In a first step we inspected height-resolved time series, which showed that in most regions the climate change signal of the UTLS is even visible by the naked eye. Figure 2 depicts, as an example, time series of geopotential height anomalies for northern hemispheric summer as simulated by one arbitrarily chosen ECHAM5 run for large-scale zonal mean bands. The time period 200l-2050 is highlighted with a frame since it was the basis for subsequent trend analysis (see Sect. 3). The model simulations show a strong signal of a geopotential height increase, reflecting the thermal expansion due to tropospheric temperature rise, through almost the whole UTLS region, typically up to about 20 hPa. The pressure levels of maximum increase depend on the region's latitude: in the tropics, the strongest signal can be found around 50 hPa, in the mid-latitudes around l50—l00 hPa and in the Arctic region even below. In the extra-tropics, stratospheric cooling leads to a decrease of geopotential height above around 20 hPa. Signals around and above this height level must be interpreted with caution, since the upper boundary layer of all investigated

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