Atmospheric Teleconnection Associated with the Atlantic Multidecadal Variability in Summer: Assessment of the CESM1 Model
Although observational studies indicate a close link between North Atlantic sea surface temperature anomalies (AMV) and circum-global climate anomalies, it does not imply that we know precisely what are the underlying physical mechanisms connecting AMV with these circum-global climate anomalies. Because the typical period of AMV is 50–80 years, the limited instrumental records could not resolve multiple cycles of AMV, thus preventing us from building a statistically significant relationship between AMV and its regional impacts. While the model-based approaches could overcome this limitation and provide a comprehensive resource for studying the global impacts of AMV (Nicolì et al. 2020), the incapability of models to simulate the typical AMV frequency hinders our ability to explore the physical mechanisms governing the AMV and it impacts on regional climate. Therefore, it is essential for us to evaluate under what conditions models could simulate the AMV and its regional impacts more faithfully since AMV is such an important source of variability in the Earth’s climate. We will also assess whether the correction of the simulated North Atlantic sea surface temperature (SST) biases can produce a better AMV teleconnection. Our focus is the summer (June–July–August, JJA) season and we evaluate how well the models can reproduce the observed time evolution of the AMV, the associated atmospheric teleconnections, and their impacts on summer rainfall and temperature.
Our results suggest that the uninitialized large ensemble simulations from all models can produce an AMV time evolution and its regional climate impacts similar to the observations to a certain degree. By initializing the observed oceanic condition in decadal prediction simulations, the simulated AMV and its regional impacts are closer to the observed ones than those in uninitialized ensemble simulations. In addition, the pacemaker simulations that nudged the time-evolving observed North Atlantic sea surface temperature anomalies produce spatiotemporal characteristics of the AMV and AMV climate impacts closer to the observed ones than the uninitialized simulations. We conclude that although coupled models can produce AMV and its regional impacts similar to observed, proper initialization and bias correction of the sea surface temperature spatial and temporal structure can improve this capability.
By analyzing four sets of free-run twentieth-century large ensemble simulations from four CMIP5-class models, and CESM-DPLE and CESM1 North Atlantic idealized and pacemaker simulations, we show that the AMV is closely associated with the climate variability in northern mid-latitude regions through a circum-global atmospheric teleconnection extending globally from the North Atlantic through Eurasia. Both CESM1 idealized AMV simulations and North Atlantic pacemaker simulations show that the AMV contribute to a circum-global teleconnection closely resembling the observational pattern. During the AMV + period, the negative geopotential height anomalies appear over North Africa, Western Europe, and northern Asia, which leads to an increase in summer precipitation, but a decrease in SAT over these regions. By contrast, positive geopotential height anomalies appear over Eastern Europe and associate with an increased SAT and a decreased summer precipitation, and vice versa for the AMV– period. The evaluation of the four CMIP5-class model’s capability in simulating the AMV-related atmospheric teleconnection via analyzing the free–run MMLEA twentieth-century historical simulations show that the MMLEA is capable of generating a teleconnection pattern similar to the observations, but with (sometimes significantly) shifted locations of the action centers in 500 hPa geopotential height field with an altered strength.