Interannual Variability of 30～ 60 Day Low-Frequency Kinetic Energy in the Lower Tropical Atmosphere
热带低层大气30～60天低频动能的年际变化与ENSO循环

Based on the NCEP reanalysis data, the relationship between the interannual variability of tropical atmospheric intraseasonal oscillation (ISO) and ENSO cycle has been studied through statisti- cal and composite analyses. The results suggest that the interannual variability of the tropical atmos- pheric ISO (30~ 60 day low-frequency oscillation) is most obvious in the central and western Pacific. During the spring and summer prior to the mature El Nino, the 30~ 60 day oscillation in the western Pacific is very active, its kinetic energy is clearly increased and the anomalous center of kinetic energy gradually propagates eastward. Along with the strengthened 30~ 60 day oscillation, there is an anoma- lous cyclone to the north of the equator and the westerly prevails in the equator. On the contrary, dur- ing the spring and summer prior to the mature La Nina, the atmospheric 30~ 60 low-frequency oscillation in the western Pacific is weak. Further analyses also suggest that the interannual variability of the winter monsoon over East Asia is reponsible for the interannual variability of tropical atmospher- ic 30~ 60 day oscillation: corresponding to the strong (weak) winter monsoon over East Asia, the cumu- lus convection in the tropical western Pacific is strengthened (weakend), therefore the atmospheric 30~ 60 day oscillation is active (weak). The data analyses further prove that as an external forcing, the interannual variability of tropical atmospheric 30~ 60 day oscillation plays an important role in the formation of E1 Nino.