What is the current status of hurricane track prediction? What caused the record number of Atlantic tropical storms in 1995? Are we witnessing a change in the number and frequency of tropical storms? Do these storms represent a changing climate? What will tropical storms be like in a greenhouse warmer world?
INTRODUCTION: Dr. James Baker, Under Secretary for Oceans and Atmosphere, Department of Commerce, National Oceanographic and Atmospheric Administration.
Dr. Jerry D. Mahlman, Director, Geophysical Fluid Dynamics Laboratory, National Oceanographic and Atmospheric Administration, Princeton, NJ
Dr. Robert W. Burpee, Director, Tropical Prediction Center/National Hurricane Center, National Oceanographic and Atmospheric Administration, Miami, FL
Dr. Kerry A. Emanuel, Director, Center for Meteorology and Physical Oceanography, Dept of Earth, Atmospheric, and Planetary Sciences, MIT, Cambridge,MA
Hurricane Prediction Aspects
Over the past two years the National Weather Service has achieved a substantial improvement in predicting the tracks of hurricanes. This breakthrough has been achieved through the vital combination of better hurricane modeling, better data from the hurricane environment, and better forecast model initialization.
These accomplishments are now being transferred to weather services, especially in the western Pacific region. The combination of better models, better data, and better theoretical understanding is also pointing the way to other improvements in prediction services. Especially promising are the potential improvements in predictions of storm intensity, ocean storm surge, and precipitation, all of which relate directly to potential reductions in loss of life and property.
Annual to Biennial Aspects
The 1995 Atlantic hurricane season witnessed 19 tropical storms, 11 of which achieved hurricane status. This is the largest number of Atlantic tropical storms since 1933, which witnessed 21 tropical storms. Consequently, 1995 (and 1933) represent the largest number of tropical storms observed in any hurricane season since 1871 when records were first compiled. 1995 also represents the second largest number of hurricanes observed (11) in one season. The 19 tropical storms observed in 1995 are more than twice the total observed during any year since 1991; a typical year has 6-8 storm events. In addition, the 11 Atlantic hurricanes of 1995 far exceed the 3-4 hurricanes observed each year since 1991.
There are at least two prominent factors associated with tropical storm and hurricane activity: 1) the El Nino-Southern Oscillation (ENSO) cycle; and 2) west African rainfall. These two factors, as well as others, have proven useful in forecasting and modeling the extent and severity of seasonal tropical storm activity in the Atlantic.
Atmospheric and oceanic conditions in 1995 were particularly favorable for enhanced Atlantic tropical storm and hurricane activity. Minimal vertical wind shear was observed during the season, from western Africa across to the Gulf of Mexico and the Caribbean Sea. The potential for storm intensification is controlled by vertical wind shear. Strong vertical shear inhibits intensification while weak shear aids intensification. Intensification can be further enhanced by large-scale patterns of abnormally warm ocean water and below normal surface pressure over the subtropical North Atlantic. In 1995 these conditions, combined with a series of intense easterly waves originating over north-central Africa, provided the primary ingredients for an intense hurricane season in the Atlantic.
During the warm phase of the El Nino/Southern Oscillation (the El Nino phase), above normal sea surface temperatures are observed throughout the central and east-central equatorial Pacific. As a consequence of increased ocean temperatures, the normal patterns of tropical cloudiness and precipitation are disrupted, and the resulting changes in wind and pressure patterns often act to inhibit Atlantic hurricane activity. Such conditions were prevalent from 1991-1995. This El Nino episode was also marked by persistent patterns of enhanced vertical shear and above normal air pressure over much of the subtropical North Atlantic. This combination of conditions resulted in a decrease in tropical storm activity over the North Atlantic from 1991-1995. These conditions, however, began to abate by March of 1995.
Long Term Climatic Aspects
Model simulations of hurricanes in a greenhouse-warmed world suggest that hurricane conditions may be generally weaker, but once formed, have the potential to be more severe. Model studies of hurricanes further suggest that hurricanes are highly sensitive to sea surface temperature.
The heat content of the atmosphere is a function of its temperature and the amount of water vapor present (its absolute humidity). In a hurricane, heat is added to the atmosphere when water evaporates from the ocean surface. The heat content of a hurricane is its energy or intensity. The amount of heat and, therefore, energy that can be put into a hurricane is proportional to the amount of sea water that can be evaporated. Evaporation can occur until the air becomes saturated with water vapor. Consequently, the amount of heat that can be put into a hurricane from the sea is proportional to how dry the air is just above the ocean surface.
Climate models and observations indicate that the water vapor content of the atmosphere will increase as the climate warms. In a greenhouse-warmed world, with more water vapor in the atmosphere and an increase in sea surface temperatures, the expectation would be greater heat energy within hurricanes and, therefore, an increase in the percentage of hurricanes that reach high intensities.
Jerry D. Mahlman: Since 1984 Dr. Mahlman has served as Director of NOAAıs Geophysical Fluid Dynamics Laboratory, one of the worldıs premier climate modeling centers. He is also a Professor in Atmospheric and Ocean Sciences at Princeton University. His interest has been directed at understanding the behavior of the atmosphere and its implications for climate change. Dr. Mahlman serves as: Chair of the Scientific Advisory Committee of NASAıs Mission to Planet Earth; member of the National Research Councilıs Board on Sustainable Development; the US Representative to the World Climate Research Programme; and a member of NASAıs Advisory Council. From 1989-1991 Dr. Mahlman served as a member of the US-USSR Joint National Academy of Sciences Committee on Global Ecology. He is the recipient of two NOAA Distinguished Authorship Awards, the Department of Commerce Gold Medal, the Presidential Distinguished Rank Award, the American Meteorological Societyıs Carl-Gustaf Rossby Research Medal, and an Honorary Alumnus Award from Colorado State University.
Robert W. Burpee: Dr. Burpee is Director of the Tropical Prediction Center, National Hurricane Center. From 1993-1995 he served as Director of the Hurricane Research Division of the Atlantic Oceanographic and Meteorological Laboratories (AOML) at NOAA. He has also served as Supervisory Meteorologist at AOML and at NOAAıs Office of Oceanic and Atmospheric Research. Dr. Burpee has also served as a Visiting Professor at MIT, and as Assistant Professor at the University of Illinois. He received his BS degree from Harvard University, and MS and Ph.D. degrees from MIT.
Kerry A. Emanuel: Since 1989 Dr. Emanuel has served as Professor and Director of the Center for Meteorology and Physical Oceanography in the Department of Earth, Atmospheric, and Planetary Sciences at MIT. His research interests involve understanding atmospheric convective systems and the implications of climate change. Dr. Emanuel is a member of the American Meteorological Society and Sigma Xi. He is also the recipient of the American Meteorological Societyıs Meisinger Award, and the Societyıs Banner I. Miller Award. He received his BS degree in Earth and Planetary Sciences from MIT, and his Ph.D. degree in Meteorology from MIT.