North Atlantic Eddy-Driven Jet in Interglacial and Glacial Winter Climates

Abstract

The atmospheric westerly flow in the North Atlantic (NA) sector is dominated by atmospheric waves or eddies generating via momentum flux convergence, the so-called eddy-driven jet. The position of this jet is variable and shows for the present-day winter climate three preferred latitudinal states: a northern, central, and southern position in the NA. Here, the authors analyze the behavior of the eddy-driven jet under different glacial and interglacial boundary conditions using atmosphere-land-only simulations with the CCSM4 climate model. As state-of-the-art climate models tend to underestimate the trimodality of the jet latitude, the authors apply a bias correction and successfully extract the trimodal behavior of the jet within CCSM4. The analysis shows that during interglacial times (i.e., the early Holocene and the Eemian) the preferred jet positions are rather stable and the observed multimodality is the typical interglacial character of the jet. During glacial times, the jet is strongly enhanced, its position is shifted southward, and the trimodal behavior vanishes. This is mainly due to the presence of the Laurentide ice sheet (LIS). The LIS enhances stationary waves downstream, thereby accelerating and displacing the NA eddy-driven jet by anomalous stationary momentum flux convergence. Additionally, changes in the transient eddy activity caused by topography changes as well as other glacial boundary conditions lead to an acceleration of the westerly winds over the southern NA at the expense of more northern areas. Consequently, both stationary and transient eddies foster the southward shift of the NA eddy-driven jet during glacial winter times.

Regional atmospheric circulation over Europe during the Last Glacial Maximum and its links to precipitation

“…The enhanced frequency of E CWTs during LGM can be attributed to the changed boundary conditions (e.g., modified Fennoscandian ice sheet) and the resulting glacial anticyclone. The enhanced frequency of S and C CWT during LGM can be attributed to the southward displacement of both the jet stream [ Merz et al , ] and storm track, and a split of the storm track over the Adriatic Sea, with one branch located north and east of the Alps [see Florineth and Schlüchter , , Figure 6]. Likewise, cyclones are apparently constrained to move along the southern flank of the Fennoscandian ice sheet, which effectively blocks their passage northeastward.…”

Influence of glacial ice sheets on the Atlantic meridional overturning circulation through surface wind change

“…From sensitivity experiments using the AGCM and the OGCM, this intensification of the wind is attributed to the topography effect of the glacial ice sheets, especially that of the North American ice sheet, which is consistent with previous atmospheric studies (e.g. Cook and Held 1988;Li and Battisti 2008;Ullman et al 2014;Löfverström et al 2014;Merz et al 2015). Our decoupled simulations also show that the strengthening of the atmospheric cyclonic circulation over the Greenland-IcelandNorwegian Seas can have a large impact on the AMOC.…”

“…However, other factors, such as surface cooling over the ice sheet, also play an important role (Cook and Held 1988;Ringler and Cook 1999;Liakka 2012;Löfverström et al 2014). Associated with the stationary wave field, the westerlies also intensify throughout the troposphere in the North Atlantic (Li and Battisti 2008;Merz et al 2015).…”

“…If the westerlies had shifted south (north), this may have led to a reduction (an increase) in eastward sea ice transport to the region of NADW formation, and thus modify the strength (or perhaps the sign) of the sea ice process. Since the meridional shift of the westerlies in response to glacial ice sheets depends on the model (Laîné et al 2009;Rivière et al 2010;Hofer et al 2012;Merz et al 2015;Beghin et al 2015) and the configuration of the ice sheets (Kageyama et al 1999, Li andBattisiti 2008;Laîné et al 2009;Beghin et al 2015), the effect of the sea ice process related to the meridional shift of the westerlies should also depend largely on individual simulations. More studies are required to understand the precise role of the sea ice process related to the meridional shift of the westerlies.…”

Daily to Decadal Modulation of Jet Variability

“…This is observed in midlatitude jets around the world using two very different jet identification methods and is exhibited across the model hierarchy from stateof-the-art climate models down to a zonally symmetric barotropic model. Although not examined here, this relationship is also consistent with the transition to a stronger, more stable North Atlantic jet during the Last Glacial Maximum (Li and Battisti 2008;Rivière et al 2010;Merz et al 2015). An additional potential application of this work is to understanding climate model biases; our results suggest that a mean state bias in jet strength could impact the jet variability in the model.…”