Xiangdong ZHANG, Xianyao CHEN, Andrew ORR, James E.OVERLAND, Timo VIHMA,

Muyin WANG4,6, Qinghua YANG7, and Renhe ZHANG8

1North Carolina State University, Asheville, NC 28801, USA

2Frontier Science Center for Deep Ocean Multispheres and Earth System, Key Laboratory of Physical Oceanography,Ocean University of China, Qingdao 266100, China

3British Antarctic Survey, High Cross, Madingley Road, Cambridge CB3 0ET, UK

4NOAA/Pacific Marine Environmental Laboratory, Seattle, WA 98115, USA

5Finnish Meteorological Institute, Helsinki FI-00560, Finland

6University of Washington, Seattle, WA 98105, USA

7School of Atmospheric Sciences, Sun Yat-sen University, and Southern Marine Science and Engineering Guangdong Laboratory (Zhuhai), Zhuhai 519082, China

8Department of Atmospheric and Oceanic Sciences, Fudan University, Shanghai 200438, China

The Arctic climate system has changed rapidly during recent decades with a two-four times faster warming rate than the global average subject to the uncertainties of analysis datasets and approaches.These changes have apparently resulted in broader and sizeable impacts within the Arctic, in the low/mid-latitudes, and globally.The importance of these changes and impacts makes the Arctic stand out within the global climate systems, drawing great attention and interests from the climate research community, the general public, and the government sector.One of the persistent, leading-edge topics in climate studies during recent decades has therefore been to improve understanding of the underlying driving mechanisms, evaluate socioeconomic and ecological impacts, and enhance the ability of the prediction and projections of Arctic climate changes, though the focal scientific questions to answer have varied with time.In particular, a relatively recent focus of the scientific community has been on how Arctic climate change influences mid-latitude climate and weather (e.g., Screen et al., 2018; Cohen et al.,2020; Overland et al., 2021; Zhang et al., 2022).To promote and contribute to the study on how Arctic climate change influences mid-latitude weather and climate, the journal Advances in Atmospheric Sciences has so far organized and published two special issues addressing different scientific questions related to this topic (Zhang et al., 2018; Mu et al., 2022).

Nevertheless, inconsistencies and open questions still remain on the influence of Arctic climate change on the mid-latitudes climate and weather.Meanwhile, new observations and studies have revealed large transient changes in the Arctic climate system, which are subject to remote forcing from the low and mid-latitude atmosphere and ocean (e.g., Zhang et al., 2013,2023; Woods et al., 2013; Tsubouchi et al., 2021).Another challenge is the ability of the state-of-the-art models to simulate the state, variability, and changes of the Arctic climate system.Model errors and biases are a source of uncertainties in studies emerging about the Arctic influence on mid-latitude climate and weather (e.g., Screen et al., 2018; Cohen et al., 2020).As a result, it is important to examine two-way interactive processes between the Arctic and the low/mid-latitudes, evaluate and improve models performance in simulating key Arctic climate system components, and assess the associated socioeconomic and ecological impacts.This special issue aims to make progresses in addressing these issues by assembling research articles covering these areas.

Eight articles in the special issue present continuing efforts to improve predictive understanding of the Arctic influence on mid-latitude climate.They mainly focus on Eurasian cold temperature events, snow cover changes, and severe convective weather.They also extend the investigation from identifying simultaneous relationships and processes to across-season linkages.The sensitivities of predictions of mid-latitude climate and circulation events to Arctic sea ice forcing have also been evaluated.Among these studies, some articles show some exceptional features.For example, Xiao et al.(2023) uses reconstructed paleoclimate data back to the late 13thcentury and finds an out-of-phase relationship between southern China cold winter events and the Barents-Kara seas sea ice anomalies.The topic of Arctic influence on mid-latitude climate emerged during recent decades when Arctic amplification (AA) became prominent.However, due to the short observational record, it is challenging to reach an agreement whether the Arctic influence on mid-latitudes results from the AA or is part of natural variability.Xiao et al.(2023)’s analysis provides an insight from paleoclimate perspectives.On the other hand, Tao et al.(2023)examines a mesoscale extreme event in the mid-latitudes and finds across-scale dynamic processes linking an Arctic potential vorticity anomaly to an eastern China extreme gale event.Extreme mesoscale, convective events typically have highly destructive impacts on daily life and infrastructures.This study provides new clues to address scale gaps and to understand the occurrence of this type of mesoscale weather extremes, as well as improve their prediction with a longer-lead time.

Wang et al.(2023) examines the mid-latitude influence on the Arctic.Specifically, how different meridional structures of the North Pacific sea surface temperature anomalies can impact the Arctic stratospheric polar vortex (SPV) strength.Changes in the strength and disturbance of SPV plays a key role in linking anomalous Arctic warming and sea ice decline to midlatitude anomalous climate events (e.g., Kim et al., 2014).This study improves understanding of SPV dynamics, in particular about how it can be modulated by mid-latitude ocean forcing.

Five other studies concentrate on evaluating model simulations and improving their physics, with a particular focus on sea ice thickness and concentration.Biases in model simulations influence radiative and turbulent heat fluxes between the ocean/sea ice and the overlying atmosphere.These fluxes contribute to Rossby wave activity, which influences the largescale atmospheric circulation (e.g., Polvani and Waugh, 2004).Liu et al.(2023b) investigates the physical reasons for model biases using a diagnostic analysis term referred to as “effective thermal inertia”, which approximates the heat content of sea ice and the upper ocean.It is suggested that differences in this term are responsible for the large spread of simulated winter sea ice cover across models.In an effort to improve models, Liu et al.(2023a) proposes a parameterization scheme incorporating wind forcing effects through sea ice in an ocean wave model.This study has implications for improving simulations of the Arctic Ocean mixing and stratification in Earth System Models.

The special issue also covers studies on the drivers of regional sea ice variability and Arctic sea level variability, and an assessment of trans-Arctic accessibility.

Rapid changes in the Arctic climate system have led it to a new state, resulting in an enhancement of nonlinear dynamic processes.As a result, extreme or rare events beyond previous records are more frequently occurred, representing a new indicator of Arctic climate change.Near future research priorities therefore are disentangling nonlinear dynamic processes, with a focus on synergetic effects of multiscale processes, feedbacks across different components of the Arctic climate system, and two-way interactions between the Arctic and low/mid-latitudes.Results from such studies would improve prediction and projections of Arctic climate change and its influence on low/mid-latitude climate and weather, benefiting policydecision making.