3. Global Warming Research Program

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1 3. Global Warming Research Program Program Director: Tatsushi Tokioka The main goal of the global warming research program is the projection and predictive understanding of global warming. The program consists of three research groups, which conduct study on global warming, carbon cycle and paleoclimate. The 4th assessment report of the Intergovernmental Panel on Climate Change (IPCC) will be published in This program aims to contribute to the report as much as possible, especially on the projection of future climate change. Selected research accomplishments during the last fiscal year are briefly described below. a. Global Warming Research Group Group Leader: T. Tokioka Members: J. Yoshimura, Y. Tsushima a-1. Climate Sensitivity and Cloud Phase Parameterizations According to the third IPCC (2001) report, the previously estimated range of the equilibrium response of global mean surface temperature to the doubling of atmospheric CO2 has not decreased substantially over the last decade and remains between 1.5 and 4.5ºC. The large range in the estimated sensitivity of surface temperature is attributable mainly to our inability to reliably determine the influence of cloud feedback upon the radiative damping of the surface temperature anomaly. Many studies have evaluated the representation of clouds and their radiative effects in GCMs. Senior and Mitchell (1993), in their study of dependency of climate sensitivity upon various cloud parameterization using GCM, noted that the phase change of cloud water from ice to liquid affects the cloud lifetime and reflectivity, thereby influences the climate sensitivity. Tsushima and Manabe (2001) analyzed cloud feedback in GCMs. On a global scale, solar cloud reflectivity increases significantly with the increase of surface temperature, in contrast to the observation. Regional analysis showed that this increase in the models seems to be related to the change in cloud water distribution in the layer between -15ºC and 0ºC where ice and water cloud coexist (Figure 1). Concentration of cloud water in this layer is caused by two reasons. (1) The phase of the cloud is determined just as a function of temperature. (2) Removal efficiency of cloud droplets is determined as a function of water phase without considering history. However, it is lower in water cloud than ice water. Figure 1: Height-latitude cross section of the changes in the water content from January to July (10-6[kg/ kg]) in the Center for Climate System Research (CCSR)/National Institute for Environmental Studies (NIES) GCM. The isotherm is added for the reference (solid for July and dotted for January). The 0 o C and - 15ºC band is intensified by blue lines. The area where cloud water increases is shaded. Emori (2003) modified the cloud parameterization. Liquid cloud melted from ice cloud is modified to become rain, which falls directly to the surface. Ogura (2003), changing the form of cloud water phase function, showed that the climate is very sensitive to such a small change. The climate sensitivity changed by as much as about 3ºC. 14

2 a-2. Tropical Cyclone Climatology of an AGCM of 20 km Resolution A super-high-resolution atmospheric GCM is being developed under the subject 4 of the MEXT Research Project for Sustainable Coexistence of Human, Nature and the Earth (Kyosei Project). During fiscal year 2003, Yoshimura investigated tropical cyclone climatology with the use of outputs from an atmospheric GCM of 20 km resolution, in collaboration with researchers at Meteorological Research Institute (MRI) and Advanced Earth Science & Technology Organization (AESTO). In some preliminary runs, formation of tropical cyclones was not active. For example, the total number of tropical cyclones with maximum low-level wind exceeding 17 m/s was only 32 in a year. This is much less than the observation (approximately 80 per year). In order to improve the representation of tropical cyclones in the model, some modifications in the prognostic Arakawa-Schubert convection scheme were made. After the modification, the number of simulated tropical cyclones significantly increased (73 in a year). Occurrence frequency of tropical cyclones as a function of maximum wind speed at 850hPa (Figure 2) is also improved substantially. Figure 2: Frequency distribution of tropical cyclone occurrence, shown as functions of maximum wind speed. Tropical cyclones are counted every 24 hours. 15

3 b. Carbon Cycle Research Group Group Leader: Y. Yamanaka Sub Leader: A. Ishida Members: M. Kishi, S. L. Smith, Y. Sasai, M. N. Aita, C. Yoshikawa b-1. Development of a Global Biogeochemical Oceanic GCM Coupled with an Ecosystem Model To predict the effects of global warming on ecosystem dynamics and the effects of those changes in ecosystem dynamics on biogeochemical cycles and oceanic CO2 uptake, we need to develop new coupled ecosystem-biogeochemical cycle models which represent explicitly the phytoplankton and recycling of nutrients. We developed an ecosystem model, North pacific Ecosystem Model Used for Regional Oceanography (NEMURO), which has 11 compartments: two phytoplankton, three zooplankton, three nutrients, two particles and dissolved matter, in the Modeling Workshop in the North Pacific Marine Science Organization (PICES), and coupled NEMURO with three-dimensional ocean general model which includes nutrient and carbon cycles with horizontal resolution of 1 x 1 degrees (3- D NEMURO) in the last fiscal year. Using 3-D NEMURO, we discussed the role of seasonal vertical migration of copepods, one of popular zooplankton, in the marine ecosystem in the subarctic region (Aita et al., 2003, Fisheries Oceanogr.). Since the global total downward flux by the vertical migration of copepods is estimated to be 5-10 % of that by settling particle, marine snow, at the 1,000 m depth, the migration would be needed for more realistic simulation of the marine biological cycles. We conducted the historical run from 1948 to 2002 by daily forcing provided by NCEP, to study interannual / decadal variability of biological production and oceanic CO2 uptake. Our preliminary result simulates the climate shift in 1970s associated with Pacific Decadal Oscillation (PDO) index well (Figure 3). The primary production in 1980s decreases in the subtropical and increases in the subarctic regions of western North Pacific compared with that in 1960s (Figure 4), which is consistent with a mechanism suggested by Chai et al. (2002). 60N 40N 20N Figure 3: Time series of (a) Pacific Decadal Oscillation (PDO) index, (b) primary production in the subarctic, and (c) primary production subtropical regions of western North Pacific from 1948 to Difference of annual primary production[mgc/m 2 /d] between and Subtropical Subarctic 120E 140E 160E W 140W 120W Figure 4: Difference of annual primary production (mgc/m 2 /d) between and in the North Pacific. Positive value represents an increase of primary production in 1980s compared with that in 1960s. 16

4 b-2. Tracer Study with an Eddy-resolving Oceanic GCM Sasai et al.(2004) investigated oceanic chlorofluorocarbons (CFCs) uptake using an eddyresolving high resolution model with 1/10 x 1/10 degrees on the Earth Simulator, to understand effects of mesoscale eddies on the CFCs distribution. The estimation of global CFC-11 inventory in the model is 5.1 x 10 8 mol in This value is within the estimated range from World Ocean Circulation Experiment (WOCE) CFC-11 concentrations (Willey et al., 2004). The model successfully reproduces the CFC-11 distribution in the deep layer ventilated by the antarctic bottom water (AABW) from the Weddell Sea and the Ross Sea (Figure 5). The model clearly shows two pathways of CFC-11 spreading from the western Weddell Sea to the section along the Greenwich Meridian (AJAX). One pathway is over the South Scotia Ridge into the Scotia Sea and continues eastward with the Antarctic Circumpolar Current (ACC) to the AJAX section. The other pathway is eastward in the northern Weddell Gyre along the South Scotia Ridge. Our model results demonstrate that global models with super high resolution have potentially useful for studying local distributions such as those observed in one time cruse. Figure 5: CFC-11 inventories (mol/m 2 ) and comparison of the CFC-11 concentrations (pmol/kg) from observation and model with potential density (solid line) along (a)-(c) Greenwich in the South Atlantic (1983) and (d)-(e) 170 o W in the South Pacific (1996). Shaded area is a range of 13 models from Dutay et al. (2002). 17

5 c. Paleoclimate Research Group Group Leader: A. Abe Sub Leader: T. Motoi Members: K. Sakai, T. Nishimura, W. L. Chan, J. Hargreaves, H. Yih, T. Segawa, R. Ohgaito c-1. Numerical Experiments for Paleo-climate and Global Warming Using Fully Coupled GCM In fiscal year 2003, the coupled atmosphere and ocean GCM, which is planned to be used for the new global warming studies in the IPCC framework, was developed in collaboration with the Ecosystem Change Research Program of FRSGC, Center for Climate System Research (CCSR), University of Tokyo and National Institute for Environmental Studies (NIES), approved as Kyosei Project under the MEXT. The coupled atmosphere and ocean model without the so-called flux adjustment with an intermediate resolution (about 250 km for atmosphere and 50 to 100 km for ocean) was developed, tuned and tested against observational data by Abe-Ouchi, Segawa and Ohgaito. Several transient experiments with CO2 increase and decrease as well as simulations for 6,000 years Before Present (BP) (6ka exp. hereafter) and 20,000 years BP (Last Glacial Maximum (LGM)) were performed by the latest model versions for about 1,000 years integration. For the 6ka exp., the model result was quite similar to the experiments by Atmospheric GCM coupled to slab ocean, showing that the role of ocean is not so large compared to the role of vegetation for determining the Greening Sahara and the environment over the monsoon region, although the ocean feedback works as a positive one at high latitude, while it worked as a negative one in the low latitude. For the lower CO2 world and LGM climate, the model result gives a very encouraging result showing that the model sensitivity of low latitude is within the uncertainty of data (Kageyama et al 2003, also including our contribution) and the Thermo- Haline circulation (THC) at LGM is shallower and slightly weaker than present as broadly shown in data. c-3. Quantifying the Uncertainties in Climate Models Using the Ensemble Kalman Filter Uncertainty in climate forecasts is largely due to uncertainty in model parameterizations (at least for a specified forcing scenario), so probabilistic climate forecasting is essentially a problem of probabilistic multivariate parameter estimation in a nonlinear model. Using the ensemble Kalman filter, we have developed an efficient probabilistic parameter estimation system which is designed to enable objective optimal probabilistic ensemble forecasting with fully coupled AOGCMs. We demonstrate its effectiveness by application to a low resolution coupled 3D ocean - 2D atmosphere Earth System Model, and present some forecasts of anthropogenically forced climate change. Using this method, we were able to improve this low resolution model to the extent that its performance is, in some respects, as good as or better than state-of-the-art GCMs. Sensitivity analyses have suggested that the THC may be intrinsically bistable and vulnerable to anthropogenic change. However, these modelling experiments cannot provide quantitative (probabilistic) answers to the important questions, since they are not based on objective estimates of the present state of the climate system and its associated uncertainty. We investigate this theme also by applying the ensemble Kalman filter to the above coupled Earth System Model. Our tentative results indicate that over short time scales (100 years) the Atlantic THC decreases rapidly under global warming, and, given an accurate model and forcing scenario, present day observations may enable us to predict the resulting drop in circulation to within about 3 Sv. Over paleo timescales (1,000 years) however, the indication is that present observations of the Atlantic THC may be insufficient to enable prediction of the long-term fate of the THC. We also showed that the action of the global carbon cycle in restoring the levels of atmospheric CO2 may be critical to the long term fate of the THC. c-2. Development of a Climate Model of Intermediate Complexity In order to help planning and/or interpreting the GCM results, development of an atmosphere-oceanicesheet climate model of intermediate complexity continued through fiscal year 2003 for paleoclimate studies and some preliminary attempts of paleocimate experiments have began. 18

6 c-4. Some Sensitivity Studies to Understand the Role of Ocean in Paleoclimate (i) Response of the Atlantic thermohaline circulation to higher atmospheric CO2 concentrations Several experiments are performed using a Geophysical Fluid Dynamics Laboratory (GFDL) climate model (R15 resolution in the atmospheric component, 4.5ºx3.75º in the oceanic component), whereby CO2 concentrations are set to four times that of the present, with and without ice sheets in the polar re g i o n s ( n a m e d C O 2 x 4 a n d C O 2 x 4 n i s r u n, respectively) and to eight times that of the present, with ice sheets intact (CO2x8 run). For the CO 2 x4 run, time evolution of the North Atlantic THC intensity is divided into 4 stages. The first is a very rapid decrease within the first 200 years. This is followed by a minimum of 2-3 Sv, after which a gradual recovery takes place over approximately 1,000 years (2nd stage). Thirdly, within a century, the intensity increases rapidly, and finally, the mean intensity appears to reach an equilibrium value, slightly larger than that of a control run, and exhibits a much larger variability. (ii) Coupled Ocean-Atmosphere Multi-Decadal Oscillation in the Southeast Pacific Basin Simulated by a Climate Model A natural period and related spatial pattern are estimated from a control run of the model stated in (i) over 10,000 years. A common regionalized multidecadal period of approximately 35 model years, in and around the Southeast Pacific Basin, is close to periods estimated from climate proxies of tree-ring, ice core, and deep-sea sediments. Composite analyses are performed in order to figure out the mechanism of the oscillation, which use band-pass filtered model variables for the period range of 22 to 66 years. Fields of sea-ice thickness and sea level pressure show strong coupling feature through evolution of the oscillation in the basin. Atmospheric surface wind plays the role of restoring factor of the coupled oscillation, which is regionalized due to closed contours of vorticity f/h (Figure 7). During the THC recovery, warming continues in the sub-surface and deep layers in the low latitudes. Warm saline water, at depths of 1,000-1,500 m, extends northward from the low latitudes. The change in density due to warmer water at these depths in high latitudes eventually leads to convective instability and resumption of a strong THC. Without ice sheets, a delay of nearly 1,000 years is seen in the full recovery of the THC, which reaches a higher value of about 23 Sv. In the CO2x8 run, no full recovery is attained until after 6,000 years. The intensity of the final state is not much different from that of the 4xCO2 run (Figure 6). Figure 7: Anomaly-evolution of sea-ice thickness (left panel; unit of cm) and sea level pressure (right panel; unit of hpa) over a half period of the multi-decadal oscillation south of 400S. The half period for assigned phases (written on the upper left corner on each frame) are from the anomaly maximum at phase 900 to the zero-crossing at 1800 to the minimum at Figure 6: Time series of the meridional overturning strength in the North Atlantic Ocean for 4 cases: control run, 4xCO2 run (CO2 concentration set to 1200 ppm), 4x CO2nis run (CO2 concentration set to 1200 ppm, ice sheets removed) and 8x CO2 run (CO2 concentration set to 2400 ppm) 19

7 (iii) Effects of Stopping the Mediterranean Outflow on the Southern Polar Region The sensitivity of the climate in the southern polar region to the stopping of the Mediterranean outflow is investigated by use of the coupled model stated in (i). Large responses in the SST due to the stopping are found in both the northern North Atlantic and the Southern Ocean. The greatest changes are found between latitudes 50ºS and 60ºS in the Pacific Ocean sector and has a maximum SST decrease of about 1.5ºC. A smaller decrease in SST of up to 0.7ºC is found at a similar latitude in the Indian Ocean sector. The largest decreases in Sea Surface Salinity (SSS) are seen in the same two locations (Figure 8). (iv) Switching of Deep Water Source from the North Atlantic to the North Pacific due to Opening of the Panamanian Gateway in a Coupled Atmosphere-Ocean-Land Surface Model Switching of deep water source from the North Atlantic to the North Pacific is found in a climate model experiment whereby the Panamanian Gateway is opened. THC collapses in the Atlantic within 500 years and develops in the Pacific within 1,000 years due to salt transport from the Atlantic to the Pacific through the open gateway (Figures 9, 10). The meridional overturning in the Antarctic bottom water exhibits little change in structure, but reduces by 2.6 Sv (20%) in intensity, brought about by the decreasing difference in density between the polar and equatorial regions, resulting from salinity anomalies. Figure 9: Streamfunction of the meridional circulation in the N Pacific and N Atlantic oceans, averaged over the 100-years period during equilibrium solutions with closed and open Panamanian gateway. (a ) N Pacific, closed gateway (b) N Pacific, open gateway (c ) N Atlantic, closed gateway (d) N Atlantic, open gateway Figure 8: Difference (NMOW-control run) in SST, averaged over the first 5,000 years of the integration. Negative values are shaded and contour lines are drawn in intervals of 0.5ºC. The 3 labels refer to minimum values. Figure 10: Time series of annual mean intensity of the streamfunction of the meridional circulation in both oceans for the experiment with open gateway. Units are in Sverdrups (10 6 m 3 /s). 20