Abstract
El Niño Southern Oscillation (ENSO) represents the major driver of interannual climate variability at global scale. Observational and model-based studies have fostered a long-standing debate on the shape and intensity of the ENSO influence over the Euro-Mediterranean sector. Indeed, the detection of this signal is strongly affected by the large internal variability that characterizes the atmospheric circulation in the North Atlantic–European (NAE) region. This study explores if and how the low-frequency variability of North Pacific sea surface temperature (SST) may impact the El Niño-NAE teleconnection in late winter, which consists of a dipolar pattern between middle and high latitudes. A set of idealized atmosphere-only experiments, prescribing different phases of the anomalous SST linked to the Pacific Decadal Oscillation (PDO) superimposed onto an El Niño-like forcing in the tropical Pacific, has been performed in a multi-model framework, in order to assess the potential modulation of the positive ENSO signal. The modelling results suggest, in agreement with observational estimates, that the PDO negative phase (PDO−) may enhance the amplitude of the El Niño-NAE teleconnection, while the dynamics involved appear to be unaltered. On the other hand, the modulating role of the PDO positive phase (PDO+) is not reliable across models. This finding is consistent with the atmospheric response to the PDO itself, which is robust and statistically significant only for PDO−. Its modulation seems to rely on the enhanced meridional SST gradient and the related turbulent heat-flux released along the Kuroshio–Oyashio extension. PDO− weakens the North Pacific jet, whereby favoring more poleward propagation of wave activity, strengthening the El Niño-forced Rossby wave-train. These results imply that there might be conditional predictability for the interannual Euro-Mediterranean climate variability depending on the background state.
Highlights
El Niño-Southern Oscillation (ENSO) is known to be a major driver of global climate variability at the interannual time scale, representing probably the most important source of seasonal predictability worldwide (e.g. Doblas-Reyes et al 2013; McPhaden et al 2006), and potentially for the North Atlantic–European (NAE) sector (e.g. Dunstone et al 2016; Mathieu et al 2004; Scaife et al 2014)
While Pacific Decadal Oscillation (PDO)− leads to a zonal wind pattern that is consistent among the three models and characterized by a weakening of the jet at its exit region, PDO+ yields a more complex and less coherent signal in the multi-model ensemble
The impact of low-frequency sea surface temperature (SST) variability over the extratropical Pacific on the El Niño teleconnection has been explored. It is well-known that ENSO represents the primary source of seasonal predictability at the global scale (e.g. Manzanas et al 2014)
Summary
El Niño-Southern Oscillation (ENSO) is known to be a major driver of global climate variability at the interannual time scale, representing probably the most important source of seasonal predictability worldwide (e.g. Doblas-Reyes et al 2013; McPhaden et al 2006), and potentially for the North Atlantic–European (NAE) sector (e.g. Dunstone et al 2016; Mathieu et al 2004; Scaife et al 2014). Brönnimann (2007) reviewed and identified the canonical El Niño signature on the NAE circulation as characterized by a dipolar structure in sea level pressure between middle (negative anomalies) and high (positive anomalies) latitudes for the late-winter season (January–March; JFM) This is in agreement with the climatic signal correlated with the positive ENSO phase over the Euro-Mediterranean region, analyzed in an observational framework by Fraedrich and Müller (1992). Non-stationarity in the El Niño impact on Euro-Mediterranean precipitation, resembling the signal in observations, has been reported in long-term preindustrial simulations (López-Parages et al 2015, 2016b) In both cases, it was linked to the modulating effect of lowfrequency modes of sea surface temperature (SST) variability, affecting the tropospheric mean circulation which in turn constraints the ENSO teleconnection.
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