Nonlinearity in the tropospheric pathway of ENSO to the North Atlantic

Abstract

Abstract. The El Niño–Southern Oscillation (ENSO) can exert a remote impact on North Atlantic and European (NAE) winter climate. This teleconnection is driven by the superposition and interaction of different influences, which are generally grouped into two main pathways, namely the tropospheric and stratospheric pathways. In this study, we focus on the tropospheric pathway through the North Pacific and across the North American continent. Due to the possible nonstationary behavior and the limited time period covered by reanalysis datasets, the potential nonlinearity of this pathway remains unclear. In order to address this question, we use a simplified physics atmospheric model forced with seasonally varying prescribed sea surface temperatures (SST) following the evolution of different ENSO phases with linearly varying strength at a fixed location. To isolate the tropospheric pathway the zonal mean stratospheric winds are nudged towards the model climatology. The model experiments indicate that the tropospheric pathway of ENSO to the North Atlantic exhibits significant nonlinearity with respect to the tropical SST forcing, both in terms of the location and amplitude of the impacts. For example, strong El Niño leads to a significantly stronger impact on the North Atlantic Oscillation (NAO) than a La Niña forcing of the same amplitude. For La Niña forcings, there is a saturation in the response, with no further increase in the NAO impact even when doubling the SST forcing, while this is not the case for El Niño. These findings may have important consequences for long-range prediction of the North Atlantic and Europe.

Stratospheric Nudging And Predictable Surface Impacts (SNAPSI): A Protocol for Investigating the Role of the Stratospheric Polar Vortex in Subseasonal to Seasonal Forecasts

“…Douville, 2009), even if only the zonally symmetric component is imposed (e.g. Simpson et al, 2011;Hitchcock and Simpson, 2014;Zhang et al, 2018;Jiménez-Esteve and Domeisen, 2020). By comparing the difference between the nudged and control ensembles, the processes that drive this downward coupling can be diagnosed in each model for a variety of events of interest.…”

“…The protocol presented here is primarily based on a zonally-symmetric nudging technique that has been used successfully to identify stratospheric influences on the tropospheric circulation in both hemispheres (Simpson et al, 2011;Hitchcock and Simpson, 2014;Zhang et al, 2018;Jiménez-Esteve and Domeisen, 2020). In essence, by comparing an ensemble hindcast in which the stratosphere is constrained to the observed evolution, to a second hindcast in which the stratospheric circulation is constrained to climatology, the tropospheric impacts of the stratospheric anomalies can be isolated.…”

Modulation of the ENSO teleconnection to the North Atlantic by the tropical North Atlantic and Caribbean

“…In addition to reanalysis data, we utilize Isca (Vallis et al, 2018), a simplified atmospheric general circulation model (AGCM), to conduct a series of sensitivity experiments. This model uses the Geophysical Fluid Dynamics Labratory (GFDL) dynamical core, and uses the same model configuration as Jiménez-Esteve and Domeisen (2020). The atmosphere does not have explicit liquid water content but instead the moisture and radiative processes are considered using evaporation from the surface and a fast condensation scheme.…”

The Role of an Indian Ocean Heating Dipole in the ENSO Teleconnection to the North Atlantic European Region in Early Winter during the Twentieth Century in Reanalysis and CMIP5 Simulations

“…This teleconnection is much weaker than the ENSO teleconnection to the Pacific-North American (PNA) region, but is nevertheless the major source for seasonal predictability in the Euro-Atlantic region (e.g., Domeisen et al 2015;Scaife et al 2017). Several pathways have been proposed for ENSO teleconnections to the North Atlantic region, including the canonical tropospheric pathway including the tropical Northern Hemisphere (TNH) pattern (e.g., Wallace and Gutzler 1981;Jiménez-Esteve and Domeisen 2018; and many others) and a stratospheric one (Ineson and Scaife 2009;Butler et al 2014;Domeisen et al 2019). Recent studies have reported that in models and reanalysis there are significant nonlinearities and asymmetries between the ENSO response in the North Pacific (Frauen et al 2014;Garfinkel et al 2019;Jiménez-Esteve and Domeisen 2019), Arctic stratosphere (Rao and Ren 2016;Trascasa-Castro et al 2019;Weinberger et al 2019), and North Atlantic (Hardiman et al 2019;Jiménez-Esteve and Domeisen 2020;Trascasa-Castro et al 2019;Weinberger et al 2019).…”

“…The upperlevel divergence may act as a Rossby wave source (Sardeshmukh and Hoskins 1988) and induce an extratropical wave train, which propagates northeastward from the source region toward the PNA region, eventually reaching the Atlantic region during boreal winter. Transient eddy feedbacks are also essential to zonally extend the anomalies to the North Atlantic (e.g., Li and Lau 2012a,b;Jiménez-Esteve and Domeisen 2018).…”