The oceans play a key role in the global climate variability. This dissertation examines climate variability in historical simulations from fourteen CMIP5 (Coupled Model Intercomparison Project Phase 5) coupled models on different time scales. Responses of oceans to the external volcanic eruption, green house gas forcing, and internally generated variability are investigated with emphasis on higher latitudes.

Chapter 2 addresses the oceanic response to tropical volcanic eruptions. Previous modeling studies have provided conflicting high latitude climate responses to volcanic eruptions, including the ocean's role. This controversy happens mainly because the response varies widely from model to model, and even varies among ensemble members of a single model. The increase in Atlantic Meridional Overturning Circulation (AMOC) after the volcanic eruption is closely linked with its internal variability.

Chapter 3 addresses the seasonal and centennial trends in the Arctic Ocean. The Arctic warming is apparent in all models, although there is considerable variability especially its seasonal cycle. Both the surface heat flux and the oceanic heat convergence contribute to the Arctic warming on centennial time scale. Meanwhile, the seasonal variation of oceanic warming is largely determined by the atmospheric heating. In models presenting a clear seasonal cycle of surface net flux increases, there is a notable retreat of sea ice extent in winter, which allows more heat loss from the ocean through turbulent fluxes.

Chapter 4 discusses the internally generated variability of high latitude water masses. Both the magnitude and the time scale of subarctic decadal variability are strikingly similar to observations. The analysis of the more realistic models provides constraints on relative roles of the oceanic heat transport and the atmospheric heat flux. One possible factor that could give rise to the different origins of ocean variability is the blocking of mid-latitude jet stream. The oceanic heat transport is more important to the decadal variability of the high latitude ocean in models where winter-time atmospheric blocking events over the Euro-Atlantic sector are more frequent.