ASYMMETRIC IMPACT OF TROPICAL SST ANOMALIES ON ATMOSPHERIC INTERNAL VARIABILITY OVER THE NORTH PACIFIC

A substantial asymmetric impact of tropical Pacific SST anomalies on t he internal variability of the extratropical atmosphere is found. A va riety of diagnoses is performed to help reveal the dynamical processes leading to the large impact. Thirty-five years of geopotential height s and 29 years of wind fields analyzed operationally at the National C enters for Environmental Prediction (NCEP), formerly the National Mete orological Center, and three sets of 10-yr-long perpetual January inte grations run with a low-resolution NCEP global spectral model are inve stigated in detail for the impact of the SST anomalies on the blocking flows over the North Pacific. The impact on large-scale deep trough f lows is also examined. Both the blocking and deep trough Bows develop twice as much over the North Pacific during La Nina as during El Nino winters. Consequently, the internal dynamics associated low-frequency variability (LFV), with timescales between 7 and 61 days examined in t his study, display distinct characteristics: much larger magnitude for the La Nina than the El Nino winters over the eastern North Pacific, where the LFV is highest in general. The diagnosis of the localized El iassen-Palm fluxes and their divergence reveals that the high-frequenc y transient eddies (1-7 days) at high latitudes are effective in formi ng and maintaining the large-scale blocking flows, while the midlatitu de transients are less effective. The mean deformation field over the North Pacific is much more diffluent for the La Nina than the El Nino winters, resulting in more blocking flows being developed and maintain ed during La Nina by the high-frequency transients over the central No rth Pacific. In addition to the above dynamical process operating on t he high-frequency end of the spectrum, the local barotropic energy con version between the LFV components and the time-mean Bows is also oper ating and playing a crucial role. The kinetic energy conversion repres ented by the scalar product between the E vector of the low-frequency components and the deformation D vector of the time-mean flow reveals that, on average, the low-frequency components extract energy from the time-mean flow during La Nina winters while they lose energy to the t ime-mean Bow during El Nino winters. This local barotropic energy conv ersion on the low-frequency end of the spectrum, together with the for cing of the high-frequency transients on blocking flows on the high-fr equency end, explain why there is a large difference in the magnitude of low-frequency variability between the La Nina and the El Nino winte rs.