Abstract
Recent studies have discovered an intriguing nonstationary relationship between El Ninõ–Southern Oscillation (ENSO) and the Western Pacific (WP) teleconnection pattern, one of the most prominent winter atmospheric circulation patterns in the North Pacific, with a regime-dependent interdecadal modulation of significant and insignificant correlations. However, the physical process underlying the observed nonstationary ENSO-WP relationship is a puzzle and remains to be elucidated, which is also essential for clarifying the still-debated nontrivial issue on whether the WP is directly forced by ENSO or by midlatitude storm tracks-driven intrinsic processes. Based on empirical orthogonal function (EOF) analysis of the upper-tropospheric teleconnection patterns and associated Rossby wave sources (RWS), we show that the nonstationarity in question is due to the regime-dependent constructive or destructive interference in meridional overturning circulation between the two leading EOFs of RWS best correlated with ENSO and WP, respectively. The observed insignificant correlation between ENSO and the WP after the 1988 regime shift can be explained by interrupted teleconnection between the tropics and high latitudes due to the collapse of the subtropical bridge pillar in the jet entrance region, consequence of the destructive interference. This suggested interference mechanism related to the regime-dependent upper-level RWS fields has significant implications for resolving the puzzle that hinders better understanding of decadal regime behaviors of the climate system in the North Pacific.
Highlights
Atmospheric teleconnections, defined as significant simultaneous correlations in meteorological parameters at remote locations on Earth[1], can be explained dynamically by the propagation of Rossby wave trains[2,3], which act like an “atmospheric bridge” between different parts of the global climate system[4,5]
Consistent with the first report of the phenomena by Wang et al.[22], a recent study[23] has pointed out that during the period 1958–1976, El Ninõ–Southern Oscillation (ENSO) exerted a strong impact on the North Pacific Oscillation (NPO), the surface expression of the WP16, while during the period 1977–2010 the ENSO-NPO relationship broke down
A more quantitative investigation of the ENSO-Western Pacific (WP) relationship is undertaken here by analyzing the wintertime upper-level circulation patterns and associated Rossby wave sources (RWS) generated by divergent flow[3], demonstrating that the empirical orthogonal function (EOF) analysis of RWS is a powerful approach to separate the respective impact of ENSO and the WP on the meridional overturning circulation, and is crucial for quantitatively evaluating their regime-dependent interferences
Summary
Large-scale upper-level circulation patterns and associated storm track activity. Before presenting the EOF analysis of RWS in the North Pacific, it is instructive to briefly review the three leading EOF modes of the winter (December–February) 300 hPa zonal wind (U300) from the National Centers for Environmental Prediction–National Center for Atmospheric Research (NCEP/NCAR) reanalysis data[27] for the 46-yr period 1970–2015 (see Methods). The anomalous TPW for 1977–1987 (Fig. 5g), obtained as combined regressions onto the two EOFs of RWS, shows a well-established tropical-extratropical teleconnection pattern in the western Pacific, with a tropical ascent in the Philippines Sea, a subtropical subsidence south of Korea and Japan, and a subpolar ascent at Kamchatka This is consistent with a previous work[13] showing significant correlations between TPW and both the NPO and ENSO in the vicinity of the above three poles of teleconnection for an extended period of 1973–1987 (Fig. 6a,c). For the post-regime shift epoch, no divergent winds originating from the tropics reach the jet entrance region west of 150°E (see the red dashed line in Fig. 7f), acting to destroy the subtropical bridge pillar Taken together, these results are practically same as those of the RWS analysis, nicely supporting the original concept of the nonstationary interference of the RWS-associated meridional overturning circulation as a key mechanism responsible for the regime-dependent nonstationary ENSO-WP relationship
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