North Atlantic Long-period Decadal Oscillation (amo), The Impact On The Climate Of The Asian Monsoon Region - Observations And Multi-mode Simulation

The Atlantic Multidecadal Oscillation (AMO), which is the fluctuation pattern of North Atlantic sea surface temperature (SST), varies over basin spatial scale and multidecadal time scale. It has a period of 65-80 years and an amplitude of 0.4oC. The AMO is found arisen from internal variability of climate system, particularly being linked to quasi-periodic fluctuation of the Thermohaline Circulation (THC). Previous studies have revealed that the AMO has significant influences on the global and regional climate of the twentieth century including Eurasia, North America, North East Brazil, African Sahel, India, as well as Atlantic hurricanes.The influence of the Atlantic Multidecadal Oscillation on Asian monsoonal climate in all four seasons is investigated by comprehensive observational analyses and ensemble experiments with three atmospheric general circulation models (AGCMs) of AM2, NCEP and CCM3. The results in both the observations and the models consistently suggest that the warm Atlantic sea surface temperature anomaly (SSTA), the warm AMO phase, gives rise to elevated air temperatures in East Asia and northern India but decreased air temperatures in much of central-southern India in all four seasons. Furthermore, the AMO acts as a pacemaker of multidecadal climate variations of the East Asia. The warm-phase AMO intensifies the East Asian summer monsoon but weakens the winter monsoon, and vise versa. This positive AMO anomaly also causes more rainfall in central and southern India in every season, particularly in summer and fall.The primary mechanism for the AMO exerting its influence is through intensifying middle and upper troposphere Eurasian heating. The heating strengthens the summer meridional land-sea thermal contrast but weakens the winter contrast, resulting in weakened Asian winter monsoons but enhanced summer monsoons.The observations show that the observed climate structure associated with warm and cold AMO phase exhibits asymmetries in the sense that the warm-phase anomalies do not but mirror images of the cold-phase anomalies. To figure out the linear and nonlinear components of the response, we carry out two sets of experiments, applying the positive or negative Atlantic SSTA. The response of warm surface temperature in East Asia and the dipole-mode of warm northern India and cold southern India was showed to be rather linear, but the response of India more rainfall in summer and fall to be highly nonlinear.This article also argued that significant warming in China in the recent two decades had been arisen from both the anthropogenic forcing of greenhouse gas and the natural forcing domained by the AMO. Along with the AMO entering a cold phase, the winter warming in China is projected to slow down, or even reverse by the middle term of 2020s.