Abstract
Climate model biases in the North Atlantic (NA) low-level tropospheric westerly jet are a major impediment to reliably representing variability of the NA climate system and its wider influence, in particular over western Europe. A major aspect of the biases is the occurrence of a prominent early-winter equatorward jet bias in Coupled Model Inter-comparison Project Phase 5 (CMIP5) models that has implications for NA atmosphere-ocean coupling. Here we assess whether this bias is reduced in the new CMIP6 models and assess implications for model representation of NA atmosphere-ocean linkages, in particular over the sub-polar gyre (SPG) region. Historical simulations from the CMIP5 and CMIP6 model datasets were compared against reanalysis data over the period 1861–2005. The results show that the early-winter equatorward bias remains present in CMIP6 models, although with an approximately one-fifth reduction compared to CMIP5. The equatorward bias is mainly associated with a weaker-than-observed frequency of poleward excursions of the jet to its northern position. A potential explanation is provided through the identification of a strong link between NA jet latitude bias and systematically too-weak model-simulated low-level baroclinicity over eastern North America in early-winter. CMIP models with larger equatorward jet biases exhibit weaker correlations between temporal variability in speed of the jet and sea surface conditions (sea surface temperatures and turbulent heat fluxes) over the SPG. The results imply that the early-winter equatorward bias in jet latitude in CMIP models could partially explain other known biases, such as the weaker-than-observed seasonal-decadal predictability of the NA climate system.
Highlights
An accurate representation of the mean state of the atmosphere in climate models is key to capturing the variability related to atmosphere-ocean coupling
We move on to address the question of whether early-winter jet latitude biases have a significant impact on the representation of North Atlantic (NA) atmosphereocean linkages. We address this question by comparing output from the comparison Project Phase 5 (CMIP5) and CMIP6 climate models with the 20th Century Reanalysis Version 3 (20 CRv3)
Winter climatological jet latitude in CMIP5 and CMIP6 we assess whether the systematic earlywinter equatorward bias in jet latitude that exists in previous model generations is present in CMIP6
Summary
An accurate representation of the mean state of the atmosphere in climate models is key to capturing the variability related to atmosphere-ocean coupling. Studies based on reanalysis data suggest that from early to late winter the climatological mid-latitude westerly jet exhibits an equatorward shift from ∼49◦ N in November to ∼46◦ N in March (Woollings et al 2014) Such a shift in jet latitude broadly follows seasonal changes in the mid-latitude storm track over the western NA, which moves equatorward from autumn to late winter in association with seasonal changes in latitude of baroclinicity over the North American continent (Hoskins and Hodges 2019b). Davini and D’Andrea (2020) show that winter blocking frequency in the European sector is still under-estimated in CMIP6, with reduced biases compared to CMIP5 and CMIP3 These studies did not consider the seasonal progression of the bias and the possible implications in terms of the representation of links between jet variability and heat fluxes (HFs) and SSTs over the NA. In terms of jet variability, we focus on speed of the mid-latitude westerly jet rather than latitude, since jet speed variability induces a stronger and more persistent imprint on NA SSTs and correlates strongly with the SPG region on seasonal to multi-decadal timescales (Woollings et al 2015, Ma et al 2020)
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