We have been conducting modeling studies to investigate the impacts of global warming on various aspects of tropical cyclones (TCs). Earlier, Tsutsui and Kasahara (1996) examined the question on how well a global climate model with the horizontal grid resolution of approximately 300 km can simulate the behaviors of TC-like disturbances in long-term climate integrations with climatological sea surface temperatures (SSTs). One interesting finding was that the seasonal and geographical variations of simulated TC (STC) formations reproduced observational statistics reasonably well even in such a coarse resolution model, where the T42 NCAR Community Climate Model Version 2 (CCM2) was used. In this study, we investigate its sensitivity regarding TC behaviors to SST changes due to doubled CO2 content in the atmosphere.
We performed two climate simulations; one is a control run using climatological SST data, and the other is a doubled CO2 run using increased SST estimates taken from the result of a transient 125-year simulation with the NCAR Climate System Model (CSM), a coupled atmosphere-ocean model, incorporating 1% per year increase of CO2. The SSTs in the CSM simulation at the time of 2xCO2 show the increase of nearly 1 K in the tropics and subtropics. The positive SST anomaly shows a maximum along the equator and tapers off in the subtropics until starting to increase toward the higher latitudes. This trend is noticeable in the Pacific during northern summer, but the SST anomaly has less spatial variability in the Atlantic.
We examined the twice-daily outputs from the two eight-year control and 2xCO2 runs by a search method developed earlier to investigate the impacts of increased CO2 on STCs, warm-core cyclonic disturbances. The frequency of STCs is calculated by counting the number of days when STCs appear at each grid point. The intensity of STCs is evaluated as the lowest sea-level pressure (SLP) recorded by the STCs. The impacts of global warming on STCs are found by comparing those statistics of eight years.
The most noticeable change of STC activity in the 2xCO2 run is observed in the western North Pacific, where the frequency increases by about 30% between 10N and 20N, and the minimum SLP deepens by 5 hPa. Large-scale circulation field and thermal structure also indicate changes favorable to TC development. This tendency, however, is not clear in other ocean basins, such as the South Pacific. Over the eastern North Pacific and North Atlantic both the frequency and intensity of STCs tend to decrease.
Presumably, the above model results depend not only on the specified SST anomaly, but also on the model's resolution and physics, particularly, cumulus parameterization. Plan is underway to use a high resolution version (T170) of CCM3 to conduct a similar experiment to investigate the dependence of our findings on different atmospheric model and cumulus parameterization.
Reference
Tsutsui, J., and A. Kasahara, 1996: Simulated tropical cyclones using
the National Center for Atmospheric Research community climate model.
Journal of Geophysical Research, 101, D10, 15013-15032.