Abstract
Numerous studies demonstrated that the Pacific Meridional Mode (PMM) can excite Central Pacific (CP) El Niño-Southern Oscillation (ENSO) events and that the PMM is mostly a stochastic phenomenon associated with mid-latitude atmospheric variability and wind-evaporation-SST feedback. Here we show that CP sea surface temperature (SST) variability exhibits high instantaneous correlations both on interannual (ENSO-related) and decadal (Pacific Decadal Oscillation (PDO)-related) timescales with the PMM. By prescribing an idealized interannual equatorial CP ENSO SST forcing in a partially-coupled atmosphere/slab ocean model we are able to generate a realistic instantaneous PMM response consistent with the observed statistical ENSO/PMM relationship. This means that CP ENSO and the PMM can excite each other respectively on interannual timescales, strongly suggesting that a fast positive feedback exists between the two phenomena. Thus, we argue that they cannot be considered two independent dynamical entities. Additionally, we show that the interannual CP ENSO SST forcing generates atmospheric circulation variability that projects strongly on the Aleutian Low and North Pacific SST anomalies that exhibit the characteristic PDO pattern.
Highlights
The Pacific is home to climate variability on sub-annual to interannual timescales associated with the El Niño-Southern Oscillation (ENSO)[1,2] and its modulation by the annual cycle[3,4], as well as on longer decadal-to-multi-decadal timescales[5,6] associated with the Pacific Decadal Oscillation (PDO)[7,8], the North Pacific Gyre Oscillation (NPGO)[9,10], and the Interdecadal Pacific Oscillation (IPO)[11]
It has been demonstrated in many studies that the Pacific Meridional Mode (PMM) is able to induce Central Pacific (CP) ENSO variability
To reconcile the co-existence of these different mechanisms, a strong positive feedback needs to be present between CP ENSO and the PMM, in which equatorial CP ENSO sea surface temperature (SST) are able to excite a PMM response without any significant delay
Summary
Numerous studies demonstrated that the Pacific Meridional Mode (PMM) can excite Central Pacific (CP) El Niño-Southern Oscillation (ENSO) events and that the PMM is mostly a stochastic phenomenon associated with mid-latitude atmospheric variability and wind-evaporation-SST feedback. In agreement with previous studies[10] we find that high-latitude Pacific decadal variability can be induced by integrating tropical CP ENSO forcing The projection of this low-frequency variability on the PMM spatial pattern might explain the observed decadal signal in CP SSTs. Note that throughout this paper we characterize the PMM by using the raw (for which the ENSO signal is not removed) SST expansion coefficient-based PMM index, which differs from the most commonly used PMM index definition that aims to remove some of the ENSO signal (for details refer to the Methods section)
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