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|Gilbert (1988)||Opal (1995)||Bonnie (1998)|
|EPIC (2001)||Isidore/Lili (2002)||Ivan (2004)|
|Jeanne (2004)||Katrina/Rita/Wilma (2005)||Juliette (2006)|
Oceanic mixed layer (ML) response to Hurricane Gilbert in the western Gulf of Mexico was investigated using the Miami Isopycnic Coordinate Ocean Model (MICOM). Three snapshots of oceanic observations indicated that a Loop Current Warm Core Eddy (LCWCE) contributed significantly to the ML heat and mass budgets. To examine the time evolution of different physical processes in the ML, MICOM was initialized with realistic, climatological, and quiescent conditions for the same realistic forcing. The ML evolves differently for the realistic background condition with the LCWCE in the domain; differences between climatological and quiescent conditions remain small. Mixed layer temperature (MLT) and ML depth (MLD) differences of up to 1°C and 30 m are directly attributed to horizontal advective processes in the LCWCE regime due to preexisting velocities. Comparison of simulated temperatures using realistic conditions in the model shows improved agreement with profiler observations. Using four entrainment-mixing parameterizations, the spatial and temporal ML evolution is investigated in MICOM simulations. Although the rates of simulated cooling and deepening differ for the four schemes, the overall pattern remains qualitatively similar.
For the three schemes that use surface-induced turbulence to predict entrainment rate, the cooling pattern extends farther away from the track. Based on linear regression analysis, MLTs simulated using the bulk Richardson number closure fit the observed temperatures better than did the other schemes. Averaged surface fluxes ranged from 10% to 30% in the directly forced region, with larger values in the LCWCE regime. Overall, entrainment mixing remains the dominant mechanism in controlling the heat and mass budgets. Top
On 4 Oct 1995, Hurricane Opal deepened from 965 hPa to 916 hPa in the Gulf of Mexico over a 14 h period upon encountering a warm ocean ring shed by the Loop Current during an upper-level atmospheric trough interaction. Oceanic features such as warm core rings (WCR), the Loop Current and the Gulf Stream, represent a source of enhanced air-sea fluxes to the atmospheric boundary layer that may cause strengthening of atmospheric disturbances. Warm layers exceeding 26°C extend to at least 100 m beneath the surface in these oceanic features, and represent high-heat content water.
Satellite altimetry data from TOPEX is a useful tool to study oceanic mesoscale dynamic processes from its coverage of the sea height anomaly (SHA) field, and provides information on the vertical ocean structure when complemented by hydrographic data. Based on historical hydrographic measurements placed within the context of a two-layer model, TOPEX-derived upper layer thickness fields indicated the presence of two WCRs in the Gulf of Mexico during September and October 1995. Hurricane Opal passed directly over one of these WCRs where the wind field increased from 35 m s-1 to 65 m s-1, and the radius of maximum wind decreased from 40 km 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 to 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 six times the threshold to sustain a hurricane. Composited AVHRR-derived SSTs indicated a 2 to 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 1°C. Thus, the WCR's effect was to provide a regime of positive feedback to the atmosphere 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. Top
The passage of hurricane Bonnie in 1998 in the western Atlantic Ocean basin underscored the uncertainties in understanding the upper ocean mixed layer response in the presence of a strong wind and vigorous surface wave field. Central to these uncertainties is determining the role of strongly forced surface waves on the mixed layer cooling and deepening patterns particularly in the right-rear quadrant of the storm where strong current shears develop (Shay et al., 1992). On August 24 an air-sea interaction experiment was conducted from the NOAA WP-3D aircraft where the mixed layer was observed from a series of Airborne eXpendable BathyThermographs (AXBTs) and concurrent observations of the directional wave spectra from the NASA Scanning Radar Altimeter (Walsh et al., 1996, Wright et al., 2001). During this period of time there was a marked upper ocean heat potential loss or approximately four times the value required to sustain a tropical cyclone when the storm had a maximum intensity of about 50m s-1 and a central pressure of 955 mb range.
The observed mixed layer response is mapped by removing a pre-storm condition from climatological profiles. Directional wave spectra are used to estimate significant slope, defined here as the root mean square wave height divided by the wavelength of the dominant wave (Huang, 1981). This slope will be compared to the observed mixed layer depth and temperatures.
MIXED LAYER STRUCTURE
The mixed layer temperature and depth response was determined by removing a climatological average of 28.5°C and 25m. Note that the mixed layer depth is defined as the depth at which the temperature change is more than 0.2°C, which is the resolving capability of the thermistor (Shay et al., 1992). As shown in Fig. 1, objectively analyzed mixed layer fields indicated maximum cooling of 2.5°C during Bonnie, and a mixed layer deepening by 45 to 50m during this period. A large fraction of the cooling is associated with current shears across the ocean mixed layer base associated with forced near inertial motions (Shay et al., 1992; Jacob et al., 2000).
SURFACE WAVE SPECTRA
By timing a radar pulse, the SRA measures sea surface topography from backscattered power at 36GHz over a swath proportional to 0.8 times the aircraft altitude, which is 1.2 km in the Bonnie case (Wright et al., 2001). The SRA directly measures range to the surface as it scans a 1° beam across +22° swath with 64 points. From the Bonnie case they found a maximum significant wave height of about 10.5m in the front-right quadrant, with a strong gradient decreasing to 6 m in the left-rear quadrant, which is reflected in Fig. 2. Over the domain, the surface wave wavelengths ranged from 175 to 200 m wavelengths, which represents a 10 to 11 sec wave period based on linear theory for the surface wave swell. Of particular interest here are the directional wave spectra in the right rear quadrant indicated a bimodal distribution with one component propagating towards about 330°T and a more energetic component moving at about 80°T. From these measurements, Wright et al. (2001) found a fairly complicated surface wave environment associated with hurricane Bonnie.
The significant slope is the root mean square wavelength divided by the wavelength of the dominant wave. Huang (1981) showed the importance of significant slope relative to mixed layer processes including mixing efficiency and dissipation. The significant slope ranges from 0.02 to 0.05 with a maximum of 0.05 in the region of the 2.5°C cooling just in back of the eye. Furthermore, there is a secondary maximum in the right-front quadrant. Note that the significant wave height is a maximum of 10.5 m in that region (Wright et al., 2001). On the left side of the storm significant slopes are less than 0.02. The spatial variations of the slopes suggest a correlation to mixed layer depth changes of 40 to 50 m as a result of the mixing. These slope issues are now being investigated since they are proportional to mixing efficiency in ocean mixed layer models (Huang, 1981).
A simple model proposed by Huang (1981) is used to examine the spatial behavior of the significant slope relative to mixed layer temperature and depth response induced by hurricane Bonnie. While these preliminary results are encouraging, upper ocean cooling is primarily induced by wind-driven current shear across the mixed layer base as observed in previous experiments. Future experiments need to use current profiles from Airborne eXpendable Current Profilers (AXCPs) to resolve both the shear and the orbital Velocities (Shay et al., 1992). Previous studies have shown orbital velocities detected by the AXCPs were correlated to those found from directional wave spectra. The importance of combining these various measurements is that we can begin to understand the effects of both currents and waves on the deepening of the mixed layer during strong winds. When used in concert with atmospheric profilers and stepped frequency microwave radiometer, the measurements may help us sort out the surface wind stress and the drag coefficient at high winds. Top
As part of a NOAA Joint Hurricane Testbed effort, an Eastern Pacific (EPAC) oceanic heat content (OHC) estimation scheme was developed for use in the Statistical Hurricane Intensity Prediction Scheme for predicting hurricane intensity. The approach follows those made operational for hurricane intensity forecasts from SHIPS in the Atlantic Ocean Basin. In the EPAC, the modified algorithm computes OHC from multiple platform radar altimeters and sea surface temperatures from TRMM microwave imager. Altimeter data are smoothed, combined and objectively analyzed to a 0.5 deg; grid to estimate isotherm depths and OHC variations based on a seasonal climatology. Estimates from 2000 to 2006 were compared to several sets of thermal structure measurements from NOAA’s Tropical Atmosphere Ocean (TAO) buoys. Top
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 post-storm 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 4 m s-1. Given pre-storm ocean heat content (OHC) levels exceeding 100 kJ cm-2 in the LC (current velocities >1 m s-1), 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 m-2, where the maximum observed wind was 49 m s-1. Spatially integrating these surface enthalpy fluxes suggested a maximum surface heat loss of 9.5 kJ cm-2 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 ~1°C but more than 2°C in the Gulf Common Water, where the maximum estimated surface enthalpy flux was 1.4 kW m-2, associated with peak surface winds of 51 m s-1. Because of Lili’s asymmetric structure and rapid translational speed of 7 m s-1, the maximum surface heat loss resulting from the surface enthalpy flux was less than 5 kJ cm-2.
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 Richardson 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. Top
The ocean response to a tropical cyclones is studied using the Hybrid Coordinate Ocean Model (HYCOM), which is being developed by NOAA/NCEP as the next ocean component of the coupled Hurricane Weather Research and Forecasting model, in an effort to improve the ocean model response. The overarching goal is to improve the capability of coupled TC forecast models to forecast intensity change, which depends in part on the temperature and thickness of the upper ocean warm layer as represented by the ocean heat content (OHC). Combined model-observational studies are critically important for evaluating and improving ocean model performance, particularly in regards to the magnitude and pattern of SST cooling driven by tropical cyclones. Simulations of the ocean response to hurricane Ivan (2004) in the northwest Caribbean and Gulf of Mexico have (1) demonstrated the importance of accurately initializing ocean eddies and boundary currents in the ocean model; (2) revealed sensitivity of the current and temperature response to the model vertical mixing parameterizations; and (3) emphasized the importance of ocean observations for both initializing and evaluating the ocean model. The ocean response was evaluated against microwave satellite SST measurements and moored ocean current observations. During the period of deployment, Hurricane Ivan passed directly over 14 Acoustic Doppler Current Profiler (ADCP moorings that were deployed as part of the Navy Research Laboratory Slope to Shelf Energetics and Exchange Dynamics (SEED) project from May through Nov 2004. These observations enable the simulated ocean current response to a hurricane in a continental shelf/slope region to be evaluated with unprecedented detail. We will present the results of additional Ivan response simulations designed to identify the optimal choice of horizontal and vertical resolution for operational TC forecasting; i.e., the coarsest resolution that can be used without significantly degrading the simulated response. With these choices identified, additional simulations of Ivan along with simulations of Katrina and Rita (2005) in the Gulf of Mexico will be performed. Goals include (1) understanding the physical processes responsible for mixed layer deepening and cooling while devising strategies for improving the mixed layer response and (2) understanding how the pre-existing quasi-geostrophic flow field modifies the ocean current response forced by storm winds and how this impacts the SST cooling pattern. Top
During the 2004/2005 hurricane season, several intense hurricanes made landfall in South Florida. In Sep 04, tropical storm (TS) Jeanne had a west to northwest trajectory along the northern tier of Caribbean Islands. The TS then moved directly northward in response to the large scale atmospheric steering pattern ( Figure 2). Along this northward trajectory, the TS became a hurricane on September 16 and made a complete circle in response to the changing atmospheric steering pattern. In this region, the SSTs were relatively warm with values of more than 29°C based on composited TMI_AMSR-E measurements. Hurricane Jeanne then moved directly towards the west and made landfall along the Broward-West Palm Beach County line as a category 2 hurricane on the Saffir-Simpson Scale. During this period, the wind field of Jeanne excited strong surface currents in the South Florida WERA domain.
Surface Current Response:
During this period, Jeanne excited an energetic coastal current response as measured by WERA. An eastward current response of 1 m s-1 emanated from the Biscayne Bay where offshore surface winds approached 22 m s-1 along the south side of Jeanne and gusts up to 25 m s-1. This current response forced an eastward bulge of 1000 km2 resulting in an apparent offshore Florida Current meander ( Figure 4). The Florida Current velocities decreased in response to the hurricane since the winds were generally orthogonal to the current. However as the hurricane moved inland, the cyclonic rotating winds were in phase with the Florida Current resulting in a stronger surface flow to the north of more than 2 m s-1.
To examine the time space variability of the current response, the surface velocity field was center differenced in time to estimate the acceleration in the currents by the surface winds of Jeanne (Figure 5). The surface current response is the largest between the western flank of the Florida Current and Biscayne Bay. The current accelerations approach 10-3 m s-2. By contrast, the acceleration rate is minimal within the Florida Current of 2 x 10-4 m s-2. This should not be a surprise in that wind driven currents over western boundary currents do not significantly alter their velocity core structure. This is analogous to the passage of hurricanes Isidore and Lili over the Loop Current where the background current (and vertical shear) was not significantly altered by these weather events (Shay and Uhlhorn 2008).
To examine the time space variability of the current response, the surface velocity field was center differenced in time to estimate the acceleration in the currents by the surface winds of Jeanne. The surface current response is the largest between the western flank of the Florida Current and the Biscayne Bay. The current accelerations approach 10-3 m s-2. By contrast, the acceleration rate is the minimal within the Florida Current of 2 x 10-4 m s-2. This should not be a surprise in that wind driven currents over western boundary currents do not significantly alter their velocity core structure. This is analogous to the passage of hurricanes Isidore and Lili over the Loop Current where the background current (and vertical shear) was not significantly altered by these weather events (Shay and Uhlhorn 2008).
Directionality of the surface winds lies in the Doppler spectra at each of the radar cells (>5000). Wind directions are estimated using the ratio of the two Bragg peaks in the spectrum. And with two sites, the ambiguity of the wind directions is removed. During the period of strong forcing, radar-derived inferred wind direction follows that measured at Fowey. The scatter suggests an optimal fit between the observed and inferred wind directions with a slope of 0.99. The histogram of directional differences indicates a normal distribution centered on a difference of -5 to -10°, consistent with a bias of 7.5°.
Inferring the surface winds requires an assumption concerning the wind only drives the local wind waves (Wyatt 2005) where the waves are derived from the second-order returns in the Doppler spectra ( Figure 7). Thus some caution has to be applied to these wind speeds in that this assumption, which is clearly violated under low wind conditions. However in these stronger winds, the assumption appears to be valid. Notwithstanding, the inferred wind speeds indicate reasonable agreement between those observed at Fowey and the inferred wind speeds during Jeanne. The scatter plot indicates a clustering between 10 to 13 m s-1, and as the wind speed increases, the scatter increases. The bias is 2.5 m s-1 and the slope is 0.75 between the observed and inferred. The histogram of the wind speed differences has a normal distribution centered on zero.
Katrina, Rita, Wilma (2005 Trifecta)
During the passage of hurricanes Katrina and Rita, measurements from oceanic profilers and satellite radar altimetry from NASA Jason-1 and TOPEX missions and NOAA Geosat Follow-On-Missions indicate that both hurricanes interacted with the Gulf of Mexico’s Loop Current (LC) and warm core ring (WCR) complex in Aug and Sept 2005. Both storms rapidly intensified to Category-5 status over these warm oceanic features where the 26°C isotherm depth and ocean heat content (OHC) exceeded 100 m and 120 kJ cm-2, respectively. Satellite-derived OHC estimates and 26°C isotherm depths agreed well with those determined from in situ data. Subsequent to Rita’s passage, a cold core ring (CCR) moved between the WCR and the LC where the ocean cooled due to enhanced mixing and upwelling processes that scaled well with observed air-sea parameters. This hurricane encounter with a CCR along with an eyewall cycle may have helped weakened Rita prior to landfall. Spatial variations of warm and cold features must be accounted for in predictive coupled atmosphere-ocean models to forecast hurricane intensity accurately. Top
The Eastern Pacific Ocean basin (hereafter referred to as EPAC) is also a region of significant upper oceanic variability given the warm pool and gradual shoaling of the oceanic thermocline from west to east, and the westward propagation of WCRs forced either by low-level jets (Hurd 1929; Kessler 2003) or current instabilities (Hanson and Maul 1991) (Figure 1). This feature moved southwestward at 13 to 15 cm s-1 and dissipated within the Eastern Pacific Investigation of Climate (EPIC) domain in late Oct 2001 (Figure 2). Over this oceanic regime, tropical cyclogenesis often begins at between 10 to 14°N and 90 to 100°W, which is characterized by SST gradients and OHC variations that impact TC intensity change (Raymond et al. 2004). Of particular importance to the maintenance of the SST and OML temperature structure are the sharp thermal gradients across the OML base starting about 30 to 35 m beneath the surface and the 20°C isotherm separates the upper from the lower layer in a two-layer model. During hurricane Juliette in Sept 01 (Shay and Jacob 2006), and the subsequent intensification to category 4 status, the SST cooling was less than 1°C in the regime with strong vertical gradients (~20 to 24 cph: cycles per hour) (Wijesekera et al. 2005). Wind-driven ocean current shear tends to be insufficient to significantly cool the upper ocean through shear instability until Juliette moved into an area with weaker stratification (~10 cph) where SST cooling was 4 to 5°C. Entrainment mixing across the OML base due to ocean current shear did not lower the bulk Richardson number to below criticality. Hence, a larger fraction of OHC was available for Juliette through air-sea fluxes during the hurricane’s rapid intensification phase.
An important aspect of this problem is that considerable variability exists in OHC estimates in differing basins owing to the temperature and salinity characteristics. Temperatures and salinities vary in response to incoming radiation and precipitation (ITCZ) as well as the air-sea fluxes (Gill 1982). As suggested in Figure 7, profiles from the LC subtropical water and the tropical EPAC illustrate marked differences in the temperatures and the resultant buoyancy frequency profile. In an OML, the vertical density gradients (N) are essentially zero by virtue of the “well-mixed” assumption in temperature and salinity. By contrast, maximum buoyancy frequency (Nmax) in the GOM is 5 to 6 cycles per hour (cph) in the LC water mass distributed over the upper 100 m of the water column. In the EPAC, however, Nmax is ≈ 20 cph due to the sharpness of the thermocline and halocline (pycnocline) located at the OML base (i.e. 30 to 35 m). Beneath this maximum, N ≥ 3 cph are concentrated in the seasonal thermocline over an approximate thermocline scale (b) of 200 m and exponentially decay with depth approaching 0.1 cph. In the Loop Current water, Nmax ranges from 4 to 6 cph and remains relatively constant, and below the 20°C isotherm depth (≈ 250 m), N decreases exponentially. Such behavior has important implications for shear-instability and vertical mixing processes via Richardson number. In the EPAC, this implies that for large N, wind-forced shears have to be significantly larger in for mixing to occur compared to the LC or the Gulf Common Water where N is typically 12 cph. Given a large N at lower latitudes (12°N) where the IP is long in the EPAC warm pool, SST cooling and OML deepening will be much less than in the GOM as observed during hurricane Juliette in Sept 2001 (Shay and Jacob 2006). Significant SST cooling of more than 5°C occurred when Juliette moved northwest where Nmax decreased to about 14 cph at higher latitudes. While for the same hurricane in the GOM, similar levels of SST cooling would be observed in the common water but not in the LC water mass because the 26°C isotherm depth is three to four times deeper. These regional to basin scale variations in the temperature and salinity structure and the resultant stratification represent a paradox for hurricane forecasters, which is the rationale underlying the use of satellite radar altimetry in mapping isotherm depths and estimating OHC from surface height anomalies (SHA) and assimilate them into oceanic models.
During the 2008 hurricane season, hurricanes Gustav and Ike moved over the Gulf of Mexico and interacted with the Loop Current (LC) and the mesoscale eddy field. As part of the National Center for Environmental Prediction (NCEP)-directed tail Doppler Radar Missions, concurrent oceanic and atmospheric measurements were acquired from sixteen NOAA WP-3D research flights for pre, during and post-storm conditions. Such coupled measurements are absolutely necessary to improve models at the National Center. Over two hundred global positioning system (GPS) sondes were deployed during the in-storm flights across the Gulf of Mexico to document the evolving atmospheric structure over warm and cool ocean features. Each research flight deployed airborne expendable bathythermographs (AXBT) to document the evolving upper ocean thermal structure across the entire Gulf of Mexico for the first time. In support of operational modeling efforts at NCEP, more than four hundred AXBTs were deployed on these research flights to document the coupled boundary layer responses. To complement these data, twenty-one drifters (10 with 150m thermistor chains) and floats were deployed by the Air Force WC-130J northwest of the LC. Over this array, forty-five GPS sondes were deployed and surface winds were mapped from Stepped Frequency Microwave Radiometer (SFMR).
These data are being processed to assess data quality and the levels of the observed ocean thermal response in the LC and Gulf Common Water, and to relate upper ocean changes to the evolving boundary layer structure. Top