The Linear Sensitivity of the North Atlantic Oscillation and Eddy-Driven Jet to SSTs

Tim Woollings,Hugh S Baker,Myles R Allen,Chris E Forest

doi:10.1175/jcli-d-19-0038.1

Abstract

Abstract The North Atlantic Oscillation (NAO) and eddy-driven jet contain a forced component arising from sea surface temperature (SST) variations. Due to large amounts of internal variability, it is not trivial to determine where and to what extent SSTs force the NAO and jet. A linear statistical–dynamic method is employed with a large climate ensemble to compute the sensitivities of the winter and summer NAO and jet speed and latitude to the SSTs. Key regions of sensitivity are identified in the Indian and Pacific basins, and the North Atlantic tripole. Using the sensitivity maps and a long observational SST dataset, skillful reconstructions of the NAO and jet time series are made. The ability to skillfully forecast both the winter and summer NAO using only SST anomalies is also demonstrated. The linear approach used here allows precise attribution of model forecast signals to SSTs in particular regions. Skill comes from the Atlantic and Pacific basins on short lead times, while the Indian Ocean SSTs may contribute to the longer-term NAO trend. However, despite the region of high sensitivity in the Indian Ocean, SSTs here do not provide significant skill on interannual time scales, which highlights the limitations of the imposed SST approach. Given the impact of the NAO and jet on Northern Hemisphere weather and climate, these results provide useful information that could be used for improved attribution and forecasting.

Highlights

The North Atlantic Oscillation (NAO) is a teleconnection pattern that dominates the climate variability over the North Atlantic (Wallace and Gutzler 1981; Hurrell 1995)
NAO time series for each reanalysis are computed by projecting each individual season onto the first EOF of the reanalysis sea level pressure (SLP) for that particular season (e.g., the ERA-20C winter NAO time series is computed by projecting the December– February (DJF) mean SLP for each year onto the first EOF of the DJF ERA-20C SLP) The NAO EOF is computed over the region 208–808N, 908W–408E)
The dominant modes of sea surface temperature (SST) variability in the Indian Ocean are associated with an El Niño–Southern Oscillation (ENSO) signal in the Pacific (Fig. S6). This suggests that in the real world, much of the SST variability in the Indian Ocean is being forced by ENSO, via the atmosphere, rather than the SSTs forcing the atmosphere. This does not preclude the Indian Ocean teleconnection that we find in the model occurring in the real world, but has implications when using Indian Ocean SSTs combined with the global teleconnection operator (GTO) to reconstruct the NAO

Summary

Introduction

The North Atlantic Oscillation (NAO) is a teleconnection pattern that dominates the climate variability over the North Atlantic (Wallace and Gutzler 1981; Hurrell 1995). In 2010 the anomalously negative observed NAO appears to have been forced in some part by the Atlantic and Pacific SSTs. Interestingly, on the fast time scales, the Indian Ocean is not correlated with the reanalysis, even though the winter NAO has a large region of sensitivity to SSTs in the Indian Ocean, and the high standard deviation in the Indian Basin reconstruction suggests that there is variability in the Indian Ocean SSTs. The trend to more positive NAO years in the later part of the time period does appear to be linked to the Indian Ocean, but we cannot confirm this as the reconstruction has no skill coming from the Indian Ocean.

Results

Conclusion