A new version of the atmospheric general circulation model of the Meteorological Research Institute (MRI), with a horizontal grid size of about 20 km, has been developed. The previous version of the 20-km model, MRI-AGCM3.1, which was developed from an operational numerical weather-prediction model, provided information on possible climate change induced by global warming, including future changes in tropical cyclones, the East Asian monsoon, extreme events, and blockings. For the new version, MRI-AGCM3.2, we have introduced various new parameterization schemes that improve the model climate. Using the new model, we performed a present-day climate experiment using observed sea surface temperature. The model shows improvements in simulating heavy monthly-mean precipitation around the tropical Western Pacific, the global distribution of tropical cyclones, the seasonal march of East Asian summer monsoon, and blockings in the Pacific. Improvements in the model climatologies were confirmed numerically using skill scores (e.g., Taylor's skill score).

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.

We document the development and simulation characteristics of the next generation modeling system for seasonal to decadal prediction and projection at the Geophysical Fluid Dynamics Laboratory (GFDL). SPEAR (Seamless System for Prediction and EArth System Research) is built from component models recently developed at GFDL—the AM4 atmosphere model, MOM6 ocean code, LM4 land model, and SIS2 sea ice model. The SPEAR models are specifically designed with attributes needed for a prediction model for seasonal to decadal time scales, including the ability to run large ensembles of simulations with available computational resources. For computational speed SPEAR uses a coarse ocean resolution of approximately 1.0° (with tropical refinement). SPEAR can use differing atmospheric horizontal resolutions ranging from 1° to 0.25°. The higher atmospheric resolution facilitates improved simulation of regional climate and extremes. SPEAR is built from the same components as the GFDL CM4 and ESM4 models but with design choices geared toward seasonal to multidecadal physical climate prediction and projection. We document simulation characteristics for the time mean climate, aspects of internal variability, and the response to both idealized and realistic radiative forcing change. We describe in greater detail one focus of the model development process that was motivated by the importance of the Southern Ocean to the global climate system. We present sensitivity tests that document the influence of the Antarctic surface heat budget on Southern Ocean ventilation and deep global ocean circulation. These findings were also useful in the development processes for the GFDL CM4 and ESM4 models.

Owing to the limited length of observed tropical cyclone data and the effects of multidecadal internal variability, it has been a challenge to detect trends in tropical cyclone activity on a global scale. However, there is a distinct spatial pattern of the trends in tropical cyclone frequency of occurrence on a global scale since 1980, with substantial decreases in the southern Indian Ocean and western North Pacific and increases in the North Atlantic and central Pacific. Here, using a suite of high-resolution dynamical model experiments, we show that the observed spatial pattern of trends is very unlikely to be explained entirely by underlying multidecadal internal variability; rather, external forcing such as greenhouse gases, aerosols, and volcanic eruptions likely played an important role. This study demonstrates that a climatic change in terms of the global spatial distribution of tropical cyclones has already emerged in observations and may in part be attributable to the increase in greenhouse gas emissions.

Possible future change in tropical cyclone (TC) activity over the North Atlantic (NA) was investigated by comparison of 25-yr simulations of the present-day climate and future change under the A1B emission scenario using a 20-km-mesh Meteorological Research Institute (MRI) and Japan Meteorological Agency (JMA) atmospheric general circulation model. The present-day simulation reproduces many essential features of observed climatology and interannual variability in TC frequency of occurrence and tracks over the NA. For the future projection, the model is driven by the sea surface temperature (SST) that includes a trend projected by the most recent Intergovernmental Panel on Climate Change (IPCC) multimodel ensemble and a year-to-year variation derived from the present-day climate. A major finding is that the future change of total TC counts in the NA is statistically insignificant, but the frequency of TC occurrence will decrease in the tropical western NA (WNA) and increase in the tropical eastern NA (ENA) and northwestern NA (NWNA). The projected change in TC tracks suggests a reduced probability of TC landfall over the southeastern United States, and an increased influence of TCs on the northeastern United States. The track changes are not due to changes of large-scale steering flows; instead, they are due to changes in TC genesis locations. The increase in TC genesis in the ENA arises from increasing background ascending motion and convective available potential energy. In contrast, the reduced TC genesis in the WNA is attributed to decreases in midtropospheric relative humidity and ascending motion caused by remotely forced anomalous descent. This finding indicates that the impact of remote dynamical forcing is greater than that of local thermodynamical forcing in the WNA. The increased frequency of TC occurrence in the NWNA is attributed to reduced vertical wind shear and the pronounced local warming of the ocean surface. These TC changes appear to be most sensitive to future change in the spatial distribution of rising SST. Given that most IPCC models project a larger increase in SST in the ENA than in the WNA, the projected eastward shift in TC genesis is likely to be robust.