Temporal and Spatial Characteristics of Intraseasonal Oscillations in the Meridional Wind Field over the Subtropical Northern Pacific

Based on the NCEP/NCAR reanalysis I daily data from 1958 to 2002,climatic characteristics of the 30-60-day intraseasonal oscillations (ISOs) of the zonal wind (u), meridional wind (v), and geopotential height(h) over global areas and especially the ISO of v over the subtropical northern Pacific are analyzed using the space-time spectrum analysis and wavelet transform methods. The results show that the ISO of v is very different from those of u and h, with the former representing the meridional low-frequency disturbances, which are the most active in the subtropics and mid-high latitudes, but very weak in the tropics. In the subtropical Northern Hemisphere, the energies of the ISOs of u and h are both concentrated on the waves with wave number of 1 and periods of 30-60 days, while the main energy of the ISO of v is concentrated on the waves with wave numbers of 4-6 and periods of 30-60 and 70-90 days. The westward propagating energies for the 30-60-day oscillations of u; v; and h are all stronger than the eastward propagating energies in the subtropics.In addition, the ISO of v is the strongest (weakest) in summer (winter) over the subtropics of East Asia and northwestern Pacific, while the situation is reversed over the subtropical northeastern Pacific, revealing a "seesaw" of the ISO intensity with seasons over the subtropics from the northwestern to northeastern Pacific.In the subtropical northwestern Pacific, the interannual and interdecadal changes of the ISO for v at 850 hPa indicate that its activities are significantly strong during 1958-1975, while obviously weak during 1976-1990, and are the strongest during 1991-2000, and its spectral energy is obviously abnormal but ruleless during the ENSO periods. However, in the 2-7-yr band-pass filtering series, the interannual changes of the v ISO over the subtropical northwestern Pacific contain distinct ENSO signals. And in the 9-yr low-pass filtering series,the v ISO changes over the subtropical northwestern Pacific are significantly out of phase with the changes of the Nieno-3.4 SST , whereas the v ISO changes in the subtropical northeastern Pacific are significantly in phase with the changes of the Nieno-3.4 SST.