Asymmetric response in Northeast Asia of summer NDVI to the preceding ENSO cycle

The possibility of an asymmetric response of summer (June-July-August; JJA) vegetation vigor over Northeast Asia (NEA), captured by Advanced Very High Resolution Radiometer Normalized Difference Vegetation Index (NDVI), to the preceding spring (February-March-April; FMA) El Nio-Southern Oscillation (ENSO) cycle was investigated. Over the period 1982-2006, negative NDVI anomalies with statistical significance appeared over NEA during El Nio episodes; whereas, during La Nia, only slightly increasing NDVI anomalies appeared, and without statistical significance. Results showed the asymmetric responses of atmospheric circulations to El Nio and La Nia to be the most likely main mechanisms responsible for the asymmetric impacts of the spring (FMA) ENSO cycle on summer (JJA) NDVI over NEA (NEA-NDVI). During El Nio, a meridional teleconnection with an anticyclone-cyclone-anticyclone poleward structure is formed from the Philippine Sea to the Sea of Okhotsk. NEA is located right at the border between the mid-latitude cyclone and the high-latitude anticyclonic anomalies, accompanied by a strong northeasterly wind anomaly and the transport of dry, cold air from Russia and East Siberia to NEA. Therefore, the vegetation vigor is distinctly inhibited during El Nio episodes. However, during La Nia episodes, there is a quasi-meridional teleconnection along the Great Circle Route from the tropical western Pacific to the North Pacific, which is quite different from the meridional teleconnection formed during El Nio episodes. Results showed that the northeastward pattern has little impact on circulation and climate factors over NEA. As a result, during La Nia, only a slightly increasing NDVI anomaly appears over NEA, and without statistical significance.