Impact of tropical SST variations on the linear predictability of the atmospheric circulation in the Atlantic/European region

Henrik Feddersen

doi:10.4401/ag-3381

Abstract

Seasonal mean values of tropical Sea Surface Temperature (SST) and Atlantic/European Mean Sea Level Pressure (MSLP) from a 301-year coupled ocean/atmosphere model run are analysed statistically. Relations between the two fields are identified on both interannual and interdecadal timescales. It is shown that tropical SST variability affects Atlantic/European MSLP in winter. In particular, there appears to be a statistically significant relation, between the leading modes of variability, the El Nino/Southern Oscillation (ENSO) and the North Atlantic Oscillation (NAO). During cold ENSO (La Nina) years the NAO tends to be in its positive phase, while the opposite is the case during warm ENSO (El Nino) years, although to a lesser extent. Similar analyses that are presented for gridded observational data, confirm this result, although here tropical Atlantic SST appears to be stronger related to the NAO than tropical Pacific SST. The linear predictability of a model simulated NAO index is estimated by making statistical predictions that are based on model simulated tropical SST. It is shown that the predictive skill is rather insensitive to the length of the training period. On the other hand, the skill score estimate can vary significantly as a result of interdecadal variability in the climate system. These results are important to bear in mind when making statistical seasonal forecasts that are based on observed SST.

Highlights

Interannual variability of the large-scale atmospheric circulation in the Atlantic/European region is dominated by variability that is internal to the atmosphere
The identified statistical links between El Niño/Southern Oscillation (ENSO) events and the phase of the North Atlantic Oscillation (NAO) hold some potential for Atlantic/European seasonal to interannual predictability, in years with large tropical Sea Surface Temperature (SST) anomalies, a study by Johansson et al (1998) found no significant predictability of surface air temperature in Northern Europe associated with ENSO
Singular Value Decomposition (SVD) analyses of observational data show differences between the two patterns that are based on the first and the last 46-year period, respectively, of the 19031994 period. These results show that the statistical relationship between tropical SST and Atlantic/European Mean Sea Level Pressure (MSLP) varies on intera b

Summary

Introduction

Interannual variability of the large-scale atmospheric circulation in the Atlantic/European region is dominated by variability that is internal to the atmosphere. Given the lack of observational data back in time, we focus our analysis in the present study on data time series from a 301-year integration of the ECHO-G coupled ocean/atmosphere general circulation model Such long time series allow us to better identify tropical/ extratropical teleconnection signals, and to study the robustness over many decades of the teleconnection relations. Teleconnections on interdecadal timescales are identified between tropical SST and atmospheric variability in the Atlantic/European region in the model time series. Teleconnections on these longer timescales will show up in interannual climatic time series as apparent trends, and they contribute (positively or negatively) to the interannual predictability.

Observational data

Model data

Interdecadal covariability

Interannual predictability

Findings

Conclusions