COMPARISON OF MONTHLY AND INTRAMONTHLY INDEXES FOR THE PACIFIC NORTH-AMERICAN TELECONNECTION PATTERN

Many applied climatological studies use teleconnection indices to char acterize atmospheric flow. Although these indices are often associated with temperature and precipitation patterns, surprisingly little atte ntion has been paid to changes in the flow patterns within the monthly period. This study analyzes the differences between the monthly and i ntramonthly indices (based on 5-day mean data) for the most important source of winter season low-frequency variability in the midtropospher ic geopotential height field over North America, the Pacific/North Ame rican (PNA) pattern. Results suggest that the monthly index adequately represents flow patterns at shorter temporal scales. Months with PNA indices suggesting a typical configuration (moderate 700-mb ridging ov er western North America with moderate troughing in eastern North Amer ica) tend to be comprised of days with the same flow, rather than an o ffsetting of days with amplified ridge-trough and a reversed trough-ri dge pattern across North America. It is found that consecutive days of amplified flow are common, and that the reverse pattern, although inf requent, can be quite persistent when it does occur. Even though there is a strong relationship between the monthly and intramonthly indices , intramonthly variability and persistence of flow must be considered if a truly accurate relationship between circulation and environmental phenomena is to be established.