H. L. Johnson, PhD thesis (University of Reading, UK).
The thermohaline circulation transports approximately 1 PW of heat northwards in the Atlantic basin. Variations in its strength could therefore have a significant impact on climate. Current understanding is based largely on steady state theories. Motivated by the results of recent climate model experiments, which suggest that the circulation could change abruptly as a result of global warming, here a conceptual model and new quantitative theory for the time-dependent thermohaline circulation are presented.
The dynamical response of the upper, warm limb of the Atlantic thermohaline circulation to a sudden change in deep water formation at high latitudes is first illustrated using an idealized shallow water model. Kelvin waves are initiated which propagate along the western boundary to the equator on a timescale of months. Adjustment in the North Atlantic is therefore rapid. The response in the southern hemisphere, governed by Rossby wave propagation in the interior, is much slower. Through a mechanism here termed the ``equatorial buffer'' the equator acts to limit the size of the response in the South Atlantic on short timescales. As a consequence, the equator behaves as a low-pass filter, restricting variability on decadal and shorter timescales to the hemispheric basin in which it is forced.
A new, quantitative theory is developed to describe this adjustment. It is based on a single variable - the thermocline thickness on the eastern boundary - from which the time-varying circulation throughout the entire basin can be calculated. The theory agrees favourably with the numerical results in both idealized and realistic Atlantic domains. It is also applied with success in a full global ocean domain, where it demonstrates that the response in the Indian and Pacific on decadal timescales is small.
Implications for abrupt climate change, the likely sources of variability in North Atlantic overturning, and the monitoring and modelling of thermohaline variability are discussed.