A negative phase shift of the winter AO/NAO due to the recent Arctic sea‐ice reduction in late autumn

Abstract

This paper examines the possible linkage between the recent reduction in Arctic sea-ice extent and the wintertime Arctic Oscillation (AO)/North Atlantic Oscillation (NAO). Observational analyses using the ERA interim reanalysis and merged Hadley/Optimum Interpolation Sea Surface Temperature data reveal that a reduced (increased) sea-ice area in November leads to more negative (positive) phases of the AO and NAO in early and late winter, respectively. We simulate the atmospheric response to observed sea-ice anomalies using a high-top atmospheric general circulation model (AGCM for Earth Simulator, AFES version 4.1). The results from the simulation reveal that the recent Arctic sea-ice reduction results in cold winters in mid-latitude continental regions, which are linked to an anomalous circulation pattern similar to the negative phase of AO/NAO with an increased frequency of large negative AO events by a factor of over two. Associated with this negative AO/NAO phase, cold air advection from the Arctic to the mid-latitudes increases. We found that the stationary Rossby wave response to the sea-ice reduction in the Barents Sea region induces this anomalous circulation. We also found a positive feedback mechanism resulting from the anomalous meridional circulation that cools the mid-latitudes and warms the Arctic, which adds an extra heating to the Arctic air column equivalent to about 60% of the direct surface heat release from the sea-ice reduction. The results from this high-top model experiment also suggested a critical role of the stratosphere in deepening the tropospheric annular mode and modulation of the NAO in mid to late winter through stratosphere-troposphere coupling.

Nonlinear response of mid-latitude weather to the changing Arctic

“…These events can weaken, or even reverse, the stratospheric winds, leading to an eventual downward propagation of the circulation feature into the troposphere 86 and a tendency for a negative phase of the AO. This mechanism establishes memory in the system, as sea ice loss and snow cover in late autumn can affect the tropospheric jet stream in late winter through lagged transfer of wave-induced disturbances involving the stratosphere 39 . Only models with realistic stratospheres are able to capture this mechanism.…”

“…Often climate models and correlation analyses of observations produce differing estimates of how the climate will respond to the ongoing AA and loss of sea ice 48,58 . For example, climate model studies have reported shifts towards both the positive or negative phases of the AO and/or NAO, or no apparent shift, in response to AA 13,19,34,39,59 . Analyses that involve averaging over large areas, long time periods and/or many ensemble members may not reveal specific atmospheric responses to AA, such as enhanced jet stream ridges and troughs that occur in specific locations.…”

“…Extensive regions of positive temperature anomalies in the Arctic may increase the persistence of weather systems 20,38 . Further, troposphere-stratosphere connections can trigger changes in the regional wind patterns 39 . Contributors to a lack of simple robust linkages include the four properties mentioned above, which are discussed in more detail in the following sections.…”

Arctic-Mid-Latitude Linkages in a Nonlinear Quasi-Geostrophic Atmospheric Model

“…There is a growing body of works relating anomalously low summer-autumn sea-ice-cover in the Arctic to severe winters over Northern Eurasia and North America based on both reanalysis data and model results analysis (e.g., [7,8]; review articles by Cohen et al [9][10][11]). Most evidence has been found for dynamical pathways in the troposphere and in the stratosphere, resulting in more frequent occurrence of the negative phase of the Arctic Oscillation (AO) or North Atlantic Oscillation (NAO) in late winter (e.g., [5,12,13]). Due to additional heat uptake by the larger sea-ice-free areas in summer and early autumn, the refreezing in autumn is delayed.…”

“…Enhanced upward propagating planetary waves may lead to wave breaking in the polar stratosphere, which in turn may weaken the polar vortex (e.g., [16,17]). The downward propagation of these stratospheric circulation anomalies may start in mid-winter and can contribute to the persistent negative NAO/AO anomalies in mid-winter to late winter [13,16,18].…”