87th AMS Annual Meeting

Monday, 15 January 2007: 4:30 PM
The Thermal and Dynamical Structure of the Atmosphere during the 1998-2000 La Nina
214C (Henry B. Gonzalez Convention Center)
Amir Shabbar, MSC, Toronto, ON, Canada; and B. Yu
Contrary to the impacts ascribed to strong ENSO cold events (La Niņa) during the last half of the 20th century, the two-year protracted La Niņa during 1998-2000 produced abnormally warm and dry winter (Dec-Mar) conditions over most of North America. Despite the presence of a moderately strong La Niņa, analyses of thermal and dynamical features show that the North American climate was more responsive to the changes in the North Pacific sector during the 1998-2000 period. A strong north-south temperature gradient across the North Pacific from the surface through a deep layer of the troposphere contributed to the strengthening of the East Asian Jet, as well as its eastward extension.

This configuration relates to the coupled mode of non-ENSO variability in the North Pacific. Stationary wave activity during the 1998-2000 La Niņa shows an apparent source region south of the Aleutian Islands, whereas composite of past La Niņas indicates the subtropical Pacific as a possible source region. Due to a lack of clear height anomaly contrast between the central Pacific Ocean and North America, the stationary wave anomalies weakened as they crossed into North America. This study highlights the importance of the North Pacific climate variability in determining North American climate anomalies. In particular, the strengthening and the extension of the East Asian Jet Stream can significantly alter canonical La Niņa-related winter climate pattern over North America.

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