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# Impacts of Oceanic Fronts and Eddies in the Kuroshio-Oyashio Extension Region on the Atmospheric General Circulation and Storm Track

• This paper reviews the progress in our understanding of the atmospheric response to midlatitude oceanic fronts and eddies, emphasizing the Kuroshio-Oyashio Extension (KOE) region. Oceanic perturbations of interest consist of sharp oceanic fronts, temperature anomalies associated with mesoscale eddies, and to some extent even higher-frequency sub-mesoscale variability. The focus is on the free atmosphere above the boundary layer. As the midlatitude atmosphere is dominated by vigorous transient eddy activity in the storm track, the response of both the time-mean flow and the storm track is assessed. The storm track response arguably overwhelms the mean-flow response and makes the latter hard to detect from observations. Oceanic frontal impacts on the mesoscale structures of individual synoptic storms are discussed, followed by the role of oceanic fronts in maintaining the storm track as a whole. KOE fronts exhibit significant decadal variability and can therefore presumably modulate the storm track. Relevant studies are summarized and intercompared. Current understanding has advanced greatly but is still subject to large uncertainties arising from inadequate data resolution and other factors. Recent modeling studies highlighted the importance of mesoscale eddies and probably even sub-mesoscale processes in maintaining the storm track but confirmation and validation are still needed. Moreover, the atmospheric response can potentially provide a feedback mechanism for the North Pacific climate. By reviewing the above aspects, we envision that future research shall focus more upon the interaction between smaller-scale oceanic processes (fronts, eddies, submesoscale features) and atmospheric processes (fronts, extratropical cyclones etc.), in an integrated way, within the context of different climate background states.
摘要: 本文综述了中纬度海洋锋面和涡旋对大气环流影响的相关研究进展，重点关注黑潮-亲潮延伸区 (KOE)的海-气相互作用。综述的海洋现象包括海洋锋面、与中尺度涡有关的海温异常、以及次中尺度变率，而大气现象则关注边界层以上的自由大气。中纬度大气受风暴轴中剧烈的瞬变涡旋活动影响，因此本文对大气平均基本流和风暴轴的响应进行了评估。风暴轴对海洋扰动的响应远远大于对基本流的响应，导致基本流的响应被掩盖且难以从观测资料中识别出来。本文首先介绍了海洋锋面对风暴轴内单个气旋结构的影响，并进一步阐述对风暴轴的整体影响。KOE区的海洋锋面有显著的年代尺度变化，因此在年代尺度上对风暴轴和大气环流产生调控，相关研究进展在文中进行了总结和比较。现有的海洋锋面对大气影响的研究结果仍存在很大的不确定性，这主要是由数据分辨率不足等因素导致。近期有模式研究强调海洋中尺度涡乃至次中尺度过程对风暴轴的作用，这是未来重要的发展方向，但仍需要更多的验证。此外，大气对海洋扰动的响应可以反馈海洋，形成闭合的北太平洋海-气反馈机制。通过对上述多个研究方向的总结，作者指出未来研究应在充分考虑不同气候背景下，系统研究并聚焦中小尺度海洋过程 (锋面、涡旋、次中尺度过程等) 和大气过程 (锋面、温带气旋等) 的相互作用。
• Figure 1.  The 1993−2016 mean (a) geostrophic current velocity (m s−1, shading) and SSH (contours; CI = 0.1 m); (b) eddy kinetic energy (m2 s−2, shading) and SSH (contours; CI = 0.1 m) based on the AVISO daily satellite altimeter data. Eddy kinetic energy is defined as $\left({u}^{'2}+{v}^{'2}\right)/2$, where the prime denotes the 300-day high-pass filtered data. Shown in (a) are also the position of the KE and OE. The 1981−2016 mean (c) SST gradient [°C (100 km)−1, shading] and SST (contours; CI = 1°C); (d) interannual standard deviation of SST (°C, shading) and SST (contours; CI = 1°C) based on the NOAA daily OISST blended satellite and in-situ data. Shown in (c) are also the position of the KEF and branches of the OEF and the approximate area of the SAFZ.

Figure 2.  (a) Time series of the strength (STR) and position (POS) indices of the KE (Q14) and the KEF (Chen, 2008). (b) Time series of the KE strength (STR), position (POS), path length (PATH), and recirculation gyre strength (GYRE) metrics from Q14. The time series are digitized from the respective references and then normalized about their respective mean and low-pass filtered with a cutoff period of one year.

Figure 3.  (a) Lagrangian storm track defined as the track density of extratropical cyclones (number of cyclones in 2° × 2° grid boxes per winter, shading) during 1948−2011 overlaid on 1948−2018 mean zonal wind at 500 hPa (contours; CI = 5 m s−1). Extratropical cyclone track data is developed by the Cooperative Institute for Climate and Satellites, North Carolina State University based on NOAA’s 20th-century reanalysis dataset (https://etcsrv.cicsnc.org/ETCv8). Zonal wind data is from the NCEP-NCAR Reanalysis 1 dataset. (b) The 1948−2018 mean winter Eulerian storm track, defined as the standard deviation of 2-8-day bandpass filtered geopotential height (m, shading), overlaid on the zonal wind (contours; CI = 5 m s−1), both at 500 hPa. Geopotential height and zonal wind data are from the NCEP-NCAR Reanalysis 1 dataset.

Figure 4.  Schematic showing the thermally direct, linear response and the eddy-mediated response to latent and sensible heating induced by large-scale SST anomaly in the KOE [Reprinted from Zhou (2019) with permission].

Figure 5.  Schematic showing the main sources of baroclinicity: the quasi-stationary planetary waves generated by large-scale orography, the ocean−continent contrast, and the oceanic fronts and eddies.

Figure 6.  Schematic showing the atmospheric mean-flow response to a mid-latitude SST front (see section 4.1).

Figure 7.  Schematic of the structure of an idealized extratropical cyclone during its mature phase, showing the cold and warm air flows (thin arrowed curves), the cold and warm fronts (black dashed curve with black triangles and red semicircles), the cloud cover (grey shading), and the cold and warm conveyor belts (faded blue and red block arrows). Based on Bjerknes and Solberg (1922) and Stull (2017).

Figure 8.  (a) Time series of KE indices from various literature studies indicated in the legend. The vertical line denotes June 2002, before which the O’Reilly and Czaja (2015) index is projected backward using SSH data. (b) Cross-correlation between the literature KE indices and the Q14 index, with the full (projected) and unprojected indices of O’Reilly and Czaja (2015) shown separately. (c) Time series of OE indices from various literature studies indicated in the legend. References: T12: Taguchi et al. (2012); O18: Okajima et al. (2018); W18: Wills and Thompson (2018); YA19: Yao et al. (2019); YU18: Yuan and Xiao (2018). Here, the Taguchi et al. (2012) index is shown as the winter (DJF) mean of the authors’ monthly indices. Yuan and Xiao (2018) provided indices for each season. Shown here is their winter index. (d) Cross-correlation of literature OE indices with the Q14 KE index. Indices are digitized from the respective references and then normalized about their respective mean and low-pass filtered with a cutoff period of one year.

Figure 9.  (a) The positive NPO-WP pattern, defined as the 2nd empirical orthogonal function (EOF) of sea level pressure anomaly (hPa). (b) The positive PNA pattern, defined as the 1st EOF of 500 hPa geopotential height anomaly (m). Anomaly is defined as the monthly deviation from the multi-year mean annual cycle. Based on the NCEP-NCAR Reanalysis 1 dataset for 1948−2018.

Figure 10.  (a) Time series of idealized KE and OE indices (thick curves), both having a period of 10 years, with KE leading OE by 2.5 years; and the time series of the idealized combined atmospheric impacts of the KE and OE with different linear weights (thin curves). (b) Cross-correlation between the idealized KE index (thick black curve) and the idealized OE index (thick red curve) and the combined atmospheric impacts of the KE and OE with different linear weighs (thin curves).

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## Manuscript History

Manuscript revised: 14 July 2021
Manuscript accepted: 02 August 2021
###### 通讯作者: 陈斌, bchen63@163.com
• 1.

沈阳化工大学材料科学与工程学院 沈阳 110142

## Impacts of Oceanic Fronts and Eddies in the Kuroshio-Oyashio Extension Region on the Atmospheric General Circulation and Storm Track

###### Corresponding author: Guidi ZHOU, guidi.zhou@hhu.edu.cn;
• 1. Key Laboratory of Marine Hazards Forecasting, Ministry of Natural Resources (MNR), Hohai University, Nanjing 210098, China
• 2. College of Oceanography, Hohai University, Nanjing 210098, China
• 3. Southern Marine Science and Engineering Guangdong Laboratory (Zhuhai), Zhuhai 519080, China

Abstract: This paper reviews the progress in our understanding of the atmospheric response to midlatitude oceanic fronts and eddies, emphasizing the Kuroshio-Oyashio Extension (KOE) region. Oceanic perturbations of interest consist of sharp oceanic fronts, temperature anomalies associated with mesoscale eddies, and to some extent even higher-frequency sub-mesoscale variability. The focus is on the free atmosphere above the boundary layer. As the midlatitude atmosphere is dominated by vigorous transient eddy activity in the storm track, the response of both the time-mean flow and the storm track is assessed. The storm track response arguably overwhelms the mean-flow response and makes the latter hard to detect from observations. Oceanic frontal impacts on the mesoscale structures of individual synoptic storms are discussed, followed by the role of oceanic fronts in maintaining the storm track as a whole. KOE fronts exhibit significant decadal variability and can therefore presumably modulate the storm track. Relevant studies are summarized and intercompared. Current understanding has advanced greatly but is still subject to large uncertainties arising from inadequate data resolution and other factors. Recent modeling studies highlighted the importance of mesoscale eddies and probably even sub-mesoscale processes in maintaining the storm track but confirmation and validation are still needed. Moreover, the atmospheric response can potentially provide a feedback mechanism for the North Pacific climate. By reviewing the above aspects, we envision that future research shall focus more upon the interaction between smaller-scale oceanic processes (fronts, eddies, submesoscale features) and atmospheric processes (fronts, extratropical cyclones etc.), in an integrated way, within the context of different climate background states.

Reference

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