Improving and Harmonizing El Niño Recharge Indices
El Niño Southern Oscillation (ENSO) is the leading mode of interannual climate variability, with large socioeconomical and environmental impacts. The main conceptual model for ENSO, the Recharge Oscillator (RO), considers two independent modes: the fast zonal tilt mode in phase with central‐eastern...
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Veröffentlicht in: | Geophysical research letters 2022-12, Vol.49 (23), p.n/a |
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Sprache: | eng |
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Zusammenfassung: | El Niño Southern Oscillation (ENSO) is the leading mode of interannual climate variability, with large socioeconomical and environmental impacts. The main conceptual model for ENSO, the Recharge Oscillator (RO), considers two independent modes: the fast zonal tilt mode in phase with central‐eastern Pacific Temperature (Te), and the slow recharge mode in phase quadrature. However, usual indices (western or equatorial sea level/thermocline depth h) do not orthogonally isolate the slow recharge mode, leaving it correlated with Te. Furthermore the optimal index is currently debated. Here, we develop an improved recharge index by objectively optimizing the RO equations fit to observations. (a) Te‐variability is regressed out, to build hind statistically‐independent from Te. Capturing the pure recharge, hind reconciles usual indices. (b) The optimum is equatorial plus southwestern Pacific hind_eq+sw (because of ENSO Ekman pumping meridional asymmetry). Using hind_eq+sw, the RO becomes more consistent with observations. hind_eq+sw is more relevant for ENSO operational diagnostics.
Plain Language Summary
El Niño and La Niña events have important impacts globally. A key element for long‐lead forecasts is the recharge state of the tropical Pacific Ocean, as captured in the Recharge Oscillator (RO) conceptual model. The RO considers two independent modes of oceanic variability, a fast adjustment process and a slow recharge/discharge process. However, usual recharge indices mix these two modes of variability, and can thus lead to ambiguous operational diagnostics of the actual oceanic recharge state. Here we develop a better recharge index, independent of the fast mode, which reconciles typical indices and allows us to go beyond the current geographical debate on the optimal metrics. We use an objective approach optimizing the RO resemblance to observations to find the optimal index: the independent sea level (or equivalently thermocline depth) averaged over the equatorial and southwestern tropical Pacific. We recommend this simple and unambiguous index for El Niño operational forecasts diagnostics.
Key Points
To clarify the debate on the Recharge Oscillator index, we develop an objective approach optimizing equation fit to observations
The recharge index must be based on the slow component of sea level/thermocline depth, taken independently from the fast zonal tilt mode: this reconciles usual metrics
The optimal index is this independent component averaged in the equ |
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ISSN: | 0094-8276 1944-8007 |
DOI: | 10.1029/2022GL101003 |