Middle atmosphere temperature changes and the duration of summer

Details Middle atmosphere temperature changes and the duration of summer Middle atmosphere temperature changes and the duration of summer

Apr 2004

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2004jastp..66..437o&link_type=abstract

Journal of Atmospheric and Solar-Terrestrial Physics, Volume 66, Issue 6-9, p. 437-450.

Physics

18

Middle Atmosphere, Temperature Trends, Semi-Annual Oscillation, Summer Duration, Growing Season, Circulation Reversal

Scientific paper

Hydroxyl proxy temperatures have been measured at Wuppertal (51°N,7°E) since 1980 in an attempt to determine a long-term trend in the upper mesosphere. No trend has been found, if the mean temperatures of the various years are intercompared. If, however, monthly means are analyzed, monthly long-term trends are obtained that are positive or negative, and of considerable magnitude. They show a systematic variation from month-to-month, the average of which is about zero. These apparently conflicting results can be reconciled if a shape parameter of the seasonal temperatures is defined that is independent of the yearly mean temperature. The equivalent summer duration (ESD) is found to be a suitable parameter. It is defined as that part of the year when the temperature is below a threshold value Ts. The value of Ts(198K) is chosen such that the resulting ESD (about 160 days) is similar to the duration of the growing season on the ground. A long-term analysis of ESD shows an increase in summer duration of about 19 days (12%) in 20 years. This is similar to a corresponding change found for the vegetation period. A change (advance) of the phase of the semi-annual oscillation of mesospheric temperature is found to be closely related to both the variations of monthly trends, and the increase of ESD, respectively.
A similar analysis has been performed in the stratopause region (1hPa) by means of data from the Stratospheric Sounding unit (SSU), with comparable results. A long-term trend of ESD is found also at this altitude (44 days/26% in 20 years), and a systematic variation in monthly trends is seen as well. Again a phase change of the semi-annual oscillation is closely connected to this. There is, however, an important difference between the upper stratosphere data and those of the upper mesosphere: the ESD change is a decrease in the stratosphere, and the phase change is a regression! This result is confirmed by a further analysis in the lower stratosphere (30hPa), where the Summer Duration is defined as the interval between circulation turn-around in spring and autumn.
Recent GCM calculations indicate dynamical and thermal changes in the middle atmosphere as consequences of anthropogenic CO2 and O3 changes. They are similar to those reported here. Our circulation reversal data go back to the year 1958. They indicate that the trend seen before year 1980 was different from the trend after that year, and during long periods even opposite. This is difficult to reconcile with anthropogenic influences, and it therefore remains to determine to what extent the long-term variations seen may be man-made, or are part of a natural climate fluctuation.

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