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One of the major globally relevant systematic biases in previous generations of climate models has been an equatorward bias in the latitude of the Southern Hemisphere (SH) mid-latitude tropospheric eddy driven westerly jet. The far-reaching implications of this for Southern Ocean heat and carbon uptake and Antarctic land and sea ice are key reasons why addressing this bias is a high priority. It is therefore of primary importance to evaluate the representation of the SH westerly jet in the latest generation of global climate and earth system models that comprise the Coupled Model Intercomparison Project Phase 6 (CMIP6). In this paper we assess the representation of major indices of SH extratropical atmospheric circulation in CMIP6 by comparison against both observations and the previous generation of CMIP5 models. Indices assessed are the latitude and speed of the westerly jet, variability of the Southern Annular Mode (SAM), and representation of the Amundsen Sea Low (ASL). These are calculated from the historical forcing simulations of both CMIP5 and CMIP6 for time periods matching available observational and reanalysis data sets. From the 39 CMIP6 models available at the time of writing there is an overall reduction in the equatorward bias of the annual mean westerly jet from 1.9° in CMIP5 to 0.4° in CMIP6 and from a seasonal perspective the reduction is clearest in austral spring and summer. This is accompanied by a halving of the bias of SAM decorrelation timescales compared to CMIP5. However, no such overall improvements are evident for the ASL. Fil: Bracegirdle, T.J.. British Antarctic Survey; Reino Unido Fil: Holmes, C.R.. British Artarctic Survey; Reino Unido Fil: Hosking, J.S.. British Antarctic Survey; Reino Unido Fil: Marshall, G.J.. British Antarctic Survey; Reino Unido Fil: Osman, Marisol. Consejo Nacional de Investigaciones Científicas y Técnicas. Oficina de Coordinación Administrativa Ciudad Universitaria. Centro de Investigaciones del Mar y la Atmósfera. Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales. Centro de Investigaciones del Mar y la Atmósfera; Argentina Fil: Patterson, M.. University of Oxford; Reino Unido Fil: Rackow, T.. Helmholtz Centre for Polar and Marine Research; Alemania

This study provides a mechanistic explanation of why the Holton-Tan (HT) effect, a phenomenon in which the strength of northern stratospheric winter polar vortex synchronizes with the equatorial quasi-biennial oscillation (QBO), was disrupted in the middle to late winters of 1978-1997. In line with recent reassessments of the HT effect, we find that an easterly QBO in the lower stratosphere leads to the formation of a midlatitude wave guide that enhances both the upward propagating planetary waves from the troposphere into the lower stratosphere (∼35-50°N, 30-200 hPa) and the northward wave propagation in the upper to middle stratosphere (∼35-60°N, 20-5 hPa). This enhanced poleward refraction of planetary waves results in a more disturbed polar vortex, causing the HT effect. The weakening of the HT effect in 1978-1997 was associated with a broader and strengthened polar vortex in November to January. The divergence of wave activity generated by eddies growing within the vortex provided the momentum source and allowed wave activity to propagate meridionally away from the vortex; this interfered with the QBO modulation of planetary wave propagation and led to a weakening of the HT effect during this period. The stronger than average polar vortex in 1978-1997 was associated with a vertically coherent cooling signature over northeastern Asia in the stratosphere. We suggest that a change of stratospheric circulation and/or a change of the stratosphere-troposphere coupling were the main causes for the disrupted HT effect in 1978-1997. © 2014 American Geophysical Union. All Rights Reserved.

To explore the mechanisms linking Arctic sea-ice loss to changes in mid-latitude surface temperatures, we conduct idealized modeling experiments using an intermediate general circulation model and with sea-ice loss confined to the Atlantic or Pacific sectors of the Arctic (Barents-Kara or Chukchi-Bering Seas). Extending previous findings, there are opposite effects on the winter stratospheric polar vortex for both large-magnitude (late twenty-first century) and moderate-magnitude sea-ice loss. Accordingly, there are opposite tropospheric Arctic Oscillation (AO) responses for moderate-magnitude sea-ice loss. However, there are similar strength negative AO responses for large-magnitude sea-ice loss, suggesting that tropospheric mechanisms become relatively more important than stratospheric mechanisms as the sea-ice loss magnitude increases. The mid-latitude surface temperature response for each loss region and magnitude can be understood as the combination of an ‘indirect’ part induced by the large-scale circulation (AO) response, and a residual ‘direct’ part that is local to the loss region.

To explore the mechanisms linking Arctic sea-ice loss to changes in mid-latitude surface temperatures, we conduct idealized modeling experiments using an intermediate general circulation model and with sea-ice loss confined to the Atlantic or Pacific sectors of the Arctic (Barents-Kara or Chukchi-Bering Seas). Extending previous findings, there are opposite effects on the winter stratospheric polar vortex for both large-magnitude (late twenty-first century) and moderate-magnitude sea-ice loss. Accordingly, there are opposite tropospheric Arctic Oscillation (AO) responses for moderate-magnitude sea-ice loss. However, there are similar strength negative AO responses for large-magnitude sea-ice loss, suggesting that tropospheric mechanisms become relatively more important than stratospheric mechanisms as the sea-ice loss magnitude increases. The mid-latitude surface temperature response for each loss region and magnitude can be understood as the combination of an ‘indirect’ part induced by the large-scale circulation (AO) response, and a residual ‘direct’ part that is local to the loss region.