Self sustained thermohaline oscillations and their implications for biogeochemical cycles

Abstract

An ocean general circulation model (OGCM) configured with a paleo ocean bathymetry such as late Permian shows that different modes of ocean circulation might exist in warm climate: a strong 'thermal mode' induced by cooling at high latitudes and a weak 'haline mode' induced by evaporation at subtropics. The 'haline mode', obtained with enhanced freshwater flux and reduced vertical diffusivity, is inherently unstable, flushed by thermally-driven polar convection every few thousand years. A 3-box model of the thermohaline circulation is developed to study the basic physical mechanism of thermohaline oscillations. By including convective adjustment and a parameterization of the localized nature of convection, the box model shows that haline mode is unstable over a certain freshwater forcing/vertical diffusivity range.

(cont.) Self-sustained oscillatory thermohaline circulations, with periods ranging from centuries to several millennia, are supported. When the amplitude of surface freshwater flux exceeds a certain threshold the haline mode stabilizes. The relationship between oscillation periods and the freshwater flux/vertical diffusivity is also studied. Biogeochemical modeling of the late Permian ocean shows that the strong 'thermal mode' leads to well oxygenated deep ocean, the weak 'haline mode' leads to depletion of deep ocean oxygen. Biogeochemical cycles driven by the thermohaline oscillation found in the 3-box model shows that: the quasi-steady 'haline mode' is correlated with lower biological productivity, depleted deep ocean oxygen, heavier surface 613C due to weak vertical mixing, the transient 'thermal mode' is correlated with higher biological productivity, oxygenated deep ocean and lighter surface 613C due to strong vertical mixing. Those correlations are consistent with rhythmic paleo records. The 613C shift during mode switch is proportional to mean ocean nutrient level.