21世紀気候変動予測革新プログラム

論文リスト

Authors
Lestari, R. K., M. Watanabe, and M. Kimoto, 2010
Title
Role of air-sea coupling in the South China Sea summer monsoon variability. J. Meteor. Soc. Japan, 89A, 283-290.
Keywords
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Abstract

A possible role of the air-sea coupling in the South China Sea (SCS) summer monsoon variability is studied using a coupled general circulation model (CGCM) and its atmospheric component (AGCM). The 50-year integration of the CGCM well reproduces the summer monsoon variability over SCS, where the precipitation anomaly is positively correlated with the low-level cyclonic circulation anomaly that accompanies enhanced surface westerlies. Negative sea surface temperature (SST) anomaly is found in SCS during the strong monsoon years, indicating the atmospheric driving SST through wind-induced evaporation. The 50-year AGCM run forced by historical SST obtained from the CGCM reveals the monsoon variability amplified by about 50 percent as compared with the CGCM. The absence of the air-sea coupling keeps SST warm in SCS, which increases the local evaporation and precipitation. The enhanced precipitation over SCS may intensify surface westerly over the remote regions, resulting in an increase in the moisture flux convergence that in turn contributes to the positive precipitation anomaly. This result suggests that the air-sea coupling works to stabilize the monsoon and hence suppress the variability via the large-scale moisture transport and the wind-induced local evaporation.

Journal
J-STAGE
Authors
Watanabe, M., M. Chikira, Y. Imada, and M. Kimoto, 2011
Title
Convective control of ENSO simulated in MIROC. J. Climate, 24, 543-562.
Keywords
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Abstract

The high sensitivity of the El Niño-Southern Oscillation (ENSO) to cumulus convection is examined by means of a series of climate simulations using an updated version of the Model for Interdisciplinary Research on Climate (MIROC), called MIROC5. Given that the preindustrial control run using MIROC5 shows a realistic ENSO, the integration is repeated with four different values of the parameter, λ, which affects the efficiency of the entrainment rate in cumuli. The ENSO amplitude is found to be proportional to λ-1 and to vary from 0.6 to 1.6 K.A comparison of four experiments reveals the mechanisms for which the cumulus convections control behavior of ENSO in MIROC as follows. Efficient entrainment due to a large λ increases congestus clouds over the intertropical convergence zone (ITCZ) and reduces the vertical temperature gradient over the eastern Pacific, resulting in a wetter ITCZ and drier cold tongue via accelerated meridional circulation. The dry cold tongue then shifts the atmospheric responses to El Nino/La Nina westward, thereby reducing the effective Bjerknes feedback. The first half of these processes is identifiable in a companion set of atmosphere model experiments, but the difference in mean precipitation contrast is quite small. On one hand, the mean meridional precipitation contrast over the eastern Pacific is a relevant indicator of the ENSO amplitude in MIROC. On the other hand, the nonlinear feedback from ENSO affects the mean state, the latter therefore not regarded as a fundamental cause for different ENSO amplitudes.

Journal
AMS Journals Online
Authors
Kim, H.-J, K. Takata, B. Wang, M. Watanabe, M. Kimoto, T. Yokohata, and T. Yasunari, 2011
Title
Global monsoon, El Niño, and their interannual linkage simulated by MIROC5 and the CMIP3 CGCMs. J. climate, 24, doi:10.1175/2011JCLI4132.1.
Keywords
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Abstract

This study evaluates the capability of coupled global climate models (CGCMs) in simulating the prime examples of the forced response (global monsoon) and internal feedback process (El Niño). Emphases are also placed on the fidelity of the year-to-year variability of global monsoon precipitation that is coordinated by the interannual sea surface temperature (SST) fluctuation over the tropics. The latest version of Model for Interdisciplinary Research on Climate (MIROC5) with advanced physical schemes are compared with the two previous versions (MIROC3s) and with the twenty CGCMs participating in the third phase of the Coupled Model Intercomparison Project (CMIP3).The climatological annual mean and cycles of precipitation and 850-hPa winds, the key components to demarcate global monsoon domain, are reproduced better in MIROC5 than in MIROC3s. As a consequence, the former considerably outperforms the latter and is generally superior to the CMIP3 CGCMs in replicating the intensity and domain of global monsoon precipitation and circulations. These results highlight the importance of the improved physical parameterization in a model. Analyses of the monthly Niño 3 index suggest that the amplitude and periodicity of El Niño are simulated better in MIROC5 than in MIROC3s. Yet the reality of nonlinear ENSO dynamics measured indirectly by the SST asymmetricity over the equatorial Pacific is unsatisfactory in the MIROC family as well as in the majority of the CMIP3 models. The maximum covariance analysis shows that a significant fraction of the interannual global monsoon rainfall variability is in concert with El Niño. The multi-model results reveal that such coupling is robust across the current CGCMs. More importantly, the fidelity of the global monsoon precipitation significantly relies on the realism of the tropical SST. Comparison among the MIROC models suggests that improved El Niño is likely attributable to the more realistic Bjerknes feedback loop, which results from the intensified convective activity over the equatorial central Pacific.

Journal
AMS Journals Online
Authors
Imada, Y., M. Kimoto, and X. Chen, 2011
Title
Impact of atmospheric mean state on tropical instability wave activity. J. Climate, in press
Keywords
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Abstract

The features of simulated tropical instability waves (TIWs) in the Pacific Ocean are compared between atmospheric models of two different resolutions coupled with a uniform oceanic model. Our results show that TIWs are more active in the high-resolution model, even though it includes atmospheric negative feedback. Such negative feedback is not identified in the low-resolution atmospheric model because of the absence of atmospheric responses. Comparison of the energetics between two models shows that the large TIW activity in the higher-resolution model is due to the difference in barotropic energy sources near the surface. A high-resolution atmosphere results in a tighter intertropical convergence zone (ITCZ) and associated stronger wind curl and shear. This causes a stronger surface current shear between the South Equatorial Current (SEC) and North Equatorial Counter Current (NECC), which is one of the main sources of TIW kinetic energy. These results indicate the important role of the atmospheric mean field on TIW activity and the advantage of using high-resolution models to represent coupling among multi-scale phenomena.

Journal
AMS Journals Online
Authors
Imada, Y., and M. Kimoto, 2011
Title
Parameterization of tropical instability waves and examination of their impact on ENSO characteristics. J. Climate, in press
Keywords
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Abstract

The impact of tropical instability waves (TIWs) on El Nino/Southern Oscillation (ENSO) characteristics is investigated by introducing a new parameterization of TIWs into an atmosphere-ocean general circulation model (AOGCM), MIROC, with a medium resolution (~1.4°) ocean model. Because this resolution is not sufficient to reproduce eddies at the spatial scale of TIWs, this approach isolates TIW effects from other factors that can affect ENSO characteristics. The parameterization scheme represents the effect of baroclinic eddy heat transport by TIWs. A 100-year integration reveals a significant role of TIWs in observed ENSO asymmetry. Asymmetric heat transport associated with TIWs that are active (inactive) during La Nina (El Nino) generates a significant asymmetric negative feedback to ENSO and explains the observed asymmetric feature of a stronger amplitude El Nino and weaker amplitude La Nina. Furthermore, the parameterized eddy heat flux also affects the mean subsurface heat balance via the shallowing and steepening thermocline. This change in subsurface stratification induces a stronger thermocline feedback and a longer ENSO period.

Journal
AMS Journals Online