Formation mechanism for the amplitude of interannual climate variability in subtropical northern hemisphere: relative contributions from the zonal asymmetric mean state and the interannual variability of SST

The Amplitude Interannual climate Variability (AIV) differs among the subtropical northern hemisphere, and the Western North Pacific (WNP) was claimed to exhibit the largest AIV. The robustness of the AIV pattern is investigated in this study with different atmospheric variables from multiple datasets. As consistently shown by the interannual variance patterns of precipitation and circulation, the AIV over subtropical northern hemisphere closely follows the mean state of precipitation, where higher (lower) AIV is located at moister (drier) regions. The largest AIV is seen over the broad area from South Asia to WNP, followed by a secondary local maximum over the Gulf of Mexico. To further investigate the formation mechanism for the AIV pattern, numerical simulations are performed by Community Atmosphere Model version 4 (CAM4). The zonal asymmetry of AIV is reduced if the interannual SST variability is removed, and it almost disappears if the zonal asymmetry of SST mean state is removed. The results suggest that the zonal asymmetric AIV pattern primarily originates from the zonal asymmetric SST mean state, and it is amplified by the interannual SST variability. The atmospheric convection–circulation feedback plays a key role in connecting the AIV with the mean state precipitation. In both observation and CAM4 simulations, stronger (weaker) convection–circulation feedback is seen in moister (drier) regions. By modulating the mean state precipitation and the associated intensity of convection–circulation feedback, the zonal asymmetric SST mean state accounts for the zonal asymmetry of AIV in the subtropical northern hemisphere.