Atmospheric oscillations on time scales of 1-2 months

Abstract

<p>Low-frequency oscillations in the troposphere and stratosphere on time scales of 1-2 months are observed in long time series of globally gridded temperature and geopotential height data. The stratosphere, rarely investigated for 1-2 month oscillations, is the main subject of this study. A combination of statistics and dynamics is used to capture three-dimensional wave motion characteristics and the influence of 1-2 month oscillations in changing the stratospheric mean zonal wind. Observations are compared with several models encompassing 1-2 month time scales;In Section II we calculate statistical significance in the power spectra of 1-2 month oscillations throughout the stratosphere from 90 hPa to 1.5 hPa; the results support other studies which show that the oscillation is significant throughout much of the stratosphere, particularly in the Southern Hemisphere. Under the hypothesis of an Indian Ocean tropical forcing, 1-2 month oscillations in the high stratosphere are presumed to originate from 1-2 month temperature fluctuations propagating out of the tropospheric heat source region; these tropical fluctuations propagate into the winter hemisphere midlatitudes, and then upward into the stratosphere, resulting in an "out-then-up" conceptual picture of the wave propagation;Section III is a continuation of Section II statistics and a previous study that discovered an extratropical 35-60 day wavetrain in 200 hPa Southern Hemisphere geopotential heights. In Section III, both troposphere and stratosphere are combined to substantiate the "out-then-up" conceptual picture from Section II. Using the statistics of correlation and coherence, the previously discovered 35-60 day 200 hPa wavetrain is shown to be statistically connected to the Indian Ocean-western Pacific Ocean tropics. Time-lag correlation plots will indicate both a forcing of the wavetrain from this region and a possible "feedback" into this region by the wavetrain itself. The coherence statistic will be shown to exhibit a dominance of 1-2 month over other periods of the spectrum. Finally, the contribution of 1-2 month eddies in changing the mean stratospheric flow is presented, both for a special case and for long-term (eight years).</p>