure. Because the largest variabil- ity is observed near the surface during hurricane pas- is used, where r is the weighted nondimensional dis- sage, spatial scales here are smaller, and they increase tance between an observation and a grid point. In the with depth (Table 6). The horizontal covariance model absence of detailed climatological information avail- ␳(r) is plotted for each layer as a function of dimen-

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TABLE 4. Same as Table 3, but for Lili storm AXCP profiles in the upper 200 m.

Time TC SZ0 V1 S1 Z1 V2 S2 Z2 V3 S3 Z3 R UTC Variable s cm sϪ1 cm sϪ1 mcmsϪ1 sϪ1 mcmsϪ1 sϪ1 mcmsϪ1 sϪ1 mcmsϪ1 0240 u 13.5 Ϫ21 25 Ϫ10 Ϫ4 Ϫ0.61 Ϫ30 5 Ϫ0.35 Ϫ45 1 0.12 Ϫ150 2.2 ␷ 8.5 64 Ϫ93 Ϫ10 4 5.13 Ϫ30 Ϫ22 Ϫ3.34 Ϫ45 Ϫ4 0.14 Ϫ150 7.9 0312 u 11.5 Ϫ56 Ϫ145 Ϫ10 26 Ϫ0.27 Ϫ60 32 Ϫ0.01 Ϫ80 31 0.02 Ϫ200 8.7 ␷ 7.0 Ϫ176 57 Ϫ10 51 0.03 Ϫ60 51 Ϫ0.07 Ϫ80 41 0.17 Ϫ200 10.4 0336 u 10.0 85 Ϫ52 Ϫ12 Ϫ3 0.86 Ϫ50 Ϫ10 Ϫ0.37 Ϫ100 Ϫ1 Ϫ0.00 Ϫ200 4.3 ␷ 14.0 Ϫ47 Ϫ17 Ϫ12 16 0.65 Ϫ50 Ϫ13 0.67 Ϫ100 Ϫ9 Ϫ0.43 Ϫ200 6.1 0342 u 14.5 Ϫ19 101 Ϫ12 Ϫ28 Ϫ2.20 Ϫ50 0 0.55 Ϫ100 0 Ϫ0.27 Ϫ200 4.9 ␷ 10.5 23 44 Ϫ12 Ϫ6 0.57 Ϫ50 Ϫ5 Ϫ0.43 Ϫ100 14 Ϫ0.17 Ϫ200 6.2 0440 u 10.5 Ϫ39 6 Ϫ8 Ϫ5 Ϫ0.71 Ϫ50 Ϫ4 0.52 Ϫ100 Ϫ12 Ϫ0.10 Ϫ200 8.4 ␷ 8.5 73 Ϫ47 Ϫ8 Ϫ2 Ϫ0.53 Ϫ50 12 Ϫ0.08 Ϫ100 14 Ϫ0.01 Ϫ200 12.2 0527 u 11.0 Ϫ87Ϫ10 Ϫ28 Ϫ0.38 Ϫ50 Ϫ16 Ϫ0.27 Ϫ80 Ϫ15 0.04 Ϫ200 3.0 ␷ 9.0 13 Ϫ6 Ϫ10 Ϫ12 1.20 Ϫ50 Ϫ19 Ϫ1.12 Ϫ80 1 Ϫ0.06 Ϫ200 5.3 0538 u 8.0 34 Ϫ48 Ϫ8 Ϫ7 0.37 Ϫ30 Ϫ6 Ϫ0.34 Ϫ60 7 Ϫ0.11 Ϫ200 2.4 ␷ 11.5 5 Ϫ27 Ϫ8 Ϫ8 0.45 Ϫ30 Ϫ4 Ϫ0.58 Ϫ60 6 Ϫ0.03 Ϫ200 6.5 0545 u 10.5 51 Ϫ53 Ϫ8 Ϫ5 1.71 Ϫ40 Ϫ13 Ϫ1.97 Ϫ60 9 Ϫ0.03 Ϫ200 2.5 ␷ 11.5 1 46 Ϫ8 Ϫ19 Ϫ0.87 Ϫ40 Ϫ15 1.06 Ϫ60 Ϫ22 Ϫ0.06 Ϫ200 2.8 0614 u 8.0 Ϫ76 Ϫ27 Ϫ10 14 0.07 Ϫ35 12 0.11 Ϫ60 11 Ϫ0.01 Ϫ200 4.4 ␷ 11.0 Ϫ4 Ϫ49 Ϫ10 19 1.96 Ϫ35 7 Ϫ1.04 Ϫ60 20 Ϫ0.00 Ϫ200 6.0 0625 u 9.5 8 14 Ϫ10 Ϫ6 Ϫ0.08 Ϫ50 6 Ϫ0.71 Ϫ80 9 0.13 Ϫ200 2.1 ␷ 10. 28 30 Ϫ10 9 0.14 Ϫ50 11 Ϫ0.30 Ϫ80 9 0.11 Ϫ200 4.6 0635 u 7.5 178 Ϫ104 Ϫ8 38 0.32 Ϫ40 24 1.21 Ϫ55 20 Ϫ0.08 Ϫ200 8.7 ␷ 8.5 15 58 Ϫ8 Ϫ17 Ϫ1.52 Ϫ40 21 Ϫ1.74 Ϫ55 21 0.18 Ϫ200 4.3 0648 u 8.0 Ϫ178 Ϫ165 Ϫ17 Ϫ12 Ϫ2.97 Ϫ55 52 Ϫ0.30 Ϫ110 38 0.51 Ϫ200 9.9 ␷ 10.0 Ϫ111 0 Ϫ17 Ϫ22 Ϫ1.90 Ϫ55 21 Ϫ0.25 Ϫ110 21 0.15 Ϫ200 4.2 0711 u 9.0 25 44 Ϫ10 14 0.01 Ϫ65 17 Ϫ0.08 Ϫ150 Ϫ6 1.03 Ϫ200 12.6 ␷ 9.0 57 Ϫ42 Ϫ10 105 Ϫ0.15 Ϫ65 104 0.13 Ϫ150 58 1.64 Ϫ200 11.7 0740 u 11.0 Ϫ28 22 Ϫ10 12 0.05 Ϫ50 22 Ϫ0.74 Ϫ80 8 0.49 Ϫ180 8.4 ␷ 7.0 55 Ϫ81 Ϫ10 106 0.67 Ϫ50 57 2.36 Ϫ80 12 0.18 Ϫ180 8.5 0749 u 11.5 9 Ϫ10 Ϫ10 22 0.47 Ϫ70 16 Ϫ0.30 Ϫ120 16 0.17 Ϫ200 10.1 ␷ 11.5 Ϫ5 Ϫ25 Ϫ10 42 0.08 Ϫ70 41 Ϫ0.06 Ϫ120 39 0.08 Ϫ200 8.6 0759 u 11.0 0 Ϫ39 Ϫ12 16 Ϫ0.06 Ϫ80 3 0.51 Ϫ140 Ϫ4 Ϫ0.28 Ϫ200 9.7 ␷ 12.0 3 Ϫ28 Ϫ12 39 0.28 Ϫ80 10 0.65 Ϫ140 Ϫ15 0.16 Ϫ200 14.7 0809 u 13.0 Ϫ13 Ϫ11 Ϫ8 Ϫ14 Ϫ0.08 Ϫ80 Ϫ21 0.92 Ϫ100 Ϫ15 Ϫ0.29 Ϫ200 5.1 ␷ 8.0 Ϫ34 6 Ϫ8 8 0.22 Ϫ80 Ϫ44 4.48 Ϫ100 Ϫ41 Ϫ0.97 Ϫ200 11.4 0853 u 12.5 Ϫ2 Ϫ10 Ϫ10 86 Ϫ0.23 Ϫ80 79 0.75 Ϫ120 51 0.34 Ϫ200 2.8 ␷ 14.0 3 18 Ϫ10 Ϫ3 0.45 Ϫ80 Ϫ32 0.63 Ϫ120 Ϫ34 Ϫ0.26 Ϫ200 6.1 0905 u 8.0 Ϫ54 Ϫ83 Ϫ10 26 0.27 Ϫ100 12 0.08 Ϫ140 2 0.27 Ϫ200 9.6 ␷ 12.0 27 Ϫ9 Ϫ10 92 0.47 Ϫ100 44 1.33 Ϫ140 10 0.27 Ϫ200 9.9 0918 u 12.0 Ϫ21 Ϫ6 Ϫ8 11 0.64 Ϫ65 Ϫ1 Ϫ0.78 Ϫ80 Ϫ5 0.16 Ϫ200 4.3 ␷ 8.0 Ϫ11 Ϫ29 Ϫ8 152 1.23 Ϫ65 86 4.10 Ϫ80 44 0.20 Ϫ200 9.7

sional distance rL (Fig. 5). For the observed profiles, previously observed response time scales (Shay et al. the grid structure used for all Lili (Isidore) analyses is 1992). 31 ϫ 31 (41 ϫ 41) nodes in the horizontal plane, en- Assuming properly chosen scales for the interpola- compassing a 3° ϫ 3° (4° ϫ 4°) domain in latitude and tion, uncertainty estimates for each gridded field are longitude, respectively. The vertical grid contains 151 available based upon the input measurement noise. For points at 5-m depth intervals from the surface to 750 m. nominal measurement errors of surface temperatures, The Isidore (Lili) grids are rotated 270° (292°) clock- maximum mapping errors are 0.4°C in the northwest wise from north to align with the storm track direction. part of the pre-Lili grid because of data sparsity in that Isidore’s (Lili’s) storm track is aligned with the 21st region compared to temperature mapping errors of (5th) grid column from the south (southwest) side of 0.2°C (not shown) over the remainder of the domain. the domain, and the peak surface wind travels along Similarly, mapping errors for 26°C isotherm depths approximately the 23rd (8th) column. Each analysis range from 1 to 3 m, whereas those associated with uses an e-folding time scale of 2.9 days, consistent with OHC values have a maximum of 12 kJ cmϪ2. Finally,

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TABLE 5. Same as Table 3, but for Lili post-storm AXCP profiles.

Time TC SZ0 V1 S1 Z1 V2 S2 Z2 V3 S3 Z3 R UTC Variable s cm sϪ1 cm sϪ1 mcmsϪ1 sϪ1 mcmsϪ1 sϪ1 mcmsϪ1 sϪ1 mcmsϪ1 1755 u 8.0 211 Ϫ44 Ϫ9 22 1.46 Ϫ35 7 Ϫ0.16 Ϫ90 10 0.02 Ϫ200 7.6 ␷ 8.5 64 Ϫ41 Ϫ9 Ϫ6 0.91 Ϫ35 Ϫ13 Ϫ0.17 Ϫ90 Ϫ7 Ϫ0.03 Ϫ200 3.2 1807 u 11.0 18 4 Ϫ8 114 Ϫ0.02 Ϫ60 79 1.17 Ϫ120 40 0.09 Ϫ200 3.2 ␷ 9.0 Ϫ39 6 Ϫ8 Ϫ28 0.48 Ϫ60 Ϫ16 Ϫ0.83 Ϫ120 1 0.20 Ϫ200 9.2 1820 u 13.0 18 Ϫ1 Ϫ8 68 0.01 Ϫ95 44 3.10 Ϫ110 22 Ϫ0.04 Ϫ200 3.5 ␷ 7.0 Ϫ10 Ϫ55 Ϫ8 13 0.39 Ϫ95 Ϫ7 0.41 Ϫ110 3 Ϫ0.30 Ϫ200 8.7 1846 u 8.5 Ϫ23 Ϫ31 Ϫ15 Ϫ6 Ϫ1.51 Ϫ50 11 0.27 Ϫ120 Ϫ4 0.14 Ϫ200 3.5 ␷ 11.0 Ϫ219Ϫ15 Ϫ15 1.09 Ϫ50 Ϫ22 Ϫ0.37 Ϫ120 Ϫ5 Ϫ0.09 Ϫ200 5.7 1929 u 9.0 19 Ϫ27 Ϫ10 Ϫ6 0.74 Ϫ25 Ϫ17 0.39 Ϫ50 Ϫ17 Ϫ0.09 Ϫ140 2.1 ␷ 7.5 37 Ϫ143 Ϫ10 53 3.01 Ϫ25 14 1.28 Ϫ50 Ϫ1 Ϫ0.03 Ϫ140 7.2 1946 u 8.0 14 Ϫ17 Ϫ10 29 Ϫ0.13 Ϫ70 22 0.72 Ϫ100 8 0.06 Ϫ200 5.7 ␷ 7.0 4 2 Ϫ10 92 Ϫ0.09 Ϫ70 78 1.10 Ϫ100 46 0.30 Ϫ200 4.7 2003 u 13.0 Ϫ17 5 Ϫ10 Ϫ20 Ϫ0.23 Ϫ80 Ϫ6 Ϫ0.33 Ϫ120 10 Ϫ0.21 Ϫ200 3.5 ␷ 10.0 Ϫ146Ϫ10 Ϫ79 0.20 Ϫ80 Ϫ40 Ϫ2.31 Ϫ120 4 0.06 Ϫ200 7.8 2047 u 9.0 Ϫ20 Ϫ19 Ϫ15 34 0.02 Ϫ70 24 0.26 Ϫ140 15 0.00 Ϫ200 2.3 ␷ 13.0 50 66 Ϫ15 35 1.11 Ϫ70 15 Ϫ0.30 Ϫ140 25 0.01 Ϫ200 4.7 2055 u 8.5 32 6 Ϫ15 65 0.50 Ϫ70 48 0.17 Ϫ100 34 0.37 Ϫ160 2.5 ␷ 9.0 3 3 Ϫ15 67 0.62 Ϫ70 33 1.11 Ϫ100 6 0.36 Ϫ160 2.6 2108 u 10.0 6 Ϫ27 Ϫ15 Ϫ5 5.02 Ϫ25 Ϫ25 Ϫ0.38 Ϫ50 Ϫ22 0.19 Ϫ65 2.2 ␷ 9.0 Ϫ56 18 Ϫ15 15 Ϫ1.94 Ϫ25 6 1.44 Ϫ50 Ϫ18 0.87 Ϫ65 1.2 2117 u 10.0 Ϫ9 Ϫ6 Ϫ22 1 Ϫ0.10 Ϫ50 0 0.10 Ϫ110 Ϫ4 0.02 Ϫ200 5.1 ␷ 12.0 223 Ϫ42 Ϫ22 36 6.45 Ϫ50 Ϫ24 Ϫ1.02 Ϫ110 13 Ϫ0.14 Ϫ200 11.1 2125 u 9.0 0 8 Ϫ8 Ϫ22 0.23 Ϫ45 Ϫ10 Ϫ0.92 Ϫ80 Ϫ5 0.19 Ϫ200 10.2 ␷ 8.0 21 17 Ϫ814Ϫ1.14 Ϫ45 33 0.12 Ϫ80 16 0.26 Ϫ200 23.7 2135 u 8.0 Ϫ92 36 Ϫ10 Ϫ42 Ϫ1.65 Ϫ60 Ϫ4 0.19 Ϫ100 0 Ϫ0.15 Ϫ200 4.9 ␷ 10.0 58 Ϫ17 Ϫ10 87 0.15 Ϫ60 89 Ϫ0.30 Ϫ100 62 0.67 Ϫ200 10.9 2233 u 11.0 12 0 Ϫ12 47 0.24 Ϫ80 30 0.44 Ϫ120 22 Ϫ0.03 Ϫ200 3.5 ␷ 10.0 3 15 Ϫ12 Ϫ21 0.35 Ϫ80 Ϫ29 Ϫ0.23 Ϫ120 Ϫ38 0.35 Ϫ200 3.3 2318 u 8.0 18 64 Ϫ10 Ϫ23 1.10 Ϫ60 Ϫ21 Ϫ1.48 Ϫ100 0 0.17 Ϫ200 5.9 ␷ 7.0 Ϫ72 Ϫ65 Ϫ10 60 Ϫ0.40 Ϫ60 75 Ϫ0.28 Ϫ100 46 0.71 Ϫ200 6.7 2333 u 15.0 Ϫ25 5 Ϫ10 Ϫ48 1.45 Ϫ25 Ϫ37 Ϫ0.59 Ϫ100 30 Ϫ0.89 Ϫ200 8.7 ␷ 9.0 Ϫ26 84 Ϫ10 Ϫ51 0.30 Ϫ25 Ϫ25 Ϫ0.75 Ϫ100 Ϫ3 0.14 Ϫ200 5.4 2353 u 8.0 Ϫ37 Ϫ13 Ϫ10 Ϫ12 Ϫ0.68 Ϫ40 5 Ϫ0.60 Ϫ60 6 0.07 Ϫ200 3.4 ␷ 8.0 48 8 Ϫ10 Ϫ38 2.06 Ϫ40 Ϫ35 Ϫ3.29 Ϫ60 Ϫ7 0.06 Ϫ200 4.3 0007 u 9.0 30 10 Ϫ8 Ϫ52 Ϫ0.20 Ϫ40 Ϫ22 Ϫ1.35 Ϫ80 Ϫ5 0.16 Ϫ200 5.5 ␷ 9.0 Ϫ71Ϫ8 79 0.18 Ϫ40 60 0.82 Ϫ80 37 0.12 Ϫ200 9.1 0020 u 8.5 Ϫ48 20 Ϫ10 Ϫ3 Ϫ0.27 Ϫ100 14 Ϫ0.28 Ϫ130 17 0.02 Ϫ200 4.8 ␷ 8.0 Ϫ86Ϫ10 5 0.35 Ϫ100 7 Ϫ1.18 Ϫ130 3 0.62 Ϫ200 5.1

current mapping errors are typically in the realm of ployed within the storm from both aircraft (Table 1), 0.1–0.2 m sϪ1, with the larger values located in the including sondes deployed from Air Force Reserve re- northwest corner of the pre-Lili domain. In all of the connaissance and NOAA G-IV synoptic flights. Sur- mapped fields, these mapping errors are much less than face winds are estimated from SFMR measurements by those in the observed oceanic signals (e.g., high signal- sensing brightness temperatures at multiple frequen- to-noise ratios). cies, as thoroughly described in Uhlhorn et al. (2007). From each GPS dropwindsonde (Hock and Franklin c. Air–sea fluxes 1999), 10-m values of temperature and specific humid- Sea surface forcing is described by the fluxes of mo- ity were acquired and objectively analyzed using the mentum, heat, and moisture. These fluxes are esti- Bretherton et al. (1976) OI method projected onto a mated from bulk formulas utilizing near-surface (10 m) storm-relative grid aligned with the direction of storm atmospheric thermodynamic and wind measurements motion. SFMR wind observations are objectively ana- and upper-ocean thermal data. Atmospheric data are lyzed using HRD’s H*Wind system (Powell and Hous- measured from the large number of GPS sondes de- ton 1996) and then interpolated bilinearly to a storm-

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FIG. 4. Examples of model fits (solid) using a three-layer approach of Sanford et al. (1987) compared to observed profiles (dots) for (a)–(c) Isidore and (d)–(f) Lili for the (a), (d) u and (b), (e) ␷ components (m sϪ1). (c), (f) Differences between observed and model profiles for both velocity components normalized by the surface wave amplitudes (from Tables 3 and 4). Note that the RMS differences are smaller in Lili, but because the surface wave is less energetic, the (c) normalized differences for Lili are larger than (f) for the Isidore profiles.

relative grid as for the GPS atmospheric thermody- |␶| ϭ ␳ | |2 ͑ ͒ aCd U10 , 6 namic variables (See Fig. 2). Finally, sea surface ϭ ␳ | |⌬ ͑ ͒ temperature observations are also optimally interpo- Qs acpCh U10 T, and 7 lated to the same storm-relative grid (as noted above), Q ϭ ␳ L␷C |U |⌬q, ͑8͒ resulting in a set of variables at a common location l a q 10 ␳ from which the spatial distribution of the bulk surface where a is the atmospheric density; Cd, Ch, and Cq are momentum, sensible, and latent heat fluxes are esti- exchange coefficients of momentum, sensible and la-

mated; that is, tent heat, respectively; U10 is the 10-m wind speed; cp is specific heat of air at constant pressure; L␷ is the latent ⌬ Ϫ TABLE 6. Horizontal and vertical correlation scales used in the heat of vaporization; and T (Ts Ta) is the difference objective analyses. between SST and 10-m air temperature. SSTs are de- fined to be near-surface temperatures within the first Depth Horizontal Vertical few meters of the sea surface from the expendables,1 range (m) scale L (°) scale (m) 0–50 0.5 20 50–100 1.5 40 1 The temperature of a bulk OML, defined as the depth where 100–150 2.0 60 the temperature decreases by more than 0.2°C, is given by a ver- 150–200 2.5 80 tical average from near-surface to this depth. During periods of 200–400 3.0 100 strong wind forcing, the upper ocean is well mixed in temperature, 400–600 4.0 150 which represents a bulk OML temperature as observed during 600–750 5.0 200 hurricanes (Sanford et al. 1987; Shay et al. 1992, 2000).

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salinity, which stabilizes the OML and reduces the rate of vertical mixing (Jacob and Kolinsky 2007). Tropical Rainfall Measuring Mission (TRMM) Microwave Im- ager (TMI)-derived rain rates (Fig. 6) were used to estimate surface precipitation fluxes for Isidore and Lili (with maximum rain rates of 35 and 20 mm hrϪ1, re- spectively). Surface flux distributions during the Isidore in-storm flight (21 September) are shown in Fig. 7. Peak enthal- py flux is found in the right rear quadrant of the storm to be Ϸ1.8 kW mϪ2 as a result of high SSTs that show negligible decrease from prestorm conditions. The maximum momentum flux (ϳ7NmϪ2) is located in the right front quadrant and is associated with a highly sym- ␳ Ϫ FIG. 5. Covariance model weighting function (r) as a function metric storm because of its fairly slow 4 m s 1 transla- of dimensional distance r ϫ L (°) applied to observations tional speed. Similarly, the estimated surface fluxes in for the objective analyses as per Table 6 for differing length scales L. Lili are shown in Fig. 8. Lili’s surface wind field (2 October) indicates marked asymmetry resulting from ⌬ Ϫ ϳ and q (qs qa) is the difference between the saturated the more rapid storm motion toward the NW ( 7 specific humidity at the SST and unsaturated 10-m at- msϪ1), and correspondingly, surface fluxes are en- mospheric specific humidity. The surface drag coeffi- hanced on the right side of the track. Compared to cient Cd is computed from the Large and Pond (1981) Isidore, the maximum surface enthalpy flux in Lili is relationship but is capped at a maximum value of 2.5 ϫ weaker (1.4 kW mϪ2) despite peak surface winds of 51 10Ϫ3, based on recent results indicating a threshold or msϪ1. This lower flux results primarily from SSTs ob- Ϫ1 saturation value of Cd at 28–33 m s wind speeds served in the LC regime that are approximately 1°C (Powell et al. 2003; Donelan et al. 2004; Shay and Jacob lower than those during Isidore. This is an important 2006; Jarosz et al. 2007). Heat exchange coefficients point that highlights how modest surface temperatures

(Ch, Cq) are set equal to Cd, which is conservative com- differences can effectively alter surface heat fluxes un- pared to the theory proposed by Emanuel (1995) (i.e., der hurricane wind conditions (Cione and Uhlhorn Emanuel’s theoretical results suggest that this enthalpy 2003). and drag coefficient ratio lies between 1.2 and 1.5 in To improve our understanding of how these esti- severe hurricanes). An additional ocean forcing mecha- mated fluxes relate to sea–air heat exchange, enthalpy nism results from the surface precipitation flux (rain (heat and moisture) fluxes are integrated in the along- rate). Freshwater input by rain can alter the ocean’s track direction to obtain a cross-track (radial) distribu- response both by direct cooling (caused by rain at a tion of the ocean heat loss through the sea surface. An lower temperature than the SST) and by reducing the along-track spatial coordinate is used to convert to

Ϫ FIG. 6. Rain rates (mm h 1) based on TRMM data during (a) Isidore and (b) Lili.

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FIG.7.Air–sea heat, moisture, and momentum fluxes derived from GPS sondes and SFMR from Isidore on 21 Sept 2002 for (a) sensible heat (W mϪ2), (b) latent heat (W mϪ2), (c) momentum or wind stress (N mϪ2), and (d) enthalpy (i.e., sensible plus latent heat) Ϫ2 (W m ) in a storm-coordinate system normalized by Rmax. time, assuming a steadily moving storm based on the speed. Such rain events induce changes in the OML observed storm speed (Table 2). Estimated surface heat salinity balance of 0.2 to 0.4 practical salinity units losses (kJ cmϪ2) for Isidore and Lili are shown in Fig. 9. (psu), as documented by conductivity, temperature, At the eyewall, surface heat loss in Isidore is 9.5 kJ and depth (CTD) measurements acquired in typhoon cmϪ2, compared to 4.5 kJ cmϪ2 during Lili. These dif- wakes in the western Pacific Ocean (Pudov and Pet- ferences result from higher enthalpy fluxes (1.8 versus richenko 2000). Thus, the OML salinity balance and the 1.4 kW mϪ2) and slower storm speeds (4 versus 7 m sϪ1) surface buoyancy flux must be accounted for in ocean in Hurricane Isidore. response models for light and strong winds (Price et al. Based on the TRMM data (see Fig. 6), net freshwater 1986; Jacob and Koblinsky 2007). input (precipitation minus evaporation, hereafter P Ϫ d. Temperature and velocity profiles E rate) is estimated for both storms by integrating these data in the along-track direction (Fig. 9b). As suggested Current and temperature profiles from the BЈ–B Ϫ by the fluxes, the P E rate in Isidore was three times transect (see Fig. 3g) along 1.5 to 2 Rmax to the right of Ϯ ϳ larger than in Lili (between 4Rmax). At levels of 300 the track are shown in Fig. 10, 2 IPs ( 63 h) following mm within Isidore’s core, rain impacted the OML bal- Lili’s passage over the domain. Current profiles along ance through the P Ϫ E rate. In contrast, the P Ϫ E rate the northern part of transect (BЈ) in the GCW indicated in Lili’s core was 115 mm because of a faster translation an anticyclonic rotation with depth suggestive of verti-

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FIG. 8. Same as Fig. 7, but for Lili on 2 Oct 2002. cal energy propagation out of the wind-forced OML In the GCW, located in the northwest corner of the (Leaman 1976). During Gilbert’s passage, this observed analysis domain, OML cooling and deepening are typi- anticyclonic rotation with depth was found to 4 times cal signatures of a stronger ocean response that often more energetic than the cyclonically rotating compo- provide a negative feedback during hurricane passage nent (Shay and Jacob 2006). This current vector rota- (Chang and Anthes 1978; Bender and Ginis 2000). tion forced strong current shears beneath the OML, However, in the LC, this negative feedback to the at- inducing cooling by entrainment mixing processes. This mosphere is minimized compared to that observed over effect lowers Richardson numbers to below criticality the GCW. Because the assumption of horizontal homo- for a deepening and cooling OML (Pollard et al. 1973; geneity is violated in the LC and WCR regimes, Halli- Price 1981; Jacob et al. 2000). In the center of the BЈ–B well et al. (2008) stress the importance of initializing transect, OML currents approach 1 m sϪ1 flowing to- three-dimensional oceanic models to accurately predict ward the east. Notice that the warmer thermal structure intensity from coupled forecasting models. In fact, approaches 100 m depth where the currents remain Falkovich et al. (2005) introduced a numerical ap- relatively constant with depth in the LC. This baroclinic proach for feature-based ocean modeling that involves current structure tends to be in geostrophic balance cross-frontal sharpening of the background tempera- with a current reversal at 500 m and with weaker cur- ture and salinity and, hence, the density fields. These rents extending to a depth of 1000 m. By contrast, the studies underscore the need for three-dimensional ex- current structure at point B is shallower, with maximum perimental data to improve oceanic model initialization OML currents of 0.35 m sϪ1. schemes.

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than 170 m in the eastern part of the Yucatan Straits because of downwelling of the isotherms. As shown in Fig. 12, mean cross-track variability in the along-track direction prior and subsequent to Lili’s passage revealed similar features except that the cur- rents and horizontal temperature (density) gradients were not impacted quite as much by the steep bottom terrain. Maximum upper-ocean currents were directed toward the north at speeds of more than 0.5 m sϪ1. There was cooling in the northern part of the domain where the 26°C isotherm depth was located at 50-m depth before Lili; the isotherm depth then decreased by about 10 m on 4 October, due in part to upwelling along the track. Spatially averaged baroclinic currents in- creased to 0.7 m sϪ1 in response to Lili. As in the oce- anic response to Isidore, there was little evidence of significant upper-ocean cooling between snapshots. Al- though the time envelope between the pre- and post- Lili measurements was nearly two weeks, there was not much evidence that this Category 3 hurricane impacted the LC even just 2 days following passage. By contrast, forced near-inertial current shears deepened and cooled the OML by about 4°C in the GCW during Hurricane Gilbert’s passage (Shay et al. 1992).

FIG. 9. Cross-track distribution of the (a) surface heat loss in- Ϫ2 duced by surface fluxes (kJ cm ) and (b) P Ϫ E rain fluxes (mm) 5. Forced response in the Loop Current observed during Hurricanes Isidore (solid) and Lili (dashed), with error bars (1 std dev) based on measurements. An important question emerging from recent studies is the upper-ocean cooling levels during hurricane pas- e. Temperature and velocity sections sage. Although early studies focused on the concept of negative feedback during hurricane passage (i.e., Background ocean flows are set up by horizontal Chang and Anthes 1978), observational evidence has pressure gradients resulting from nonzero temperature suggested that the STW mass associated with the LC and salinity gradients and may play a significant role in and WCR does not significantly cool compared to the affecting the development of strong wind-driven cur- GCW. Based on profiler measurements before and af- rent shears within the LC and WCR regimes (Fig. 11). ter Rita, the oceanic cooling was minimized, suggesting Pre- and post-Isidore measurements across the Yucatan less negative feedback even though Rita was a Cat- Straits indicate strong density and pressure gradients egory 5 hurricane over the LC (Shay 2008). Historical associated with the LC. Pre-Isidore measurements sug- records suggest that once a hurricane enters the GOM gest a northward-flowing LC of 1 m sϪ1 skewed toward basin it will likely intensify prior to making a landfall the western boundary of the Yucatan Straits. This is the (Marks and Shay 1998). Contrasting the details of the region where the horizontal density and pressure gra- oceanic response differences of the LC and WCR ver- dients sharpen because of a strong bottom slope. The sus the GCW water masses has implications with re- initial 26°C isotherm depth was a maximum of ϳ150-m spect to negative feedback to the atmosphere. The fo- depth in the straits but was near the bottom over the cus here is on examining the observed oceanic response Yucatan Shelf. These spatial variations were sharpened to Hurricanes Isidore and Lili. because Isidore cooled the shelf waters by more than 4.5°C compared to ϳ1°C across the Yucatan Straits. Ϫ a. SST The LC response was an increase of 0.4 m s 1, consis- tent with the expected current response of 0.38 m sϪ1 as Pre- and post-Isidore and Lili SST fields are shown in per scaling arguments in Table 2. On the western side of Fig. 13. Pre-Isidore SSTs ranged from 28.5° to 29.5°C the straits, the 26°C isotherm upwelled toward the sea over the experimental domain. During the post-Isidore surface, whereas this isotherm depth increased to more experiment on 23 September, SST changes were ob-

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Ј Ϫ1 FIG. 10. Section B–B current vector stick plot (cm s ) and temperature (°C) at 1.5 Rmax from Lili’s track on 4 Oct (poststorm). Time is scaled in terms of inertial period. Black dots represent the OML depth in the upper panel extending from the surface to 150-m depth.

FIG. 11. (top) Pre- and (bottom) post-Isidore along-track section of temperature (°C, color) and geostrophic velocity (m sϪ1, dashed contours) across the Yucatan Straits. The heavy dashed black line depicts the depth of the 26°C isotherm.

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FIG. 12. Same as Fig. 11, but for (top) pre- and (bottom) post-Lili.

FIG. 13. (a) Pre- and (b) post-Isidore SSTs; (c) ⌬SST for Isidore; (d) pre- and (e) post-Lili SSTs; and (f) ⌬SST for Lili (all in °C) relative to Isidore’s or Lili’s track (black line) across the Yucatan Straits or the southeast GOM relative to bottom topography (dotted lines) for the 200- and 1000-m-depth contours. Storm motion is indicated by arrows in (a) for Isidore and Ϫ1 ⌳Ϫ1 (d) for Lili, respectively. Coordinate system (cross-track: xR max; along-track: y ) dimen- sions are relative to the mean storm locations from in-storm flights (for which the Rmax and ⌳ values are given in Table 2).

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FIG. 14. Same as Fig. 13, but for the 26°C isotherm depth (m). served along these bottom topographical gradients, strained by strong cross-stream topographical gradients with the largest SST changes of 4.5°C occurring over separating the Yucatan Straits from the Yucatan Shelf. the Yucatan Shelf (Fig. 13c). That is, SSTs decreased to Consistent with these large SST changes over the shelf, less than 25°C, because along-shelf wind stress driving a isotherm depths decreased along these bottom topo- net surface offshore flow resulted in upwelling of a shal- graphical gradients after Isidore. However, in the cen- low seasonal thermocline. Although significant SST ter and eastern part of the deeper Yucatan Straits, iso- cooling and upward isotherm displacements occurred therm depth increased by 20 m (Fig. 14c), which might over the shelf just prior to landfall, only small thermal be a manifestation of the downwelling cycle. This alter- structure and isotherm depth changes were observed nating upwelling cycle along the steep bottom slope and across the Straits to the western tip of Cuba. Thus, SSTs over the shelf and downwelling cycle in the straits is an remained above 28.5°C in the straits a day after Isidore, integral part of the oceanic response to hurricane forc- suggestive of less negative feedback to the storm. Given ing (Geisler 1970). These processes result in a tighten- the 10-day interval between Isidore and Lili, pre-Lili ing of the isotherm depths (and hence the thermocline) SSTs were 29° to 29.5°C over most of the experimental and their gradients across the abrupt topographical domain. After Lili’s passage, SSTs decreased to only changes. 28.5°C in the LC; however, along the northwest part of The corresponding 26°C isotherm depths for pre-Lili the measurement domain, SSTs cooled to less than conditions were located at more than 150-m depth in 27°C in the GCW. Consistent with the National Data the southeast part of the LC (Fig. 14d) and decreased to Buoy Center (NDBC) 42001 measurements (see Fig. 50 m along the northwest periphery in the GCW. Com- 1), this observed SST change equated to more than 2°C pared to a monthly climatology (Teague et al. 1990), cooling (Fig. 13f) as Lili intensified to a Category 4 these isotherm depths seem to be anomalously deep, storm in the south-central GOM (Pasch et al. 2004). but they were consistent with satellite-derived isotherm depths derived from satellite altimetry based on a sea- sonal climatology (Halliwell et al. 2008). Given Lili’s b. 26°C isotherm depths rapid translation speed, poststorm isotherm depths Prior to Isidore, a strong horizontal gradient of the changed little compared to the pre-Lili values in the LC 26°C isotherm depth was observed, such that depths (Fig. 14e); that is, isotherm depths ranged between 90– were found to be ranging from more than 150 m in the 140 m in the LC compared to about 100–150 m before Yucatan Straits to less than 30 m over the Yucatan the storm. The isotherm displacements induced by Lili Shelf (Fig. 14). These horizontal differences were con- were about 10 m, which is consistent with scaling argu-

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Ϫ FIG. 15. Same as Fig. 13, but for OHC (kJ cm 2) relative to the depth of the 26°C isotherm in Fig. 14. ments using both the maximum wind stress and trans- of more than 100 kJ cmϪ2 prior to Lili (Fig. 15d). Maxi- lation speed values in Table 2. mum OHC levels from profiler measurements ex- ceeded 140 kJ cmϪ2 compared to satellite-inferred val- c. OHC variability ues of ϳ130 kJ cmϪ2 (not shown). By contrast, in the Pre- and poststorm OHC distributions (Fig. 15) re- GCW, OHC values after Lili decreased by more than Ϫ2 Ϫ2 flect these SSTs and 26°C isotherm depths. Pre-Isidore 35 kJ cm , compared to less than 15 kJ cm in the LC OHC in the northwest Caribbean basin and through the regime. This larger OHC loss, associated with an SST eastern part of the Yucatan Straits exceeded 160 kJ decrease of more than 2°C, may have been caused by cmϪ2, in accord with satellite-derived OHC values de- enhanced vertical shears because the surface enthalpy Ϫ2 rived from radar altimetry (Halliwell et al. 2008). Over fluxes only accounted for about 4–5kJcm of heat the Yucatan Shelf, pre-Isidore OHC values were about loss through the surface. Given that the post-Lili survey 40 kJ cmϪ2 (Fig. 15a), suggestive of a shallow seasonal was conducted 1.5 IPs (48 h) after passage (Fig. 15e), thermocline maintained by the trade winds (Gill 1982). the OHC loss may have been greater than this value In the post-Isidore distributions, the OHC values were because a major contributor to the oceanic heat budget less than those observed during prestorm conditions, is associated with the northward advection of heat by although by less than 20 kJ cmϪ2 along the western the LC. Differencing pre- and post-Lili OHC fields in- parts of the straits; along the eastern part, the OHC dicates that average fluxes were significantly less than Ϫ Ϫ Ϫ increased by about 20 kJ cmϪ2, consistent with a down- 17 kJ cm 2 d 1 (ϳ2.0 kW m 2) because of the hurri- welling signal (i.e., deepening of the 26°C isotherm; Fig. cane passage. Along the northwest part of the Lili do- 15c). Over the shelf, however, the OHC losses were main (i.e., the GCW), thin OMLs cool and deepen more than 40 kJ cmϪ2 where SSTs cooled by 4.5°C. In quickly during hurricane passage where the SST de- the center of the straits, there was essentially no OHC creases typically range from 3 to 6°C (Price 1981; Shay change, which was presumably due to northward heat et al. 1992; Jacob et al. 2000) and induce a negative transport by the LC. Evidently, these large spatial gra- feedback (Chang and Anthes 1978; Bender and Ginis dients in SSTs and OHC across the Yucatan Straits 2000). Price et al. (1994) argued that the North Atlantic impacted the enthalpy fluxes that affected Isidore’s in- subtropical front over which Hurricanes Josephine tensification to Category 3 status. Because isotherm (1984) and Gloria (1985) passed is inconsequential to depths decreased by about 20 m in the LC with a 1°C the simulated ocean response. In the three-dimensional SST cooling, the LC essentially maintained OHC levels LC regime, however, the horizontal pressure gradients

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TABLE 7. Observed current shear from AXCP profiles deployed more current and shear measurements are needed to in TCs. Means and standard deviations. fully assess model parameterizations.

Ϫ1 Ϫ2 Storm Shear (s ϫ 10 ) e. Bulk Richardson number Norbert (1984) 2.2 Ϯ 1.3 The bulk Richardson number is estimated from the Gilbert (1988) 3.5 Ϯ 1.7 Isidore (2002) 2.0 Ϯ 1.4 following expression: Lili (2002) 1.4 Ϯ 1.3 gh␣⌬T Ri ϭ , ͑9͒ b ␦V2 where ␣ is the coefficient of thermal expansion (2 ϫ and balanced currents are considerably stronger than in Ϫ Ϫ 10 4 °C 1; Kraus and Businger 1994), h is the OML the North Atlantic Ocean subtropical front. Thus, to depth, ⌬T is the temperature difference between the simulate the response, ocean models must be initialized bulk OML and averaged temperature in layer 2, and with these basic state flows (Marks and Shay 1998; Hal- the magnitude of the bulk current shears (|␦V|) are liwell et al. 2008). determined from the model-fitted mean current differ- ences between layers 1 and 2 as per Tables 3, 4, and 5. d. Vertical current shear Pollard et al. (1973) used a value of unity for the bulk The most effective process for OML cooling and de- Richardson number as a condition for the onset of mix- creasing SST is by entrainment mixing (downward heat ing processes at the OML base, whereas Ellison and flux) across the base of the OML associated with vig- Turner (1959) found that more appropriate values orous near-inertial current shears (e.g., Price 1981; ranged between 0.4 and 0.8 for the initiation of vertical Schade and Emanuel 1999). This process is parameter- mixing. Price (1981) used a Rib of 0.8 as the critical ized in numerical models and has been shown to pro- value for mean current shear-induced mixing based on duce widely varying results depending on the chosen a scaled entrainment law from experimental laboratory mixing parameterization (Jacob et al. 2000; Jacob and results. Shay 2003). The results presented here suggest that Small values of Rib indicate regions where shear- strong prestorm current regimes may limit the devel- induced mixing is likely to overwhelm the damping ef- opment of wind-forced near-inertial currents and their fect of stratification during the cooling process (Fig. vertical shears. 17). However, in-storm measurements in Isidore indi- Ͼ Current profiles are used to assess the vertical shear cate Rib 1 over most of the domain. These values in using profiles with a vertical resolution of 2 m after the LC are consistent with the observed 1°C cooling Ϫ removal of the orbital velocities (see section 4a). Based and the OHC change of 20 kJ cm 2. Even where the on (4), current shears at the OML base are estimated maximum SST cooling of 4.5°C was observed, bulk from the model-fitted shear coefficients in layer 2 (S ) Richardson numbers were above critical values (Figs. z2 from Tables 3, 4, and 5. The means and standard de- 17a,b) suggesting that upwelling of colder thermocline viations of S are compared to Norbert and Gilbert water (induced by the wind stress curl) was the domi- z2 shear measurements in Table 7. Within measurement nant mechanism over the shelf. The results for Lili are error (Gregg et al. 1986), weaker shears were observed similar (Figs. 17c,d), although they are more evident in in the current profiles acquired during two severe hur- the poststorm analysis of current shear measurements. ricanes. These estimated shear values in layer 2 are Both of these cases point to a reduction in shear- objectively analyzed for both in- and poststorm fields induced mixing events in regions of strong background only (Fig. 16). Evident in these fields is the weaker currents; that is, the presence of deep warm layers Ϫ shear observed in the LC regime compared to mea- coupled with strong background flows of 1 m s 1 sured shears in the GCW. Within the LC, for example, (caused by horizontal density and pressure gradients) current shears ranged from 0.5–1.5 ϫ 10Ϫ2 sϪ1, but out- precludes the generation of strong shears observed in side the LC, the shears were 2.0–5.0 ϫ 10Ϫ2 sϪ1,or2to the near-inertial wave wake (Shay et al. 1998). These 4 times larger. This effect is obvious in the in-storm results point to a physical mechanism that must be well Isidore and poststorm Lili current fields (Figs. 16a,d). understood in coupled models that predict hurricane The lagged current shear response in Lili may also re- intensity. sult from rapid storm motion and its asymmetric wind stress distribution compared to the symmetric and 6. Summary and concluding remarks slower-moving Isidore. Given these wide-ranging re- The goal of the aircraft-based experiment was to sults between two distinct water masses, considerably measure the three-dimensional current, temperature,

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Ϫ Ϫ FIG. 16. In-storm and poststorm current shears (ϫ10 2 s 1) in layer 2 based on the Sanford et al. (1987) model for (a), (b) Isidore and (c), (d) Lili relative to the storm tracks (solid lines) and the bottom topography (dotted lines) for the (a), (c) 200- and (b), (d) 500-m depth contours. and salinity responses excited by hurricane passage measurements to improve our understanding of their with a focus on assessing the responses in and over the mutual response. LC. The aircraft-based sampling strategy resulted in The ocean response to the passage of Hurricanes Isi- several snapshots of upper-ocean current, temperature, dore and Lili was investigated using in situ observations and salinity structure, required to examine the response from oceanic and atmospheric sondes and remotely to the passage of two Category 3 hurricanes moving sensed ocean surface winds (Uhlhorn et al. 2007). over the same domain over a 10-day period. Given the These hurricanes intensified to major hurricane status inherent uncertainties of storm-track prediction for the (Category 3 and above) over the LC, and Lili’s deep- pre-Isidore flights, this experimental objective was ening cycle continued into the central GOM basin as achieved with a high degree of success for Isidore and the storm reached Category 4 status just northwest of Lili. This is one of only a few datasets in which currents the LC boundary. Atmospheric conditions were condu- and shears were directly measured during hurricane cive for deepening, as noted by Pasch et al. (2004), over passages (Sanford et al. 1987; Shay et al. 1992; Price et an ocean where the extent of the cooling was O(1°C) al. 1994; Sanford et al. 2005). In this case, the GPS and the upper-ocean heat loss was less than 20 kJ cmϪ2. sondes and SFMR data complement these in situ ocean Even after both hurricanes, the upper ocean was still

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FIG. 17. Same as Fig. 16, but for Rib. warm, with SSTs of 28.5° to 29°C and OHC levels ex- 3) maximum surface heat loss from the ocean was less Ϫ2 Ϫ2 ceeding 100 kJ cm . These results are similar to ob- than 10 kJ cm at Rmax, where enthalpy fluxes servations acquired prior and subsequent to Hurricane ranged from 1.4 to 1.8 kW mϪ2 for Lili and Isidore, Rita, which followed a track close to Katrina over the respectively. LC and WCR (Shay 2008). Based on these profiler The first point is important with regard to initializing data, lessons learned from Isidore and Lili included the ocean models with realistic background conditions following: (Halliwell et al. 2008; Mainelli et al. 2008). Emanuel 1) the three-dimensional LC precludes the develop- (2001) argues that the upper ocean can be treated as a ment of strong vertical current shears to force mix- series of one-dimensional column models for coupled ing and deepening of the OML despite applied wind hurricane forecasting. The results reported, and other stresses of7NmϪ2 due to the strength of the current recent studies such as Wu et al. (2007), imply that three- and the depth of the warm isotherms; dimensional advective tendencies must be accounted 2) cooling of 4.5°C over the Yucatan Shelf to Isidore for in such models. Because the ocean response is was due to wind-forced upwelling, and more than weakest in strong frontal regimes, the negative feed- 2°C cooling during Lili was due to shear-induced back to the atmosphere is much less than over quies- mixing events in the GCW; and, cent ocean regimes. The third conclusion has direct rel-

Unauthenticated | Downloaded 09/27/21 04:17 PM UTC 3272 MONTHLY WEATHER REVIEW VOLUME 136 evance to the threshold value for sustaining a hurri- Acknowledgments. LKS acknowledges the support of cane’s intensity. Leipper and Volgenau (1972) the National Science Foundation for both the experi- suggested a value of 17 kJ cmϪ2 dayϪ1, but this thresh- ment and data analysis through Grants ATM-01-08218 old must be revisited to understand how much oceanic and ATM 04-44525; flight hours were provided by heat loss is related to surface enthalpy fluxes versus NOAA’s Hurricane Research Division (HRD) and the entrainment heat loss to the thermocline. In this con- National Hurricane Center. E. Uhlhorn was supported text, Cione and Uhlhorn (2003) argue that only inner- by both NOAA and ONR-CBLAST under the leader- core SSTs are necessary for intensity forecasting, but ship of Dr. Peter Black. We gratefully acknowledge Dr. their results are inconclusive because the OHC held Jim McFadden of the Office of Aircraft Operations constant even though SSTs were changing. With tem- (OAO) and Drs. Hugh Willoughby and Frank Marks perature profiles, OHC changes relative to the 26°C (NOAA HRD) in orchestrating these aircraft experi- isotherm can be estimated at finer scales than currently ments. Dr. Paul Chang of NOAA/NESDIS directed the available from coarse altimeter tracks (Cheney et al. Ocean Winds flights on N42RF and provided support 1994). for both the Isidore and Lili flights. Mr. Michael Black The scientific issue is not just the OHC magnitude; (HRD field director) directed all field support for these the depth of the 26°C isotherms in the LC and WCR flights. We appreciate the extraordinary efforts of the regime are of equal importance in determining the OAO pilots, engineers, and technicians during these value of the OHC as per Eq. (1). The deeper this layer experimental efforts. Tom Cook and Scott Guhin as- of warm water, the more turbulent mixing is required to sisted in the aircraft-based experiments, and Jodi Brew- overturn and cool the OML. Significant internal wave ster provided graphical support. Reviews from three shears associated with near-inertial motions were not anonymous reviewers significantly improved the manu- observed in the LC during Isidore and Lili because of script. the strength of the horizontal pressure gradients that force the background currents. If shear-induced mixing REFERENCES is arrested, significant OML cooling and deepening will Baker, W., S. Bloom, J. Woollen, M. Nestler, E. Brin, T. Schlatter, not occur. In contrast, when current shears are large, and G. Branstator, 1987: Experiments with a three- they lower the bulk Richardson numbers to below criti- dimensional statistical objective analysis scheme using FGGE cal values and cool the SST (a proxy for OML tempera- data. Mon. Wea. Rev., 115, 272–296. tures under high winds) as the OML deepens (Price Bender, M., and I. Ginis, 2000: Real-case simulations of hurri- 1983; Shay et al. 1992). Entrainment heat fluxes into the cane–ocean interaction using a high-resolution coupled model: Effects on hurricane intensity. Mon. Wea. Rev., 128, thermocline will result in lower surface enthalpy fluxes 917–946. that feed the hurricane and impact hurricane intensity Bosart, L., C. S. Velden, W. E. Bracken, J. Molinari, and P. G. (Chang and Anthes 1978; Schade and Emanuel 1999, Black, 2000: Environmental influences on the rapid intensi- Bender and Ginis 2000). For coupled model forecast- fication of Hurricane Opal (1995) over the Gulf of Mexico. ing, currents and shears (momentum responses) are as Mon. Wea. Rev., 128, 322–352. Bretherton, F. P., R. E. Davis, and C. B. Fandry, 1976: A tech- important as thermal profiles for simulating OML cool- nique for objective analysis and design of experiments ap- ing and deepening processes. Given the number of ver- plied to MODE-73. Deep-Sea Res., 23, 559–582. tical mixing models using bulk and turbulence closure Chang, S. W., and R. A. Anthes, 1978: Numerical simulations of schemes (Jacob and Shay 2003), exploring parameter the ocean’s nonlinear, baroclinic response to translating hur- space under differing ocean conditions (water masses) ricanes. J. Phys. Oceanogr., 8, 468–480. Cheney, R., L. Miller, R. Agreen, N. Doyle, and J. Lillibridge, will require more current (and shear) measurements 1994: TOPEX/POSEIDON: The 2-cm solution. J. Geophys. than have been previously acquired under hurricane Res., 99, 24 555–24 563. conditions. Once obtained, these could be integrated Cione, J. J., and E. W. Uhlhorn, 2003: Sea surface temperature into the NOAA Intensity Forecasting Experiments variability in hurricanes: Implications with respect to inten- (Rogers et al. 2006). The ocean’s momentum response sity change. Mon. Wea. Rev., 131, 1783–1796. D’Asaro, E. A., 2003: The ocean boundary layer below hurricane to hurricane forcing has been used to determine the Dennis. J. Phys. Oceanogr., 33, 561–579. behavior of the surface drag coefficient (Shay and Ja- Donelan, M. A., B. K. Haus, N. Reul, W. Plant, M. Stiassnie, H. cob 2006; Jarosz et al. 2007). These data are needed to Graber, O. Brown, and E. Saltzman, 2004: On the limiting test and evaluate innovative forecasting schemes in- aerodynamic roughness of the ocean in very strong winds. volving both air–sea and vertical mixing parameteriza- Geophys. Res. Lett., 31, L18306, doi:10.1029/2004GL019460. Elliot, B. A., 1982: Anticyclonic rings in the Gulf of Mexico. J. tions. This is a crucial test for coupled models designed Phys. Oceanogr., 12, 1292–1309. for operational hurricane intensity forecasts (Marks Ellison, T. H., and J. S. Turner, 1959: Turbulent entrainment in and Shay 1998). stratified flows. J. Fluid Mech., 6, 424–448.

Unauthenticated | Downloaded 09/27/21 04:17 PM UTC SEPTEMBER 2008 SHAYANDUHL HORN 3273

Emanuel, K. A., 1995: Sensitivity of tropical cyclones to surface Pun, 2005: The interaction of Supertyphoon Maemi (2003) exchange coefficients and a revised steady-state model incor- with a warm ocean eddy. Mon. Wea. Rev., 133, 2635–2649. porating eye dynamics. J. Atmos. Sci., 52, 3969–3976. Mainelli, M., M. DeMaria, L. K. Shay, and G. Goni, 2008: Appli- ——, 2001: Contributions of tropical cyclones to meridional heat cation of oceanic heat content estimation to operational fore- transport by the oceans. J. Geophys. Res., 106, 14 771–14 781. casting of recent category 5 hurricanes. Wea. Forecasting, 23, Falkovich, A., I. Ginis, and S. Lord, 2005: Ocean data assimilation 3–16. and initialization procedure for the coupled GFDL/URI Mariano, A. J., and O. B. Brown, 1992: Efficient objective analysis hurricane prediction system. J. Atmos. Oceanic Technol., 22, of dynamically heterogeneous and nonstationary fields via 1918–1932. the parameter matrix. Deep-Sea Res., 39, 1255–1271. Forristall, G., K. Schaudt, and C. K. Cooper, 1992: Evolution and Marks, F., and L. K. Shay, 1998: Landfalling tropical cyclones: kinematics of a Loop Current eddy in the Gulf of Mexico Forecast problems and associated research opportunities. during 1985. J. Geophys. Res., 97, 2173–2184. Bull. Amer. Meteor. Soc., 79, 305–323. Freeland, H. J., and W. J. Gould, 1976: Objective analysis of me- Marquardt, D., 1963: An algorithm for least-squares estimation of soscale ocean circulation features. Deep-Sea Res., 23, 915– nonlinear parameters. J. Soc. Indust. Appl. Math., 11, 431– 923. 441. Geisler, J. E., 1970: Linear theory of the response of a two-layer Maul, G., 1977: The annual cycle of the Gulf Loop Current. Part ocean to a moving hurricane. Geophys. Fluid Dyn., 1, 249– I: Observations during a one-year time series. J. Mar. Res., 35, 272. 29–47. Gill, A. E., 1982: Atmosphere–Ocean Dynamics. Academic Press, Oey, L., T. Ezar, and H.-C. Lee, 2005: Loop Current, rings, and 661 pp. related circulation in the Gulf of Mexico: A review of nu- ——, 1984: On the behavior of internal waves in the wakes of merical models and future challenges. Circulation in the Gulf storms. J. Phys. Oceanogr., 14, 1129–1151. of Mexico: Observations and Models, Geophys. Monogr., Vol. Gregg, M., E. A. D’Asaro, T. J. Shay, and N. Larson, 1986: Ob- 161, Amer. Geophys. Union, 31–55. servations of persistent mixing and near-inertial internal Pasch, R. J., M. B. Lawrence, L. Avila, J. L. Beven, J. L. Franklin, waves. J. Phys. Oceanogr., 16, 856–885. and S. R. Stewart, 2004: Atlantic hurricane season of 2002. Halliwell, G., L. K. Shay, S. D. Jacob, O. M. Smedstad, and E. Mon. Wea. Rev., 132, 1829–1859. Uhlhorn, 2008: Improving ocean model initialization for Pollard, R. T., P. B. Rhines, and R. Thompson, 1973: The deep- coupled tropical cyclone forecast models using GODAE ening of the wind-mixed layer. Geophys. Astrophys. Fluid nowcasts. Mon. Wea. Rev., 136, 2576–2591. Dyn., 4, 381–404. Hock, T. F., and J. L. Franklin, 1999: The NCAR GPS dropwind- Powell, M. D., and S. H. Houston, 1996: Hurricane Andrew’s sonde. Bull. Amer. Meteor. Soc., 80, 407–420. landfall in south Florida. Part II: Surface wind fields and Hong, X., S. W. Chang, S. Raman, L. K. Shay, and R. Hodur, potential real-time applications. Wea. Forecasting, 11, 329– 2000: The interaction of Hurricane Opal (1995) and a warm 349. core ring in the Gulf of Mexico. Mon. Wea. Rev., 128, 1347– ——, P. J. Vickery, and T. A. Reinhold, 2003: Reduced drag co- 1365. efficient for high wind speeds in tropical cyclones. Nature, Jacob, S. D., and L. K. Shay, 2003: The role of oceanic mesoscale 422, 279–283. features on the tropical cyclone–induced mixed layer re- Price, J. F., 1981: Upper ocean response to a hurricane. J. Phys. sponse: A case study. J. Phys. Oceanogr., 33, 649–676. Oceanogr., 11, 153–175. ——, and C. Koblinsky, 2007: Effects of precipitation on the up- ——, 1983: Internal wave wake of a moving storm. Part I: Scales, per ocean response to a hurricane. Mon. Wea. Rev., 135, energy budget, and observations. J. Phys. Oceanogr., 13, 949– 2207–2225. 965. ——, L. K. Shay, A. J. Mariano, and P. G. Black, 2000: The 3D ——, R. A. Weller, and R. Pinkel, 1986: Diurnal cycling: Obser- oceanic mixed layer response to Hurricane Gilbert. J. Phys. vations and models of the upper ocean response to diurnal Oceanogr., 30, 1407–1429. heating, cooling, and wind mixing. J. Geophys. Res., 91 (7), Jarosz, E., D. A. Mitchell, D. W. Wang, and W. J. Teague, 2007: 8411–8427. Bottom-up determination of air–sea momentum exchange ——, T. B. Sanford, and G. Forristall, 1994: Forced stage response under a major tropical cyclone. Science, 315, 1707–1709. to a moving hurricane. J. Phys. Oceanogr., 24, 233–260. Kraus, E. B., and J. A. Businger, 1994: Atmosphere–Ocean Inter- Pudov, V., and S. Petrichenko, 2000: Trail of a typhoon in the action. Oxford Monogr. on Geology and Geophysics, No. 27, salinity field of the oceanic upper layer. Izv. Acad. Sci., 24, Oxford University Press, 362 pp. 700–706. Large, W. G., and S. Pond, 1981: Open ocean momentum flux Rogers, R., and Coauthors, 2006: The intensity forecasting experi- measurements in moderate to strong winds. J. Phys. Ocean- ment: A NOAA multiyear field program for improving tropi- ogr., 11, 324–336. cal cyclone intensity forecasts. Bull. Amer. Meteor. Soc., 87, Leaman, K. D., 1976: Observations of vertical polarization and 1523–1537. energy flux of near-inertial waves. J. Phys. Oceanogr., 6, 894– Sanford, T. B., P. G. Black, J. Haustein, J. W. Fenney, G. Z. For- 908. ristall, and J. F. Price, 1987: Ocean response to a hurricane. Leben, R. R., 2005: Altimeter-derived Loop Current metrics. Cir- Part I: Observations. J. Phys. Oceanogr., 17, 2065–2083. culation in the Gulf of Mexico: Observations and Models, ——, J. H. Dunlap, J. A. Carlson, D. C. Webb, and J. B. Girton, Geophys. Monogr., Vol. 161, Amer. Geophys. Union, 181– 2005: Autonomous velocity and density profiler: EM-APEX. 201. Proc. Eighth IEEE/OES Working Conf. on Current Measure- Leipper, D., and D. Volgenau, 1972: Hurricane heat potential of ment Technology, Southampton, United Kingdom, IEEE, the Gulf of Mexico. J. Phys. Oceanogr., 2, 218–224. 152–156. Lin, I., C.-C. Wu, K. A. Emanuel, I.-H. Lee, C.-R. Wu, and I.-F. Schade, L., and K. Emanuel, 1999: The ocean’s effect on the in-

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tensity of tropical cyclones: Results from a simple coupled Sturges, W., and R. Leben, 2000: Frequency of ring separations atmosphere–ocean model. J. Atmos. Sci., 56, 642–651. from the Loop Current in the Gulf of Mexico: A revised Shay, L. K., 2008: Upper ocean structure: A revisit of the response estimate. J. Phys. Oceanogr., 30, 1814–1819. to strong forcing events. Encyclopedia of Ocean Sciences, J. Teague, W. J., M. J. Carron, and P. J. Hogan, 1990: A comparison Steele et al., Eds., Elsevier, in press. between the Generalized Digital Environmental Model and ——, and R. L. Elsberry, 1987: Near-inertial ocean current re- Levitus climatologies. J. Geophys. Res., 95 (C5), 7167–7183. sponse to hurricane Frederic. J. Phys. Oceanogr., 17, 1249– Uhlhorn, E. W., P. G. Black, J. L. Franklin, M. Goodberlet, J. 1269. Carswell, and A. Goldstein, 2007: Hurricane surface wind ——, and S. D. Jacob, 2006: Relationship between oceanic energy measurements from an operational stepped frequency micro- fluxes and surface winds during tropical cyclone passage. At- wave radiometer. Mon. Wea. Rev., 135, 3070–3085. mosphere–Ocean Interactions, Vol. 2, Advances in Fluid Me- Walker, N., R. R. Leben, and S. Balasubramanian, 2005: Hurri- chanics, W. Perrie, Ed., WIT Press, 115–142. cane-forced upwelling and chlorophyll a enhancement within ——, P. G. Black, A. J. Mariano, J. D. Hawkins, and R. L. Els- cold core cyclones in the Gulf of Mexico. Geophys. Res. Lett., berry, 1992: Upper ocean response to Hurricane Gilbert. J. 32, L18610, doi:10.1029/2005GL023716. Geophys. Res., 97 (12), 20 227–20 248. Wright, C. W., and Coauthors, 2001: Hurricane directional wave ——, A. J. Mariano, S. D. Jacob, and E. H. Ryan, 1998: Mean and spectrum spatial variation in the open ocean. J. Phys. Ocean- near-inertial ocean current response to Hurricane Gilbert. J. ogr., 31, 2472–2488. Phys. Oceanogr., 28, 859–889. Wu, C.-C., C.-Y. Lee, and I.-I. Lin, 2007: The effect of the ocean ——, G. J. Goni, and P. G. Black, 2000: Effect of a warm oceanic eddy on tropical cyclone intensity. J. Atmos. Sci., 64, 3562– feature on Hurricane Opal. Mon. Wea. Rev., 128, 1366–1383. 3578.

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