Abstract
AbstractClear decadal variations exist in the predictability of the El Niño–Southern Oscillation (ENSO), with the most recent decade having the lowest ENSO predictability in the past six decades. The Bjerknes Feedback (BF) intensity, which dominates the development of ENSO, has been proposed to determine ENSO predictability. Here we demonstrate that decadal variations in BF intensity are largely a result of the sensitivity of the zonal winds to the zonal sea level pressure (SLP) gradient in the equatorial Pacific. Furthermore, the results show that during low‐ENSO predictability decades, zonal wind anomalies over the equatorial Pacific are more linked to SLP variations in the off‐equatorial Pacific, which can then transfer this information into surface temperature and precipitation fields through the BF, suggesting a weakening in the ocean‐atmosphere coupling in the tropical Pacific. This result indicates that more attention should be paid to off‐equatorial processes in the prediction of ENSO.
Highlights
The results show that during low-El Niño–Southern Oscillation (ENSO) predictability decades, zonal wind anomalies over the equatorial Pacific are more linked to sea level pressure (SLP) variations in the off-equatorial Pacific, which can transfer this information into surface temperature and precipitation fields through the Bjerknes Feedback (BF), suggesting a weakening in the ocean-atmosphere coupling in the tropical Pacific
The anomalously high pressure center in the northeastern Pacific between the 1959–1968 period and the 1972–1981 period found in the HadSLP2 data set is not as strong as that found in the NCEP data set (Figures S4b and S4e)
This possibility is supported by the fact that the mean SLP differences calculated from NCEP and HadSLP2 become similar for the recent decades (i.e., 1992–2001 and 2004–2013; Figures 4a and 4d) when SLP observations become more available over oceans
Summary
Developing the ability to predict El Niño–Southern Oscillation (ENSO) events one or more seasons in advance is of great societal importance but remains a challenging task [Latif et al, 1998; Kirtman and Schopf, 1998; Chen et al, 2004; Chen and Cane, 2008; Jin et al, 2008; Barnston et al, 2012]. Recalling the simple signal-to-noise principle, a larger standard deviation (SD) in the Niño3.4 (5°S–5°N; 120°–170°W) SST index (i.e., signal) constitutes a larger signal-to-noise ratio, which implies that the ENSO anomalies cannot be disrupted by the noise and results in a more persistent signal [McPhaden, 2003; Yu and Kao, 2007] To quantitatively verify this inference, the standardized decadal variations in the Niño3.4 SD and the ENSO persistence forecast skill are presented, which clearly shows that these forecasts have a high coherency across all six decades, with the correlation coefficient between the Niño3.4 SD and the averaged one to three leading months persistence forecast skill being as high as 0.84. The correlation coefficient between the BF intensity and the persistence forecast skill at 1–3 months lead is as high as 0.63 (passing the 10% significance level with 8 degrees of freedom on the basis of a Student’s t test), which supports the hypothesis that the decadal variations in ENSO predictability are strongly associated with the variations in the BF intensity in the tropical Pacific (Table S1 in the supporting information). Since reliable precipitation data are available only after 1979, we first contrast these three subprocesses in the lowest predictability decade (2004–2013; see Figure 1) with those in the neighboring decade (1992–2001) that has high predictability
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