Abstract
The Kuroshio Extension (KE) exhibits prominent decadal fluctuations that enhance the low-frequency variability of North Pacific climate. Using available observations, we show evidence that a preferred decadal timescale in the KE emerges from the interaction between KE and the central tropical Pacific via Meridional Modes. Specifically, we show that changes in the KE states apply a persistent downstream atmospheric response (e.g. wind stress curl, 0–12 months timescales) that projects on the atmospheric forcing of the Pacific Meridional Modes (PMM) over 9 months timescales. Subsequently, the PMM energizes the central tropical Pacific El Niño Southern Oscillation (CP-ENSO) and its atmospheric teleconnections back to the Northern Hemisphere (1–3 months timescale), which in turn excites oceanic Rossby waves in the central/eastern North Pacific that propagate westward changing the KE (~3 years timescales). Consistent with this hypothesis, the cross-correlation function between the KE and the PMM/CP-ENSO indices exhibits a significant sinusoidal shape corresponding to a preferred spectral power at decadal timescales (~10 years). This dynamics pathway (KE→PMM/CP-ENSO→KE) may provide a new mechanistic basis to explain the preferred decadal-timescale of the North Pacific and enhance decadal predictability of Pacific climate.
Highlights
To quantify how the KE signal propagates through the sequence we examined the signal to noise (SNR = 1/[1 − R2]) ratio of the KE index against indices of the wind stress curl, PMM, Central Pacific ENSO (CP-ENSO), NPGO, and KE using optimal lag correlations (R) at 12, 28, 30, 34, 60 months, which are consistent with the timescale of propagation of the sequence
We cannot exclude that the central Pacific SSTa have some impact on, and explain some of the variance (R = 0.1–0.2) of, the KE wind stress curl response pattern, as evident from correlating the index of the curl response pattern with concurrent and leading SSTa in the tropical Pacific (Supplementary information Fig. 6c). This is consistent with some studies suggesting that the CP-ENSO can influence the initiations of the North Pacific PMM in the following season by altering the North Pacific Oscillation (NPO) variability[52]
The observational evidence in this study provides a mechanistic hypothesis for exploring a new set of decadal climate interactions between the North Pacific western boundary current and the tropical Pacific, which could explain the quasi-preferred decadal peak in the observed spectrum of Pacific climate
Summary
Received: 11 February 2019 Accepted: 27 August 2019 Published: xx xx xxxx timescale oscillations in Pacific climate. The atmospheric anomalies in the Aleutian Low or NPO excite and drive the westward-propagating oceanic Rossby waves (e.g. the PDO and NPGO SSHa signals) that reach and impact the KE state on timescales of 2.5–3 years[20,22,23,30,47] This sequence of teleconnections is summarized in Hypothesis 2 in Fig. 3: KE (12 months)→ KE downstream response (9 months) → PMM/CP-ENSO (1–3 months)→ NPGO (36 months)→ KE (note that while the low-frequency expression of this sequence reveals a CP-ENSO/NPGO signatures, on interannual timescales we don’t exclude contributions from eastern Pacific ENSO/PDO). The spatial progression of the KE index in wind stress curl and SSH field reveals that the KE atmospheric/oceanic downstream response patterns (Fig. 5h,i at lag 0–12 month) are not correlated to the NPGO forcing patterns, showing a significant structural difference in the eastern North Pacific region (e.g. Fig. 5h at lag 0–12 month vs Fig. 5e). The temporal interactions between the KE and PMM/CP-ENSO (Fig. 6a) along with the spatial signatures inferred from lead/lag correlations with the KE index (Fig. 6b) support our hypothesis that decadal fluctuations of KE can emerge through a two-way climate coupling between the North Pacific and the tropics
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