Mechanisms of North Pacific Jet on Winter North American Temperature Dipole
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摘要:
为理解北美冬季“西暖‒东冷”温度偶极子的成因，基于再分析数据对影响北美温度的大气环流、西风急流和海温背景进行分析.结果表明，北美温度偶极子主要受正位相的太平洋北美遥相关（PNA+）环流影响，弱的西风急流下北美温度偶极子最强.此外，北太平洋海温通过调制急流变化，对PNA+期间的北美温度偶极子结构和强度产生影响.在正位相太平洋年代际振荡冬季，中纬度西风急流偏南，PNA+环流在北美地区呈经向结构，引起“西北暖‒东南冷”温度异常.而正位相维多利亚海温模态有利于中纬度西风急流减弱，导致PNA+在北美西部的高压系统向西移动，同时东部低压持续存在，形成水平波列结构，进而加剧“西暖‒东冷”温度偶极子异常.
Abstract:To understand the causes of extreme temperature dipoles in North America during winter, we analyse atmospheric circulation, the westerly jet stream, and the sea surface temperature (SST) background based on reanalysis data. Our study reveals the role of the positive Pacific-North American pattern (PNA+) in driving sub-seasonal temperature dipoles on North America. The results indicate that a weakened North Pacific jet favors the formation of the temperature dipole. Additionally, North Pacific SSTs can modulate the structure and strength of the temperature dipole by influencing the jet stream. Specifically, during the positive phase of the Pacific decadal oscillation (PDO) in winter, a meridionally oriented wave train structure related to PNA+ events is more easily formed, with the North Pacific westerly jet stream shifting southward. This structure contributes to a "warm northwest-cold southeast" temperature anomaly pattern on North America. Conversely, during the positive phase of the Victoria SST pattern, the mid-latitude jet stream is weakened. As a result, the high-pressure system associated with PNA+ shifts westward, while the eastern low-pressure system persists. This forms a horizontal wave train structure, which strengthens the intensity of the "west warm-east cold" surface air temperature dipole anomaly on North America associated with PNA+.
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图 1 1950-2019冬季PNA和TWE指数标准化时间序列
Fig. 1. Time series of the normalized PNA and TWE index during 1950-2019 winters
图 2 1950-2019冬季（120°E~120°W，10°N~60°N）北太平洋U500距平场前两个EOF模态空间分布和标准化的主成分序列
等值线代表U500气候态；红色（蓝色）点代表大于（小于）0.5标准差
Fig. 2. Leading and second EOF (EOF1 and EOF2) modes (a, b) of the DJF-mean U500 anomaly over the North Pacific (120°E-120°W, 10°N‒60°N) and their corresponding (c) PC1 and (d) PC2 time series during 1950-2019 winters
图 3 1950-2019年太平洋中纬度西风急流偏南（a、c）和偏北（b、d）冬季合成的U500异常场及PNA+期间时间平均的（Lag‒10~Lag 10天）SAT（填色，单位：K）和Z500（等值线，单位：gpm）异常场.PNA+的Lag‒10~Lag 10天40°N~65°N经向平均的Z500异常变化场（e, f）
图e和f中蓝线代表最大值；打点区域表示合成结果通过0.05显著性水平的双边Student’s t检验
Fig. 3. Composite DJF-mean U500 anomalies and time-mean composite daily Z500 (contours, unit: gpm) and SAT (colored shading, unit: K) anomalies averaged from Lag‒10 to Lag 10 of PNA+ events for southward-shifted (a, c) and northward-shifted (b, d) North Pacific jet winters during 1950-2019. Time-longitude evolution (e, f) of the meridional mean (40°N‒65°N) Z500 anomalies from Lag‒10 to Lag 10 of PNA+
图 5 偏南、偏北、偏弱和偏强的北太平洋中纬度西风急流在1950-2019年冬季发生的PNA+事件期间TWE（a）、Tw（b）和TE（c）指数（单位：K）随时间变化
圆点表示指数通过0.05显著性水平的双边Student’s t检验
Fig. 5. Temporal evolution of the composite daily TWE (a), TW (b), and TE (c) index (unit: K) of PNA+ events for the southward-shifted, northward-shifted, weak and strong mid-latitude North Pacific jet winters during 1950-2019
图 6 1950-2019年冬季北太平洋地区U500异常前两个EOF时间系数回归得到的SST异常场（单位：K）
Fig. 6. Regressed DJF-mean SST anomalies (unit: K) against the DJF-mean U500 PC1 and PC2 time series during 1950-2019
图 7 1950-2019冬季PDO（9年低通滤波）（a）和VM的标准化时间序列（b）；PDO+和VM+冬季合成的SST（单位：K）（c，d）和U500（单位：m/s）异常场（e，f）
Fig. 7. Time series of the normalized DJF-mean PDO (9-yr low pass filter) (a) and VM indices (b) during 1950-2019; DJF-mean SST anomalies (unit: K) (c, d) and U500 (unit: m/s) anomalies (e, f) for the PDO+ and VM+ winters
图 8 PDO+（a）和VM+（b）冬季发生的PNA+事件期间逐日Z500（等值线）和SAT（填色，单位：K）异常的演变场
Fig. 8. The variation of daily Z500 (contours) and SAT (color shading, unit: K) anomalies from Lag‒8 to Lag 8 days of the PNA+ events and for the PDO+ (a) and VM+ (b) winters
图 9 PDO+冬季（红线）、VM+冬季（蓝线）和全部冬季（虚线）的PNA+事件期间TWE指数（单位：K）变化曲线
灰色区域代表红线和蓝线之间的差通过0.05显著性水平的MC检验
Fig. 9. Temporal evolution of the compo site daily TWE index (unit: K) of PNA+ events for the PDO+ (red line) and VM+ (blue line) patterns
图 10 PDO+和VM-，PDO-和VM+冬季下，DJF平均的SST异常场（单位：K）（a~c）及PNA+事件期间（Lag‒10~Lag 10）时间平均的Z500（等值线，单位：gpm）和SAT（填色，单位：℃）异常场（d~f）
Fig. 10. (a‒c) Composite DJF-mean SST anomalies (unit: K) and (d‒f) time-mean fields of composite daily Z500 (contours, unit: gpm) and SAT (color shading, unit: K) anomalies averaged from Lag‒10 to Lag 10 days of PNA+ events for PDO+ and VM+ combination, PDO- and VM+ combination and their difference
表 1 1950―2019年冬季北太平洋地区U500异常前两个EOF时间系数（PC1，PC2）与PDO、9年低通滤波PDO、VM之间的相关系数
Table 1. Correlation coefficient (Corr.) of the DJF-mean U500 PC1 and PC2 time series and oceanic variabilities (PDO, 9-yr low-pass PDO, VM) during 1950-2019
Corr. PDO PDO (9年低通滤波) VM U500 PC1 0.45** 0.34** 0.35** U500 PC2 ‒0.44** ‒0.13 0.68** 注：**代表相关系数通过0.05显著性水平的双边Student’s t检验. 
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