A Study Of The Similarities And Differences Between The North American Dipole And The North Atlantic Oscillation

The El Ni?o-Southern Oscillation(ENSO)phenomenon,which occurs in the tropical Pacific,is the strongest interannual climate signal.Its prediction is one of the key and difficult points in research.This paper uses the NCEP-NCAR reanalysis data,ERSST v3 b sea surface temperature dataset,CMAP precipitation and CMIP5 data to study the differences and connection between North American dipole(NAD)and North Atlantic Oscillation(NAO)in their time variability,spatial distribution and climate impact from the perspective of the impact of the air-sea system in the extra-tropical Pacific on ENSO forecast through Empirical Orthogonal Function,correlation and composite analyses.The study of the paper will provide an important reference for establishing a more sophisticated statistical model of the extra-tropical signals affecting ENSO and improving the ENSO forecast skill.The main results were as following:(1)In terms of their temporal variations,NAD and NAO exhibit tight phase-locking characteristics,and they both attain the maximum variance in winter.In addition,the fluctuations of the NAD are not completely consistent with those of the NAO,and that the NAD has its own temporal variability that is independent of the NAO.In terms of their spatial structure,the sea level pressure anomalies associated with the NAD are located in more western and southern areas than those associated with the NAO.The first mode of standardized EOF from North Atlantic to North America presents the distribution characteristics of NAD,and after removing NAO and ENSO signals,the first mode of EOF still presents the distribution characteristics of NAD,indicating that the existence of NAD does not depend on NAO and ENSO.(2)Compared with the NAO,the NAD has a more significant effect on the temperature and precipitation in the tropical North Atlantic(NTA).For the North Pacific,the NAD can more effectively in forcing SST warming during spring in the northeastern subtropical Pacific(NESP).This can extend equatorward to reach the equatorial central Pacific in the autumn,finally leading to a central Pacific(CP)-type El Ni?o event.In contrast,the NAO induces only weak SST warming over the NESP,so that a CP-type El Ni?o event does not occur.There are two possible reasons why the NAD can significantly impact the tropical Pacific.Compared with the NAO,the NAD influences the NTA SST more than the NAO.The NTA SST cooling during spring can induce an anticyclonic flow over the NESP region.Therefore,the NAD can exert a stronger effect on the NESP SST warming by influencing the NTA SST.Additional analysis indicates that the influence of the NAO can pass to the tropical Pacific only when the NAD and NAO have the same sign,suggesting that the NAD may serve as an important bridge linking the NAO to ENSO.(3)Most of the CMIP5 models can simulate the spatial distribution characteristics of the dipole sea-level pressure field of NAD in the North Atlantic-North America region in winter,and the effect of NAD on NTA region is more significant than that of NAO.However,most of the models are faile to simulate the impact of NAD on the North Pacific.