400 JOURNAL OF PHYSICAL OCEANOGRAPHY VOLUME 42 Loop Current Mixed Layer Energy Response to Hurricane Lili (2002). Part I: Observations ERIC W. UHLHORN NOAA/AOML/Hurricane Research Division, Miami, Florida LYNN K. SHAY Division of Meteorology and Physical Oceanography, Rosenstiel School of Marine and Atmospheric Science, University of Miami, Miami, Florida (Manuscript received 4 May 2011, in final form 3 October 2011) ABSTRACT The ocean mixed layer response to a tropical cyclone within and immediately adjacent to the Gulf of Mexico Loop Current is examined. In the first of a two-part study, a comprehensive set of temperature, salinity, and current profiles acquired from aircraft-deployed expendable probes is utilized to analyze the three-dimensional oceanic energy evolution in response to Hurricane Lili’s (2002) passage. Mixed layer temperature analyses show that the Loop Current cooled ,18C in response to the storm, in contrast to typically observed larger decreases of 38–58C. Correspondingly, vertical current shear associated with mixed layer currents, which is responsible for entrainment mixing of cooler water, was found to be up to 50% weaker, on average, than observed in previous studies within the directly forced region. The Loop Current, which separates the warmer, lighter Caribbean Subtropical Water from the cooler, heavier Gulf Common Water, was found to decrease in intensity by 20.18 6 0.25 m s21 over an approximately 10-day period within the mixed layer. Contrary to previous ocean response studies, which have assumed approximately horizon- tally homogeneous ocean structure prior to storm passage, a kinetic energy loss of 5.8 6 6.4 kJ m22,or approximately 21 wind stress-scaled energy unit, was observed. By examining near-surface currents derived from satellite altimetry data, the Loop Current is found to vary similarly in magnitude over such time scales, suggesting storm-generated energy is rapidly removed by the preexisting Loop Current. In a future study, the simulated mixed layer evolution to a Hurricane Lili–like storm within an idealized preexisting baroclinic current is analyzed to help understand the complex air–sea interaction and resulting energetic response. 1. Introduction have examined the relationship between the intensity of TCs and the SST. Malkus and Riehl (1960) derived Hurricanes, and more generally tropical cyclones (TCs), a relationship between the decrease in central pressure are among the most intense organized vortical systems and the increase in equivalent potential temperature in observed in the atmosphere. These cyclones derive their the eyewall region (due to imported enthalpy from the energy primarily from the release of latent heat upon ocean). Further studies by Emanuel (1986) and Betts condensation of water vapor (Ooyama 1969). Thus, it is and Simpson (1987) confirmed this relationship to be necessary that a large moisture source be present, such as approximately constant. More recent research has stud- the ocean, and that the sea surface temperature (SST) is ied not only the influence of SST on TC intensity but also sufficiently warm to maintain a moist enthalpy flux from the relationship between intensity and the upper-ocean the sea to the atmospheric boundary layer, as was first thermal energy (Shay et al. 2000). recognized by Palmen (1948). Numerous other studies Because the underlying ocean significantly modulates TC intensity, much attention has been drawn toward gaining a better understanding of the physical interac- Corresponding author address: Dr. Eric W. Uhlhorn, NOAA/ AOML/Hurricane Research Division, 4301 Rickenbacker Cswy., tion between the atmosphere and ocean during these Miami, FL 33149. events. Unfortunately, because of limited observational E-mail: [email protected] data at the air–sea interface in high-wind conditions, the DOI: 10.1175/JPO-D-11-096.1 Ó 2012 American Meteorological Society Unauthenticated | Downloaded 09/29/21 08:47 AM UTC MARCH 2012 U H L H O R N A N D S H A Y 401 understanding of momentum and energy transfer pro- cesses has not progressed enough to significantly in- crease forecast accuracy. The recent Office of Naval Research sponsored Coupled Boundary Layer Air–Sea Transfer (CBLAST) field experiment in 2003–04 (Black et al. 2007) was designed to address questions regarding poorly understood air–sea exchange processes in TCs such as the maturity of the sea state (Donelan et al. 1993), sea spray (Fairall et al. 1994), and boundary layer roll vortices (Foster 2005) and their possible impacts on TC intensity. In sum, the various conclusions generally now agree that the bulk enthalpy and momentum ex- change coefficients at high wind speeds are not as large as previously assumed (Drennan et al. 2007; French et al. 2007). Further complicating TC air–sea interactions is the sometimes large variability in the upper ocean typical of FIG. 1. SST (8C) on 4 Oct 2002 from combined Advanced large-scale ocean current systems (e.g., Gulf Stream, Microwave Scanning Radiometer for Earth Observing System Kuroshio) that exist near continental western bound- (AMSR-E)/Tropical Rainfall Measuring Mission Microwave Im- aries where TCs often make landfall. A particularly in- ager (TMI) microwave imagery, 2 days after Hurricane Lili passed teresting region to examine the effects of variable ocean over the southeast GOM. Arrow points to area of significant sur- face cooling, which is delineated from the LC’s intrusion into the structure on TC intensity is the Gulf of Mexico (GOM). Gulf. Courtesy of Remote Sensing Systems, Inc. Studies of Hurricanes Gilbert (Shay et al. 1992), Opal (Shay et al. 2000), Ivan (Walker et al. 2005), Katrina, and Rita (Shay 2009; Jaimes and Shay 2009, 2010) have air–sea fluxes decrease (Chang and Anthes 1978; Price demonstrated the potential influence GOM upper- 1981; Shay et al. 1992; Schade and Emanuel 1999; ocean variability can have on TC intensity. As revealed Bender and Ginis 2000). These observations demand a by microwave satellite imagery, SST cooling (indicated detailed understanding of the physical mechanisms re- by the arrow in Fig. 1) was associated with Hurricane sponsible for preventing the expected cooling. In this Lili’s (2002) passage through the GOM. However, this first of a two-part study, the analysis of these observa- cooler water is clearly confined to an area outside of the tions in Lili is expanded beyond primarily the thermal GOM Loop Current (LC), which is located in the extreme response previously reported by the authors. Particular southeast portion of the GOM. While Lili traversed the attention is given to analyzed fields of mixed layer cur- GOM, it underwent a period of rapid intensification rents and resulting mechanical energy. In section 2, mass from Saffir–Simpson category 1 to 4, immediately fol- and current field objective analyses are constructed from lowed by a weakening to category-1 intensity, over a 2.5- observations obtained in the GOM Loop Current region day period (Pasch et al. 2004). Particularly relevant to around the time of Lili’s passage. From these data, mixed understanding air–sea energy exchange over the LC, the layer depth and geostrophic current fields are estimated. resulting modulation of SST cooling, and ultimately In section 3, the evolution of mixed layer mechanical feedback to TC intensity is the large horizontal transport energy is computed from the analyses, and section 4 ex- and its impact on shear-induced mixing. Adding to un- amines the variability of the Loop Current system in the certainties in forecasting hurricane intensity in this region southeast GOM to help understand the observational is the often-complex atmospheric environment, which results. A summary of results and conclusions are pre- also exerts an influence on intensity (Bosart et al. 2000). sented in section 5. In a future study, details of the energy During 2002, life cycles of Hurricanes Isidore and Lili interactions are examined with the aid of an idealized in the northwest Caribbean Sea and GOM were exten- numerical model. sively observed (Shay and Uhlhorn 2008), which indi- cated deep, warm ocean structures cooled relatively little 2. Observed fields and provided a relative positive thermal feedback to these storms. This is in contrast to more typical negative As reported in Shay and Uhlhorn (2008), a large set of feedback on hurricane intensity when shear-induced temperature, salinity, and current vertical profiles was mixing at the ocean mixed layer (OML) base cools and obtained prior to, during, and after passage of Hurricane deepens this layer and causes a cold ocean wake where Lili (2002) across the southeast GOM during a joint Unauthenticated | Downloaded 09/29/21 08:47 AM UTC 402 JOURNAL OF PHYSICAL OCEANOGRAPHY VOLUME 42 FIG. 2. Locations of the ocean probes deployed prior to the passage of Hurricane Lili used to estimate the initial temperature conditions. AXBTs (circles), AXCTDs (3’s), and AXCPs (pluses). Lili’s observed ‘‘best track’’ is indicated by the solid line, and marked at 6-h intervals. The storm travels from southeast to northwest. The solid box indicates the analysis region for this study, and the dotted contours identify the 200- and 1000-m isobaths. National Oceanic and Atmospheric Administration– (AXCP) profile locations over the course of the exper- National Science Foundation (NOAA–NSF) aircraft- iment. The fairly dense horizontal coverage of vertical based field research experiment. These data underwent profiles provided an opportunity to compute optimally an extensive processing and quality-control effort to ana- interpolated objective analyses of three-dimensional lyze details about the upper-ocean evolution in response (3D) synoptic fields for relevant quantities to examine to a Saffir–Simpson category-3 storm. Figures 2–6 show the mass, momentum, and energy evolution over a 10-day aircraft expendable bathythermograph (AXBT) probe, period. Details regarding experimental goals, method- aircraft expendable conductivity–temperature–depth ology, and data processing are provided in the above (AXCTD) probe, and aircraft expendable current probe referenced article.