Asian Origin of Interannual Variations of Summer Climate over the Extratropical North Atlantic Ocean

The authors have identified an interannual relationship between Asian tropospheric temperature and the North Atlantic Ocean sea surface temperature (SST) during summer (May–September) and discussed the associated features of atmospheric circulation over the Atlantic–Eurasian region. When tropospheric temperature is high (low) over Asia, positive (negative) SST anomalies appear in the extratropical North Atlantic. This relationship is well supported by the changes in background atmospheric circulation and ocean–atmosphere–land thermodynamic processes. When heat transfer from the land surface to the atmosphere over Asia strengthens, local tropospheric temperature increases and positive temperature anomalies propagate westward from Asia to the North Atlantic, leading to an increase in summer tropospheric temperature over the Atlantic–Eurasian region. Accordingly, a deep anomalous ridge occurs over the extratropical North Atlantic Ocean, with low-level southerly anomalies over the western portion of the ocean. Sensitivity experiments with climate models show that the interannual variations of the North Atlantic–Eurasian atmospheric circulation may not be forced by the extratropical Atlantic SST. Instead, experiments with changing Asian land surface heating capture the above observed features of atmospheric circulation anomalies, westward propagation of tropospheric anomalies, and Atlantic SST anomalies. The consistency between the observational and model results indicates a possible impact of Asian land heating on the development of atmospheric circulation and SST anomalies over the Atlantic–Eurasian region.