The influence of global warming on the climatology of tropical cyclones in the western North Pacific basin is examined using the high-resolution International Pacific Research Center (IPRC) regional climate model forced by ocean temperatures and horizontal boundary fields taken from the NCAR Community Climate System Model version 2 (CCSM2) coupled global climate model. The regional model is first tested in 10 yr of simulation with boundary forcing taken from observations and is shown to produce a reasonably good representation of the observed statistics of tropical cyclone numbers and locations. The model was then run for 10 yr with forcing from a present-day control run of the CCSM2 and then for 10 yr with forcing fields taken from the end of a long run with 6 times the present-day atmospheric CO 2 concentration. The global-mean surface air temperature warming in the perturbed run is 4.5 K, while the surface warming in the tropical western North Pacific is about 3 K. The results of these experiments reveal no statistically significant change in basinwide tropical cyclone numbers in the peak season from July to October in response to the CO 2 increase. However, a pronounced and statistically significant increase in tropical cyclone occurrence in the South China Sea is found. While the basinwide total number of storms remains nearly unchanged in the warm climate, there is a statistically significant increase in the average strength of the cyclones and in the number of the storms in the strongest wind categories.

Recent diagnostics with the Geophysical Fluid Dynamics Laboratory Climate Model, version 2.1 (GFDL CM2.1), coupled model’s twentieth-century simulations reveal that this particular model demonstrates skill in capturing the mean and variability associated with the South Asian summer monsoon precipitation. Motivated by this, the authors examine the future projections of the mean monsoon and synoptic systems in this model’s simulations in which quadrupling of CO2 concentrations are imposed. In a warmer climate, despite a weakened cross-equatorial flow, the time-mean precipitation over peninsular parts of India increases by about 10%–15%. This paradox is interpreted as follows: the increased precipitation over the equatorial western Pacific forces an anomalous descending circulation over the eastern equatorial Indian Ocean, the two regions being connected by an overturning mass circulation. The spatially well-organized anomalous precipitation over the eastern equatorial Indian Ocean forces twin anticyclones as a Rossby wave response in the lower troposphere. The southern component of the anticyclone opposes and weakens the climatological cross-equatorial monsoon flow. The patch of easterly anomalies centered in the southern Arabian Sea is expected to deepen the thermocline north of the equator. Both these factors limit the coastal upwelling along Somalia, resulting in local sea surface warming and eventually leading to a local maximum in evaporation over the southern Arabian Sea. It is shown that changes in SST are predominantly responsible for the increase in evaporation over the southern Arabian Sea. The diagnostics suggest that in addition to the increased CO2-induced rise in temperature, evaporation, and atmospheric moisture, local circulation changes in the monsoon region further increase SST, evaporation, and atmospheric moisture, leading to increased rainfall over peninsular parts of India. This result implies that accurate observation of SST and surface fluxes over the Indian Ocean is of urgent need to understand and monitor the response of the monsoon in a warming climate. To understand the regional features of the rainfall changes, the International Pacific Research Center (IPRC) Regional Climate Model (RegCM), with three different resolution settings (0.5° × 0.5°, 0.75° × 0.75°, and 1.0° × 1.0°), was integrated for 20 yr, with lateral and lower boundary conditions taken from the GFDL model. The RegCM solutions confirm the major results obtained from the GFDL model but also capture the orographic nature of monsoon precipitation and regional circulation changes more realistically. The hypothesis that in a warmer climate, an increase in troposphere moisture content favors more intense monsoon depressions is tested. The GFDL model does not reveal any changes, but solutions from the RegCM suggest a statistically significant increase in the number of storms that have wind speeds of 15–20 m s−1 or greater, depending on the resolution employed. Based on these regional model solutions a possible implication is that in a CO2-richer climate an increase in the number of flood days over central India can be expected. The model results obtained here, though plausible, need to be taken with caution since even in this “best” model systematic errors still exist in simulating some aspects of the tropical and monsoon climates.

Vertical profiles of the most important species of nocturnal total reactive nitrogen (NOy = NO2 + HNO3 + ClONO2 + 2 N2O5 + HO2NO2) together with its source gas N2O were retrieved from infrared limb emission spectra measured by the Michelson Interferometer for Passive Atmospheric Sounding, Balloon‐borne version (MIPAS‐B) instrument inside the late winter arctic vortex from Kiruna (Sweden, 68°N) on 24 March 1997 and in summer midlatitudes from Gap (France, 44°N) on 2 July 1997. The measured data were compared to calculations performed with the three‐dimensional chemistry transport model (CTM) Karlsruhe Simulation model of the Middle Atmosphere (KASIMA). The results show that in the late winter arctic vortex most of the available nitrogen and chlorine is in the form of HNO3 and ClONO2, respectively. An anomalous N2O‐NOy correlation observed in March 1997 appears to be caused to a large extent by quasi‐horizontal mixing of air masses across the vortex edge. However, near 20 km some denitrification of ∼1.5 to 2 ppbv NOy could be observed. The N2O profile measured in July 1997 indicates remnants of polar vortex air and is not reproduced by the CTM at the same location. However, the profile shapes of the individual compounds of the NOy family as well as the NOx/NOy ratio are reproduced fairly well by the model.

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