Principal Research Results

Potential Changes in Typhoon Intensity in the Near Future

Background
The IPCC Fourth Assessment Report (AR4), published in 2007, states with certainty that warming of about 0.2 ℃ per
decade is projected for near-future (~ 2030) climate change, irrespective of any variation in carbon dioxide emissions from now. TheAR4, however, does not provide useful information about possible changes in the frequency and intensity of typhoons caused byglobal warming . It is generally difficult to distinguish global warming signals about typhoon frequency and intensity from relatively
large fluctuations affected by natural variations of sea surface temperature and large-scale atmospheric circulation. Toward useful
future projection, a need exists not only to improve numerical climate models but also to develop analysis methods to examine climate
model output from various aspects relevant to typhoon changes.

Objectives
This study aims to quantify near-future potential changes in typhoon intensity around Japan in the western North Pacificbased on thermodynamic aspects of results from climate model experiments for the 20th century and future projection. The experiments were conducted by CCSM3, one of the latest atmosphere-ocean coupled general circulation models.

Principal Results
1. Relationship between sea surface temperature and potential typhoon intensity
The upper limit of typhoon development (or the lower limit of central pressure)＊ 1, which is a measure of potential typhoon
intensity, can be theoretically estimated from thermodynamic environment given by sea surface temperature and vertical profile ofupper temperature (Fig. 1). This study compares the present (1980-1999 average) and near-future (2010-2029 average) climate regarding the potential intensity based on the CCSM3 experiments, resulting in the following.
(1) In typhoon development regions of the western North Pacific, the lower limit of central pressure decreases on average by 5-7 hPa
for 0.6-0.7 ℃ warming of sea surface temperature during 30 years (Table 1). This change is equivalent to about 3 m/s increase in
the maximum wind speed at the mature stage of a typhoon.
(2) The relationship between central pressure and sea surface temperature differs between the present and near-future climate. Possible
typhoon intensification due to global warming is modest compared to intensity change inferred from sea surface temperaturechange alone (Fig. 2).
2. Impact of upper air temperature change
Influence of upper air temperature change on the potential typhoon intensity is found as follows.
(1) Typhoon development regions are characterized by the fact that upper troposphere (about 10 km) warming is about two times
greater than sea surface warming (Fig. 3). Lowering central pressure at a typhoon center directly results from the above airwarming relative to the environment. It is, therefore, understood that global-scale greater warming contributes to suppression oftyphoon intensification.
(2) Relatively large changes in mid-latitude sea surface temperatures lead to a tendency of smaller meridional temperature gradient in
the upper air. This environment change suggests an influence on keeping intensity and structural change of typhoons that approachor make landfall over Japan.

Future Developments
Potential typhoon intensity will be further improved with probabilistic information derived from multiple climate model
projections including different biases and uncertainties. Surface wind models, used for strength design of infrastructure, will be also
improved with the advanced information about typhoons approaching or making landfall over Japan.
Main Researcher: Junichi Tsutsui,Senior Research Scientist, Environmental Physics Sector, Environmental Science Research Laboratory
Reference
J. Tsutsui, 2008, “Maximum potential intensity of tropical cyclones derived from numerical experiments using the Community
Climate System Model (CCSM3)”, J. Disaster Research, vol.3, pp.25-32＊1：The upper limit of typhoon intensity is theoretically calculated from upper-air temperature increase (density decrease) relative to the environment, leading to a central pressure drop.

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2. Environment

The air flow in a typhoon or hurricane features anaxisymmetric intense vortex and transverse secondary
circulation. Thermodynamic model for the secondary
circulation gives a theoretical upper limit of cyclone
intensity as minimum central pressure. This study uses atheoretical model formulated by Holland (1997, J. Atmos.
Sci.).

Fig.1 Conceptual model of a tropical cyclone (oneside of vertical cross section at the center)2
Table 1 Mean sea surface temperature (SST) and minimum central pressure (Pc)in the typhoon genesis region in the western North PacificSeasonLatitudeJun-Jul
Aug-Sep
Oct-Nov

6.3-23.1N
6.3-25.9N
6.3-21.7N

PresentSST
Pco( C)
(hPa)
28.5
926
28.8
911
28.5
925

Near-future change
SST
Pco( C)(hPa)
0.64
5.1 (3.5-10.0)
0.66
7.3 (5.3-10.8)
0.73
7.2 (4.2-11.5)

Near-future (2010-2029) changes represent differences from the present (1980-1999) state, where positive
numbers indicate an increase for sea surface temperature and a decrease for central pressure. 120-160E
averages are shown. Numbers in parentheses indicate the min-max range over each latitude region.

Fig.2 Relationship between minimum centralpressure and sea surface temperature
Numerical values at each season and latitude for the present
and near future are plotted by markers. Large stars indicate theaverage of points below 990 hPa, shown by dotted line, foreach climate state. A curve representing the near-future
relationship shifts from a similar curve for the present climate
toward a higher temperature and slightly lower centralpressure.

Fig.3 Near-future change in thermodynamic
environment over the western North Pacific
Change in the near-future relative to the present state for (a)vertical cross section of air temperature, (b) its 5-25N average, and(c) sea surface temperature. Greater warming occurs in the tropicalupper troposphere.

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