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Natural Climate Variability on Decade-to-Century Time Scales
year integration of a coupled ocean-atmosphere model is used to study the stability and variability of this meridional overturning (hereafter referred to as the thermohaline circulation, or THC). The output from this model, developed at NOAA's Geophysical Fluid Dynamics Laboratory, forms the basis for the analyses presented here. Several previous modeling studies (Mikolajewicz and Maier-Reimer, 1990; Weaver and Sarachik, 1991b; Weaver et al., 1991; Winton and Sarachik, 1993) have used ocean-only models to study the variability and stability of the thermohaline circulation, in contrast to the fully coupled ocean-atmosphere model employed in the present investigation. Since the mechanisms governing the fluxes of heat and fresh-water across the ocean surface differ substantially between the two types of models, the models might be expected to have different time scales for variations in the THC. While the quantitative results presented here will depend on the details of the model formulation and the physical parameterizations employed, it is hoped that the physical processes identified in this model study are robust and are responsible for interdecadal variability in the North Atlantic. The present study demonstrates that substantial variability can occur in the coupled ocean-atmosphere system on time scales of decades in the absence of external forcings such as changing concentrations of greenhouse gases.
MODEL AND EXPERIMENTAL DESIGN
The oceanic component of the coupled model solves the primitive equations of motion with an approximate horizontal resolution of 3.75° longitude by 4.5° latitude, and 12 unevenly spaced vertical levels. A simple thermodynamic balance is used to predict the presence and thickness of sea-ice, which is advected by surface currents as long as the sea ice thickness is less than 4 m. The atmospheric model solves the primitive equations using the spectral transform technique, and has an approximate horizontal resolution of 7.5° longitude by 4.5° latitude. The atmospheric model, which has 9 unevenly spaced levels in the vertical, incorporates seasonally varying insolation and terrestrial radiation. Cloud cover is predicted whenever the relative humidity exceeds a critical value (99%). At continental surfaces, the heat and water budgets and their interaction with the atmosphere are computed.
The coupled model is global in domain, incorporating realistic geography smoothed to the model resolution. The atmosphere and ocean interact through the surface fluxes of heat, fresh-water, and momentum. The surface fluxes of heat and fresh-water into the ocean are adjusted at each grid point in order to obtain a more realistic simulation of climate. The surface flux adjustments are derived from preliminary integrations of the separate oceanic and atmospheric models. The derived flux adjustments do not vary from year to year and are not dependent on the anomalies of surface temperature and salinity. Thus, they do not explicitly affect the strength of the feedback processes that reduce the anomalies of temperature and salinity at the ocean surface. The model and the flux adjustments are described in more detail in Manabe et al. (1991).
Starting from an initial condition in quasi-equilibrium, the coupled model is time-integrated over a period of 200 years. The mean rate of change of global mean sea surface temperature for the model over this period is very small (0.01°C per century).
Comparison with Observed Variability
An assessment of the model's ability to simulate climate variability is indicated in Figure 1 by comparing a spectrum of model generated sea surface temperature (SST) to a spectrum of observed SST (Frankignoul and Hasselmann, 1977). The observed SSTs are from ocean weather ship India in the North Atlantic (59°N, 19°W), while the model SSTs are from the model grid point closest to the location of ocean weather ship India. Although the model tends to underestimate the total variance (proportional to the area beneath the spectrum), the overall agreement between the two spectra is good.3 One should note, however, that the present model does not resolve mesoscale eddies, which may contribute significantly to the variability of sea surface temperature at seasonal to annual time scales.
Meridional Overturning Index and Variability
The stream function describing the meridional circulation in the Atlantic basin (200-year mean) is shown in Figure 2. The circulation follows the direction of the arrows; its magnitude is proportional to the gradient of the contours. Water flows northward in the upper layers, sinks at high latitudes, and flows southward at depth. The region in which most of the sinking occurs in the model North Atlantic is principally confined to the latitudinal belt from 52°N to 72°N; hereafter, the term "sinking region" will refer to the
In subsequent figures and the text, it is demonstrated that the intensity of the meridional overturning in the model North Atlantic has enhanced variance at a time scale of 40 to 50 years, but the spectrum of model SST in Figure 1 is not characterized by enhanced variance in this frequency band. As shown in Figure 4a, there is a spatial pattern of SST anomalies associated with these overturning variations. The point chosen for the SST spectrum in Figure 1 is not located in this region, and thus does not have enhanced variance at the 40-to-50-year time scale. Spectra of model SSTs from the region of maximum SST change shown in Figure 4a are characterized by enhanced variance at the 40-to-50-year time scale.