Relationship Between AO/NAO And ENSO And Their Impact On Climate Anomalies In China

ENSO is the leading pattern of interannual climate variability of the tropical Pacific ocean-atmosphere system, while AO/NAO is the primary mode of the atmospheric low-frequency variability in the extratropical Northern Hemisphere, which has considerable variability on intraseasonal time scales. They both exert crucial impact on the East Asian winter and summer climate anomalies. Based on low-pass filtered daily data, the intraseasonal variability of AO and its relationship with ENSO were analyzed. Then their influence on the surface air temperature (SAT) and precipitation anomalies in China during the boreal winter were studied. By using monthly data, the annual relationship between AO/NAO and East Asian summer monsoon (EASM) was investigated. This study also focused on the physical mechanisms responsible for the impact of AO/NAO on the winter and summer climate anomalies in China. The main conclusions are summarized as follows:(1) The intraseasonal variability of AO and its relationship with ENSO in boreal winter.The temporal evolution of the AO can be interpreted as being a Markov process with an e-folding timescale of about 6.6 days. Most of the AO events are characterized by a length of about 1 to 2 weeks and those with length of about 3 to 4 weeks also occur frequently. During the cold (warm) phase of ENSO, the occurrences of positive (negative) phase of AO are about 15% more frequent than those of the negative (positive) phase of AO. In addition, the cold phase of ENSO benefits the occurrence of positive AO events with length of about 2 to 4 weeks, while the warm phase of ENSO benefits the occurrence of negative AO events with length up to one month or more (about 4 to 7 weeks). Further analysis shows that ENSO exerts considerable influence on the temporal evolution and spatial pattern of the AO. The Pacific-Northern American pattern (PNA) plays an important role in linking the ENSO and the AO.(2) The impact of AO and ENSO on the winter SAT anomalies in northern China and its physical mechanism.In boreal winter, the SAT anomalies over the region northwards of 35°N in China, especially in the northeast China, are significantly related to the phase of AO. In addition, AO can explain about 12% of the variance of the regional averaged SAT anomalies over northeast China. During the different phases coupling of the AO and the ENSO, the SAT anomalies in northern China are mainly determined by the phase of AO. However, the frequency of extreme low SAT occurrence in northeast China is highest (about 31%) during the negative phase of AO combined with the warm phase of ENSO. Further analysis reveals that the influence of AO on the SAT anomalies in northern China is directly related to the 500-hPa East Asian Trough (EAT) anomaly, rather than relies on the indirect effect of the Siberian high. The EAT anomaly associated with the AO maintains primarily by the horizontal temperature advection induced by the zonal and meridional wind anomalies and the climatological mean temperature in the middle and low troposphere.(3) The impact of AO and ENSO on the winter SAT and precipitation anomalies in southern China and their physical mechanisms.In boreal winter, the positive phase of AO is usually followed by the below-normal SAT in southwest China and the above-normal precipitation in the middle and south of the Yangtze River valley and vice versa. During the different phases coupling the AO and the ENSO, the precipitation anomalies in southeast China are mainly controlled by the phase of ENSO, while the path of the water vapor transport anomalies as well as the center of the precipitation anomalies are modulated significantly by the phase of AO.The impacts of AO on the winter SAT and precipitation anomalies in southern China are closely related to a wave train from the Western Europe to the Middle East and northern Arabian Sea (referred as the Eurasian subtropical wave train) in the troposphere. During the positive phase of AO, the Middle East jet stream in the upper troposphere is intensified and the northern Arabian Sea is controlled by an anomalous anticyclone in the lower troposphere, accompanied by the intensified subtropical westerlies over the southern Tibetan Plateau and strengthened trough embedded in this subtropical westerlies. The strengthened trough can intensify the flow from the Bay of Bengal into China, which benefits the above-normal precipitation in southern China. The intensified subtropical westerlies over the southern Tibetan Plateau also benefits the formation of stratus clouds through intensifying the ascending motion again the lee side of the plateau. The increased amount of stratus clouds can cool the land surface through radiative forcing. Further analysis indicates that this Eurasian subtropical wave train primarily induces by the Ekman pumping associated with AO in the Mediterranean region. Barotropic modeling results suggest that the upper-level divergence anomaly in the Mediterranean region associated with AO can force a wave train almost the same as the observed one.(4) Change of the relationship between spring AO/NAO and EASM and its possible mechanism.The relationships between EASM and monthly AO/NAO were examined by using correlation analysis for the period from 1948 to 2009. It was showed that the EASM was well correlated with the previous spring (April-May) AO/NAO on annual time scale. But their relationship was characterized by large decadal variations, with change of sign from negative to positive in the 1970s. Based on previous knowledge of the role of the North Atlantic SSTA (sea surface temperature anomaly) triple in linking the spring AO/NAO and the EASM, observational analysis in this study revealed that the change of the spring AO/NAO-EASM relationship was closely related to the pre-winter (December-March) AO/NAO-SSTA tripole coupling mode, which has an asymmetric effect on the spring SSTA tripole during the different phases of spring AO/NAO. Before the 1970s, the pre-winter AO/NAO-SSTA tripole coupling mode only has a considerable weakening effect on the spring SSTA tripole during the positive phase of spring AO/NAO. After the 1970s, the effect of the former on the latter during both the positive and negative phases of the spring AO/NAO was negligible. Therefore, the symmetric effect of spring AO/NAO was combined with the asymmetric effect of pre-winter AO/NAO-SSTA tripole coupling mode on the spring SSTA tripole caused the decadal unstationary relationship between the spring AO/NAO and SSTA tripole, resulting in change of the AO/NAO-EASM relationship in the 1970s.(5) The possible mechanism responsible for the impact of winter and spring AO/NAO on the EASM.Previous research has shown that the North Atlantic SSTA tripole provides memory allowing the winter and spring AO/NAO to affect the EASM, which is supported by the observational results in this paper. Due to the change of the combined effect of the pre-winter and spring AO/NAO on the spring SSTA tripole in the 1970s, the amplitude and spatial structure of the summer SSTA tripole, which is closely related to the spring SSTA tripole, both change obviously in the 1970s. And thus the atmospheric circulation responses to the SSTA tripole in the summer are remarkably different before and after the 1970s. Furthermore, the influence of the summer SSTA tripole on the EASM is closely related to waveguide of the North Atlantic and the Asia subtropical jet streams (STJS). Whether the former can exert considerable impact on the latter depends on the meridional position of forcing relative to the North Atlantic STJS. These results suggest that the impact of the winter and spring AO/NAO on the EASM is sensitive to the spatial structure of the North Atlantic SSTA tripole induced by the former.