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Masato Sugi and Jun Yoshimura

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 temperature and water vapor, leading to increases in radiative cooling and tropical precipitation. When SST is uniformly raised 2°C without changing the atmospheric CO2 concentration, the tropical precipitation is increased by about 6%.

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Quanzhen Geng and Masato Sugi

Abstract

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.

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Quanzhen Geng and Masato Sugi

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 hemispheres in the control run is examined—by comparing the cyclone activities simulated in the AGCM and those analyzed from the NCEP–NCAR reanalysis data of the same period from 1979 to 1998. An objective cyclone identification and tracking algorithm is used to analyze the cyclone activity. The results show that the model can reproduce the cyclone activity reasonably well.

Second, the possible change in cyclone activity due to enhanced greenhouse gases and sulfate aerosols is examined. The main results are summarized as follows. 1) The total cyclone density (number of cyclones in a 4.5° × 4.5° area per season) tends to decrease significantly in the midlatitudes of both of the Northern and Southern Hemispheres during the December–January–February (DJF) and June–July–August (JJA) seasons. The decrease of cyclone density in the midlatitudes of both of the Northern and Southern Hemispheres in the DJF season is about 7%. In the JJA season, the decreases of cyclone density in the Northern and Southern Hemispheres' midlatitudes are about 3% and 10%, respectively. 2) Although weak and medium-strength cyclones decrease, the density of strong cyclones increases by more than 20% in the Northern Hemisphere in JJA and in the Southern Hemisphere in both DJF and JJA. 3) The density of strong cyclones in the Northern Hemisphere summer (JJA) increases over the eastern coasts of Asia and North America. In the Southern Hemisphere, the density of strong cyclones increases over the circumpolar regions around Antarctica in both summer (DJF) and winter (JJA) seasons. The density of strong cyclones also increases over the southeastern coasts of South Africa and South America.

Finally, the possible reasons for the change in cyclone activity due to enhanced greenhouse gases and sulfate aerosols are examined. It is shown that the changes in the extratropical cyclone activity are closely linked to the changes in the baroclinicity in the lower troposphere, which are mainly related to the changes in the horizontal and vertical temperature distributions in the atmosphere due to enhanced greenhouse gases and sulfate aerosols. It is shown that, in the Northern Hemisphere midlatitudes, the decrease of baroclinicity is mainly caused by the decrease of meridional temperature gradient, while in the Southern Hemisphere midlatitudes, the decrease of baroclinicity is mainly caused by the increase of static stability caused by the enhanced greenhouse gases and sulfate aerosols.

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Ryuichi Kawamura, Masato Sugi, and Nobuo Sato

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 wave train traversing the North Pacific and North America, which produced warm temperature anomalies along the west coast of North America, as well as western Canada. However, an increase in observed wintertime surface temperature over northern Eurasia at almost the same period can little be explained by anomalous SST forcing from the Tropics. The internal variability of the extratropical atmosphere itself is suggested to contribute much more to the circulation regime over the Eurasian continent.

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Ryuichi Kawamura, Masato Sugi, and Nobuo Sato

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 in the 1976/77 winter. By contrast, this mode, deduced from the climatological SST control run, has intermonthly and short-term interannual variability but no pronounced interdecadal variability.

It is strongly suggested that the anomalous SST forcing exerts strong influence on the PNA mode and modulates its amplitude, and as a consequence, longer-tem variability, such as interdecadal variability, has appeared in the time sequence of this mode. It is confirmed from the T42 GCM experiment that the interdecadal variations of the wintertime extratropical atmosphere over the North Pacific are substantially controlled by the anomalous SST forcing in the Tropics, and that, in particular, the tropical forcing is primarily responsible for the abrupt change of the midlatitude circulation regime in the 1976/77 winter.

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Ryuichi Kawamura, Masato Sugi, and Nobuo Sato

Abstract

Interdecadal and interannual) variations of a model atmosphere in the northern extratropics is examined using a T42 GCM forced with observed near-global SSTs from January 1955 to December 1988.

The leading mode of summertime 500-hPa height field deduced from the real SST experiment is found to he dominated by interdecadal variability. This mode shows a zonally elongated pattern with prominent loadings in low-latitude regions and accounts for an increase of the zonal, summertime 500-hPa heights in subtropical regions from the 1970s to the 1980s. Simulated springtime leading mode, which is dominated by interdecadal variability, exhibits a mixed pattern with the wintertime PNA mode and the summertime zonally elongated mode, whereas the zonally elongated pattern like the summertime EOFI cannot be found in northern fall.

From an investigation based on the seasonality of tropical response of the model atmosphere, it is found that the summertime and springtime leading modes with a pronounced zonally symmetric component depend largely upon the tropical SST anomalies of interdecadal variability. The weakness of tropical response in fall contributes largely to the absence of the zonally elongated mode with definite interdecadal variability in this season.

The regional and tempers features of the observed decadal surface air temperature anomalies are well simulated by the real SST experiment. The time sequence of the above summertime EOFI, which accounts for a strong dependence of tropical atmosphere to SST anomaliess, is found to coincide well with the summertime mean hemispheric land surface air temperature. It is inferred, therefore, that the tropical SSTs of interdecadal variability contribute a great deal to the decrease and increase in the Northern Hemispheric land surface temperature observed in recent decades.

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Sachie Kanada, Akiyoshi Wada, and Masato Sugi

Abstract

Recent 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 lower level around the leading edge of thinner near-surface inflows, where the equivalent potential temperature substantially increases in the future climate. Emanuel’s potential intensity theory suggests that about half of the intensification (increase in central pressure fall) is explained by the changes in the atmospheric environments and sea surface temperature, while the remaining half needs to be explained by other processes. It is suggested that the structural change projected by NHM2, which is significant within a radius of 50 km, is playing an important role in the intensification of extremely intense tropical cyclones in simulations of the future climate.

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Yohei Yamada, Masaki Satoh, Masato Sugi, Chihiro Kodama, Akira T. Noda, Masuo Nakano, and Tomoe Nasuno

Abstract

Future changes in tropical cyclone (TC) activity and structure are investigated using the outputs of a 14-km mesh climate simulation. A set of 30-yr simulations was performed under present-day and warmer climate conditions using a nonhydrostatic icosahedral atmospheric model with explicitly calculated convection. The model projected that the global frequency of TCs is reduced by 22.7%, the ratio of intense TCs is increased by 6.6%, and the precipitation rate within 100 km of the TC center increased by 11.8% under warmer climate conditions. These tendencies are consistent with previous studies using a hydrostatic global model with cumulus parameterization.

The responses of vertical and horizontal structures to global warming are investigated for TCs with the same intensity categories. For TCs whose minimum sea level pressure (SLP) reaches less than 980 hPa, the model predicted that tangential wind increases in the outside region of the eyewall. Increases in the tangential wind are related to the elevation of the tropopause caused by global warming. The tropopause rise induces an upward extension of the eyewall, resulting in an increase in latent heating in the upper layers of the inclined eyewall. Thus, SLP is reduced underneath the warmed eyewall regions through hydrostatic adjustment. The altered distribution of SLP enhances tangential winds in the outward region of the eyewall cloud. Hence, this study shows that the horizontal scale of TCs defined by a radius of 12 m s−1 surface wind is projected to increase compared with the same intensity categories for SLP less than 980 hPa.

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Hiroyuki Murakami, Yuqing Wang, Hiromasa Yoshimura, Ryo Mizuta, Masato Sugi, Eiki Shindo, Yukimasa Adachi, Seiji Yukimoto, Masahiro Hosaka, Shoji Kusunoki, Tomoaki Ose, and Akio Kitoh

Abstract

New 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, the projected frequency of TC occurrence shows a consistent decrease in the western part of the western North Pacific (WNP) and in the South Pacific Ocean (SPO), while it shows a marked increase in the central Pacific. Both versions project a future increase in the frequency of intense TCs globally; however, the degree of increase is smaller in version 3.2 than in version 3.1. This difference arises partly because version 3.2 projects a pronounced decrease in mean TC intensity in the SPO. The 20-km-mesh model version 3.2 projects a northward shift in the most intense TCs (category 5) in the WNP, indicating an increasing potential for future catastrophic damage due to TCs in this region.

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Thomas Knutson, Suzana J. Camargo, Johnny C. L. Chan, Kerry Emanuel, Chang-Hoi Ho, James Kossin, Mrutyunjay Mohapatra, Masaki Satoh, Masato Sugi, Kevin Walsh, and Liguang Wu

Abstract

An assessment was made of whether detectable changes in tropical cyclone (TC) activity are identifiable in observations and whether any changes can be attributed to anthropogenic climate change. Overall, historical data suggest detectable TC activity changes in some regions associated with TC track changes, while data quality and quantity issues create greater challenges for analyses based on TC intensity and frequency. A number of specific published conclusions (case studies) about possible detectable anthropogenic influence on TCs were assessed using the conventional approach of preferentially avoiding type I errors (i.e., overstating anthropogenic influence or detection). We conclude there is at least low to medium confidence that the observed poleward migration of the latitude of maximum intensity in the western North Pacific is detectable, or highly unusual compared to expected natural variability. Opinion on the author team was divided on whether any observed TC changes demonstrate discernible anthropogenic influence, or whether any other observed changes represent detectable changes. The issue was then reframed by assessing evidence for detectable anthropogenic influence while seeking to reduce the chance of type II errors (i.e., missing or understating anthropogenic influence or detection). For this purpose, we used a much weaker “balance of evidence” criterion for assessment. This leads to a number of more speculative TC detection and/or attribution statements, which we recognize have substantial potential for being false alarms (i.e., overstating anthropogenic influence or detection) but which may be useful for risk assessment. Several examples of these alternative statements, derived using this approach, are presented in the report.

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