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Benjamin Jaimes
,
Lynn K. Shay
, and
Eric W. Uhlhorn

Abstract

Using dropsondes from 27 aircraft flights, in situ observations, and satellite data acquired during Tropical Cyclone Earl (category 4 hurricane), bulk air–sea fluxes of enthalpy and momentum are investigated in relation to intensity change and underlying upper-ocean thermal structure. During Earl’s rapid intensification (RI) period, ocean heat content (OHC) variability relative to the 26°C isotherm exceeded 90 kJ cm−2, and sea surface cooling was less than 0.5°C. Enthalpy fluxes of ~1.1 kW m−2 were estimated for Earl’s peak intensity. Daily sea surface heat losses of , , and kJ cm−2 were estimated for RI, mature, and weakening stages, respectively. A ratio of the exchange coefficients of enthalpy (C K ) and momentum (C D ) between 0.54 and 0.7 produced reliable estimates for the fluxes relative to OHC changes, even during RI; a ratio overestimated the fluxes.

The most important result is that bulk enthalpy fluxes were controlled by the thermodynamic disequilibrium between the sea surface and the near-surface air, independently of wind speed. This disequilibrium was strongly influenced by underlying warm oceanic features; localized maxima in enthalpy fluxes developed over tight horizontal gradients of moisture disequilibrium over these eddy features. These regions of local buoyant forcing preferentially developed during RI. The overall magnitude of the moisture disequilibrium (Δq = q s − q a ) was determined by the saturation specific humidity at sea surface temperature (q s ) rather than by the specific humidity of the atmospheric environment (q a ). These results support the hypothesis that intense local buoyant forcing by the ocean could be an important intensification mechanism in tropical cyclones over warm oceanic features.

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Lynn K. Shay
,
Gustavo J. Goni
, and
Peter G. Black

Abstract

On 4 October 1995, Hurricane Opal deepened from 965 to 916 hPa in the Gulf of Mexico over a 14-h period upon encountering a warm core ring (WCR) in the ocean shed by the Loop Current during an upper-level atmospheric trough interaction. Based on historical hydrographic measurements placed within the context of a two-layer model and surface height anomalies (SHA) from the radar altimeter on the TOPEX mission, upper-layer thickness fields indicated the presence of two warm core rings during September and October 1995. As Hurricane Opal passed directly over one of these WCRs, the 1-min surface winds increased from 35 to more than 60 m s−1, and the radius of maximum wind decreased from 40 to 25 km. Pre-Opal SHAs in the WCR exceeded 30 cm where the estimated depth of the 20°C isotherm was located between 175 and 200 m. Subsequent to Opal’s passage, this depth decreased approximately 50 m, which suggests upwelling underneath the storm track due to Ekman divergence.

The maximum heat loss of approximately 24 Kcal cm−2 relative to depth of the 26°C isotherm was a factor of 6 times the threshold value required to sustain a hurricane. Since most of this loss occurred over a period of 14 h, the heat content loss of 24 Kcal cm−2 equates to approximately 20 kW m−2. Previous observational findings suggest that about 10%–15% of upper-ocean cooling is due to surface heat fluxes. Estimated surface heat fluxes based upon heat content changes range from 2000 to 3000 W m−2 in accord with numerically simulated surface heat fluxes during Opal’s encounter with the WCR. Composited AVHRR-derived SSTs indicated a 2°–3°C cooling associated with vertical mixing in the along-track direction of Opal except over the WCR where AVHRR-derived and buoy-derived SSTs decreased only by about 0.5°–1°C. Thus, the WCR’s effect was to provide a regime of positive feedback to the hurricane rather than negative feedback induced by cooler waters due to upwelling and vertical mixing as observed over the Bay of Campeche and north of the WCR.

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Benjamin Jaimes
,
Lynn K. Shay
, and
George R. Halliwell

Abstract

The response of quasigeostrophic (QG) oceanic vortices to tropical cyclone (TC) forcing is investigated using an isopycnic ocean model. Idealized oceanic currents and wind fields derived from observational data acquired during Hurricane Katrina are used to initialize this model. It is found that the upwelling response is a function of the curl of wind-driven acceleration of oceanic mixed layer (OML) currents rather than a function of the wind stress curl. Upwelling (downwelling) regimes prevail under the TC’s eye as it translates over cyclonic (anticyclonic) QG vortices. OML cooling of ~1°C occurs over anticyclones because of the combined effects of downwelling, instantaneous turbulent entrainment over the deep warm water column (weak stratification), and vertical dispersion of near-inertial energy. By contrast, OML cooling of ~4°C occurs over cyclones due to the combined effects of upwelling, instantaneous turbulent entrainment over regions of tight vertical thermal gradients (strong stratification), and trapping of near-inertial energy that enhances vertical shear and mixing at the OML base. The rotational rate of the QG vortex affects the dispersion of near-inertial waves. As rotation is increased in both cyclones and anticyclones, the near-inertial response is shifted toward more energetic frequencies that enhance vertical shear and mixing. TC-induced temperature anomalies in QG vortices propagate westward with time, deforming the cold wake. Therefore, to accurately simulate the impact of TC-induced OML cooling and feedback mechanisms on storm intensity, coupled ocean–atmosphere TC models must resolve geostrophic ocean eddy location as well as thermal, density, and velocity structures.

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Johna E. Rudzin
,
Lynn K. Shay
, and
William E. Johns

Abstract

Multiple studies have shown that reduced sea surface temperature (SST) cooling occurs under tropical cyclones (TCs) where a fresh surface layer and subsurface halocline exist. Reduced SST cooling in these scenarios has been attributed to a barrier layer, an upper-ocean feature in the tropical global oceans in which a halocline resides within the isothermal mixed layer. Because upper-ocean stratification theoretically reduces ocean mixing induced by winds, the barrier layer is thought to reduce SST cooling during TC passage, sustaining heat and moisture fluxes into the storm. This research examines how both the inclusion of salinity and upper-ocean salinity stratification influences SST cooling for a variety of upper-ocean thermal regimes using one-dimensional (1D) ocean mixed layer (OML) models. The Kraus–Turner, Price–Weller–Pinkel, and Pollard–Rhines–Thompson 1D OML schemes are used to examine SST cooling and OML deepening during 30 m s−1 wind forcing (~category 1 TC) for both temperature-only and temperature–salinity stratification cases. Generally, the inclusion of salinity (a barrier layer) reduces SST cooling for all temperature regimes. However, results suggest that SST cooling sensitivities exist depending on thermal regime, salinity stratification, and the 1D OML model used. Upper-ocean thermal and haline characteristics are put into context of SST cooling with the creation of a barrier layer baroclinic wave speed to emphasize the influence of salinity stratification on upper-ocean response under TC wind forcing.

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Luna Hiron
,
David S. Nolan
, and
Lynn K. Shay

Abstract

The Loop Current (LC) system has long been assumed to be close to geostrophic balance despite its strong flow and the development of large meanders and strong frontal eddies during unstable phases. The region between the LC meanders and its frontal eddies was shown to have high Rossby numbers indicating nonlinearity; however, the effect of the nonlinear term on the flow has not been studied so far. In this study, the ageostrophy of the LC meanders is assessed using a high-resolution numerical model and geostrophic velocities from altimetry. A formula to compute the radius of curvature of the flow from the velocity field is also presented. The results indicate that during strong meandering, especially before and during LC shedding and in the presence of frontal eddies, the centrifugal force becomes as important as the Coriolis force and the pressure gradient force: LC meanders are in gradient-wind balance. The centrifugal force modulates the balance and modifies the flow speed, resulting in a subgeostrophic flow in the LC meander trough around the LC frontal eddies and supergeostrophic flow in the LC meander crest. The same pattern is found when correcting the geostrophic velocities from altimetry to account for the centrifugal force. The ageostrophic percentage in the cyclonic and anticyclonic meanders is 47% ± 1% and 78% ± 8% in the model and 31% ± 3% and 78% ± 29% in the altimetry dataset, respectively. Thus, the ageostrophic velocity is an important component of the LC flow and cannot be neglected when studying the LC system.

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Johnny C. L. Chan
,
Yihong Duan
, and
Lynn K. Shay

Abstract

The interaction between a tropical cyclone (TC) and the underlying ocean is investigated using an atmosphere–ocean coupled model. The atmospheric model is developed from the Pennsylvania State University (Penn State)–National Center for Atmospheric Research (NCAR) mesoscale model version 4 MM4 and the ocean model consists of a mixed layer and an inactive stagnant layer beneath. Coupling between the atmosphere and the ocean models is achieved through wind stress and surface heat and moisture fluxes that depend on the sea surface temperature (SST). In the absence of a background flow, the atmospheric component consists of only a predefined vortex with an initial central pressure and the radius of the 15 m s−1 wind. The basic control experiments demonstrate that the coupled model can simulate the development of a TC and its interaction with the ocean.

Changes in TC intensity are sensitive to those of SST and the response is almost instantaneous. An SST of ∼27°C is found to be the threshold for TC development. In addition, the initial depth of the ocean mixed layer has an appreciable effect on TC intensity, which also depends on the movement of the TC. Furthermore, the vertical structure of ocean (vertical temperature gradient in the stagnant layer and temperature differential between the two layers) plays a significant role in modulating TC intensity.

In the presence of a warm core eddy (WCE), a TC intensifies before its center reaches the edge of the WCE. Although the TC attains maximum intensity at the center of the WCE, it does not weaken to its original intensity after leaving the WCE. During the entire passage of the TC, the SST at the center of the WCE decreases by about only 1°C, and the WCE generally maintains its original characteristics. However, two cold pools are observed around its periphery. A similar intensification process occurs when a TC moves over a sharp SST gradient and a locally deep ocean mixed layer. These results are explained by the interaction between the ocean and the TC circulation as well as the change in the total surface heat flux.

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Lynn K. Shay
,
Edward J. Walsh
, and
Pen Chen Zhang

Abstract

During the third intensive observational period of the Surface Wave Dynamics Experiment (SWADE), an aircraft-based experiment was conducted on 5 March 1991 by deploying slow-fall airborne expendable current profilers (AXCPs) and airborne expendable bathythermographs (AXBTs) during a scanning radar altimeter (SRA) flight on the NASA NP-3A research aircraft. As the Gulf Stream moved into the SWADE domain in late February, maximum upper-layer currents of 1.98 m s−1 were observed in the core of the baroclinic jet where the vertical current shears were O(10−2 s−1). The SRA concurrently measured the sea surface topography, which was transformed into two-dimensional directional wave spectra at 5–6-km intervals along the flight tracks. The wave spectra indicated a local wave field with wavelengths of 40–60 m propagating southward between 120° and 180°, and a northward-moving swell field from 300° to 70° associated with significant wave heights of 2–4 m.

As the AXCP descended through the upper ocean, the profiler sensed orbital velocity amplitudes of 0.2–0.5 m s−1 due to low-frequency surface waves. These orbital velocities were isolated by fitting the observed current profiles to the three-layer model based on a monochromatic surface wave, including the steady and current shear terms within each layer. The depth-integrated differences between the observed and modeled velocity profiles were typically less than 3 cm s−1. For 17 of the 21 AXCP drop sites, the rms orbital velocity amplitudes, estimated by integrating the wave spectra over direction and frequency, were correlated at a level of 0.61 with those derived from the current profiles. The direction of wave propagation inferred from the AXCP-derived orbital velocities was in the same direction observed by the SRA. These mean wave directions were highly correlated (0.87) and differed only by about 5°.

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Yonggang Liu
,
Robert H. Weisberg
, and
Lynn K. Shay

Abstract

To assess the spatial structures and temporal evolutions of distinct physical processes on the West Florida Shelf, patterns of ocean current variability are extracted from a joint HF radar and ADCP dataset acquired from August to September 2003 using Self-Organizing Map (SOM) analyses. Three separate ocean–atmosphere frequency bands are considered: semidiurnal, diurnal, and subtidal. The currents in the semidiurnal band are relatively homogeneous in space, barotropic, clockwise polarized, and have a neap-spring modulation consistent with semidiurnal tides. The currents in the diurnal band are less homogeneous, more baroclinic, and clockwise polarized, consistent with a combination of diurnal tides and near-inertial oscillations. The currents in the subtidal frequency band are stronger and with more complex patterns consistent with wind and buoyancy forcing. The SOM is shown to be a useful technique for extracting ocean current patterns with dynamically distinctive spatial and temporal structures sampled by HF radar and supporting in situ measurements.

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Lynn K. Shay
,
Thomas M. Cook
, and
P. Edgar An

Abstract

Over a 29-day time series in July 1999, an ocean surface current radar (OSCR) in very high frequency (VHF) mode mapped the surface velocity field at 250-m resolution at 700 cells off Fort Lauderdale, Florida. During the experiment, autonomous underwater vehicles (AUVs), equipped with upward- and downward-looking 1.2-MHz acoustic Doppler current profilers (ADCPs), measured subsurface current structure over four to six radar cells during two mixed layer patterns on 9 and 27 July 1999. As these AUV sampling patterns were conducted over 500 m × 500 m and 500 m × 750 m areas, these missions required about 80–90 min (four radar sample intervals) to form four and seven synoptic snapshots, respectively.

Based on autocorrelation analyses of the profiler data, along-AUV-track subsurface profiles were averaged at 10-s intervals, mapped to a surface from 1.5–6.5 m, and compared to surface currents at more than 500 points for each snapshot. Comparisons between the surface and subsurface currents from the AUV revealed spatially averaged differences ranging from 4 to 26 cm s−1 during these two experiments. The largest differences occurred when the surface and subsurface current vectors were orthogonal; otherwise, differences were O(10 cm s−1). Scatterplots between 2-m and radar-derived surface currents indicated a consistent relationship with mooring data. From the seven spatial snapshots acquired during the second experiment, current profiles suggested a time-dependent oscillation that was corroborated by radar and moored ADCP data. Least squares fits of these profiles from sequential AUV snapshots to a simple model isolated an ∼9.2 ± 1 h oscillation where the along-shelf current was O(50 cm s−1).

Spatially averaged current profiles from four and seven snapshots were subsequently time averaged to form a mean profile from each experiment. In the downwind directions, these mean profiles were compared to a wind-driven, logarithmic layer profile in the upper 6.5 m based on a 10-m surface winds. Regression analyses suggest a slope of ≈1.16 between the theoretical and observed mean profiles with a bias of about 3 cm s−1. In this context, the averaged winds played a role in driving the coastal ocean circulation. These results further suggest that the spatial averaging by the radar is consistent when subsurface current variations are averaged over similar time and space scales.

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Lynn K. Shay
,
Simon W. Chang
, and
Russell L. Elsberry

Abstract

During the passage of hurricane Frederic in 1979, four ocean current meter arrays in water depths of 100–950 m detected both a baroclinic and a depth-independent response in the near-inertial frequency band. Although the oceanic response was predominately baroclinic, the hurricane excited a depth-independent component of 5–11 cm s−1.

The origin and role of the depth-independent component of velocity is investigated using a linear analytical model and numerical simulations from a 17-level primitive equation model with a free surface. Both models are forced with an idealized wind stress pattern based on the observed storm parameters in hurricane Frederic. In an analytical model, the Green's function (K 0) is convolved with the wind stress curl to predict a sea surface depression of approximately 20 cm from the equilibrium position. The near-inertial velocities are simulated by convolving the slope of the sea surface depression with a second Green's function. The barotropic current velocities rotate inertially with periods shifted above the local inertial period by 1%–2% and the maximum amplitude of 11 cm s−1 is displaced to the right of the track at x = 2R max (radius of maximum winds).

The free surface depression simulated by the primitive-equation model is also about 18–20 cm. The primitive equation model simulations indicate that the vertical mean pressure gradient excites 10–11 cm s−1 depth-averaged currents at x = 3R max. The net divergence and convergence of the horizontal velocities induces vertical deflections of the sea surface. The spatial pattern of the barotropic amplitudes simulated by the numerical and analytical models differ by less than 2 cm s−1 in the region 0 < x < 4R max, which suggests that the barotropic response to the passage of a moving hurricane is governed by linear processes.

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