Cool HP

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FIGURE 6 Same as Fig. 5, but for the tropical PC index (a-c) and North Pacific PC index (d-f), 191588, respectively.

b c f tions (and perhaps in southernmost South America) than is the tropical PC, whereas precipitation and streamflow in most other regions more strongly express the tropical PC series. These parallels and connections can also be identified by computing the leading PCs of streamflow and precipitation in the Americas and then correlating the variations of those leading modes with global SSTs (Dettinger et al., 2000) to obtain SST correlation patterns that closely resemble the combined ENSO and decadal ENSO-like SST patterns studied here.

1.5.2. Surface Air Temperature

Positive CT and GR indices are associated with warmer than normal surface conditions in much of the Americas. The patterns indicated for CT temperatures (Fig. 5c) and GR temperatures (Fig. 5f), however, are not as similar as the precipitation patterns discussed previously. Positive CTs (El Niños) are associated with warm temperatures in Canada, the western United States, Central America, much of tropical South America (on both sides of the Andes), Paraguay-Uruguay-

Patagonia, and southeasternmost Brazil (see also Diaz and Kiladis 1992). El Niños bring cool temperatures to the southeastern United States and to the eastern Amazon basin. Positive GRs are associated with warmer than normal conditions in northwestern North America and cooler than normal conditions across southeastern and eastern North America. Temperature conditions elsewhere are not as closely related to the GR index as they are to CT. These temperature relations are largely echoed in regression coefficients for the tropical and North Pacific PCs (Figs. 6c and 6f). As was the case with CT and GR, the spatial patterns of temperature relations with the two PC series are not as similar as the relations for precipitation and streamflow; the pattern correlation between Figs. 6c and 6f is only +0.16.

These temperature responses to CT and GR are as expected from the atmospheric circulation changes associated with the two indices. When positive, both CT and GR tend to route anomalously southerly winds over western North America as circulation patterns become more zonal over much of the North Pacific. The winds then tend to be anomalously northward near the west coast of North America from northern México to

Alaska (Fig. 4a). Warmer than normal American tropics derive from warmer SSTs and drier conditions in tropical South America (Diaz and Kiladis 1992). In subtropical South America, poleward deviations of the westerlies also result in warmer temperatures overall. Cooler temperatures in the southeastern United States with positive CTs and along the entire eastern United States are consistent with more storminess and anomalous northerly winds, respectively, as indicated by the details of the 500-mbar height regression coefficients shown in Figs. 4a and 4c.

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