We began by examining the correlation between the summer (JJA) NEA-NDVI and the preceding (FMA) ENSO index, and calculated lines of least-squares fit between the El Niño and La Niña episodes of the ENSO cycle and the corresponding NEA-NDVI values. The results showed that, approximate 30.3% of the summer vegetation variance could be explained by the preceding ENSO variation, while the correlation coefficients between the summer (JJA) NEA-NDVI and the preceding (FMA) ENSO index were −0.53 (raw data), and −0.55 (detrended data), both statistically significant (α 0.01), as estimated using the standard Student’s t-test. However, we found that there was indeed an asymmetric response of NEA-NDVI to the preceding ENSO cycle. For El Niño episodes, the correlation coefficient between NEA-NDVI and the warm phase of Niño3 index was −0.52. Meanwhile, the correlation between NEA-NDVI with the cold phase of Niño3 index was 0.25. That is, over the period 1982–2006, negative NDVI anomalies with statistical significance appeared over NEA during El Niño episodes; whereas, during La Niña episodes, only slightly increasing NDVI anomalies appeared, without statistical significance, over NEA (Fig.1). These asymmetric responses might enable the Niño3 index to serve as an accurate predictor of subsequent summer vegetation vigor over NEA.
The possible reasons for the asymmetric relationship between the summer (JJA) NEA-NDVI and the preceding (FMA) ENSO cycle can be summarized as follows:
(1) The climate conditions for vegetation growth may go far beyond the various summertime thresholds for different climate factors. Therefore, the above-threshold variation of climate factors could not affect the vegetation vigor, in terms of the statistical significance of high-value JJA NEA-NDVI years. Whereas, if the variation of climate factors is below the threshold, it would lead to a significant adverse impact on vegetation growth in low-value JJA NEA-NDVI years.
(2) Compared with a noticeable drop during El Niño, the summer NEA-NDVI during La Niña was found to slightly increase. However, because the negative impact of rainfall and soil is more significant during La Niña, the increase in summer NEA-NDVI would be partly inhibited during La Niña and the growth rate would become less significant as a result.
(3) The greater intensity of the SSTA over the eastern Pacific during El Niño compared with La Niña could contribute to the asymmetry of ENSO and further influence climate factors asymmetrically. During 1982–2006, the spring (FMA) SSTA during El Niño events was +1.25°C, even as high as 2°C (the peak appeared in 1998). However, the cold FMA SSTA during La Niña events showed consistently smaller amplitude at −0.65°C.
(4) The inconsistency in the development periods of El Niño and La Niña episodes may lead to different atmospheric circulation anomalies and further influence vegetation vigor to some extent. In this study, the El Niño cases were in the decaying stage, while the La Niña cases were in the maintenance stage (Fig.2).
(5) Different structures of the EAP teleconnection in the summer atmospheric circulation may have different impacts on climate factors, and even vegetation vigor, over NEA (Fig.3).
The possible physical mechanisms for the asymmetric and non-simultaneous ENSO–NDVI relationship uncovered and discussed in this study are summarized in the block diagram shown as Fig.4. During the El Niño episodes, the anticyclonic anomaly over the Philippine Sea (PSAC) was associated with the EAP teleconnection, with a distinct poleward wave-activity flux (anticyclone–cyclone–anticyclone meridional structure) from the Philippine Sea to the Sea of Okhotsk in the lower–middle troposphere. NEA is located right at the border of the mid-latitude cyclone and high-latitude anticyclone, accompanied by a strong northeasterly wind anomaly and the transport of cold, dry air from Russia and East Siberia to the region. In addition, local thermal changes, negative sensible heat flux, and positive latent heat flux anomalies, also play a vital role in the low-temperature anomalies over NEA. Therefore, the vegetation vigor is distinctly inhibited during El Niño episodes. However, during La Niña episodes, owing to the persistent negative SSTA and Matsuno–Gill pattern over the equatorial western Pacific from spring to summer, a significant anticyclonic anomaly can be found in the area north of the Solomon Islands. In summer, the anticyclonic anomaly suppresses local convection and further excites a quasi-meridional teleconnection with a northeastward anticyclone–cyclone–anticyclone structure along the Great Circle Route from the tropical western Pacific to the North Pacific. Therefore, the northeastward atmospheric circulation anomalies have little impact on the climate of NEA. Besides, cloudless and positive net solar flux anomalies, without statistical significance, could be found over NEA. As a result, these conditions led to a slight increase temperature and decrease in precipitation over NEA, and further favored vegetation vigor to a small degree during the La Niña episodes.