The role of deep sea heat storage in the secular response to climatic forcing

Abstract

The influence of the world oceans on climatic response is considered here with emphasis on the heat transferred to waters beneath the well-mixed surface layer and to polar bottom water forming zones. An upwelling-diffusing model is formulated to treat this problem whose effective transport properties are calibrated from the steady state vertical profiles of radiocarbon, potential temperature and other tracers measured by chemical oceanographers. The key issue with regard to the question of atmospheric temperature response to external climatic forcing is whether heat is exchanged between the surface mixed layer and deep sea at rates comparable to heat transfer rates between the planetary radiation field and the atmosphere-mixed layer system. An important model parameter appearing in the analysis is the polar sea warming coefficient FI equal to the rate of change of polar sea temperature relative to changes in areally averaged mixed layer temperature. For FI values in the range of 0 to 2 the models predicts response times in the range of 8 to 20 years to attain 63% of the equilibrium temperature change for a step function climatic forcing, and 50 to 1000 years to get 90% of the equilibrium response. These may be compared with the roughly 4 year response time one gets with an oceanic mixed layer only model. To study the carbon dioxide climate problem, a more realistic time-dependent forcing function is used based on the historical growth of fossil fuel CO2 and a logarithmic scaling law for the temperature increment which would obtain at any instant if the system were in radiative-convective equilibrium. Our results suggest the influence of deep sea thermal storage could delay the full value of temperature increment predicted by equilibrium models by 10 to 20 years in 1980 to 2000 A.D. time frame. Also considered is the model response to periodic forcing, the sensitivity of the results, and the implications of the model results with regard to climatic changes on a decadal to millenial timescale.

Can El Nińo amplify the solar forcing of climate?

“…On one hand, there is an l 1-year signal in the Earths global temperature spectrum [Mann and Park, 1994]. On the other hand, this signal is noisy and transient [Pitrock, 1978] and is attenuatcd by the inertia of oceans [Hoffert et al, 1980]. The gaps between missing data points were linearly interpolated.…”

The effect of reduced ocean overturning on the climate of the last glacial maximum

“…No flow of water across the equator takes place. The mean upwelling velocity in the current climatic state is 4 m yr P1 (Hoffert et al 1980), which in our model outline implies overturning rates of 17.2 and 20.2 Sv in the Northern (NH) and the Southern Hemisphere (SH), respectively. These overturning rates are in reasonable agreement with ob- served values (e.g.…”

Solar Irradiance Variations and Global Ocean Temperatures

“…Vertical advection of heat also takes place through an overturning thermohaline circulation driven by high-latitude processes such as sea-ice formation around the Antarctic continent, and the basic model output is a timevarying solution of the coupled differential equations that govern heat balance at the surface and heat transport in the deep ocean. Details can be found in HOFFERT et al (1980) and in REID and GAGE (1988). Figure 2 shows the model output for a sinusoidal variation in solar irradiance with an amplitude of 1% and a period of 83 years, corresponding approximately to the quasi-period of the Gleissberg cycle of solar activity.…”

“…REID of magnitude estimate can be found by using a relatively simple one-dimensional model that calculates the temperature response of the upper mixed layer of the ocean to variable solar heating. The model of HOFFERT et al (1980) has been chosen for this purpose, and will be described only briefly here.…”