Intellectual Merit. This study will contribute to and extend a systematic framework that has been developed over the last decade to understand the nonlinear dynamics and different regimes of the ocean circulation, by including a detailed view on how the wind and thermohaline components of the ocean circulation interact. It is unknown how a more accurate modeling of the wind- and buoyancy-driven circulation, including details in downwelling and boundary currents, influences the internal decadal-to-interdecadal variability of the model solutions. Without this knowledge the theory of the ocean circulation will be incomplete. This study of the physics of decadal-to-interdecadal variability in the North Atlantic ocean circulation will focus on the interaction of the wind- and buoyancy-driven circulations in the low-viscosity limit. Recent numerical and theoretical studies indicate that decreasing the viscosity results in fundamentally different mean circulations, modes of heat transport, and regions of deep convection and downwelling at high latitudes compared to those in low-resolution models typically used for climate studies. It is anticipated that these changes will influence in a fundamental way how the role of the North Atlantic Ocean circulation in decadal-to-interdecadal climate variability is viewed.
The main hypothesis is that the decrease in viscosity in ocean models and the concomitant changes in the boundary currents, sea surface temperature, regions of deep mixing, surface currents, mechanisms of heat transport, and the surface heat flux within the subpolar gyre, will provide a more accurate representation of oceanic processes involved in decadal-to-interdecadal variability of the North Atlantic climate. Therefore, the study of the low-viscosity limit will reveal new mechanisms of internal oceanic decadal-to-interdecadal variability. The tools will consist of numerical bifurcation methods applied to stratified ocean models at relatively high resolution and direct numerical simulations of the ocean circulation.
Broader Impacts. The study is aimed at gaining a better understanding of the ocean variability on time scales relevant to the societally significant question of global climate predictability. The results will be of interest for both climate change predictability and ocean state monitoring. The results will be disseminated by publications in leading oceanographic journals, and through presentations in a variety of settings. Both PIs maintain web pages on which this research project will be described and results will be made available. Both PIs are on staff at educational institutions within which they are (and will continue to be) active in advising, supervising, and teaching graduate students and summer student fellows (undergraduates). Two graduate students will be trained under this project in modern numerical methods and climate dynamics.
