A study of ocean-atmosphere interaction and low-frequency variability of the climate system

A combination of empirical data analysis techniques and simplified coupled ocean-atmosphere modeling is applied to investigate empirical and theoretical characteristics of organized decadal-to-century timescale climate variability. A multivariate time series methodology for isolating and reconstructing significant quasi-oscillatory spatiotemporal signals in empirical climate data is developed and applied to a variety of instrumental and high-quality long-term climate proxy datasets. A simplified model of the coupled ocean-atmosphere system is developed to investigate dynamical mechanisms which may be important in describing observed signals.; The empirical analyses demonstrate consistent evidence for organized modes of large-scale climatic variability on interdecadal (15-30 year) and century (60-100 year) timescales. The interdecadal mode exhibits a pattern dominated by inter-related variations in atmospheric circulation and surface temperature in the Pacific which bears resemblance to a delayed-oscillator mode observed in previous coupled ocean-atmosphere general circulation model (GCM) experiments. The century-scale mode shows a pattern of high-amplitude variability in the polar North Atlantic, and North Atlantic signature in both temperature and inferred atmospheric circulation anomalies bearing some similarity to a century-scale signal related to the ocean thermohaline circulation which has been isolated in another coupled model study.; The simplified theoretical model used to investigate possible dynamical mechanisms describes the zonally-averaged thermohaline and wind-driven circulation of an idealized global ocean. The effects of horizontal gyre circulations, atmospheric responses to ocean surface temperature changes, and the delayed response of gyre circulation variations to changes in atmospheric windstress are each parameterized. The internal dynamics of the model is investigated by long stochastically forced integrations. In the absence of a gyre dynamics representation, a 200-300 year oscillation is isolated that is associated with advection of salinity and temperature anomalies by the meridional overturning. When gyre-scale dynamics are parameterized, a 70-100 year oscillation that is more consistent with observed century timescale climate oscillations is isolated, involving a combination of meridional overturning and gyre-scale dynamics. When the delayed response of gyres variations to wind variations is accounted for, modes with decadal (10-20 year) timescales are identified, somewhat similarity to the delayed-oscillator mode described earlier.