Analysis of tropospheric temperature gradients via radiosonde wind data in the western tropical Pacific

Recent climate change detection studies that have focused on the vertical profile of temperature change show a warming troposphere and a cooling stratosphere, which is consistent with global climate models forced with a combination of anthropogenic and natural factors. Concern is raised in the Tropics, however, where radiosondes and one of the satellite analyses (UAH) show significantly less warming in the lower troposphere relative to the surface. This contradicts climate model predictions, which show a maximum warming in the tropical mid-troposphere. This discrepancy may be due to artificial (i.e. non-climatic) effects in the satellites and radiosondes.

Analysis of the wind field offers an alternative approach to the monitoring of temperature patterns and their change over time. Assuming geostrophic and hydrostatic balance, the thermal wind equation (TWE) relates the vertical wind shear to the horizontal gradient of temperature. Upon integration of the TWE, a relationship between the geostrophic wind at two pressure levels and the horizontal gradient of the mean temperature in the layer (or the geopotential difference between layers) is obtained.

The accuracy of the TWE was investigated using 70 radiosondes in the western tropical Pacific. The TWE was subsequently used, along with the 850 hPa and 300 hPa winds, to determine the meridional gradient of the geopotential difference (Δ Φ) between these two pressure surfaces. Preliminary results show that for most latitude bands, the meridional Δ Φ gradient based on observed temperature exhibits a negative trend over the satellite era (1979-2004), implying that the depth-averaged (tropospheric) temperature near the equator has decreased relative to that at higher latitudes. The corresponding meridional Δ Φ gradient based on the TWE, however, shows negligible change.