Seiji Yukimoto

Future Changes in Tropical Cyclone Activity Projected by the New High-Resolution MRI-AGCM*

AbstractNew 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, t...

Climate Change Projections

This chapter assesses the effects of global warming on the climate of Japan. Japan is an archipelago extending in a southwest-northeast direction to the east of the Eurasian continent embracing the Japan Sea. High mountain ranges in the central part of the biggest island, Honshu, cause diverse climate conditions over Japan. The major features of Japan’s climate are the winter and summer monsoons, the two rainy seasons such as the Bai-u season (the early-summer rainy period) and the Shurin season (autumn rainy period) and the typhoons (tropical storms). Possible changes in these features due to global warming are of much concern. Our current knowledge is not so advanced as to make definite assessments on the changes in the above mentioned features or the regional-scale climate change over Japan due to global warming. In this chapter, however, we tried to review state-of-the-art studies as far as possible and to assess the potential effects of global warming on Japan’s climate, including qualitative estimates. The principal results of this chapter are summarized as follows.

Interannual variability in the stratospheric‐tropospheric circulation in a Coupled Ocean‐Atmosphere GCM

Relationships between the northern winter stratospheric and tropospheric circulations and the sea surface temperature (SST) simulated by a global coupled ocean-atmosphere general circulation model are investigated. Two modes are extracted for interannual variability of the zonal-mean zonal wind by an empirical orthogonal function analysis. The first mode is related with interannual variations of the stratospheric polar vortex. This mode has a significant correlation with the North Pacific SST through tropospheric circulation changes. The second mode is the variation of the tropospheric subtropical jet and is related with the simulated El Nino. Model results imply no strong relationship between El Nino and the polar vortex.

ENSO-like interdecadal variability in the Pacific Ocean as simulated in a coupled general circulation model

Spatial and temporal structures of interdecadal variability in the Pacific Ocean are investigated using results from an atmosphere-ocean coupled general circulation model (AOGCM). The model shows a basin-wide spatial pattern of the principal sea surface temperature (SST) variability similar to the observed one. Both interdecadal and interannual temporal structures of the SST variability agree well between the observation and the AOGCM. On the other hand, a slab ocean model coupled to the same atmospheric model as the AOGCM fails to simulate the observed temporal structure. Therefore the timescale of the coupled variability is associated with dynamical processes in the ocean. A distinct interdecadal mode of the coupled atmosphere-upper ocean temperature variability is found in the AOGCM, with a spatiotemporal structure coherent with the SST variability. The mode accompanies an El Nino-Southern Oscillation (ENSO)-like spatial pattern of SST and the surface wind and behaves like a delayed oscillator in ENSO. A wedge-shaped anomaly pattern of the upper thermocline temperature is formed in the eastern Pacific, and its northern subtropical signal propagates westward, enhanced by a subtropical wind forcing at the central basin. Arrival of the subtropical signal at the western Pacific around 20°N switches the anomaly of subsurface temperature in the equatorial region through anomalous oceanic heat transport along the western boundary. The travel time of the trans-Pacific signal in the subtropics appears to be responsible for the timescale of this mode. The AOGCM successfully simulated the second mode of SST with a major variation in the midlatitude North Pacific as in the observed SST. In the upper ocean heat content we found another distinct mode, which is characterized by a midlatitude-subtropics dipole pattern migrating around the North Pacific subtropical gyre. However, the associated SST variation of this mode shows a poor correspondence in the dominant interdecadal modes for the observed SST.

Future Change in Extratropical Cyclones Associated with Change in the Upper Troposphere

AbstractFuture changes in Northern Hemisphere wintertime storm activity as a consequence of global warming are investigated using the AGCM of Meteorological Research Institute (MRI-AGCM) with horizontal grid sizes of 60 and 20 km. A future (2075–99) climate experiment, in which the change in sea surface temperature (SST) derived from the Coupled Model Intercomparison Project phase 3 (CMIP3) multimodel ensemble mean is added to observed SST, is compared with a present-day (1979–2003) climate experiment. Results of three-member simulations using the 60-km model are presented. A single simulation using the 20-km model is also presented, showing that similar results are obtained.In the future climate experiment, the number of intense cyclones (sea level pressure below 980 hPa) shows a significant increase whereas the number of total cyclones shows a significant decrease, similar to the results obtained from the CMIP3 models themselves. The increase in intense cyclones is seen on the polar side and downstream ...

Kelvin and Rossby‐gravity wave packets in the lower stratosphere of some high‐top CMIP5 models

We analyze the stratospheric Kelvin and Rossby-gravity wave packets with periods of a few days in nine high-top (i.e., with stratosphere) models of the fifth Coupled Model Intercomparison Project (CMIP5). These models simulate realistic aspects of these waves and represent them better than the tropospheric convectively coupled waves analyzed in previous studies. There is nevertheless a large spread among the models, and those with a quasi-biennial oscillation (QBO) produce larger amplitude waves than the models without a QBO. For the Rossby-gravity waves this is explained by the fact that models without a QBO never have positive zonal mean zonal winds in the lower stratosphere, a situation that is favorable to the propagation of Rossby-gravity waves. For the Kelvin waves, larger amplitudes in the presence of a QBO is counter intuitive because Kelvin waves are expected to have larger amplitude when the zonal mean zonal wind is negative, and this is always satisfied in models without a QBO. We attribute the larger amplitude to the fact that models tuned to have a QBO require finer vertical resolution in the stratosphere. We also find that models with large precipitation variability tend to produce larger amplitude waves. However, the effect is not as pronounced as was found in previous studies. In fact, even models with weak precipitation variability still have quite realistic stratospheric waves, indicating either that (i) other sources can be significant or that (ii) the dynamical filtering mitigates the differences in the sources between models.

Evaluating CMIP5 ocean biogeochemistry and Southern Ocean carbon uptake using atmospheric potential oxygen: Present-day performance and future projection

Observed seasonal cycles in atmospheric potential oxygen (APO~O2 + 1.1 CO2) were used to evaluate eight ocean biogeochemistry models from the Coupled Model Intercomparison Project (CMIP5). Model APO seasonal cycles were computed from the CMIP5 air-sea O2 and CO2 fluxes and compared to observations at three Southern Hemisphere monitoring sites. Four of the models captured either the observed APO seasonal amplitude or phasing relatively well, while the other four did not. Many models had an unrealistic seasonal phasing or amplitude of the CO2 flux, which in turn influenced APO. By 2100 under RCP8.5, the models projected little change in the O2 component of APO but large changes in the seasonality of the CO2 component associatedwith ocean acidification. Themodels with poorer performance on present-day APO tended to project larger net carbon uptake in the Southern Ocean, both today and in 2100.

Evaluation of CMIP5 upper troposphere and lower stratosphere geopotential height with GPS radio occultation observations

We present a detailed comparison of geopotential height fields between the Coupled Model Inter-Comparison Project phase 5 (CMIP5) models and satellite observations from GPS radio occultation (RO). Our comparison focuses on the annual mean, seasonal cycle, and interannual variability of 200 hPa geopotential height in the years 2002–2008. Using a wide sample of atmosphere-only model runs (AMIP) from the CMIP5 archive, we find that most models agree well with the observations and weather reanalyses in the tropics in both the annual means and interannual variabilities. However, the agreement is poor over the extratropics with the largest model spreads in the high latitudes and the largest bias in the southern middle to high latitudes that persist all seasons. The models also show excessive seasonal variability over the Northern midlatitude land areas as well as the Southern Ocean but insufficient variability over the tropics and Antarctica. While the underlying causes for the model discrepancies require further analyses, this study demonstrates that global observations from GPS RO provide accurate benchmark-quality measurements in the upper troposphere and lower stratosphere through which biases in climate models as well as weather reanalyses can be identified.

Interannual Variability in Low Stratiform Cloud Amount over the Summertime North Pacific in Terms of Cloud Types

AbstractUsing long-term (1958–2008) ship-based cloud observations and reanalysis data, interannual variability in the low stratiform cloud (LSC) amount of stratocumulus (Sc), stratus (St), and sky-obscuring fog (FOG) is examined over the summertime North Pacific. The correlation between the LSC amount and the estimated inversion strength is positive but relatively weak, compared with the well-known linear relationship for their seasonal variabilities. This reflects the regional contrast: the correlations are stronger in the southeastern North Pacific (SE NP) and weaker in the northwestern North Pacific (NW NP). Regarding the LSC types, variations in Sc amount are large over the SE NP and correlated with the inferred capping inversion strength. Variations in FOG amount are large over the NW NP and correlated with the inferred surface-based inversion strength. The compensating variations between the Sc and FOG amounts result in an apparent small variation in the total LSC amount in this region. Variations i...