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Sea Surface Temperature and Ocean Heat Content during Tropical Cyclones Pam (2015) and Winston (2016) in the Southwest Pacific Region

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Abstract

The sea surface temperature (SST) and upper-ocean heat content (OHC) have been explored along the track of two tropical cyclones (TCs): TC Pam (2015) and TC Winston (2016). These TCs severely affected the islands of Vanuatu and Fiji, in the South Pacific region (8°–30°S, 140°E−170°W). The SST decreased by as much as 5.4°C along the tracks of the TCs with most cooling occurring to the left of the TCs tracks relative to TC motion. SST cooling of 1°–5°C has generally been observed during both the forced and relaxation stages of TC passage. Argo profiles near the TCs revealed observable temperature-based mixed layer deepening. Subsurface warming was also observed post-TC passage from the temperature profile of one of the floats after the passage of both TCs. The OHC and heat fluxes are seen to play an important part in TC intensification as both these TCs intensified after passing over regions of high OHC and enhanced heat fluxes. Apart from the traditionally used OHC obtained up to the depth of the 26°C isotherm (QH), the OHC was also determined up to the depth of the 20°C isotherm (QH,20). The QH and QH,20 values decreased in the majority of cases post TC passage while QH,20 increased in one instance post-TC passage for both the TCs. QH,20 was also used to identify heat energy changes at deeper levels and it correlated well with the traditionally used OHC during the weaker stages of the TCs.

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Lightning data from the World Wide Lightning Location Network along with tropical cyclone (TC) track and intensity data from the China Meteorological Administration are used to study lightning activity in TCs over the northwest Pacific from 2005 to 2009 and to investigate the relationship between inner core lightning and TC intensity changes. Lightning in TCs over the northwest Pacific is more likely to occur in weak storms at tropical depression (10.8–17.1 m s−1) and tropical storm (17.2–24.4 m s−1) intensity levels, in agreement with past studies of Atlantic hurricanes. The greatest lightning density (LD) in the inner core appears in storms undergoing an intensity change of 15–25 m s−1 during the next 24 h. Lightning is observed in all storm intensity change categories: rapid intensification (RI), average intensity change (AIC), and rapid weakening (RW). The differences in LD between RI and RW are largest in the inner core, and the LD for RI cases is larger than for RW cases in the inner core (0–100 km). Lightning activity there, rather than in the outer rainbands, may be a better indicator for RI prediction in northwest Pacific storms. There was a marked increase in the lightning density of inner core during the RI stage for Super Typhoon Rammasun (2008). Satellite data for this storm show that the RI stage had the highest cloud top height and coldest cloud top temperatures, with all the minimum black body temperature values being below 200 K in the inner core. Lightning in TCs over NP is more likely to occur in TD and TS intensity levelLightning in the inner core may be a better indicator for NP RI predictionA different pattern of lightning and TC intensity change exists among basins
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The spatial structure and temporal evolution of the sea surface temperature (SST) anomaly (SSTA) associated with the passage of tropical cyclones (TCs), as well as their sensitivity to TC characteristics (including TC intensity and translation speed) and oceanic climatological conditions (represented here by latitude), are thoroughly examined by means of composite analysis using satellite-derived SST data. The magnitude of the TC-generated SSTA is larger for more intense, slower-moving, and higher-latitude TCs, and it occurs earlier in time for faster-moving and higher-latitude storms. The location of maximum SSTA is farther off the TC track for faster-moving storms, and it moves toward the track with time after the TC passage. The spatial extension of the cold wake is greater for more intense and for slower-moving storms, but its shape is quite independent of TC characteristics. Consistent with previous studies, the calculations show that the mean SSTA over a TC-centered box nearly linearly correlates with the wind speed for TCs below category 3 intensity while for stronger TCs the SSTA levels off, both for tropical and subtropical regions. While the linear behavior is expected on the basis of the more vigorous mixing induced by stronger winds and is derived from a simple mixed-layer model, the level-off for intense TCs is discussed in terms of the dependence of the maximum amplitude of the area-mean SSTA on TC translation speed and depth of the prestorm mixed layer. Finally, the decay time scale of the TC-induced SSTA is shown to be dominated by environmental conditions and has no clear dependence on its initial magnitude and on TC characteristics.
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An oceanic climatology to calculate upper-ocean thermal structure was developed for application year-round in the North Atlantic Ocean basin. The Systematically Merged Atlantic Regional Temperature and Salinity (SMARTS) Climatology is used in a two-layer model to project sea surface height anomalies (SSHA) into the water column at ¼° resolution. SMARTS blended monthly temperature and salinity fields from the World Ocean Atlas 2001 (WOA01) and Generalized Digital Environmental Model (GDEM) version 3.0 based on their performance compared to in situ measurements. Daily mean isotherm depths of 20°C (D20) and 26°C (D26) (and their mean ratio), reduced gravity, and mixed layer depth (MLD) were estimated from the climatology. This higher-resolution climatology resolves features in the Gulf of Mexico (GOM), including the Loop Current (LC) and eddy shedding region. Using SMARTS with satellite-derived SSHA and SST fields, daily values of isotherm depths, mixed layer depths, and ocean heat content (OHC) were calculated from 1998 to 2012. OHC is an important scalar when determining the ocean’s impact on tropical cyclone intensification, because it is a better predictor of SST cooling during hurricane passage. Airborne- and ship-deployed expendable bathythermographs (XBT), long-term moorings, and Argo profiling floats provided over 50 000 thermal profiles to assess satellite retrievals of isotherm depths and OHC using the SMARTS Climatology. The OHC calculation presented in this document reduces errors basinwide by 20%, with a 35% error reduction in the GOM.
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[1] The Main Development Region (MDR) for tropical cyclones (TCs) in the western North Pacific Ocean is the most active TC region in the world. Based on synergetic analyses of satellite altimetry and gravity observations, we found that the subsurface ocean conditions in the western North Pacific MDR has become even more favorable for the intensification of typhoons and supertyphoons. Compared to the early 1990s, a 10% increase in both the depth of the 26°C isotherm (D26) and Tropical Cyclone Heat Potential (TCHP) has occurred in the MDR. In addition, the areas of high TCHP (≥ 110 kJ cm−2) and large D26 (≥ 110 m) have 13% and 17% increases, respectively. Because these high TCHP and large D26 regions are often associated with intensification of the most intense TCs (i.e. supertyphoons), this recent warming requires close attention and monitoring.
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The regional upper-ocean dynamical and thermodynamical responses to two consecutive, strong tropical cyclones (TCs) – 04B (10/15-10/19) and 05B (10/25-11/3) in 1999 (hereafter, TC1 and TC2) in the Bay of Bengal (BoB) and the associated oceanic processes are investigated using an eddy-permitting Hybrid Coordinate Ocean Model. The TC winds induce positive sea surface height anomalies (SSHA) along the northern BoB coastline and Andaman Sea due to onshore convergence, and negative SSHA along the TC tracks due to Ekman divergence, which in turn induce geostrophic flow. The TC-associated radiation and precipitation have negligible effects on the northwestern BoB top 30m-averaged temperature (T0-30m), while the strong TC winds significantly enhance turbulent heat flux causing T0-30m decrease. Due to the existence of the barrier layer and subsurface warm advection in the northwestern BoB, vertical mixing may induce near-surface warming by entraining warm water from below. As a result, the proportion of the T0-30m cooling caused by turbulent heat flux is likely elevated in the 6°×6° footprints of the TCs. Both TC wind-induced vertical mixing and upwelling significantly cool T0-30m in TC1’s wake, while upwelling dominates the maximum cooling region in TC2’s wake, likely due to the preceding deepening of the mixed layer by TC1. The near-surface cooling for both TCs has rightward bias because of the higher winds and the resonant response on the right, and Ekman divergence extends the cooling areas outward. TC1 and TC2’s sizes and pre-storm oceanic conditions are found to be the most influential factors for near-surface cooling.
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The synoptic-scale flow during tropical cyclogenesis and cyclolysis over the North Atlantic Ocean is investigated using compositing methods. Genesis and lysis are defined using the National Hurricane Center (NHC, now known as the Tropical Prediction Center) best-track data. Genesis (lysis) occurs when NHC first (last) identifies and tracks a tropical depression in the final best track dataset. Storm-centered composites are created with the Analysis of the Tropical Oceanic Lower Level (ATOLL; ∼900 hPa) and 200-hPa winds for June-November produced by NHC for the years 1975-93. Results show that significant regional differences exist in 200-hPa flow during genesis across the Atlantic basin. Composites of genesis in the western part of the basin show a 200-hPa trough (ridge) located to the west (east) of the ATOLL disturbance. In the eastern half of the basin composites of genesis show a sprawling 200-hPa ridge centered northeast of the ATOLL disturbance. The major axis of this elliptically shaped 200-hPa anticyclone extends zonally slightly poleward of the ATOLL level disturbance. Another composite of relatively rare genesis events that are associated with the equalorward end of frontal boundaries show that they generally occur in the equatorward entrance region of a jet streak in conjunction with an ATOLL cyclonic vorticity maximum in a region where vertical shear is minimized. An approximation of the Sutcliffe-Trenberth form of the quasigeostrophic omega equation is used to estimate the forcing for vertical motion in the vicinity of developing tropical cyclones. Forcing for ascent is found in all three genesis composites and is accompanied by a nonzero minimum in vertical shear directly above the ATOLL cyclonic vorticity maximum. Vertical shear over developing depressions is found to be near 10 m s -1, suggestive that weak shear is necessary during tropical cyclogenesis to help force synoptic-scale ascent. Composites of tropical cyclone lysis show much weaker ATOLL cyclonic vorticity when compared to the genesis composites. The magnitude of the vertical shear and the forcing for ascent above the lysis ATOLL disturbance are stronger and weaker, respectively, than in the genesis composites. These differences arise due to the presence of a jet-streak and a longer half-wavelength between the trough and ridge axes in the lysis 200-hPa flow composite. The genesis flow patterns are decomposed by crudely removing the signature of the developing cyclone and its associated convection. Two separate and very different flow patterns commonly observed during genesis over the eastern and western Atlantic Ocean are found to be very similar once the flows are decomposed. Both flows are characterized by strong deformation at low levels and at 200 hPa with an upper-level jet exit region near the developing depression.
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During the passage of hurricane Norbert in 1984, the Hurricane Research Division of NOAA conducted a Planetary Boundary Layer Experiment that included the deployment of Airborne eXpendable Current Profilers (AXCP). A total of. 16 AXCPs provided for the fist time high-resolution vertical profiles of currents and temperatures in hurricane wind conditions. This study focuses on the vertical structure of the near-inertial baroclinic current excited by the passage of this hurricane. The transient hurricane-induced currents are isolated from the AXCP profiles in Norbert by subtracting a spatially-averaged current. Near the center of hurricane Norbert, the WKBJ-scaled vertical wavenumber spectra are a decade greater than the Garrett-Munk spectra (GM75). The fist ten linear, baroclinic free modes are calculated from the spatially-averaged Brunt–Vaisala frequency. To allow a more direct comparison with the AXCP observations in the current wind regime, the near-inertial response for the three dimensional v...
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This study examines the local memory of atmospheric and oceanic changes associated with a tropical cyclone (TC). The memory is quantified through anomalous maximum potential intensity (MPI) evolution for 20 days prior to the arrival of a TC through 60 days after the TC passage. The local MPI weakens and is not restored to the evolving climatology until well after the TC has departed. Stabilization occurs through warming of the atmosphere and cooling of the ocean surface on different time scales. The time scale of MPI stabilization following TC passage is approximately 30-35 days for a tropical storm to 50-60 days for a category 3-5 hurricane, with significant storm-specific and basin-specific variability. The atmospheric stabilization (warming with respect to SST) begins with TC arrival and continues for approximately 7-10 days after passage, when the troposphere cools below normal. The rewarming of SST and the subsequent rewarming of the atmosphere occurs within approximately 35 days for all intensities, despite a positive (weakened) MPI anomaly through two months. This suggests that the atmosphere retains anomalous warmth beyond what can be attributable to sensible heating from the rewarmed SST. The maintenance of a positive MPI anomaly beyond 35 days is thus attributed to a feedback on larger scales that requires considerable further research. A TC's passage through a region does not always lead to a weakening of the MPI. In regions poleward of the sharp SST gradient, the MPI one month after TC passage is often several millibars stronger than climatology. There are also mesoscale regions of destabilization one month after TC passage that may result partially from salinity changes driven by oceanic mixing as well as changes in precipitation and evaporation.
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The center of Hurricane Felix passed 85 km to the southwest of the Bermuda Testbed Mooring (BTM; 31°44′N, 64°10′W) site on 15 August 1995. Data collected in the upper ocean from the BTM during this encounter provide a rare opportunity to investigate the physical processes that occur in a hurricane's wake. Data analyses indicate that the storm caused a large increase in kinetic energy at near-inertial frequencies, internal gravity waves in the thermocline, and inertial pumping, mixed layer deepening, and significant vertical redistribution of heat, with cooling of the upper 30 m and warming at depths of 30–70 m. The temperature evolution was simulated using four one-dimensional mixed layer models: Price-Weller-Pinkel (PWP), K Profile Parameterization (KPP), Mellor-Yamada 2.5 (MY), and a modified version of MY2.5 (MY2). The primary differences in the model results were in their simulations of temperature evolution. In particular, when forced using a drag coefficient that had a linear dependence on wind speed, the KPP model predicted sea surface cooling, mixed layer currents, and the maximum depth of cooling closer to the observations than any of the other models. This was shown to be partly because of a special parameterization for gradient Richardson number (RgKPP) shear instability mixing in response to resolved shear in the interior. The MY2 model predicted more sea surface cooling and greater depth penetration of kinetic energy than the MY model. In the MY2 model the dissipation rate of turbulent kinetic energy is parameterized as a function of a locally defined Richardson number (RgMY2) allowing for a reduction in dissipation rate for stable Richardson numbers (RgMY2) when internal gravity waves are likely to be present. Sensitivity simulations with the PWP model, which has specifically defined mixing procedures, show that most of the heat lost from the upper layer was due to entrainment (parameterized as a function of bulk Richardson number RbPWP), with the remainder due to local Richardson number (RgPWP) instabilities. With the exception of the MY model the models predicted reasonable estimates of the north and east current components during and after the hurricane passage at 25 and 45 m. Although the results emphasize differences between the modeled responses to a given wind stress, current controversy over the formulation of wind stress from wind speed measurements (including possible sea state and wave age and sheltering effects) cautions against using our results for assessing model skill. In particular, sensitivity studies show that MY2 simulations of the temperature evolution are excellent when the wind stress is increased, albeit with currents that are larger than observed. Sensitivity experiments also indicate that preexisting inertial motion modulated the amplitude of poststorm currents, but that there was probably not a significant resonant response because of clockwise wind rotation for our study site.
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Wind-induced near-inertial energy has been believed to be an important source for generating the ocean mixing required to maintain the global meridional overturning circulation. In the present study, the near-inertial energy budget in a realistic (1)/(12)degrees model of the North Atlantic Ocean driven by synoptically varying wind forcing is examined. The authors find that nearly 70% of the wind-induced near-inertial energy at the sea surface is lost to turbulent mixing within the top 200 m and, hence, is not available to generate diapycnal mixing at greater depth. Assuming this result can be extended to the global ocean, it is estimated that the wind-induced near-inertial energy available for ocean mixing at depth is, at most, 0.1 TW. This confirms a recent suggestion that the role of wind-induced near-inertial energy in sustaining the global overturning circulation might have been overemphasized.
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The Rossby adjustment process in the wake of a storm is studied with a view to finding, within the context of linear theory, how wave dispersion in both the vertical and meridional directions spreads energy which is initially confined entirely to the mixed layer. Comparisons are made between three cases 1) a periodic storm on the f-plane (where dispersion is purely vertical); 2) a bounded storm on the f-plane; and 3) a bounded storm centered 2700 km from the equator on an equatorial beta-plane. Particular attention is paid to the initial rate of loss of energy from the mixed layer, and some simple formulas which work very well in the cases studied are derived. These show that the rate of loss goes up when the mixed-layer depth is increased, and also that the rate scales as the square of the wavenumber. Values of the rates are sufficient to provide a major source of energy for internal waves below the mixed layer. The often-observed tendency for phase lines to propagate upward is found in all cases, but the analysis also shows possible shortcomings in the way observations are interpreted. Some attention is also paid to the closer of the ocean floor on the results. Other observed properties of near-inertial waves which are found in the model studies are the following 1) intermittency; 2) storm effects on currents are largest just below the storm track; 3) the horizontal and vertical scales tend to decrease with time after the storm has passed; 4) vertical group propagation on scales comparable with the mixed layer is very slow, and 5) some tendency for bottom intensification is found. Another result is that beta-dispersion can be quite important, and some effect are transmitted to the equator quite rapidly (typically two weeks).
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A two-layer model of the upper ocean is used to simulate the thermal response to hurricane passage. Mixed layer temperature and depth equations similar to those of Kraus and Turner (1967) and Denman (1973) are used to describe the upper layer., Horizontal and vertical advection are calculated from Ekman theory, with an empirical partitioning of the input stress between the mechanical generation and entrainment mixing terms. A compensating return flow below the Ekman depth, but within the upper 100 m of the seasonal thermocline, is imposed. The surface heat and momentum fluxes are calculated with the symmetrical hurricane model of Elsberry et al. (1974). The response to both a stationary and a moving hurricane of constant intensity is simulated. The model produces regions of near-surface cooling, subsurface cooling induced by upwelling, and an intermediate layer of warming due to entrainment and convective mixing. A decrease in mixed layer depth near the center and an increase outside the radius of maximum winds are obtained. Oceanic heat budget calculations suggest that advective processes, rather than heat loss to the storm, play the dominant role in modifying the ocean thermal structure near the center of the storm. Effects of varying both oceanic and atmospheric model parameters are tested to illustrate the sensitivity of the model.
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During favorable atmospheric conditions, Hurricanes Katrina and Rita deepened to category 5 over the Loop Current's (LC) bulge associated with an amplifying warm core eddy. Both hurricanes subsequently weakened to category 3 after passing over a cold core eddy (CCE) prior to making landfall. Reduced (increased) oceanic mixed layer (OML) cooling of;1°C (4.5°C) was observed over the LC (CCE) where the storms rapidly deepened (weakened). Data acquired during and subsequent to the passage of both hurricanes indicate that the modulated velocity response in these geostrophic features was responsible for the contrasts in the upper-ocean cooling levels. For similar wind forcing, the OML velocity response was about 2 times larger inside the CCE that interacted with Katrina than in the LC region affected by Rita, depending on the prestorm OML thickness. Hurricane-induced upwelling and vertical mixing were increased (reduced) in the CCE (LC). Less wind-driven kinetic energy was available to increase vertical shears for entrainment cooling in the LC, as the OML current response was weaker and energy was largely radiated into the thermocline. Estimates of downward vertical radiation of near-inertial wave energies were significantly stronger in the LC (12.1 × 10-2 W m-2) than in the CCE (1.8 × 10-2 W m-2). Katrina and Rita winds provided O(1010)Wto the global internal wave power. The vertical mixing induced by both storms was confined to the surface water mass. From a broader perspective, models must capture oceanic features to reproduce the differentiated hurricane-induced OML cooling to improve hurricane intensity forecasting.
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It has been demonstrated that a large input of energy from the ocean is necessary to establish and maintain hurricane force winds over the sea. However, there has been no suitable data which could serve as a basis for calculating this input. Now, observations are available to show that, early in the hurricane season, there are varying initial conditions in the Gulf of Mexico which could lead to significantly different total heat exchanges. The sea can provide some seven days of energy flow into a hurricane at some times and at some locations, but less than one day in others depending upon the amount of heat initially available in the Gulf waters. In the four summers represented by the data, a quantity defined as hurricane heat potential was found to vary from a low of 700 cal cm−2 column north of Yucatan to a high of 31,600 in the central east Gulf. Synoptic data on hurricane heat potential, if made regularly available to forecasters, might serve as a basis for improved forecasts of changes in In...
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1] I show that the latest series of climate models reproduce the observed mid-depth Southern Ocean warming since the 1950s if they include time-varying changes in anthropogenic greenhouse gases, sulphate aerosols and volcanic aerosols in the Earth's atmosphere. The remarkable agreement between observations and state-of-the art climate models suggests significant human influence on Southern Ocean temperatures. I also show that climate models that do not include volcanic aerosols produce mid-depth Southern Ocean warming that is nearly double that produced by climate models that do include volcanic aerosols. This implies that the full effect of human-induced warming of the Southern Ocean may yet to be realized. Citation: Fyfe, J. C. (2006), Southern Ocean warming due to human influence, Geophys. Res. Lett., 33, L19701, doi:10.1029/2006GL027247.
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At first, the description of the energy cycle of the mature tropical storm is amplified on the basis of recent upper air observations. Air particles passing through such a storm at first undergo isothermal expansion as they move toward a center. Then they ascend with release of condensation. At high levels they move outward and mix with the environment giving off heat to the surrounding colder air. Several requisites for maintenance of the observed temperature field are stated. After a discussion of previous theories of hurricane formation the proposed model is described. The initial intensification of the wind field is brought about by mass divergence at high levels that imposes a pressure reduction on the surface layers. This divergence is the result of interaction between the large‐scale disturbances of the upper air inside and outside the tropics. A solenoidal circulation is initiated that acts in the kinetic energy producing sense. But this circulation contains an internal mechanism for its own destruction and is maintained only under certain special conditions which are stated.
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The ocean's baroclinic response to a steadily moving storm is analyzed using a numerical model for an inviscid, multi-layered fluid. This first part of a two-part study gives a detailed account of the response to a rapidly moving hurricane, while parameter dependence is examined in the second part. A central theme of both parts is the coupling between wind-forcing, the surface mixed layer, and the thermocline. The baroclinic response is made up of a geostrophic component and a three-dimensional wake of inertial-internal waves which is emphasized. An important qualitative result is that the vertical penetration scale is large compared to the thermocline thickness. The initial isopycnal displacement is almost uniform through the thermocline, and the associated pressure field couples the mixed layer to the entire thermocline. Vertical energy propagation is thus very rapid near the storm track, (100 m day SUP - SUP 1), and largely responsible for a rapid post-storm decay of mixed layer inertial motion (e-folding in 5 inertial periods). Measurements made by buoy EB-10 in the wake of Hurricane Eloise provide a semi-quantitative check on the model results. (from author's abstract)
Article
The upper ocean response to a moving hurricane is studied using historical air-sea data and a three-dimensional numerical ocean model. Sea surface temperature (SST) response is emphasized. The model has a surface mixed-layer (ML) that entrains according to a velocity dependent parameterization, and two lower layers that simulate the response in the thermocline. The passage of Hurricane Eloise (1975) over buoy EB-10 is simulated in detail. Model results indicate that entrainment caused 85% of the irreversible heat flux into the ML; air-sea heat exchange accounted for the remainder. The rightward bias occurs in the model solution because the hurricane wind-stress vector turns clockwise with time on the right side of the track and is roughly resonant with the ML velocity. High ML velocities cause strong entrainment and thus a strong SST response. Model comparisons with EB-10 data suggest that a wind-speed-dependent drag coefficient is appropriate for hurricane conditions. Numerical experiments show that the response has a lively dependence on a number of air-sea parameters. Nonlocal processes are important to some aspects of the upper ocean response. (from author's abstract)