A Flexible Coupled Ocean-Atmosphere General Circulation Model

Based on the National Center for Atmospheric Research (NCAR) Climate System Model version 1 (CSM-1), a Flexible coupled General Circulation Model version 0 (FGCM-0) is developed in this study through replacing CSM-1’s oceanic component model with IAP L30T63 global oceanic general circulation model and some necessary modifications of the other component models. After the coupled model FGCM-0 is spun up for dozens of years, it has been run for 60 years without flux correction. The model does not only show the reasonable long-term mean climatology, but also reproduce a lot of features of the interannual variability of climate, e.g. the ENSO-like events in the tropical Pacific Ocean and the dipole mode pattern in the tropical Indian Ocean. Comparing FGCM-0 with the NCAR CSM-1, some common features are found, e.g. the overestimation of sea ice in the North Pacific and the simulated double ITCZ etc. The further analyses suggest that they may be attributed to errors in the atmospheric model.

Long-term Behaviors of Two Versions of FGOALS2 in Preindustrial Control Simulations with Implications for 20th Century Simulations

Climate drift in preindustrial control （PICTL） simulations can lead to spurious climate trends and large uncertainties in historical and future climate simulations in coupled models. This study examined the long- term behaviors and stabilities of the PICTL simulations in the two versions of FGOALS2 （the Flexible Global Ocean-Atmosphere-Land System model Version 2）, which have been submitted to the Coupled Model Inter- comparison Project Phase 5 （CMIP5）. As verified by examining time series of thermal fields and their linear trends, the PICTL simulations showed stable long-term integration behaviors and no obvious climate drift [the magnitudes of linear trends of SST were both less than 0.04℃ （100 yr）-1] over multiple centuries. The changed SSTs in a century （that corresponded to the linear trends） were less than the standard deviations of annual mean values, which implied the internal variability was not affected. These trend values were less than 10~0 of those of global averaged SST from observations and historical runs during the periods of slow and rapid warming. Such stable long-term integration behaviors reduced the uncertainty of the estimation of global warming rates in the historical and future climate projections in the two versions of FGOALS2. Compared with the trends in the Northern Hemisphere, larger trends existed in the SST and sea ice extents at the middle to high latitudes of the Southern Hemisphere （SH）. To estimate the historical and future climate trends in the SH or at some specific regions in FGOALS2, corrections needed to be carried out. The similar long-term behaviors in the two versions of FGOALS2 may be attributed to proper physical processes in the ocean model.

Long-term surface air temperature trend and the possible impact on historical warming in CMIP5 models

Whether large trends exist in pre-industrial control （PICTL） runs is critically important for evaluating simulations of present climate change. This study examined the long-term trends in PICTL surface air temperature （SAT） in CMIP5 models. Small trends （〈0.06 ℃/100 year） in the globally averaged SAT （GASAT） exist in most CMIP5 models. Of these, positive （negative） trends result from positive （negative） net radiation fluxes at the TOA. This conclusion was further confirmed by the significant positive correlations between the TOA and the SAT tendency. The PICTL GASAT trends constitute less than 10% of the historical trends, indicating that such trends are of negligible importance in estimates of historical global warming in most models. Spatially, relative to the historical trends, the PICTL trends comprise a nontrivial fraction （〉20%） in the Southern Ocean between 50°S and 65°S and in the northern Atlantic and Pacific oceans north of 40°N, with large inter-model spread. The long-term behavior of SAT is significantly related to ocean circulation adjustment in the mid-high latitudes.

Reducing the climate shift in a new coupled model

Climate drift refers to spurious long-term changes that may be inherent in coupled models when external forcing factors are fixed. Understanding the sources of this drift and tuning the drift are crucial for obtaining reasonable simulations from coupled models. To prepare for the upcoming Coupled Model Intercomparison Project Phase 6, a new coupled model has been constructed based on the Community Earth System Model and the Grid-point Atmospheric Model of IAP LASG version 2. However, the surface temperature predicted by the new model is too underestimated, and this underestimation is caused by a type of climate drift, i.e., ‘‘initial shock.'' This study analyzes the source of the cold surface temperature from the perspective of energy balance and attempts to reduce the surface temperature drift by tuning the relative humidity threshold for low cloud.