8.4.4 Cold Ocean-Warm Land Pattern

The Cold Ocean-Warm Land (COWL) pattern indicates that the oceans are relatively cold and the continents are relatively warm poleward of 40°N when the NH is relatively warm. The COWL pattern results from the contrast in thermal inertia between the continents and oceans, which allows continental temperature anomalies to have greater amplitude, and thus more strongly influence hemispheric mean temperature. The COWL pattern has been simulated in climate models of varying degrees of complexity (e.g., Broccoli et al., 1998), and similar patterns have been obtained from cluster analysis (Wu and Straus, 2004a) and EOF analysis (Wu and Straus, 2004b) of reanalysis data. In a number of studies, cold season trends in NH temperature and sea level pressure during the late 20th century have been associated with secular trends in indices of the COWL pattern (Wallace et al., 1996; Lu et al., 2004).

In their analysis of AOGCM simulations, Broccoli et al. (1998) found that the original method for extracting the COWL pattern could yield potentially misleading results when applied to a simulation forced by past and future variations in anthropogenic forcing (as is the case with most other patterns, or modes, of climate variability). The resulting spatial pattern was a mixture of the patterns associated with unforced climate variability and the anthropogenic fingerprint. Broccoli et al. (1998) also noted that temperature anomalies in the two continental centres of the COWL pattern are virtually uncorrelated, suggesting that different atmospheric teleconnections are involved in producing this pattern. Quadrelli and Wallace (2004) recently showed that the COWL pattern can be reconstructed as a linear combination of the first two EOFs of monthly mean December to March sea level pressure. These two EOFs are the NAM and a mode closely resembling the PNA pattern. A linear combination of these two fundamental patterns can also account for a substantial fraction of the winter trend in NH sea level pressure during the late 20th century.