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Natural Climate Variability on Decade-to-Century Time Scales
Ocean model in which the appropriate data can be assimilated to help define the mechanisms responsible for the fluctuations in Atlantic Ocean circulation.6
The ACCP is an interdisciplinary program that involves atmospheric scientists, oceanographers, and paleoclimatologists. Currently the program shows an appropriate balance between modeling efforts and analysis of the historical proxy and instrumental data. Furthermore, observational programs have been launched to help determine the link between intermediate-time-scale SST anomalies and variability in the thermohaline circulation by means of long-term monitoring of the deep Western Boundary Current. Observational studies that are ongoing or anticipated include tracer inventories, hydrologic monitoring of Fram Strait, and monitoring of the upper ocean thermal structure through the Atlantic Voluntary Observing Ships Special Observing Project and surface-drifter deployments.
Two different patterns of variability in the North Atlantic atmosphere-ocean system have been identified in the historical data. Deser and Blackmon (1991; 1995, in this volume), using the historical observational data of sea-surface temperature, sea-level pressure, and the zonal wind over the Atlantic Basin, have demonstrated that there is a complex wintertime atmosphere-ocean interaction in the North Atlantic with a preferred time scale of about 10 years. The anatomy of this decadal variability appears to be somewhat different from that of the interannual climate anomaly in the North Atlantic documented in Wallace et al. (1992).
Using the same surface fields and focusing on the changes in the Atlantic atmosphere-ocean system over multidecadal periods during the last century, Kushnir (1992) found the relationship between changes in the SST and changes in the overlying surface atmospheric circulation significantly different from that associated with higher-frequency variability in the Atlantic climate system. In this multidecadal transition, the atmospheric circulation changes are consistent with a local quasigeostrophic response to the changes in the ocean SST (see Figure 3). The hydrographic record for the same period indicates that the SST changes are echoed in the deep ocean, where concomitant changes in salinity are found (Lazier, 1988; Levitus, 1989a,b; see also Figure 1). Thus, as is not true for the higher-frequency "modes," evidence suggests that changes in the thermohaline circulation and sea-ice export from the Arctic Ocean are associated with this multidecadal transition.
In order to identify the climate variability in and around the Atlantic basin on decade-to-century and longer time scales, ACCP-sponsored studies have been begun that will determine the extent to which the instrumental records used in the studies mentioned earlier can be augmented (e.g.,
Decadal time-scale climate anomalies in the wintertime (December-April) in the North Atlantic Panel (a): the difference between the average wintertime SSTs between 1950 and 1964 and between 1970 and 1984. The former period (1950-1964) was characterized by warmer-than-normal water in the winter in the North Atlantic, the latter (1970-1984) by anomalously cold water. The contour interval is 0.2°C. Panel (b): as in (a), except for the attendant atmospheric surface variables: sea-level pressure (contour interval 0.5 mb) and vector winds (a 1.0-m/s vector is found below the panel). (From Kushnir, 1994; reprinted with permission of the American Meteorological Society.)
An extended presentation of the Atlantic Climate Change Program Science Plan can be found in NOAA (1992a).