Spatiotemporal Divergence of the Warming Hiatus over Land Based on Different Definitions of Mean Temperature

Existing studies of the recent warming hiatus over land are primarily based on the average of daily minimum and maximum temperatures (T(2)). This study compared regional warming rates of mean temperature based on T(2) and T(24) calculated from hourly observations available from 1998 to 2013. Both T(2) and T(24) show that the warming hiatus over land is apparent in the mid-latitudes of North America and Eurasia, especially in cold seasons, which is closely associated with the negative North Atlantic Oscillation (NAO) and Arctic Oscillation (AO) and cold air propagation by the Arctic-original northerly wind anomaly into mid-latitudes. However, the warming rates of T(2) and T(24) are significantly different at regional and seasonal scales because T(2) only samples air temperature twice daily and cannot accurately reflect land-atmosphere and incoming radiation variations in the temperature diurnal cycle. The trend has a standard deviation of 0.43 °C/decade for T(2) and 0.41 °C/decade for T(24), and 0.38 °C/decade for their trend difference in 5° × 5° grids. The use of T(2) amplifies the regional contrasts of the warming rate, i.e., the trend underestimation in the US and overestimation at high latitudes by T(2).