TRANSIENT TEMPERATURE AND SEA-LEVEL RESPONSE OF A 2-DIMENSIONAL OCEAN-CLIMATE MODEL TO GREENHOUSE-GAS INCREASES

A two-dimensional dynamical ocean model is coupled to an energy balance climate model and used to investigate the transient surface temperature and sea level response to greenhouse gas increases. For most experiments a step function surface-troposphere heating perturbation of 6 W m-2 is applied. A transient reduction in the thermohaline overturning flux of 15-30% occurs in most experiments, although in some cases a near-total circulation collapse occurs within the first 100 years and lasts 700-1000 years. The transient circulation decrease is attributed to the greater rate of downward penetration of the heating anomaly in downwelling rather than in upwelling regions and is not a result of reduced convection. For experiments in which the steady state circulation is qualitatively unchanged after a heating perturbation, the surface temperature response is roughly uniform with latitude, in the absence of ice and snow. However, in some cases a heating perturbation induces the transition from a one-cell to a two-cell overturning circulation, or causes a complete reversal in the direction of overturning when a single cell spans both hemispheres, causing marked latitudinal variations in the surface temperature response. The e-folding response time scale tau(e) for global mean surface temperature ranges from 10 to 142 years as the vertical diffusion coefficient K(nu) varies from 1 x 10(-5) m2 s-1 to 5 x 10(-4) m2 s-1, but the variation of tau(e) with K(nu) is not uniform due to changes in the transient circulation response as K(nu) is varied. The one-dimensional upwelling diffusion and pure diffusion models have a significantly faster surface transient response than the two-dimensional (2-D) model, due to transient weakening of thermohaline overturning in the 2-D model, which dampens surface temperature warming. The presence of convection has a small effect on the transient response. Transient sea level rise is highly sensitive to K(nu), with faster sea level rise associated with larger K(nu) and hence with slower surface warming. The steady state ocean warming is approximately uniform with depth and equal to the global mean surface warming (3-degrees-C for 6 W m-2 forcing) in most experiments, which produces a sea level rise of 2 to 3 m due to thermal expansion. The times required to reach 50% and 75% of the steady state sea level rise are 200-1100 years and 500-2400 years, respectively. However, if the unperturbed circulation consists of a single overturning cell and a second thermohaline overturning cell develops in response to a heating perturbation, vertical mean steady state deep ocean warming and associated sea level rise can be reduced by almost a factor of 10.