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Near-real time estimates of upper ocean heat content (UOHC) and tropical cyclone heat potential (TCHP) from altimetry
The close relationship that exists between the dynamic height and the mass field of the ocean 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 20°C isotherm. 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 tropical cyclone heat potential, is defined here as a measure of the integrated vertical temperature from the sea surface to the depth of the 26°C 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 Tropical Rainfall Measuring Mission's (TRMM) Microwave Imager (TMI) fields, (b) the altimeter-estimates of the 20°C isotherm within a two-layer reduced gravity scheme (Goni et al, 1996), (c) the depth of the 26°C isotherm from a climatological relationship between the depths of the 20°C and 26°C isotherm.
We show here examples of intensification of three Atlantic hurricanes and one Pacific typhoon that passed over areas with very high values of TCHP: Hurricanes Opal, Mitch and Bret, and typhoon Imbudo.
Hurricane Opal in the Gulf of Mexico, August-September 1995 (left):
This TC intensified from hurricane-1 (74-95 mph winds) to hurricane 4 (131-155 mph winds) while traveling over a number of warm features in the Gulf of Mexico. In particular, this TC suddenly intensified from hurricane-2 (96-110 mph winds) to hurricane-4 in a period of 10 hours when its track went over a very well defined ring with a mean radius of 150 km that had been shed by the Loop Current. Altimeter-derived fields indicate that the increase in TCHP associated with this warm ring was approximately 30 kJ/cm2. The most striking information of the ocean conditions during the life span of this hurricane over the Gulf of Mexico was that this warm ring was not detected using the AVHRR-derived sea surface temperature fields.
Hurricane Mitch in the Caribbean Sea, October 1998 (center):
This cyclone intensified from hurricane 2 to hurricane 5 (winds above 155 mph) when its track traveled over a region of warm surface waters, experiencing an intensification from hurricane 3 (111-130 mph winds) to 5 with an increase in values of TCHP approximately 80 kJ/cm2 under the track of the TC in 22 hours.
Hurricane Bret in the Gulf of Mexico, August 1999 (right):
This hurricane intensified several times in the SW Gulf of Mexico in a period of approximately 36 hours while traveling over two warm features remnants of one warm ring that had been shed by the Loop Current several months earlier. The increase in TCHP under the track of the TC during this period was approximately 80 kJ/cm2.
In these cases an association was observed between the increase in TC intensity and a raise in the value of TCHP under the track of each of the TCs. Preliminary evaluation of the upper ocean thermal conditions during the intensification of 32 of the 36 strongest TCs in the tropical Atlantic from 1993 to 2000 indicates that their intensification can be associated with the passage of their tracks over regions, with increased TCHP of at least 20 kJ/cm2.
|Global_Map||Gulf of Mexico||Caribbean||Atlantic||Northeast_Pacific||Tropical_Pacific||Southwest_Pacific||Northwest_Pacific||Southwest_Indian||Southeast_Indian||North_Indian|
|Sea Surface Temperature 26°C (20°C) isotherm in red(blue)
||Sea Height Anomaly|
|Depth 26°C Isotherm||Depth 20°C Isotherm
The algorithms to make the weekly and daily estimates are analogous . However, the data used differ in that the former uses the weekly Optimum Interpolation SST Analysis fields from NOAA/NCEP (daily SST is obtained from TMI) and in that the SHA fields are estimated from interpolating SHA data corresponding to a 10-day period ending on the last day of each week.
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