Tropical cyclones (known as hurricanes in the tropical Atlantic, western Pacific and northern Indian oceans; as typhoons in the eastern Pacific Ocean; and as cyclones in the southern Pacific and Indian oceans) are characterised by very high waves and strong winds, that can be measured by altimeters (provided that the satellite flies close enough to the area affected by the cyclone) and assimilated in real-time in some forecasting models. However, altimetry can also help identify the warm features that can cause these storms to intensify. Thus sea surface height anomalies can be used as proxies of the warm currents,that provide the hurricanes with their energy source.
Tropical Cyclone Heat Potential field for 12 August 2006. Regions with high (> 50 kJ/cm²) TCHP values are coloured yellow, green and red.
Some ocean features, such as warm ocean currents and eddies, have been linked to tropical cyclone intensification [Shay et al]. The intensive heating of the ocean’s upper centimetres which occurs during the summer months usually makes these extensive reservoirs of high thermal energy invisible when observed using sea surface temperature alone. However, they can easily be observed by altimeters as they are also characterised by greater sea surface heights [Goni et al].
Tropical Cyclonic Heat Potential computed from altimetry on 28 August 2005, with Hurricane Katrina’s trajectory and intensity overlaid. Katrina’s intensification seems to coincide with its crossing over the Loop Current.
Altimetry data in combination with historical hydrographic observations are currently used to estimate synthetic upper ocean temperature profiles. These profiles are then used to compute the integrated vertical temperature from the sea surface down to the 26°C isotherm, the temperature needed to sustain a tropical cyclone. This quantity is usually referred to as the Hurricane or Tropical Cyclone Heat Potential (TCHP) [Leipper and Volgenau] and represents the amount of heat in the upper ocean available for tropical cyclone intensification.
– Goni, G. J., S. L. Garzoli, A. J. Roubicek, D. B. Olson and O. B. Brown. Agulhas ring dynamics from TOPEX/POSEIDON satellite altimeter data, J.Mar. Res., 55, 861-883, 1997.
– Leipper, D. and D. Volgenau. Hurricane heat potential of the Gulf of Mexico, J. Phys. Oceanogr., 2, 218-224, 1972.
– Shay L. K., G. J. Goni and P. G. Black. Effect of a warm ocean ring on hurricane Opal. Mon. Weath. Rev., 128, 1366-1383, 2000.
– Goni, G., P. Black, J. Trinanes, Using satellite altimetry to identify regions of hurricane intensification, Aviso Newsletter, 9, 2003
– Scharroo, R., W. H. F. Smith, and J. L. Lillibridge, Satellite altimetry and the intensification of Hurricane Katrina, Eos Trans. AGU, 86 (40), 366, 2005.
– http://www.aoml.noaa.gov/phod/cyclone/data/ (NOAA/AOML)
Assimilation in wave models
Altimetry can also play a part in cyclonic event warnings using significant wave height measurements in near-real time (three hours). Once they have been assimilated in sea state forecast models, wave heights from altimetric satellites significantly improve their predictions. With assimilated data from two satellites, Jason-1 (CNES/NASA) and Envisat (ESA), they become even more accurate.
Difference between significant wave height from Météo France’s wave model, with and without assimilation of altimetry data (Jason-1 and ERS-2) on 25 September 2004, with Hurricane Jeanne approaching the Florida coasts.
(Credits Météo France)