Hiroshi Koide

A simulation of the Last Glacial Maximum with a coupled atmosphere‐ocean GCM

A simulation of the Last Glacial Maximum (LGM) climate has been performed by a global coupled atmosphere-ocean general circulation model (AOGCM). Two simulations are conducted for the LGM with different initial conditions: one with the present-day initial condition and the other with a pre-conditioning of fresh water flux over the North Atlantic. After more than 200 year integration both LGM simulations attained a similar quasi-equilibrium state. The global mean surface air temperature dropped 3.9 ◦ C and precipitation decreased 11% compared to the present day simulation. The sea surface temperature dropped 1.7 ◦ C in the tropics, with larger decreases in the Atlantic than in the Indo-Pacific. There is a region with warmer than present sea surface temperatures over the subtropical eastern Pa- cific. The thermohaline circulation in the North Atlantic in the LGM simulation is slightly stronger than in the present day simulation.

Spatial and seasonal characteristics of recent decadal trends in the northern hemispheric troposphere and stratosphere

Linear trends in the northern hemispheric 500 hPa geopotential height, lower tropospheric temperature, and zonal-mean zonal winds are investigated for the period 1965–1993. There is a general opinion that decadal trends in the atmospheric circulation are produced by changes in the equatorial sea surface temperatures (SSTs). In order to determine other factors, a trend analysis is conducted in the present study to remove the variabilities related to the equatorial SSTs as “known factors” by means of a linear regression. The results of the analysis suggest that during the winter, decadal trends originate from two different sources of variability. Trends over Pacific Ocean are related to changes in the equatorial SST. The decadal trends over Eurasian-Atlantic sector, however, are related to the stratospheric polar-night jet and the meridional propagation of planetary waves, which can be considered internal modes of variability in the winter circulation. During the summer, trends in the subtropical lower-tropospheric temperature are explained as a direct response to changes in the equatorial SST. Trends over eastern Siberia, however, can not be explained in a similar manner. Instead, a relation to trends in the extent of spring snow cover is suggested. Accordingly, the possibility of producing long-term variations through interactions among the atmosphere and land surface processes is also discussed.

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.

Northern hemisphere winter circulation associated with the North Atlantic Oscillation and stratospheric polar‐night jet

The interannual variability of the atmospheric winter circulation as characterized by the geopotential height at 500 hPa and temperature at 850 hPa, associated with the North Atlantic Oscillation (NAO) and stratospheric polar night jet (PNJ) is examined. It is suggested that the wintertime atmospheric circulation related to the NAO index can be separated into two different modes of variability. One is a regional mode, characterized by a meridional dipole type pattern in the 500 hPa geopotential height anomaly field over the Atlantic region. The other, having more zonal structure, extends over wider region of the northern hemisphere; this is also related to the lower stratospheric polar night jet. Air temperature near the surface over the eastern part of the Eurasian continent is more closely correlated with the above-mentioned hemispheric mode of variability than the regional NAO.

SST Variability and Its Mechanism in a Coupled Atmosphere-Mixed Layer Ocean Model

Abstract Interannual SST variability in a coupled atmosphere–mixed layer ocean model is investigated. This model has no El Nino but shows a large interannual SST variability in the tropical Pacific. The basin-scale feature of SST variation has some common characteristics shared with that obtained by a global ocean–atmosphere coupled GCM and observational data in the subtropical to the midlatitude Pacific. Both the latent heat flux and shortwave radiation have their roles in producing the SST anomalies. There is no large contrast in the total heat flux between the eastern and the western Pacific. However, their main components, the shortwave radiation and the latent heat flux, have a remarkable contrast between the cold tongue in the east and the warm pool region in the west. In the east, the ocean is warmed by shortwave radiation and cooled by latent heat. This shortwave radiation is negatively correlated with low-level clouds. When the SST is warmer than normal in the eastern Pacific, there is less low-l...