Chapter 2. Hydrology
25
Figure 7. Track of Hurricane Nora in Septem-
ber 1997, showing recurvature towards the
southwestern U.S. (From Farfan and Zehnder
2001.)
Influence of El Nio
The El Nio/Southern Oscillation (ENSO), an unusual warming of Pacific
Ocean waters, has been directly linked to increased precipitation in California
and decreased precipitation in the Pacific Northwest as the North Pacific frontal
storm trek is depressed farther south (Schonher and Nicholson 1989, Piechota et
al. 1997, Gershunov and Barnett 1998). However, no similar link is seen between
ENSO and winter rainfall in New Mexico and Arizona (Andrade and Sellers
1988). A correlation between ENSO and summer monsoonal precipitation is
uncertain (Adams and Comrie 1997); some studies suggest a positive correlation
(Harrington et al. 1992, Hereford and Webb 1992), while others cite no such
trend (Andrade and Sellers 1988, Carleton et al. 1990). Increased fall and spring
precipitation is experienced in the southwestern U.S. (principally Arizona and
New Mexico) during ENSO events (Andrade and Sellers 1988). ENSO appears
to be related to decadal-scale changes in precipitation patterns and flood
frequency in the Southwest (i.e., the southern part of the study area defined
here)(Ely 1997). The influence of ENSO events on other portions of the study
area is not well studied. Relationships between long-term patterns in streamflow
Previous Page
