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ABSTRACT
The mechanisms responsible for the decadal variability of the tripole mode of North Atlantic SST during the latter half of the twentieth century are diagnosed using a new technique. The SST and associated ocean variability are reconstructed by forcing an interactive ensemble coupled GCM by the surface fluxes resulting from weather noise. The weather noise surface fluxes are obtained from the NCEP-NCAR reanalysis by removing the simulated atmospheric feedback to the observed SST evolution. Simulations are performed to reconstruct and estimate the contributions of the local weather noise heat flux and wind stress to the observed evolution of the tripole pattern. The results indicate that the North Atlantic tripole pattern is forced primarily by the local weather noise surface heat flux. The roles of several types of ocean circulation variability, including gyres forced by the wind stress weather noise, the wind stress feedback to the SST, and the meridional overturning circulation, are also examined. Conclusions from this approach are expected to be model dependent. Further analysis, in the context of a simple model, of the mechanisms producing the tripole variability is presented in Part II.
1. Introduction
There are two dominant modes of low-frequency SST variability in the North Atlantic observed in the twentieth century, the decadal time scale tripole mode and the multidecadal time scale monopole mode. This study concentrates on attempting to understand the interplay of mechanisms that is responsible for the tripole mode.
Deser and Blackmon (1993) and Kushnir (1994) found an SST anomaly (SSTA) dipole spatial pattern in the NorthAtlantic with biennial and decadal time scales, and a monopole spatial pattern with an interdecadal time scale. The dipole becomes part of a tripole pattern if the domain is extended farther south (e.g.,Wuand Liu 2005). The tripole pattern has centers east of Newfoundland, near the southeastern coast of the United States, and in the tropical eastern Atlantic.
The variability of this pattern can be represented using the tripole index of Czaja and Marshall (2001, hereafter CM01), which we will call DTs, and which is defined as the SST anomaly averaged over a northern box (408-558N, 608-408W) minus that in a southern box (258-358N, 808-608W). Figure 1 illustrates the regions and shows the January-March (JFM) tripole index (Fig. 1b) and associated...