Global Warming Projection Studies at the Meteorological Research Institute/JMA

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1 Present and Future of Modeling Global Environmental Change: Toward Integrated Modeling, Eds., T. Matsuno and H. Kida, pp by TERRAPUB, Global Warming Projection Studies at the Meteorological Research Institute/JMA Tatsushi TOKIOKA 1 and Akira NODA 2 1 Japan Meteorological Agency (JMA), Tokyo , Japan 2 Meteorological Research Institute (MRI), Ibaraki , Japan STUDIES WITH THE USE OF A COUPLED ATMOSPHERE MIXED LAYER OCEAN MODEL Outline of the model Global warming studies at the Meteorological Research Institute (MRI) were started in the late 1980s. A coupled atmosphere-mixed layer ocean model was developed for this purpose and was applied for a quasi-equilibrium experiment under a doubled atmospheric CO 2 condition. The atmospheric model (Tokioka et al., 1984) used has its top at 100 hpa and 5 layers and a regular 5 by 4 longitude/ latitude resolution. The model incorporates the radiative model by Katayama (1972), Arakawa-Schubert cumulus parameterization (Arakawa and Schubert, 1974), the planetary boundary layer (PBL) model proposed by Randall and Arakawa and the ground surface model by Katayama (1972). The effect of the ocean is included simply as a 50 m slab without horizontal heat transport. Sea ice is parameterized following the energy-balanced zero layer model by Semtner (1976). Because the heat flux by the ocean is completely neglected, the coupled model is not free from model biases caused by the neglect of the oceanic heat transport. The meridional temperature gradient is certainly larger than the observed one. There must be regional and localized departures of the model climate from the observed one also, due, for example, to the unrealistic sea ice extent. However, the realized global model climate, such as geographical climate variations over the globe and seasons, has considerable similarity to the observed one. Thus the model could still be used to study the climate sensitivity of the earth and the qualitative characteristics of climate changes due to doubling of the atmospheric CO 2 concentration, as one of the simplest and most economical tools. We will review two studies at the MRI in the following with the use of the model; one is a study of the characteristic changes in precipitation (Noda and Tokioka, 1989) and the other those of snowfall (Saito and Tokioka, 1994). 1

2 2 T. TOKIOKA and A. NODA Characteristic changes in precipitation under the doubling of CO 2 The model is time-integrated under two conditions with seasonal cycles. In one case (1 CO 2 experiment), the atmospheric concentration of CO 2 is assumed to be 320 ppmv. In the other (2 CO 2 experiment), it is 640 ppmv. The annual global mean surface air temperature and precipitation increase by 4.3 C and 7.4%, respectively, due to the CO 2 doubling. If we look more closely at the results, the total precipitation increase is relatively small in low latitudes compared to that in mid and high latitudes. We find an increase in the cumulus-type precipitation in low latitudes throughout the year and in summer in the northern mid latitudes, an increase in stratus-type precipitation in high latitudes especially in winter, and a decrease in stratus-type precipitation in mid and low latitudes throughout the year. Figure 1 shows a scatter diagram where the abscissa is the ratio (%) of the total grid area of non-zero precipitation in an hour to the global surface, and the ordinate is the precipitation rate (mm/day). Crosses and small dots show data sampled from the 1 CO 2 and 2 CO 2 experiments, respectively. Ellipses are drawn to show the root mean square scattering in the domain for the respective cases. This figure shows the January case. The two ellipses have no overlap and the center of the ellipse shifts towards increasing precipitation intensity but decreasing area coverage. Such a characteristic change is interpreted as the result of increasing cumulus precipitation and the decreasing stratus-type precipitation in the globally averaged sense under the global warming. Cumulative frequency distribution for precipitation rates (mm/hour) were also studied. The occurrence of high intensity precipitation increases in the 2 CO 2 case especially in mid and high latitudes caused mainly by cumulus-type precipitation. This implies a potential increase in the meteorological disasters due to severe rain under the global warming. Characteristic changes in snowfall under the doubling of CO 2 When precipitation occurs in the form of snow, the surface albedo changes drastically. Snow affects the water budget of the ground surface also by holding water at the surface until melting causes runoff. So, the characteristic changes in snowfall under the global warming condition were studied. Changes in snowfall are classified into four types (Saito and Tokioka, 1994). Snowfall increases in Type + throughout the snow seasons, although it disappears in Type Y. In Type X, it starts to fall late and disappears fast, and the snow mass is less in 2 CO 2 than in 1 CO 2. In Type Z, there are no characteristic changes both in the snow amount and the snow period. The snowfall change at each grid is classified into one of these four types. Although the detailed geographical distribution of snowfall changes is not meaningful because of model biases due to the neglect of oceanic heat transport, the qualitative nature of the changes might be meaningful in considering the actual changes. We find Type + in the polar regions, and Type Y in the lowest latitudinal snowfall zone in 1 CO 2. In between Type + and Type Y zones, we find Type X and Type Z grid points.

3 Global Warming Projection Studies at the Meteorological Research Institute/JMA 3 Fig. 1. Scatter diagram of the precipitation rate (mm/day) versus the ratio (%) of the precipitation grid area to the global domain for January 1 to 10. Ellipses drawn with thick solid line and thin solid line denote the root mean square scattering for 1 CO 2 and 2 CO 2, respectively. Data points for 1 CO 2 and 2 CO 2 are denoted by crosses and dots, respectively (Noda and Tokioka, 1989). STUDIES WITH THE USE OF THE MRI-CGCM1 To improve the simulated global and regional climate and to study the transient climate response to a gradual increase in atmospheric CO 2, the mixed layer ocean model was replaced by a global ocean general circulation model (OGCM) developed at the MRI (Nagai et al., 1992). The OGCM has a realistic bottom topography, 21 vertical layers, 2.5 longitudinal resolution, and variable latitudinal resolution ranging from 0.5 at the equator to 2.0 at 12 latitude and further poleward. The AGCM adopted is described in Kitoh et al. (1995). The horizontal resolution is 5 by 4 in the longitudinal and latitudinal directions, respectively. There are 15 vertical levels with a top at 1 hpa. The physical processes adopted are summarized in Table 1. The model produced not only ENSO (El Nino and Southern Oscillation) like variabilities as expected but also inter-decadal Pacific variability. The latter had a close resemblance to the observations (Yukimoto et al., 1996).

4 4 T. TOKIOKA and A. NODA Table 1. Comparison between the MRI-CGCM1 and the MRI-CGCM2. Aspect MRI-CGCM1 MRI-CGCM2 [Atmospheric component] Horizontal resolution 5 (lon.) 4 (lat.) T42 (~ ) Layer (top) 15 (1 hpa) 30 (0.4 hpa) Solar radiation Lacis and Hansen (1974) Shibata and Uchiyama (1992) (SW) H 2 O, O 3 H 2 O, O 3, aerosol Terrestrial radiation Shibata and Aoki (1989) Shibata and Aoki (1989) (LW) H 2 O, CO 2, O 3 H 2 O, CO 2, O 3, CH 4, NO 2 Convection Arakawa and Schubert (1974) Prognostic AS Randall and Pan (1993) PBL bulk layer (Tokioka et al., 1988) Mellor and Yamada (1974) Gravity wave drag Palmer et al. (1986) Iwasaki et al. (1989) Rayleigh friction Rayleigh friction Cloud type penetrative convection, penetrative convection middle level convection, large scale condensation, large scale condensation stratus in PBL Cloudiness saturation function of relative humidity Cloud overlap random for non-convective clouds random + correlation 0.3 for convective clouds Cloud water content function of pressure and temperature function of temperature [Land process] 4-layer diffusion model 3-layer SiB [Oceanic component] Horizontal resolution 2.5 (lon.) (lat.) Layer (min. thickness) 21 (5.2 m) 23 (5.2 m) Eddy viscosity h. visc m 2 s 1 h. visc m 2 s 1 v. visc m 2 s 1 v. visc m 2 s 1 Eddy mixing horizontal-vertical mixing isopycnal mixing + Gent and McWilliams (1990) h. diff m 2 s 1 isopycnal m 2 s 1 v. diff m 2 s 1 diapycnal m 2 s 1 Vertical viscosity and diffusivity Mellor and Yamada (1974, 1982) [Sea ice] Mellor and Kantha (1989) [Atmosphere-ocean coupling] Coupling interval 6 hours 24 hours Flux adjustment heat, salinity heat, salinity + wind stress (12 S-12 N) The model was applied to a transient CO 2 experiment, where atmospheric CO 2 is assumed to increase at the compound rate of 1%/yr (Tokioka et al., 1995). The experiment shows a 1.6 C increase in the globally averaged surface air temperature in the first 70-yr period. The delay in temperature rise in the Southern Hemisphere, especially around 50 S, is dominant, as already pointed out by Stouffer et al. (1989) and others. The model was applied further to another experiment where the direct effect of sulfate aerosol increase is added to the effect of CO 2, which is assumed to increase at the compound rate of 1%/yr (JMA, 1999). The aerosol forcing is not globally uniform due to the short lifetime of sulfate aerosols in the troposphere. Therefore, it has been inferred that their effects are mostly limited near the

5 Global Warming Projection Studies at the Meteorological Research Institute/JMA 5 emission regions, and hence the characteristic response patterns will also reflect such distributions. However, the MRI-CGCM1 shows a global scale response, which is similar to that caused by the CO 2 -only forcing and is contrary to the previous studies by Mitchell and Johns (1997). STUDIES WITH THE USE OF THE MRI-CGCM2 Outline of the MRI-CGCM2 The MRI-CGCM1 had several drawbacks as pointed out in Yukimoto et al. (2001). Firstly, the thermohaline circulation in the Atlantic Ocean was very weak, if any. Because of this defect, the slow and small surface temperature increase in the North Atlantic was not simulated in the transient CO 2 run, although it is typically seen in results from many other models. Secondly, the model simulated an unrealistic sea ice cover in the Norwegian Sea which is not observed. This caused a spurious large temperature increase through sea ice melting in the CO 2 increase experiment. Finally, even though the MRI-CGCM1 successfully simulates the ENSO, the amplitude of the Sea Surface Temperature (SST) variation was smaller than that observed, and the equatorial SST anomaly maximum was found around the date line, more westward than that observed. A new version of a coupled atmosphere-ocean general circulation model (MRI-CGCM2) was developed to improve these unsatisfactory aspects. The model characteristics of both the MRI-CGCM1 and the MRI-CGCM2 are compared in Table 1. The MRI-CGCM2 has actually achieved a better performance in reproducing the mean climate and the climate variability than the MRI-CGCM1 (Yukimoto et al., 2001). Major improvements in the model performance Meridional overturning The meridional deep overturning in the Atlantic Ocean produced by the models is shown in Fig. 2. After the model integration started, the transport by the deep overturning immediately vanished in the MRI-CGCM1 (Fig. 2a), and after that, it remained at a small negative value in the entire model integration (Fig. 2c). The MRI-CGCM2 reasonably simulates the meridional overturning (Fig. 2b) of the mixed structure of both shallow and deep cells. The deep overturning cell with sinking near 60 N is associated with the North Atlantic Deep Water (NADW). Its mean transport is approximately 17 Sv that roughly agrees with the estimation of 13 Sv by Schmitz and McCartney (1993). The improvement seems to be related to the change in the calculation of flux adjustment. The deep overturning cell near the Antarctica, which is known as the origin of the Antarctic Bottom Water (AABW), is reproduced with 8 Sv maximum transport. ENSO Geographical distribution of the SST anomaly (SSTA) regressed on the NINO3 SST is shown in Fig. 3, for the observation, the MRI-CGCM1 and the MRI-CGCM2. In the MRI-CGCM2, a prominent positive anomaly is seen in the

6 6 T. TOKIOKA and A. NODA Fig. 2. Annual mean meridional overturning stream-functions for the global ocean in (a) the MRI- CGCM1 and (b) the MRI-CGCM2. Time series of the maximum (annual mean) meridional overturning in the North Atlantic Ocean for (c) the MRI-CGCM1 and (d) the MRI-CGCM2. Units are in Sv (Yukimoto et al., 2001).

7 Global Warming Projection Studies at the Meteorological Research Institute/JMA 7 Fig. 3. Sea surface temperature anomaly (SSTA) regressed on the normalized time series of the SST in NINO3 region for (a) observation, (b) the MRI-CGCM1 and (c) the MRI-CGCM2 (Yukimoto et al., 2001).

8 8 T. TOKIOKA and A. NODA Fig. 4. Time series of global annual-mean surface temperature in response to various scenarios of trace gases and aerosol. Control run (CNTL): trace gas (CO 2, CH 4, N 2 O and O 3 ) concentrations are fixed at the present (around 1990) values for 200 years, CO 2 incr run (CMIP2): CO 2 annual increase 1% compound for 150 years, IS92a run: CO 2 only for 150 years, for observed value, for IS92a emission scenario (IPCC, 1992), IS92a + aerosol: IS92a run plus the direct effect of sulfate aerosol based on the scenario of Mitchell and Johns (1997), SRES A2: IPCC/SRES A2 scenario ( ), SRES B2: IPCC/SRES B2 scenario ( ). central eastern equatorial Pacific extending to the coast of Peru. This pattern is similar to that observed, although it has shifted westward too much in the MRI- CGCM1. The unrealistic negative SSTA in the eastern equatorial Indian Ocean in the MRI-CGCM1 is caused by an unrealistic equatorial upwelling in the eastern Indian Ocean. This aspect is improved in the new model in association with improvement of the eastward gradient of the equatorial thermocline in the Indian Ocean. Results from various greenhouse gases and aerosol scenario runs Global mean response Figure 4 shows the time evolution of the globally-averaged, annual-mean surface air temperature and precipitation for the CNTL run and the other scenario

9 Global Warming Projection Studies at the Meteorological Research Institute/JMA 9 Fig. 5. Monthly averaged precipitation over Japan calculated by the nested RCM40 when it was time-integrated from 1986 to 1991 under the lateral and lower boundary conditions provided by the JMA s objective analyses (Mabuchi et al., 2000). The observed data are based on the very dense surface observational network AMeDAS. runs. Because there is little trend in the CNTL run, the time-dependent response is evaluated by subtracting a 200-year average of the CNTL run. Simulation of climate change using an idealized CMIP2 scenario has become a standard experiment, and therefore, results from such experiments by many other CGCMs are available (e.g., IPCC, 1996). For the MRI-CGCM2, the 20-year average (61 80 or in Fig. 4) globally-averaged, annual-mean differences in surface air temperature and precipitation around the time of CO 2 doubling are 1.1 C and 1.2%, respectively. This can be compared to an equilibrium doubled-co 2 experiment with the atmospheric model (adopted in the MRI- CGCM2) coupled to a slab mixed layer (50 m depth) with a globally averaged temperature increase of 2.0 C and precipitation increase of 3.4%. Compared to the other models listed by the IPCC (IPCC, 1996), the MRI-CGCM2 shows the lowest sensitivity to the CO 2 doubling. One of the main causes for this might be due to cloud feedback processes in the model. The IS92a and IS92a + aerosol runs were made to evaluate the response to historical and future radiative forcing due to increased CO 2 and the direct effect of sulfate aerosols. Figure 4 indicates that the direct effect of aerosols is small over the whole integration period and that the historical response is within the range of natural variability till the 1960s. The former response is smaller than that reported by Mitchell and Johns (1997).

10 10 T. TOKIOKA and A. NODA The global mean response to the SRES A2 and B2 scenarios is also shown in Fig. 4. The slowest and smallest response for the SRES A2 run is expected from the scenario. In addition to this, a similar warming rate is found between the SRES A2 and B2 runs till about 2030, which may be partly attributable to the gradually reducing aerosol emissions assumed in the SRES B2 scenario. Scenario dependence of geographical response The differences in annual-mean surface air temperature for the period relative to the period between the CNTL and various history plus scenario runs were studied. The greatest warming at high latitudes, particularly in the Northern Hemisphere, as well as less warming over the southern oceans and the northern Atlantic Ocean year-round due to deep mixing there, is consistent with other CGCMs (IPCC, 1996). Besides these global features, regional response patterns are also very similar among the scenario runs. Such similarities can be consistently seen in the response patterns of other CGCMs. This fact suggests that, for a fixed CGCM, its global warming pattern is robust, and therefore, the amplitude of local response is basically determined by globally averaged forcing, rather than a local one. STUDIES ON LOCAL CLIMATE CHANGES AROUND JAPAN For considering possible and effective ways to alleviate adverse effects caused by the global warming, each government and climate sensitive sectors really necessitate precise information about local climate changes. Japan is composed of small islands. However, it has various types of climate zones because it is located in an area influenced by both summer and winter Asian monsoons and the topography of each island. To obtain detailed information about possible future climate changes in Japan, a model that reproduces such detailed local climate characteristics is required. Yasuo Sato and his group have developed a method where a locally high-resolution atmospheric model can reproduce the current climate well when both lateral and surface boundary conditions are prescribed with coarse spatial resolution, i.e., several hundred kilometers (Mabuchi et al., 2000; Sasaki et al., 2000). A local model of Japan of 40 km resolution (RCM40) is used to simulate the climate around Japan. The East Asian model of 120 km resolution (RCM120) is introduced in between the global model and the RCM40 to allow smooth spatial interpolations. Both RSM120 and RSM40 were originally developed at the JMA for operational forecasts. The whole system was tested by replacing the boundary conditions with the analyzed values of coarse resolutions (about 200 km), and the system was run from 1986 to 1991 for 6 years. Figure 5 compares the monthly averaged precipitation calculated by the model and the observed data of a very dense network AMeDAS (the JMA s automated surface meteorological observation system with about 20 km resolution). The agreement between them is satisfactory except in a few cases. The same group has applied the system to the global warming problem, where the results obtained in the transient CO 2 run with the MRI-CGCM1 are

11 Global Warming Projection Studies at the Meteorological Research Institute/JMA 11 used to prescribe boundary and surface conditions. In January, RCM40 simulates in the control run realistic precipitation along the Japan Sea side of the mountain range, which is completely missing in the MRI-CGCM1. In July, precipitation corresponding to the Baiu front is improved substantially in RSM40. Preliminary results of the climate changes at the time of CO 2 doubling for July show substantial reductions in precipitation except in the western part of Japan. To increase confidence in the results, we have to improve the model climate further through the improvement of both the spatial resolution and the physical processes of the model. STUDIES ON TROPICAL CYCLONE ACTIVITIES Another big concern for Japan concerning the global warming is tropical cyclones (TCs), because they are closely connected with both meteorological disasters and water resources in summer and fall. Will the number of TCs increase (Bengtsson et al., 1996)? Will they intensify as was suggested by Emanuel (1987)? How will the paths of the TCs change? Sugi et al. (1997) ran a JMA Fig. 6. Averaged total number of simulated tropical cyclones per year (Yoshimura et al., 1999). Cumulus parameterizations of both Arakawa-Schubert (AS) and Kuo (Kuo) were adopted. Runs were repeated by changing SST, i.e., climatological SST, typical El Nino SST, typical La Nina SST, SST where climatological SST is uniformly increased by 2K (case 2K), and SST where the first EOF model in SST in the MRI-CGCM1 run was added/subtracted further for case 2K.

12 12 T. TOKIOKA and A. NODA operational model (T106L21) using both the climatological SST and the SST where the SST increase obtained in the transient CO 2 run with the MRI-CGCM1 is added to the climatological SST. The simulated tracks of TCs in the climatological run reproduce climatological distributions basically, although the model resolves them marginally. They analyzed the behavior of TCs and obtained a decrease in the number in the globally averaged sense, especially in the equatorial western Pacific in the increased SST case. Further increase in resolution will certainly be required to obtain conclusive results. To explore the changes in TCs under the global warming further, Yoshimura et al. (1999) repeated sensitivity experiments by changing the cumulus parameterization scheme from that of Kuo (case Kuo) to that of Arakawa- Schubert (case AS), and SSTA distributions. To study the occurrence of TCs under the current climate, not only the climatological SST (case CL) but also the cases where typical El Nino and La Nina type SSTAs are added (cases EN/LN) are tested. In one sensitivity run, SST is increased by +2K uniformly from the climatological SST in the low and mid latitudes (case 2K). In another run, a natural variational pattern in the transient CO 2 run with the MRI-CGCM1 (the first EOF mode in SST) is added/subtracted further (cases MR/GF). Figure 6 shows the averaged total number of simulated TCs per year for the respective cases. Although the total numbers differ substantially depending on the cumulus parameterization schemes even for the same SST, the difference in SSTA does not seem to cause big differences in the total number of TCs for the same cumulus scheme. It is noted also that the total number of TCs reduces significantly in both the Arakawa-Schubert scheme and the Kuo scheme when the SST is increased. Yoshimura et al. (1999) discusses a possible cause of the reduction in TCs under the global warming. They point out that the mean precipitation rate near the TC centers increases by about 10 30% in the runs with increased SSTs. If a TC is regarded as a system to convert latent heat into sensible heat in the tropical atmosphere, fewer TCs are required to convert the same amount of heat. REFERENCES Arakawa, A. and W. H. Schubert, 1974: Interaction of a cumulus cloud ensemble with the large scale environment, Part I. J. Atmos. Sci., 31, Bengtsson, L., M. Botzet and M. Esch, 1996: Will greenhouse gas-induced warming over the next 50 years lead to higher frequency and greater intensity of hurricanes? Tellus, 4BA, Emanuel, K. A., 1987: The dependence of hurricane intensity on climate. Nature, 326, Gent, P. R. and J. C. McWilliams, 1990: Isopycnal mixing in ocean circulation models. J. Phys. Oceanogr., 20, IPCC (Intergovernmental Panel on Climate Change), 1992: Climate Change 1992: The Supplementary Report to the IPCC Science Assessment. eds. Houghton, J. T., B. A. Callander and S. K. Varney, Cambridge University Press, UK, 200 pp. IPCC (Intergovernmental Panel on Climate Change), 1996: Climate Change 1995: The Science of Climate Change, Contribution to Working Group I to the Second Assessment Report of the IPCC. eds. Houghton, J. T., L. G. Meira Filho, B. A. Callander, N. Harris, A. Kattenberg and K. Maskell, Cambridge University Press, UK, 572 pp.

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