Using the Model for Interdisciplinary Research on Climateversion 3.2 (MIROC 3.2), we investigated the physicalnature of regional sea level changes due to enhanced greenhousewarming. The regional sea level changes were notspatially uniform, and their patterns were principally determinedby the baroclinic component (density change) dueto surface fluxes of heat, freshwater, and wind stress. Sealevel changes in the barotropic circulation were mainly confinedto the Southern Ocean. We decomposed the baroclinicresponse into vertical modes of ocean climatological stratification,considering the vertical structure of the baroclinicpressure change. The first baroclinic mode was responsiblefor about 78% of the variance in the baroclinic response,suggesting that the regional distribution of sea level changeunder global warming is mainly determined by displacementof the main pycnocline. The second and third modes wereresponsible for about 12% and 4% of the variance, respectively,some of which was related to subduction of theglobal warming signal. The decomposition of the baroclinicresponse mentioned above is suggestive of sea level changesdue to global warming as results of region‐by‐region physicalprocesses.
Using a gridded ocean temperature and salinity fieldbased on observations, the regional distribution of sea levelchanges during the last few decades was investigated. Wecalculated the baroclinic sea level change and decomposedit into vertical modes of pressure perturbation for internalwaves, considering the vertical structure of the baroclinicpressure change. The first mode is associated with verticaldisplacement of the main pycnocline, which is generally adynamical response to wind forcing, as demonstrated byprevious studies using shallow water models. Regional sealevel variations associated with this mode have largemagnitudes, especially in the tropics, where, as shown byprevious studies, interannual variability is relatively largecompared to sea level changes. Other factors affecting sealevel include changes in water mass density and horizontalmovement of water masses. These are characterized assecond and higher modes, and their combined effect on theamplitude of regional sea level changes is comparable withthat of the first mode, especially in subtropical gyre regions.In the North Pacific, the combined second and higher ordermodes give rise to significant positive sea level trends.These long‐term sea level trends are induced by stericcontributions resulting from warming and freshening of thesubtropical mode waters. This result suggests that changesin water mass properties such as temperature and salinityare also important for replicating local sea level change,especially in the subtropical gyre region.
The many studies investigating the future change of the Greenland Ice Sheet surface mass balance fromclimate model output exhibit a wide range of projections. This study makes projections from the CoupledModel Intercomparison Project phase 3 models used in the Intergovernmental Panel on Climate ChangeFourth Assessment Report and explores the underlying physical processes behind their spread. The projections are made for three Special Report on Emissions Scenarios, B1, A1B, and A2, with a focused analysis on the A1B scenario. The estimate in the study suggests that about 60% of the intermodel difference in the twenty-first-century ablation rate change under the A1B scenario is accounted for by the global annual mean temperature change. In the current study, other processes responsible for the spread in model projections are investigated after excluding this global effect. A negative correlation (-0.60) was found between the simulated summer temperature bias over the Greenland Ice Sheet under present-day conditions and the ablation rate increase during the twenty-first century, partly because maximum warming over ice is approximately limited to the melting temperature. Models with relatively larger ablation rate increase during the twenty-first century exhibit greater warming with a greater reduction in sea ice cover. The authors found that these models also simulate relatively cooler summer conditions in high latitudes with more sea ice cover in the late twentieth century, suggesting the importance of sea ice feedbacks. Also, an anticorrelation (-0.75) is found between weakening of the Atlantic meridional overturning circulation and the ablation rate increase during the twenty-first century. The relation in the model spread between the twenty-first-century ablation change and the late twentieth-century climate conditions found is then used to investigate the impact of model bias on the multimodel ensemble of projections. The result suggests that the models’ underestimation of present-day sea ice concentration near the coast of Greenland may cause an underestimation of future Greenland Ice Sheet ablation rate increase in the ensemble projection. These results emphasize the importance of correctly simulating present-day conditions and understanding the underlying multiple physical processes behind the intermodel difference to reduce the uncertainty in future projections.