One-to-Two Month Oscillations: Observed High-Latitude Tropospheric and Stratospheric Response to Tropical Forcing

Abstract

Careful spectral, correlation and coherence analyses of low-frequency fluctuations in global geopotential height data are presented. Attention is paid to proper statistical assessments. The main points are: 1) one-to-two month oscillating quasi-stationary wavetrains have recently been reported in the extratropical Southern Hemisphere troposphere, as far south as the edge of Antarctica. However, only weak correlations were observed with the supposed tropical forcing region, leading to the question of whether the wavetrain is a response to tropical forcing or possibly due to in situ instabilities on 1?2 month time scales. The present paper clears up this enigma with analyses of other tropical datasets which reveal clear correlation between low latitude source regions and the SH extratropical troposphere. 2) An earlier investigation found strong correlations between 1?2 month oscillations in the upper stratosphere and tropical troposphere, yet no vertical propagation was found directly above the tropics. This is explained in the present work with evidence of temperature fluctuations propagating initially quasi-horizontally towards higher latitudes from the Indonesian tropical troposphere, along the bottom of the tropopause to near 35°S. At this latitude, stratospheric winter westerlies allow vertical propagation of the 1?2 month perturbations up to the middle stratosphere where the wavetrain arches equatorward and upward to the stratopause. 3) Finally, Eliassen?Palm flux diagnostics for the SH stratosphere reveal that while the 1?2 month perturbations occasionally cause significant forcing of the zonal mean wind ?, on the long term average only about 10% of ??/?t can be attributed directly to these low-frequency eddies.