Wind Changes over Time and Space as a Climate Metric to Diagnose Temperature Trends

In Pielke et al. 2001: Analysis of 200 mbar zonal wind for the period 1958-1997. J. Geophys. Res., 106, D21, 27287-27290, we demonstrated that temporal and spatial trends in upper tropospheric winds can be used to diagnosis the trends in the tropospheric temperatures below the level of the wind observations. This concept uses what is called the “thermal wind relation” and is a robust, well-established relationship between the change of wind with altitude and the horizontal temperature gradient.

In that paper, we showed as an example, that a surface (1000 hPa) to 200 hPa layer-mean horizontal north-south temperature gradient of 1 degree Celsius (using an average latitude of 43 degrees) would produce a 200 hPa wind speed increase of 4.6 meters per second. This means that if there were a 0.1-0.2 degree Celsius decrease in the zonally-averaged gradient between the high- and mid-latitudes over a period of a decade or more, we would see a 0.46-0.92 meter per second decrease in the wind speed over the same time period. Such a magnitude of change in the tropospheric layer-averaged temperatures has been observed (e.g., see http://vortex.nsstc.uah.edu/data/msu/t2lt/uahncdc.lt).

There is an important message to these numbers. First, the wind speed change that must result from the observed reduction of the zonally-averaged layer-average temperature gradient is quite small in comparison to the typical upper tropospheric wind speeds associated with synoptic weather features that can be 50 meters per second and more. This small trend places the use of global- and zonally-averaged tropospheric temperature trends in an appropriate perspective, in that they are a poor metric to diagnose climate variability and change.

Secondly, in order to assess the accuracy of satellite and radiosonde assessments of tropospheric temperature trends, the monitoring of the trends in wind speed and direction provides an independent metric to assess the temperature trends. If the Arctic troposphere is really warming relative to the mid- and tropical latitudes, we should see a weakening of the zonally-averaged wind speeds. However, for the period 1958-1997, in
Chase et al. 2002: A proposed mechanism for the regulation of minimum midtropospheric temperatures in the Arctic. J. Geophys. Res., 107(D14), 10.10291/2001JD001425, we actually found that the 200 hPa winds had become somewhat stronger at higher latitudes.

The finding of the relatively small magnitude of observed zonally- and globally- averaged tropospheric temperature trends with respect to the upper tropospheric winds further illustrates why we need to focus on regional tropospheric temperature changes. Figure 11 in Pielke et al. (2005), shows spatial trends for 1979-2001 in the 300 hPa winds from the NCEP and ERA-40 Reanalyses. Areas with relatively large anomalies are diagnosed. It is the larger regional trends that have the much more direct effect on our weather. The stronger winds across the north Pacific, for example, are just one example of a trend in regional circulation patterns that directly affect the climate of North America.

Such regional assessments of tropospheric temperature trends should be a major initiative within the IPCC and other climate assessments. This needs to be completed on seasonal as well as annually averaged time scales. Our July 28, 2005 blog on What is the Importance to Climate of Heterogeneous Spatial Trends in Tropospheric Temperatures? provides further discussion as to why the regional spatial scale is so important to our understanding of climate variability and change.

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