Quasi-decadal variations in the North Atlantic

Basin-wide and regional decadal variations in the North Atlantic are investigated using the output from a prognostic coupled ice-ocean model. The study is divided into two parts. The basin-wide decadal variation in the North Atlantic is discussed in Chapter 2, and the decadal variation of the convective activity in the Labrador Sea, especially focused on the contribution of preconditioning, is discussed in Chapter 3. In this chapter, it is also shown how the convective intensity is modified and is linked to the basin-scale oscillation. Both of the phenomena involve the dominant role of the temperature variations. Chapters 2 and 3 are intended as separate reports and therefore contain their respective introductions and conclusions. The general conclusion in Chapter 4 draws together the results of this entire study.; In the North Atlantic there exist 12–14 year or quasi-decadal oscillations. Temperature anomalies that represent the upper heat content anomalies circulate around the North Atlantic basin at these timescales. It is shown from the model results that the Thermohaline Circulation (THC) is a central component in inducing the 12–14 year basin-wide oscillations. Responding to the changes in the THC the horizontal circulations adjust to a new oceanic state. During this transition, wave dynamics in the form of Rossby waves and advection/propagation along the western boundary play a role, influencing the horizontal circulations.; The basin-wide phenomenon suggests the important role of the convection process in the region of deep-water formation that is a driving source of the THC. In the following study, the dynamics of the decadal variability of the convective Activity in the Labrador Sea is discussed. The mixed-layer depths (MLD) in the convective region show a clear decadal oscillation. It turns out that most of the variability in the MLD is accounted for by a simple mathematical model. Various analyses using the simple model lead to a conclusion that oceanic stratification before the convection more significantly contributes to the convective activity than the atmospheric forcing over the region of interest. The oceanic stratification is strongly controlled by the anomalies in the upper ocean heat transport which are advected/propagated from the subtropics as part of the basin-wide oscillations. This may indicate that the decadal oscillations are the ocean-only mode.