Masato Sugi

Future Changes in Tropical Cyclone Activity Projected by the New High-Resolution MRI-AGCM*

AbstractNew versions of the high-resolution 20- and 60-km-mesh Meteorological Research Institute (MRI) atmospheric general circulation models (MRI-AGCM version 3.2) have been developed and used to investigate potential future changes in tropical cyclone (TC) activity. Compared with the previous version (version 3.1), version 3.2 yields a more realistic simulation of the present-day (1979–2003) global distribution of TCs. Moreover, the 20-km-mesh model version 3.2 is able to simulate extremely intense TCs (categories 4 and 5), which is the first time a global climate model has been able to simulate such extremely intense TCs through a multidecadal simulation. Future (2075–99) projections under the Intergovernmental Panel on Climate Change (IPCC) A1B scenario are conducted using versions 3.1 and 3.2, showing consistent decreases in the number of TCs globally and in both hemispheres as climate warms. Although projected future changes in basin-scale TC numbers show some differences between the two versions, t...

Possible Change of Extratropical Cyclone Activity due to Enhanced Greenhouse Gases and Sulfate Aerosols—Study with a High-Resolution AGCM

Abstract To investigate the possible impacts of enhanced greenhouse gases and sulfate aerosols on extratropical cyclone activity, two 20-yr time-slice experiments—the control run and the global warming run—are performed with a high-resolution AGCM (T106) of the Japan Meteorological Agency. In the control run, the atmosphere is forced by the observed SST and sea ice of 1979–98 and present-day CO2 and sulfate aerosol concentrations. In the global warming run, the atmosphere is forced by the observed SST and sea ice of 1979–98 plus the monthly mean anomalies of SST and sea ice at about the year 2050 obtained from a transient climate change experiment with the Geophysical Fluid Dynamics Laboratory (GFDL) coupled ocean–atmosphere model with a low resolution of R15. The equivalent amounts of CO2 and sulfate aerosol concentrations at about the year 2050 as used in the GFDL R15 model are prescribed. First, the performance of the high-resolution AGCM (T106) in reproducing the extratropical cyclone activity of both...

Variability of the North Atlantic Cyclone Activity in Winter Analyzed from NCEP–NCAR Reanalysis Data

Principal component analysis is applied to the cyclone density over the North Atlantic in winter analyzed with an objective cyclone identification and tracking algorithm by using the 6-h National Centers for Environmental Prediction reanalysis data from 1958 to 1998. Regressions of the cyclone density, deepening rate, moving speed, and central pressure gradient with the first principal component show that the cyclone activity over the northern North Atlantic exhibits a significant intensifying trend along with a decadal timescale oscillation in winter during the past 40 yr. All these variables vary consistently with larger (smaller) cyclone density corresponding to stronger (weaker) cyclone intensity, faster (slower) moving speed, and stronger (weaker) deepening rate. Analysis shows that the variations of the cyclone activity over the North Atlantic are closely related to the changes of large-scale baroclinicity at the lower troposphere and the North Atlantic oscillation. The relationships with the change of the North Atlantic SST are also discussed.

Climate Change Projections

This chapter assesses the effects of global warming on the climate of Japan. Japan is an archipelago extending in a southwest-northeast direction to the east of the Eurasian continent embracing the Japan Sea. High mountain ranges in the central part of the biggest island, Honshu, cause diverse climate conditions over Japan. The major features of Japan’s climate are the winter and summer monsoons, the two rainy seasons such as the Bai-u season (the early-summer rainy period) and the Shurin season (autumn rainy period) and the typhoons (tropical storms). Possible changes in these features due to global warming are of much concern. Our current knowledge is not so advanced as to make definite assessments on the changes in the above mentioned features or the regional-scale climate change over Japan due to global warming. In this chapter, however, we tried to review state-of-the-art studies as far as possible and to assess the potential effects of global warming on Japan’s climate, including qualitative estimates. The principal results of this chapter are summarized as follows.

A Mechanism of Tropical Precipitation Change due to CO2 Increase

Abstract A recent GCM study indicates that a weakening of tropical circulation associated with a slight increase in tropical precipitation may occur when atmospheric CO2 is increased. To further understand the mechanism of atmospheric temperature and precipitation changes associated with the greenhouse gas increase, a numerical experiment was conducted using an atmospheric general circulation model to investigate the separate effects of CO2 increase and sea surface temperature (SST) increase. It has been shown that the effect of CO2 increase is a reduction of radiative cooling in the lower troposphere, leading to a reduction of tropical precipitation. When atmospheric CO2 concentration is doubled (quadrupled) without changing the SST, the tropical precipitation is reduced by about 3% (6%) in the model. The reduction of radiative cooling is a result of the overlap effect of the CO2 15-μm and water vapor absorption bands. On the other hand, the effect of SST increase is the increase in atmospheric temperatu...

Recent abnormal changes in wintertime atmospheric response to tropical SST forcing

An ensemble of climate experiments has been performed using a T42 GCM version of the Japan Meteorological Agency global model to examine abnormal changes in the wintertime extratropical atmospheric response to anomalous sea surface temperature (SST) forcing in recent decades. Three independent 39-year integrations for the period 1955–1993 are forced by the same observed SST boundary condition. Although the tropical Pacific SST anomalies since the late 1980s indicate El Nino-like patterns, the Pacific/North American teleconnection (PNA) pattern is found to be hardly simulated, which stands in sharp contrast with that for the period 1977–1983. The SSTs in the vicinity of the maritime continent (the Indonesian region), including the tropical Indian Ocean, are very high during the period 1987–1993 and associated model atmosphere does undergo pronounced zonal heating over almost the entire tropics. Such tropical heating does not necessarily favor the barotropic Rossby wave dispersion into the extratropics emanating out of convective forcing since weakened Walker circulation is not capable of producing localized strong divergence anomalies, hence inhibiting generation of apparent vorticity sources in the subtropics. If the prevailing tendency of high SSTs in the vicinity of the maritime continent persists from now on, it is anticipated that a good correlation between El Nino-Southern Oscillation (ENSO) and PNA, which has been pointed out by previous numerous studies, becomes insignificant. This anomalous feature may be viewed as a signature of global warming.

Future Changes in Structures of Extremely Intense Tropical Cyclones Using a 2-km Mesh Nonhydrostatic Model

AbstractRecent studies have projected that global warming may lead to an increase in the number of extremely intense tropical cyclones. However, how global warming affects the structure of extremely intense tropical cyclones has not been thoroughly examined. This study defines extremely intense tropical cyclones as having a minimum central pressure below 900 hPa and investigates structural changes in the inner core and thereby changes in the intensity in the future climate. A 2-km mesh nonhydrostatic model (NHM2) is used to downscale the 20-km mesh atmospheric general circulation model projection forced with a control scenario and a scenario of twenty-first-century climate change. The eyewall region of extremely intense tropical cyclones simulated by NHM2 becomes relatively smaller and taller in the future climate. The intense near-surface inflow intrudes more inward toward the eye. The heights and the radii of the maximum wind speed significantly decrease and an intense updraft area extends from the lowe...

Interdecadal and Interannual Variability in the Northern Extratropical Circulation Simulated with the JMA Global Model. Part I: Wintertime Leading Mode

Abstract Interdecadal and interannual atmospheric variability in the extratropical Northern Hemisphere is investigated using an atmospheric GCM. The model used for this research is a T42 GCM version of the Japan Meteorological Agency (JMA-GSM89) global model. The 34-yr integration from January 1955 to December 1988 has been performed employing the real observed near-global SST condition. To estimate internal variability of the tropical and extratropical atmospheres, another 34-yr integration was conducted using the seasonally varying, climatological SST without interannual variability. Using the rotated EOF analysis, the authors made an intercomparison of the Pacific/North American (PNA) wintertime teleconnection patterns prevailing in the observed and simulated extratropical atmospheres in the two experiments. The polarity of PNA derived from the real SST experiment is indicative of definite interdecal variability. particularly an abrupt change of the midlatitude circulation regime over the North Pacific...

Interdecadal and Interannual Variations over the North Pacific Simulated by a Set of Three Climate Experiments

Abstract A set of three climate experiments is performed using a T42 GCM version of the Japan Meteorological Agency global model to examine extratropical interdecadal and interannual variations over the North Pacific region associated with the anomalous SST forcing in the Tropics. Three independent 34-yr integrations from January 1955 to December 1988 are forced by the same SST boundary condition observed on the global scale. The set of these integrations provides clear evidence that the tropical SST impact upon the wintertime extratropical model atmosphere in the North Pacific is very significant. It is also concluded that the abrupt change of midlatitude circulation regime that occurred in the winter of 1976/77 was primarily caused by very localized tropical heating in the central Pacific. This anomalous SST forcing was most likely responsible for persistent negative height anomalies over the central North Pacific during at least the period from 1977 to 1983, which formed a part of the extratropical wav...

Projection of future changes in the frequency of intense tropical cyclones

Recent modeling studies have consistently shown that the global frequency of tropical cyclones will decrease but that of very intense tropical cyclones may increase in the future warmer climate. It has been noted, however, that the uncertainty in the projected changes in the frequency of very intense tropical cyclones, particularly the changes in the regional frequency, is very large. Here we present a projection of the changes in the frequency of intense tropical cyclones estimated by a statistical downscaling of ensemble of many high-resolution global model experiments. The results indicate that the changes in the frequency of very intense (category 4 and 5) tropical cyclones are not uniform on the globe. The frequency will increase in most regions but decrease in the south western part of Northwest Pacific, the South Pacific, and eastern part of the South Indian Ocean.