Comparison of local precipitation-SST relationship between the observation and a reanalysis dataset.

Arakawa, O. and A. Kitoh (2004)

Geophys. Res. Lett., 31, L12206, doi:10.1029/2004GL020283.

Abstract.

We defined a grid-scale time-phase relationship between precipitation (PR) and underlying sea surface temperature (SST) as the local PR-SST relationship, and compared the spatial distribution of the relationship in the observation with that in a reanalysis dataset. An analysis using observed pentad mean data shows precipitation lagging SST for two pentads over large areas of tropical oceans. A reanalysis dataset, on the other hand, has a shorter time-phase difference between precipitation and SST than that in the observation and does not match the spatial distribution of the observed local PR-SST relationship over the tropical Indian Ocean and the tropical western Pacific. A reanalysis dataset may not capture the observed local PR-SST relationship in terms of either time-phase difference or spatial distribution because a numerical model in a data assimilation system uses SST as a lower boundary condition as well as the inhomogeneity of observations the system used.

Effects of mountain uplift on East Asian summer climate investigated by a coupled atmosphere-ocean GCM.

Kitoh, A. (2004)

J. Climate, 17, 783-802.

Abstract.

To study the effects of progressive mountain uplift on East Asian summer climate, a series of coupled general circulation model (CGCM) experiments were performed. Eight different mountain heights were used: 0% (no mountain), 20%, 40%, 60%, 80%, 100% (control run), 120%, and 140%. The land-sea distribution is the same for all experiments and mountain heights are varied uniformly over the entire globe.
Systematic changes in precipitation pattern and circulation fields as well as sea surface temperature (SST) appeared with progressive mountain uplift. In summertime, precipitation area moves inland on the Asian continent with mountain uplift, while the Pacific subtropical anticyclone and associated trade winds become stronger. The mountain uplift resulted in an SST increase over the western tropical Pacific and the Maritime Continent and an SST decrease over the western Indian Ocean and the central subtropical Pacific. There is a drastic change in the East Asian circulations with the threshold value at the 60% mountain height. With the mountain height below 60%, the southwesterly monsoon flow from the Indian Ocean becomes strong by uplift and transports moisture toward East Asia, forming the baiu rainband. With higher mountain heights, intensified subtropical trade winds transport moisture from the Pacific into the Asian continent.
In order to investigate how the SST change affected the results presented herein, additional experiments were performed with the same experimental design but with the atmospheric GCM (AGCM). A comparison between CGCM and AGCM experiments revealed that major features such as a shift in precipitation inland and an appearance of the baiu rainband by higher orography were reproduced similarly in both the AGCM and the CGCM. However, there was a qualitatively as well as quantitatively different feature. The anticyclonic circulation anomalies in the lower troposphere, which appeared by mountain uplift in the tropical western Pacific in the CGCM associated with lowered SST, fed more moisture over East Asia and resulted in a stronger baiu rainband in the CGCM than that in the AGCM. An extent of the monsoon westerly flow is regulated by competition between the Pacific subtropical anticyclone and the southwest monsoon. The confluence zone was located near the Philippines throughout the mountain uplift in the AGCM, but it shifted backward to the west via mountain uplift in the CGCM associated with simulated SST changes. Overall the CGCM showed a larger sensitivity to mountain uplift than the AGCM due to the SST changes, thus warranting an examination of the importance of air-sea coupling and a need for the use of coupled models for such sensitivity studies.

Figure

June mean precipitation (left) and SST (right) for the observations and for eight CGCM experiments with different mountain heights.

South and East Asian summer monsoon climate and variation in MRI coupled model (MRI-CGCM2).

Rajendran, K., A. Kitoh and S. Yukimoto (2004)

J. Climate, 17, 763-782.

Abstract.

Simulations of the major characteristics of the summer monsoon climate over South Asia, East Asia, and the western North Pacific by the new version of the Meteorological Research Institute coupled GCM (MRI-CGCM2) are analyzed. In addition to assessing the simulation of mean summer monsoon rainfall and its association with SST and basic circulation parameters, the model's performance in reproducing the seasonal variation, climatological onset, peak, withdrawal, and subseasonal variation of the monsoon is also studied.
The mean rainfall distribution and the maximum rainfall centers over South Asia and the western North Pacific, including the monsoon rainbelt over central India, though shifted southward of its observed position by about 5‹, and the baiu rainband across Japan, are well simulated. However, the model underestimates the monsoon rainfall over southeast China. The biases in model mean rainfall are found to be associated with biases in wind, moisture convergence, and vertical stability of the atmosphere. The relationship between mean SST and organized convection in the model is close to the observations, with a propensity for deep convection increasing with an increase in SST above the threshold value. The simulated summer anomalies of SST and surface fluxes imply the dominance of the SST-wind-evaporation feedback system over most of the Indian and western Pacific Oceans on intraseasonal time scales. In addition, in the model, equatorial eastern Pacific SST anomalies strongly influence the Indian summer monsoon predominantly on biennial time scales. The model captures the basic monsoon seasonal cycle, onset, peak, and withdrawal over India and the western North Pacific across Japan. However, the simulation shows an early onset of the monsoon over mainland China and the early occurrence of peak rainfall over southeast China. The active-break spells in Indian monsoon rainfall and meridional propagations of rainbelts over India and the western North Pacific are realistically simulated. But, over southeastern China, the model is unable to simulate northward propagation of rainbelts, which contributes to the poor simulation of seasonal mean rainfall.

Figure

Observed (Xie-Arkin) and simulated (MRI-CGCM2) onset dates of the rainy season based on the relative climatological pentad mean rainfall.

Monsoon low-frequency intraseasonal oscillation and ocean-atmosphere coupling over the Indian Ocean.

Rajendran, K., A. Kitoh and O. Arakawa(2004)

Geophys. Res. Lett., 31, L02210, doi:10.1029/2003GL019031.

Abstract.

The impact of coupled interaction over Indian Ocean on the monsoon low-frequency intraseasonal oscillation is investigated by analysing the simulations of MRI coupled general circulation model (CGCM) and its stand alone atmospheric version (AGCM). Composites of northward propagations constructed based on objective criteria show that the characteristics of CGCM simulated low-frequency intraseasonal oscillation are close to observation. In CGCM, the atmosphere impacts SST through wind, evaporation, and radiation anomalies and the ocean modulates atmospheric convection through intraseasonal SST fluctuations. But, in AGCM, SST acts only as a boundary forcing without any significant contribution from atmosphere in modulating SST. As a result, the simulated low frequency oscillation lacks many of the important features associated with its amplitude, phase and life-cycle. These results warrant the use of coupled GCMs for simulating and studying monsoon lowfrequency oscillation.

Effects of Large-Scale Mountains on Surface Climate - A Coupled Ocean-Atmosphere General Circulation Model Study

Kitoh, A. (2002)

J. Meteor. Soc. Japan, 80, 1165-1181.

Abstract.

Effect of mountain uplift on climate is investigated by a global coupled ocean-atmosphere general circulation model with an emphasis on surface temperature changes. Results of the no-mountain run (NM) are compared with those of the control run with the present-day orography (M). When the lapse-rate effect is eliminated, continent interior becomes warmer with mountain uplift because clouds become fewer and the surface drier due to decreased moisture transport. On the other hand, South Asia and East Asia become cooler because summer monsoon precipitation is stronger, which makes the land surface wetter and increases clouds. Over the ocean, the existence of orography has a role to reduce sea surface temperatures (SST), particularly over the subtropical eastern oceans. This occurs because evaporation is larger due to stronger trade winds and also less solar radiation reaches the surface due to more low-level clouds, both associated with stronger subtropical anticyclones in M. The subtropical gyre is stronger in M than in NM, and therefore, the Kuroshio Current is stronger in M. When the effect of the ocean general circulation is not included, the SST over the western north Pacific becomes much lower in M than in NM because of stronger cold air outbreak from Siberia in winter in M. Thus, the ocean circulation changes act to reduce the SST changes by heat transport.

Tropical Pacific climate at the Mid-Holocene and the Last Glacial Maximum simulated by a coupled ocean-atmosphere GCM.

Kitoh, A. and S. Murakami(2002)

Paleoceangraphy, 17(3), 1047, doi:10.1029/2001PA000724.

Abstract.

Simulations for the mid-Holocene (6,000 years before present: 6ka) and the last glacial maximum (LGM: 21ka) have been performed by a global ocean-atmosphere coupled GCM. After the initial spin-up periods, both runs were integrated for about 200 years. For 6ka, the model shows an enhanced seasonal variation in surface temperature and a northward shift of the African and the Indian summer monsoon rain area. Overall circulation features in the tropics correspond to a strong Walker circulation state with negative sea surface temperature (SST) and precipitation anomalies in the central Pacific, and positive precipitation anomalies over the Indian and Australian monsoon regions. It is noted that there is about a 0.35‹C cooling of the global mean SST. In contrast to the 6ka result, the simulated tropical climate anomaly at 21ka corresponds to a weak Walker circulation state. The simulated LGM SST decrease is about 2‹C over the tropical western Pacific. It is larger (about 5‹C) over the Caribbean Sea. This SST anomaly is in broad agreement with the observed proxy data. Interestingly, the model simulates a warmer than present SST over the subtropical Pacific. This may be related to a weaker subtropical anticyclone due to weakened monsoon circulation at the LGM.

Figure

Annual mean anomalies for (left) 6ka and (right) 21ka. (a) SST and ground temperature, (b) precipitation, and (c) surface wind.

Effect of Orography on Land and Ocean Surface Temperature

A. Kitoh (2001)

Present and Future of Modeling Global Environmental Change: Toward Integrated Modeling, Eds., T. Matsuno and H. Kida, TERRAPUB, pp. 427-431

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

Kitoh, A., S. Murakami and H. Koide (2001)

Geophys. Res. Lett., 28, 2221-2224.

Abstract.

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 Pacific. The thermohaline circulation in the North Atlantic in the LGM simulation is slightly stronger than in the present day simulation.

Figure

Differences between the LGM and the present-day simulation.

The Eddy-mean Flow Interaction and the Tropical-extratropical Interaction in a GCM and their Dependence on Model Horizontal Resolution

Moon J. -K., K. -J. Ha, A. Kitoh (1999)

Kor. J. Atmos. Sci., Korea , Vol. 2, 15-28

Abstract.

The eddy-mean flow interaction and the tropical-extratropical interaction are investigated by analyzing GCM simulations with different horizontal resolutions of 2 x 2.5 (high), 3 x 3.3 (medium), and 4 x 5 (low) latitude/longitude. The northern hemispheric winter circulation of simulation is more in agreement with observation when the horizontal resolution increases. The synoptic storm tracks in mid-latitudes are better simulated for a high resolution case than the lower resolution ones. The upper-level Rossby wave source term are compared to show the tropical- extratropical teleconnection in barotropical sense. The largest positive forcing of tropical heating is located at the exit region of the Asian Jet in all resolutions. But, the forcings for the high and medium resolution models have stronger intensity than the observation and seem to be associated with a strong vorticity gradient. In this study, the transient eddy forcing is estimated from the E-vector formulation, similar to an extended Eliassen-Palm (EP) flux. It is found that the eddy growth due to the strong zonal component of E-vector occurs over the exit region of the East Asian Jet (EAJ) and the North Atlantic Jet (NAJ). From the analysis of the growth of kinetic energy associated with the eddies during northern hemisphere winter (DJF season) at 200hPa, it is found that the low frequency fluctuations obtain their energy from the barotropic instability of the mean flow in the exit of EAJ and the entrance of NAJ and the Arabian Jet. As compared to the observation, the magnitude and the location of maximum values in each resolution model seem to have some biases. These results suggest that the interaction between the mean and the transient eddies is not much affected by the increase of model horizontal resolution.

Tropospheric Biennial Oscillation of ENSO-Monsoon System in the MRI Coupled GCM

Ogasawara, N., A. Kitoh, T. Yasunari and A. Noda (1999)

J. Meteor. Soc. Japan, 77, 1247-1270.

Abstract.

The mechanism of the tropospheric biennial oscillation (TBO) of the South Asian monsoon system is investigated by an MRI coupled atmosphere-ocean general circulation model. The biennial variability of the South Asian monsoon affects the global scale climate variability through interactions with the air-sea coupled system over the Pacific and/or the extratropical circulation. In the strong phase of the TBO, the area of relatively strong monsoon convective maximum over South Asia in the spring to summer season moves southeastward to Indonesia in the autumn to winter season. This movement superimposes on its climatological seasonal cycle. It suggests that the northern winter monsoon convection tends to be strong around Indonesia to northern Australia when the summer Asian monsoon is strong. In the Pacific sector, anomalous state of the air-sea coupled system which forms in the summer season seems to dissipate from its eastern edge by local atmosphere-ocean coupling process through a large scale Walker circulation. The convection anomalies persist during the entire monsoon season over Indonesia and northern Australia. As a response to this equatorial monsoon convection anomaly, a Matsuno-Gill type stationary Rossby wave is established over the South Asian region. The appearance of upper level anticyclonic circulation and lower level cyclonic circulation anomaly in strong monsoon year is a favorable condition for bringing the cold air advection over the Eurasian continent. Cold air advection after the strong monsoon persists through the whole winter to spring season, to form the cold tropospheric temperature around the Central to South Asia. Then reduced land-sea or north-south temperature contrast sets up the following weak South Asian summer monsoon.The simulated TBO of the South Asian monsoon is tightly phase locked with a seasonal cycle. The phase of the TBO changes in northern spring, which suggests that the extratropical-tropical interaction is realized mainly during winter to spring through the onset of South Asian monsoon. Our results imply that the TBO is an inherent feature in land-monsoon-ocean coupled system and emphasize a more active role of monsoon-extratropical interaction in the Indian sector in winter to spring season for regulating the TBO cycle.

Figure

Schematic of the TBO in the MRI Coupled GCM.

ENSO-Monsoon Relationship in the MRI Coupled GCM.

Kitoh, A., S. Yukimoto and A. Noda (1999)

J. Meteor. Soc. Japan, 77, 1221-1245.

Abstract.

Climatological features and interannual variability of the Asian summer monsoon and its relationship with equatorial Pacific sea surface temperature (SST) anomalies simulated by a global coupled atmosphere-ocean GCM (CGCM) is investigated. The coupled model results are compared with the observation as well as the simulations by an atmospheric general circulation model (AGCM). Overall features of the model climatology and variability of the Asian summer monsoon in the CGCM is as close to the observed one as in the AGCM, although the simulated SST and its variability in the CGCM shows some bias compared to the observation. The monsoon region in the models as defined by the seasonal change of wind direction and convection proxy agrees with the observed. The models are less successful in simulating the summertime wind system in the western Pacific region. The CGCM reproduces reasonably well the observed ENSO-related interannual variability of the tropical circulation system. Its main deficit is associated with a westward displacement of simulated SST variability and an underestimation of precipitation around the Philippines. Differences are found between the CGCM and the AGCM in the variability over the Indian monsoon region. The AGCM responds well to the prescribed SST anomaly in the Pacific, but behaves erroneously over the Indian Ocean. This may be related to the fact that the AGCM is only responding to the prescribed SST fields, while the CGCM includes two-way atmosphere-ocean interactions. The CGCM results show that the simulated Indian summer monsoon rainfall anomalies are negatively correlated with the equatorial Pacific SST anomalies, being consistent with the observed one, i.e., good monsoon is associated with La Nina. As a precursory signal, ground temperature is significantly warmer in central Asia in spring preceding a good monsoon. It is noted that the snow cover anomalies are negative in the above region but its significance is marginal.

Figure

The leading multiple EOF mode for the simulated SST, precipitation and zonal wind at 850 hPa based on the June-August average data of the 150-year CGCM run.

On overestimation of tropical precipitation by an atmospheric general circulation model with prescribed SST

Kitoh, A. and O. Arakawa (1999)

Geophys. Res. Lett., 26, 2965-2968.

Abstract.

We investigate how an atmospheric general circulation model (AGCM) can reproduce the atmospheric aspects of climate, when the boundary condition are prescribed. The AGCM is integrated with sea surface temperature (SST) specified from an integration of a coupled general circulation model (CGCM). A systematic difference in the mean climate is found between the AGCM and the CGCM in the tropical Pacific. This difference is apparent when and where the local SST is warmest. In the CGCM, precipitation does not last long due to SST and cloud-radiation feedback. Precipitation in the AGCM, however, tends to persist longer. Results of this experiment imply that caution is needed when using an AGCM with boundary conditions derived from a CGCM for climate change studies.

Figure

Above: Hovmoller diagrams of (a) SST, (b) precipitation, (c) evaporation, (d) total heat flux, and (e) wind stress magnitude simulated by CGCM averaged for 14S-6S from October of yr4 to March of yr5. Bold solid lines denote the region where zonal gradient of SST is 0.1C per 1 longitude.

Below: Hovmoller diagrams of (a) precipitation, (b) evaporation, (c) total heat flux, and (d) wind stress magnitude from ensemble means simulated by AGCM. The AGCM is forced by SST obtained from CGCM.

Effect of Horizontal Resolution on the Simulation of Asian Summer Monsoon using the MRI GCM-II

Chandrasekar, A., D. V. Bhaskar Rao and Akio Kitoh (1999)

Pap. Met. Geophys., 50, 65-80.

Abstract.

The MRI atmospheric general circulation model has been used to study the effect of horizontal resolution on the simulation of Asian summer monsoon. The model has been integrated for an eight year period with three different horizontal resolutions designated as Low (4 lat X 5 long), Medium (3 lat X 3.3 long) and High (2 lat X 2.5 long), respectively. The results indicate that the increase of horizontal resolution improved the simulation of mean sea level pressure and precipitation patterns. The high resolution model simulates the heat low and monsoon trough in agreement with the observations while the low resolution model simulates an abnormal heat low over north India. The high resolution model simulates the precipitation maxima over the Bay of Bengal and the south Indian ocean and a dry region over Tarim basin better while the low resolution model better simulates the precipitation maximum over the Arabian sea. All the models simulate weaker monsoon circulation while the increase of horizontal resolution leads to further weakening of low level monsoon westerlies. The results of this study conclude that increase of horizontal resolution improved the simulation of many observed features of the Asian summer monsoon circulation but certain features deteriorated indicating that the model parameterizations may need to be suitably modified.

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

Kitoh, A., T. Motoi and H. Koide (1999)

J. Climate, 12, 1221-1239.

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 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-level stratus cloud cover and more shortwave radiation reaching the surface. In the western Pacific, the ocean is warmed by less evaporation due to weaker winds. When the ocean becomes warm, it is cooled by less shortwave radiation due to stronger activity in cumulus convection.

Figure

(a) Spatial pattern of the first EOF base don the 60-yr annual mean SST of the coupled atmosphere-mixed layer ocean model. Correlation coefficient of local SST on the time series of the first EOF are plotted. (b) Time series of the first EOF of SST.

Impact of Localized Sea Surface Temperature Anomalies over the Equatorial Indian Ocean on the Indian Summer Monsoon

Chandrasekar, A. and A. Kitoh (1998)

J. Meteor. Soc. Japan, 76, 841-853.

Abstract.

Observations indicate two favorable locations for the Tropical Convergence Zone (TCZ) for the Indian summer monsoon, one over the continent and the other over the equatorial Indian Ocean. An active spell of one TCZ coincides with a weak spell of the other TCZ. Observations also show the presence of positive sea surface temperature (SST) anomalies south of the equator over the Indian Ocean during the weak Indian summer monsoon years. The impact of such SST anomalies on the Indian summer monsoon is investigated through general circulation model ensemble experiments. The results indicate significant response over the Indian region and the same is manifested as a decrease in the monsoon precipitation and the weakening of the mean monsoon circulation. A series of identical experiments with a negative SST anomaly prescribed over the same region with the same magnitude confirm the above findings.

Simulated Changes in the Asian Summer Monsoon at Times of Increased Atmospheric CO2

Kitoh, A., S. Yukimoto, A. Noda and T. Motoi (1997)

J. Meteor. Soc. Japan, 75, 1019-1031.

Abstract.

Possible changes in the Asian summer monsoon due to increased atmospheric CO2 are investigated by an MRI global coupled atmosphere-ocean general circulation model. The summer (June-August) monsoon rainfall in India increases significantly with the global warming. On the other hand, the monsoon wind shear index, defined as the difference between 850 hPa and 200 hPa zonal winds over the northern Indian Ocean, decreases. At 850 hPa, the westerly wind shifts northward and intensifies from Sahel to northwest of India, but the monsoon westerly over the Arabian Sea weakens. It is found that increased moisture content in the warmer air leads to larger moisture flux convergence, contributing to the increased rainfall. Therefore, the monsoon wind shear index is not a good indicator for identifying the change of monsoon accompanied by the global warming. In contrast to the increased rainfall in India, changes in rainfall is little over China where soil moisture becomes drier at times of increased CO2. It is also noted that the northern Eurasian continent becomes wetter in the increased CO2 climate. Magnitude of the interannual variability of the Asian summer monsoon rainfall becomes larger in the CO2 experiment than in the control experiment, particularly in the later stage of the experiment after CO2 doubling. However it should be noted that the decadal variation of this interannual variability is also large both in the control and the CO2 experiments.

Investigation of the role of the Tibetan plateau by climate model

Kitoh, A. (1997)

Journal of Geography, 106(2) 270-279 (in Japanese with English abstract)

Abstract.

Effects of mountains on climate are investigated by an atmospheric general circulation model coupled with a 50-m depth slab ocean model. A model simulation with the present-day orography reproduces most of the principal features of the observed climate. Another simulation without mountains (NM) is done and is compared with the one with mountains (M). Orography-originated stationary waves are separated from those induced by the existence of land-sea distribution. In northern winter, about two thirds of the stationary wave is explained by orography in this study. The wintertime subtropical jet in the upper troposphere in NM is more zonal and weaker than in M. The summertime subtropical jet in NM does not shift northward much and is located over China and Japan. The Asian summer monsoon in NM is substantially weaker than in M. Surface monsoon westerlies are weak, a rain belt stays south and the precipitation over India and China is less in the model without orography. On the other hand, the precipitation in the continental interior is large and the ground is wet in NM due to large moisture transport from the ocean. The global mean sea surface temperature in NM is warmer than in M. This occurs mainly from increased solar radiation into the ocean due to decreased lower tropospheric clouds over the subtropical eastern Pacific. Land surface temperature in NM is warmer than in M due to lapse-rate effect. However, the summertime surface temperature in Europe, Russia and Canada is cooler in NM than in M because ground surface in NM is wetter.

Mountain uplift and surface temperature changes

Kitoh, A. (1997)

Geophys. Res. Lett., 24, 185-188.

Abstract.

Mountain uplift significantly affects both sea surface temperature (SST) and land surface temperature. This was studied using a coupled atmosphere/mixed layer ocean model with and without mountains. The global mean SST dropped 1.4ĄC with mountain uplift, mainly due to increased lower tropospheric clouds in the subtropical eastern Pacific. Increased clouds hinder solar radiation into the ocean and lower SST. The increased frequency of stratus incidence is related to subtropical anticyclones intensified by a strong temperature contrast between the continent and ocean in mountainous regions. Land surface temperature drops due to the lapse-rate effect. When this effect is eliminated, the continental interior does not become as cool with mountain uplift because clouds become fewer and the surface drier due to decreased moisture transport. Southern Asia becomes cooler because monsoon-induced precipitation wets the ground and increases clouds. Our result showed greater northern high latitude temperature changes than the previous study, indicating the importance of cloud-related feedback in paleoclimate modeling.

Figure

(a) Annually averaged surface temperature difference (deg C) between mountain and no-mountain runs. (b) As for (a) except for that temperature for the mountain run is adjusted by 6.5 K/km, the environmental lapse rate.

Interannual variability in the stratospheric-tropospheric circulation in a coupled ocean-atmosphere GCM

Kitoh, A., H. Koide, K. Kodera, S. Yukimoto, and A. Noda (1996)

Geophys. Res. Lett., 23, 543-546.

Abstract.

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.

Figure

Simulated correlations between the time coefficients of (top) first and (bottom) second EOF modes of zonally averaged zonal wind and (left) 50 hPa height, (middle) 500 hPa height and (right) SST in DJF. Data for 70 winters are used. Contour intervals: 10 %.