Details

Autor(en) / Beteiligte

• Saffin, Leo
• McKenna, Christine M.
• Bonnet, Rémy
• Maycock, Amanda C.

Titel

Large Uncertainties When Diagnosing the “Eddy Feedback Parameter” and Its Role in the Signal‐To‐Noise Paradox

Ist Teil von

• Geophysical research letters, 2024-06, Vol.51 (11), p.n/a

Ort / Verlag

Washington: John Wiley & Sons, Inc

Erscheinungsjahr

2024

Link zum Volltext

Quelle

Wiley Blackwell Single Titles

Beschreibungen/Notizen

• A too‐weak eddy feedback in models has been proposed to explain the signal‐to‐noise paradox in seasonal‐to‐decadal forecasts of the winter Northern Hemisphere. We show that the “eddy feedback parameter” (EFP) used in previous studies is sensitive to sampling and multidecadal variability. When these uncertainties are accounted for, the EFP diagnosed from CMIP6 historical simulations generally falls within the reanalysis uncertainty. We find the EFP is not independent of the sampled North Atlantic Oscillation (NAO). Within the same dataset, a sample containing larger NAO variability will show a larger EFP, suggesting that the link between eddy feedbacks and the signal‐to‐noise paradox could be due to sampling effects with the EFP. An alternative measure of eddy feedback, the barotropic energy generation rate, is less sensitive to sampling errors and delineates CMIP6 models that have weak, strong, or unbiased eddy feedbacks, but shows little relation to NAO variability. Plain Language Summary Model forecasts on seasonal‐to‐decadal timescales have recently been shown to have significant skill in predicting the North Atlantic Oscillation (NAO, a large‐scale pattern of variability). However, these forecasts are undermined by signal‐to‐noise ratios that are lower than expected given the skill, meaning the models are underconfident. This problem is known as the “signal‐to‐noise paradox”. Previous work has shown that models underestimate the strength of feedback from atmospheric eddies onto the midlatitude circulation, but models with a stronger eddy feedback suffer less from the signal‐to‐noise paradox. However, we find that the “eddy feedback parameter” (EFP) used in these studies exhibits large sampling uncertainty that has not previously been taken into account. When accounting for this sampling uncertainty, the EFP in models is generally consistent with reanalysis data. Furthermore, across samples, the EFP correlates with the variability of the NAO, meaning they are not independent, which makes the EFP problematic for understanding the causes of the signal‐to‐noise paradox. Samples with larger NAO variability are diagnosed with a larger EFP, even within the same dataset. An alternative measure of eddy feedback is less sensitive to sampling and better identifies models which have weak, strong, or unbiased eddy feedbacks. Key Points The “eddy feedback parameter” is a highly non‐stationary quantity, making reanalysis and model comparisons problematic on short time periods Sampling uncertainty in the eddy feedback parameter from reanalysis data is comparable to the intermodel spread Barotropic energy generation rate is a more stable quantity, but does not explain model spread in North Atlantic climate variability