LAUSR

Abstract The temporal evolution of the temperature of the lower troposphere, middle troposphere, tropopause and lower stratosphere on a global, hemispheric and latitudinal basis is studied using the recent Version 6 of the UAH MSU/AMSU global satellite temperature data set. The analysis showed that any long-term increase or decrease in temperature is not smooth and uniform, i.e., it is not slow and monotonous. In contrast, it grows or decreases monotonously in a short period, then almost stabilizes for some time, and then repeats the same pattern. In other words, the long-term temperature trend is not linear but follows the stepwise increase or decrease. In more concrete, the temperature time series over the period 12/1978–07/2018 is composed by a noisy pattern superposed on a stepwise function with abrupt shifts in 1986, 1994 and 2014. In addition, we noticed that the superposition of the stepwise function on the ENSO time series provides a new composite time series, which fits very well with the observed temperature. If the latter is delayed for almost 4 months this fit is even better. This pattern applies to the temperature of the troposphere on a global, hemispheric and tropical basis. It does not apply to the temperature of the troposphere over the extratropics and polar regions, as well as to the temperature of the lower stratosphere. In particular, the extra-tropics of the northern hemisphere displays an Arctic Oscillation stair-shaped behaviour. Perhaps the stepwise pattern in temperature evolution may be due to the fact that the gradual changes in the periodic or quasi-periodic tropospheric temperature components may trigger sudden changes which with positive feedbacks, lead to a self-enhancing change. This can be so strong that the temperature reaches critical values and then with sudden changes goes from one equilibrium state to another.