Altimetry and Hurricane Intensification

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Altimetry and Hurricane Intensification: Upper Ocean Conditions in the Tropical Atlantic, Caribbean Sea and Gulf of Mexico

Background

The intensification of hurricanes involves a combination of different favorable atmospheric conditions such as atmospheric trough interactions and vertical shear, which lead to good outflow conditions aloft. As a result of this, inflow conditions in the near-surface layer are enhanced. Clearly, as this process continues over the scale of the storm, the upper ocean provides the heat to the atmospheric boundary layer and the deepening process. In this scenario, the upper ocean thermal structure has been thought to be a parameter that only played a marginal role in hurricane intensification. However, after a series of events where the sudden intensification of hurricanes occurred when their path passed over oceanic warm features, it is now being speculated that it could be otherwise. While the investigation of the role of these rings and eddies is a topic of research in a very early stage, preliminary results have shown their importance in the intensification of hurricane Opal (Shay et al, 2000). Therefore, the monitoring of the upper ocean thermal structure has become a key element in the study of hurricane-ocean interaction with respect to the prediction of sudden hurricane intensification. These warm features, mainly anticyclonic rings and eddies shed by the Loop Current, are characterized by a deepening of several tens of meters of the isotherms towards their centers and with different temperature and salinity structure than the surrounding waters.

The specific research objectives of this work include to determine the location and hurricane heat potential of warm core rings, Florida Current and the Gulf Stream, and to examine the forcing by warm ocean features on surface stresses. To achieve these objectives this research is developing both an active ocean monitoring program based on altimeter techniques to detect warm mesoscale ocean features, and in situ oceanic and atmospheric sampling component from the NOAA WP-3D in collaboration with NOAA/AOML HRD scientists. The research effort addresses specific objectives of the NSF/NOAA United States Weather Research Program.

Hurricane Opal

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-hurricane heat potential 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.

Hurricane Bret

Similarly, during August 1999, Hurricane Bret intensified twice in the western Gulf of Mexico over two regions associated with very high (values larger than 90 KJ/cm-2) hurricane heat potential.

Methodology

The close relationship that exists between the dynamic height and the ocean mass field allows these two parameters to be used within a two-layer reduced gravity ocean model to monitor the upper layer thickness (Goni et al., 1996), which is defined in this study to go from the sea surface to the depth of the 20C isotherm (Figure 2) This isotherm is chosen because it lies within the center of the main thermocline and is often used as an indicator of the upper layer flow in the western tropical Atlantic and Gulf of Mexico waters. Although there are other factors controlling the sea height anomaly, it is assumed here that most of its variability is due to changes in the depth of the main thermocline and of barotropic origin. The hurricane heat potential, Q, is defined here as a measure of the integrated vertical temperature from the sea surface to the depth of the 26C isotherm. This parameter is computed from the altimeter-derived vertical temperature profiles estimates in the upper ocean. The temperature profiles are estimated using four points: (a) the sea surface temperature obtained from the Reynolds near real time weekly fields, (b) the altimeter-estimates of the 20C isotherm within a two-layer reduced gravity scheme (Goni et al, 1996), (c) the depth of the 26C isotherm from a climatological relationship between the depths of the 20C and 26C isotherm, (d) climatological estimates of the mixed layer depth.

What do these maps mean ?

The sea height anomaly represents the deviation of the sea height with respect to the 1993-1997 mean. For analysis and corrections of altimeter data, please refer to: Cheney et al, TOPEX/POSEIDON: The 2-cm Solution, J. Geophys. Res., 99, 24555-24564, 1994.

The upper layer thickness is the altimeter estimate of the depth of the 20C isotherm (Goni et al., 1996).

The hurricane heat potential, is a measure of the integrated vertical temperature between the sea surface and the estimate of the 26C isotherm (Shay et al., 2000).

What can these maps be used for ?

These maps can be used to identify warm anticyclonic features, usually characterized by sea height anomalies and upper layer thickness larger than their surrounding waters, and to monitor regions of very high (larger than 90 KJ/cm-2) hurricane heat potential. These regions have been associated with the sudden intensification of hurricanes.

Acknowledgements

The sea height anomaly fields are provided by Dr. Robert Cheney (NOAA/NESDIS). The Reynold's sea surface temperature weekly fields are provided by NOAA/NCEP.

A Note on POSEIDON cycles

POSEIDON shares the same antenna as TOPEX, thus only one altimeter operates at any given time. During 2000 POSEIDON will be on for cycles 278 (Mar 31 - Apr 10, 2000), 289 (Jul 18 - Jul 28, 2000) and 295 (Sep 16 - Sep 26, 2000) and near-real time data will not be available during these periods. The maps shown during these periods correspond to the latest TOPEX available data and the current SST fields. The Hurricane Heat potential fields are computed from these two data fields.

References

Goni, G. J., Kalmholz, S., S. Garzoli and D. Olson, 1996. Dynamics of the Brazil-Malvinas Confluence Based on Inverted Echo Sounders and Altimetry, J. Geophys. Res., 101, 16273-16289.

Shay, L. K., G. J. Goni, and P. G. Black, 2000. Effect of a Warm Ocean Ring on Hurricane Opal. Mon. Weath. Rev., 128, 1366-1383.

This research is funded by ONR (CBLAST Program) and NOAA.

Please address your questions or comments about these maps to Gustavo Goni

W3 development by Joaquin A. Trinanes