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Many previous studies have shown the role of the boundary forcing of SST in causing persistent climatic anomalies (e.g., Wu et al., 2009; Li et al., 2012; Zhang et al., 2022; Yu and Sun, 2023) due to an SST known to have low variance. Therefore, the relationships between the EAT EEOF1 and the SST anomaly are analyzed.
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The correlation coefficients between PC1 and the SST anomalies in March and April are shown in Fig. 5. From March to April, we see a persistent dipole SST pattern over the North Atlantic (Figs. 5a, b). Accordingly, an index measuring the dipole SST pattern over the North Atlantic is defined as the difference between the area-averaged SST anomalies over the regions bounded by (42.5°–57.5°N, 60.5°–20.5°W) and (20.5°–37.5°N, 80.5°–40.5°W) (boxes in Fig. 5a), marked as the NASSTI. The correlation coefficient between PC1 and the March (April) NASSTI is 0.40 (0.40), significant at the 95% (95%) confidence level. In addition, the correlation coefficient of the EAT index with the NASSTI in March (April) is 0.37 (0.32), significant at the 95% (95%) confidence level. The correlation between the March and April NASSTI is 0.83, indicating that the North Atlantic dipole SST pattern may impact the EAT EEOF1 through its robust monthly persistence from March to April.
Figure 5. Correlation coefficients between PC1 and SST anomalies in (a) March and (b) April during 1961–2020. (c–d) Same as in (a–b) but between PC1_res and SST anomalies. Values significant at the 95% confidence level are dotted. The red rectangles are the domains used to define the specific SST indices.
To find more SST signals related to the EAT EEOF1, but independent of the dipole SST pattern over the North Atlantic, we further conduct the following analysis. The linear regression method is applied to remove the March NASSTI-related part from PC1 (marked as PC1_res), and the remaining signal is independent of the variability of the dipole SST pattern over the North Atlantic. The correlation coefficients between PC1_res and SST anomalies in March and April are then calculated. As shown in Figs. 5c and 5d, significant correlation coefficients exist over the tropical Indian Ocean in March and April. Accordingly, another index measuring the tropical Indian Ocean SST variation is defined as the area-averaged, sign-reversed SST anomalies over the region bounded by (15.5°S–5.5°N and 50.5°–100.5°E) (box in Fig. 5c), marked as the TIOSSTI. The correlation coefficient between PC1_res and the March (April) TIOSSTI is 0.31 (0.31), significant at the 95% (95%) confidence level. The correlation coefficient of the EAT index with the TIOSSTI in March (April) is 0.19 (0.33), which is insignificant (significant) at the 95% confidence level. The reason for the insignificant correlation coefficient in March will be explained in the following analysis. The correlation between the March and April TIOSSTI is 0.92, suggesting that the persistent tropical Indian Ocean SST anomalies likely contribute to the persistent EAT anomalies in March and April.
Aside from the SST anomalies over the North Atlantic and tropical Indian Oceans, there are also significant large-scale SST signals over the North Pacific in April (Figs. 5b and 5d). The SST anomalies over the region could be related to the EAT variations in April, but they cannot influence the preceding March EAT; therefore, we will not discuss the influence of the SST anomalies over the North Pacific because this study aims to explore the possible mechanism for the persistent variations of the EAT from March to April.
Further correlation analysis shows that the coefficients between the NASSTI and TIOSSTI in March and April are only –0.16 and –0.06, which indicates that the dipole SST pattern over the North Atlantic and anomalous SST over the tropical Indian Ocean are independent factors impacting the EAT EEOF1.
The physical processes responsible for the influence of the aforementioned two SST patterns on the EAT EEOF1 are then analyzed. Figure 6 displays the regressed anomalous horizontal winds, Rossby wave source, geopotential heights against the NASSTI, and the associated wave activity flux in March and April. Related to the positive NASSTI, is a negative NAO-like pattern with an equivalent barotropic structure over the North Atlantic (Fig. 6). Concurrently, a dipole pattern of the anomalous Rossby wave source is over the region slightly downstream of the negative NAO for these two months (Figs. 6b, d). As a result, a Rossby wave train is triggered, emanating from the mid-latitude North Atlantic, propagating eastward to East Asia, and finally strengthening the EAT. The persistent dipole SST pattern over the North Atlantic through this physical process could cause persistent EAT anomalies and contribute to the EEOF1 formation.
Figure 6. (a) Regressed 500-hPa geopotential height anomalies (color; units: gpm) and 850-hPa horizontal wind anomalies (vector; units: m s–1) against the standardized NASSTI in March during 1961–2020. (b) Same as in (a) but for the regressed 300-hPa geopotential height (contour; units: gpm) anomalies, anomalous Rossby wave source (color; units: 10–11 s–2), and the associated wave flux activity (vector; units: m2 s–2). (c–d) Same as in (a–b) but for the regression results in April. Values significant at the 95% confidence level are dotted. Only the anomalous 300-hPa Rossby wave source over the North Atlantic sector that is significant at the 95% confidence level is drawn. Black vectors in (a) and (c) are significant at the 95% confidence level. Vectors less than 0.05 m2 s–2 in (b) and (d) are not shown.
The atmospheric circulation anomalies associated with the TIOSSTI are shown in Fig. 7. Associated with the cold tropical Indian Ocean SST anomalies are regionally depressed vertical motions over the tropical Indian Ocean and intensified upward motion around the western tropical Pacific, which occur in March (Fig. 7a). Over the tropical regions, the vertical motion generally reflects the variation of the convective activity. Accordingly, the convective activity is depressed above the cold SST anomaly region in the tropical Indian Ocean and is strengthened over the western tropical Pacific. Such a change in the Walker circulation favors a low-level anomalous cyclone over the western tropical Pacific. In addition, the downward vertical velocity anomalies are associated with low-level divergence and further induce the low-level westerly wind anomalies over the tropical Indian Ocean. The resultant westerly wind prevails along the equator from the western Indian Ocean extending to the west of the Maritime Continent, contributing to the anomalous cyclone over the western tropical Pacific. The anomalous cyclone can cover the southern part of the EAT (Fig. 7b), leading to significant changes in southern EAT. Similar to in March, the cold tropical Indian Ocean SST anomalies in April can also strengthen the southern part of the EAT by changing the Walker Circulation over the tropical Indian Ocean to the tropical western Pacific (Figs. 7c, d). Accordingly, the persistent tropical Indian Ocean SST anomaly contributes to the EAT EEOF1.
Figure 7. Regressed (a) 500-hPa omega (units: Pa s–1) and (b) 500-hPa geopotential height anomalies (color; units: gpm) and the 850-hPa horizontal wind anomalies (vector; units: m s–1) against the standardized TIOSSTI in March during 1961–2020. (c–d) Same as in (a–b) but for the regressed results in April. Values significant at the 95% confidence level are dotted. Vectors in (b) and (d) are significant at the 95% confidence level.
Compared to the regressed 500-hPa geopotential height anomalies over the EAT key region, we can find that the results with respect to the NASSTI are larger than (comparable to) the results with respect to the TIOSSTI in March (April). Such results are due to the different relationships of the EAT with the NASSTI and TIOSSTI in March and April. Overall, the impact of the dipole SST pattern over the North Atlantic on the EAT EEOF1 is larger than that of the tropical Indian Ocean SST anomaly.
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The above subsection demonstrated the relationship between the EAT EEOF1 and the underlying SST anomalies. Aside from SST, other factors could potentially impact the EEOF1. As reviewed in the introduction section, the Eurasian snow cover variation can significantly influence the winter EAT. Along with the gradual warming in spring, the Eurasian snow cover substantially changes, especially over the Eurasian midlatitudes. Does the snow cover variability also influence the EAT EEOF1? To answer this question, we explore the key snow cover regions in this subsection.
Figure 8 depicts the regressed Eurasian snow cover anomalies against PC1 in March and April. In March, the positive EEOF1 is significantly related to the increased snow cover over central Europe and Northeast Asia (Fig. 8a). In April, only Northeast Asia is a key region with a significant snow cover-EEOF1 connection (Fig. 8b). To focus on the persistence characteristic, we regard Northeast Asia as the key region of the snow cover. Accordingly, a snow cover index measuring the Northeast Asian snow cover variation is defined as the area-averaged snow cover concentration anomalies over the region bounded by (42.5°–55°N and 110°–135°E) (box in Fig. 8a), marked as the NEASCI. The correlation coefficient between PC1 and the March (April) NEASCI is 0.46 (0.39), significant at the 95% (95%) confidence level. The correlation between the March and April NEASCI is 0.61, indicating good persistence of the Northeast Asian snow cover variation from March to April. In addition, Han and Sun (2021) have also reported the predictive value of the March Northeast Asian snow cover on the April EAT.
Figure 8. Regressed (a) March and (b) April snow cover concentration anomalies (units: %) against PC1 during 1961–2020. Values significant at the 95% confidence level are shaded by dots. The black rectangle is the domain used to define the snow cover indices.
We further analyze the relationship between the Northeast Asian snow cover and the two SST factors introduced in section 4.1. The absolute correlation coefficients between the simultaneous NEASCI and TIOSSTI are less than 0.1 in March and April, indicating their independent variations. The correlation between NEASCI and NASSTI in March is insignificant, with a value of 0.26; while the correlation coefficient between NEASCI and NASSTI in April is 0.28, which is barely significant at the 95% confidence level. This result indicates a significant linkage between the Northeast Asian snow cover and the North Atlantic SST anomalies in April. Therefore, in the following mechanism analysis, we will also discuss the influence of the Northeast Asian snow cover in April after removing the signal of the dipole SST pattern over the North Atlantic.
The longitude-pressure cross-section of the regressed air temperature anomalies averaged along 42.5°–55°N against the NEASCI in March is shown in Fig. 9a. Significant cold air temperature anomalies are observed from the surface up to 300 hPa, with the minimum anomalies existing in the lower troposphere. Northeast Asia becomes a cold source when the regional snow cover is above normal. As a consequence, the 1000–500-hPa thickness significantly decreases (Fig. 9b), causing the northern part of the EAT to deepen (Fig. 9c). When the increased snow cover anomalies persist into April, the air temperature below 300 hPa (Fig. 9d) and the 1000–500-hPa thickness also decrease over Northeast Asia (Fig. 9e), also strengthening the northern part of the EAT (Fig. 9f). As a result of this process, the Northeast Asian snow cover could contribute to the EAT EEOF1 in March and April.
Figure 9. (a) Longitude-pressure cross-section of the regressed air temperature anomalies (units: °C) averaged along 42.5°–55°N against the standardized NEASCI in March during 1961–2020. Regressed (b) 1000–500-hPa thickness anomalies (units: gpm) and (c) 500-hPa geopotential height anomalies (units: gpm) against the standardized NEASCI in March. (d–f) Same as in (a–c) but for the regressed results in April. Values significant at the 95% confidence level are dotted.
The above correlation analysis shows a significant simultaneous correlation between the NASSTI and the NEASCI in April. To distinguish the individual influence of the Northeast Asian snow cover, we remove the signals associated with the April NASSTI from the NEASCI and re-calculate the regression results in Figs. 9d–f. The re-calculated regression results (figures not shown) are highly consistent with that in Figs. 9d–f, indicating that the pronounced influence process is mainly attributed to the snow cover anomalies over Northeast Asia.
Summarizing the above results, we can see that the EAT EEOF1 is substantially influenced by the SST anomalies over the North Atlantic and tropical Indian Oceans as well as the snow cover anomalies over Northeast Asia. To further investigate their combined effects, we construct an index (referred to as the Mult_JRA55 index) by regressing PC1 against the NASSTI, TIOSSTI, and NEASCI in March. Limited by the length of the snow cover data, the available period of the Mult_JRA55 index is from 1967 to 2020. The Mult_JRA55 index is significantly correlated with PC1, with a coefficient of 0.60, significant at the 95% confidence level. In addition, the correlation coefficients of the Mult_JRA55 index with the March and April EAT indices are 0.49 and 0.46, respectively, both significant at the 95% confidence level. Noting that only March information is used to construct the Mult_JRA55 index, the Mult_JRA55 index not only describes the status of the March EAT but also provides the prediction information for the April EAT.
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The above analyses have shown that the predominant mode of the March−April EAT variation features a persistent anomaly from March to April. Such a mode is closely related to two SST factors and one snow cover factor. To further consolidate the reanalysis results, we analyze the CESM-LENS simulation, derived from the ensemble mean result of 40 ensembles during the entire simulation period of 1920–2005 in this subsection.
Before analyzing the EAT EEOF1 in the CESM-LENS simulation, we first calculate the correlation coefficient between the EAT indices in March and April, which is 0.51 and significant at the 95% confidence level. Such a result confirms the possible persistence of the EAT anomaly from March to April. Figure 10 displays the EEOF1 of the EAT variation from March to April in the CESM-LENS simulation. There are visible differences in the EEOF1 details between simulation (Fig. 10) and reanalysis (Figs. 3a, b). For example, the centers of the anomalous EAT in EEOF1 are located further east in the model simulation than in the reanalysis, especially in April. However, the CESM-LENS simulation can generally reproduce a persistent mode of the EAT variation from March to April. We further analyze the relationship of PC1 with the East Asian temperature and precipitation in the CESM-LENS simulation. The corresponding regression results are shown in Fig. S1 in the Electronic Supplementary Material (ESM). In the CESM-LENS simulation, the persistent strengthened EAT in March−April is associated with the persistent cold surface air temperature anomalies over East Asia (Figs. S1a and S1b in the ESM), consistent with the observational results (Figs 4a, b). For precipitation, the positive EEOF1 could lead to significantly less precipitation over central-eastern China, South Korea, and southern Japan in March (Fig. S1c in the ESM), consistent with the observational results (Fig. 4c). In April, the EEOF1-related precipitation signals over southern Japan are maintained; however, significant signals are largely weakened over central-eastern China (Fig. S1d in the ESM), which differs from the observational result (Fig. 4d). This is possibly due to the simulated eastward displacement of the EEOF1 center in April (Fig. 10b).
Figure 10. EEOF1 patterns of 500-hPa geopotential height anomalies over the EAT key region in (a) March and (b) April in the CESM-LENS simulation.
To confirm the influences of the SST and snow cover factors on the EAT EEOF1, we use the same index definitions as those in the reanalysis. The correlation coefficients between PC1 and the NASSTI, TIOSSTI, and NEASCI in March and April in the model simulation are shown in Table 1, all of which are significant at the 95% confidence level. Regarding the relationships among the impact factors, the SST anomalies over the tropical Indian Ocean are independent of the other two factors in March and April, as evidenced by their maximum absolute correlation coefficient being less than 0.13. The dipole SST pattern over the North Atlantic is closely related to the downstream Northeast Asian snow cover, with a correlation coefficient of 0.35 and 0.29 in March and April, respectively, both significant at the 95% confidence level. Therefore, in the following analysis, when conducting regression analysis against the NEASCI, the part related to the NASSTI will be linearly removed from the NEASCI (denoted as NEASCI_res) to concentrate on the individual influence of the Northeast Asian snow cover.
Corr. Coeff. NASSTI TIOSSTI NEASCI March / April March / April March / April PC1 0.35* / 0.32* 0.48* / 0.45* 0.55* / 0.50* Table 1. Correlation coefficients between PC1 and NASSTI, TIOSSTI, and NEASCI in the CESM-LENS simulation. Values with an asterisk are significant at the 95% confidence level.
In the CESM-LENS simulation, a negative NAO-like pattern appears in March and April consistent with the reanalysis; this is associated with the positive dipole SST pattern over the North Atlantic (Fig. 11). As a consequence, a Rossby wave train is triggered, which propagates eastward to strengthen the EAT in both March and April. Such a process is also consistent with the reanalysis.
Figure 11. (a) Regressed 500-hPa geopotential height anomalies (color; units: gpm) against the standardized NASSTI and the associated wave flux activity (vector; units: m2 s–2) in (a) March and (b) April in the CESM-LENS simulation. Values significant at the 95% confidence level are dotted. Vectors less than 2.5 × 10–3 m2 s–2 are not shown.
The cold SST anomaly over the tropical Indian Ocean could cause anomalies of sinking motion over the tropical Indian Ocean and anomalies of rising motion over the western tropical Pacific (Figs. 12a, c). An anomalous cyclone is therefore established over the western tropical Pacific, which can strengthen the southern part of the EAT (Figs. 12b, d).
Figure 12. Regressed (a) 500-hPa omega (units: Pa s–1) and (b) 500-hPa geopotential height anomalies (color; units: gpm) and the 850-hPa horizontal wind anomalies (vector; units: m s–1) against the standardized TIOSSTI in March in the CESM-LENS simulation. (c−d) Same as in (a−b) but for the regressed results in April. Values significant at the 95% confidence level are dotted. Vectors in (b) and (d) are significant at the 95% confidence level.
The increase in Northeast Asian snow cover leads to a regional cold source (Figs. 13a, d), significantly cooling down the air column above, as indicated by a decrease in the 850–500-hPa thickness (Figs. 13b, e) and causing negative 500-hPa geopotential height anomalies (Figs. 13c, f). The 850 hPa level is selected as the lowest layer for analysis to avoid the missing values beneath it due to the terrain in the model simulation. Accordingly, the persistent Northeast Asian snow cover could cause the anomalous EAT in March−April.
Figure 13. (a) Longitude-pressure cross-section of the regressed air temperature anomalies (units: °C) averaged along 42.5°–55°N against the standardized NEASCI_res in March in the CESM-LENS simulation. Regressed (b) 850–500-hPa thickness anomalies (units: gpm) and (c) 500-hPa geopotential height anomalies (units: gpm) against the standardized NEASCI_res in March in the CESM-LENS simulation. (d–f) Same as in (a–c) but for the regressed results in April. Values significant at the 95% confidence level are dotted.
From the above analysis, we can see that the CESM-LENS simulation well reproduces the EAT EEOF1 from March to April. The impacts of the dipole SST pattern over the North Atlantic, the SST anomaly over the tropical Indian Ocean, and the snow cover anomaly over Northeast Asia on the EAT EEOF1 are also effective in the simulation. Such results further confirm the important roles of the three factors in the persistent variations of the EAT from March to April.
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Observations and the CESM-LENS simulation have shown the significant impacts of the two SST factors and the one snow cover factor on the persistent EAT variation in March−April. In this subsection, we design more experiments to confirm the impacts of these factors on the EAT.
Three experiments are conducted to verify the influence of SSTs by the Community Atmospheric Model version 5 (CAM5). In the model configuration, we select the “F_2000_CAM5” as the component set and finite volume grid which has a horizontal resolution of 1.9°(lat) × 2.5°(lon) with 30 hybrid sigma pressure levels. The first CAM5 experiment is a control run (referred to as CAM5_CTL) and has a 45-year integration forced by the global climatological SST and sea ice concentration. The first five years are used for model spin-up, and the remaining 40-year simulation is then analyzed. The second CAM5 experiment is a North Atlantic SST-sensitivity experiment (referred to as CAM5_NASST). CAM5_NASST has an ensemble of 40 members; each starting from different initial conditions and integrated for three months from 1 February to 30 April. The integration with a one-month lead is designed to make a more coordinated connection between the atmosphere and boundary conditions in March−April. The 40 initial conditions are taken from the restart files of CAM5_CTL (every 1 February of each year the 40-year EXP_CTL). The SST boundary condition in CAM5_NASST is constructed by superimposing the monthly SST anomalies over the North Atlantic (20.5°–57.5°N, 80.5°–19.5°W) on the climatological monthly SST from February to April (Fig. S2 in the ESM). The superimposed monthly SST anomalies are the regressed SST anomalies against the standardized March NASSTI, which are then multiplied by 2 to highlight the SST forcing. In addition, to avoid unrealistic discontinuities, the superimposed anomalies are linearly decreased to 0°C across five grid points over the four borders. The third CAM5 experiment is a tropical Indian Ocean SST-sensitivity experiment (referred to as CAM5_TIOSST). This experiment is the same as CAM5_NASST, but the superimposed monthly SST anomalies are the regressed SST anomalies over the tropical Indian Ocean (15.5°S–5.5°N, 50.5°–100.5°E) against the standardized March TIOSSTI and then multiplied by 2 (Fig. S3 in the ESM).
The difference between the SST-sensitivity experiment and CAM5_CTL reflects the impacts of the specific SST forcing. As shown in Figs. 14a and 14b, we can see that the persistent positive dipole SST pattern over the North Atlantic could cause the persistent strengthened EAT in March and April through the triggered Rossby wave train, despite the relatively weak significance of the EAT response in March. The atmospheric circulation responses to the anomalous cooling tropical Indian Ocean are shown in Figs. 14c and 14d. The cold tropical Indian Ocean SST anomaly facilitates an anomalous cyclone over the western North Pacific and strengthens the EAT in March and April, albeit the EAT response is only weakly significant in April.
Figure 14. Composite simulated (a) March and (b) April geopotential height (color; units: gpm) at 500 hPa between CAM5_NASST and CAM5_CTL and the associated 300-hPa wave activity fluxes (vector; units: m2 s–2). Composite simulated (c) March and (d) April 500-hPa geopotential height (color; units: gpm) and 850-hPa horizontal wind (vector; units: m s–1) between CAM5_TIOSST and CAM5_CTL. Responses of the (e) March and (f) April 500-hPa geopotential height to the diabatic forcing in LBM_NEASC. Values in (a–d) significant at the 95% confidence level are dotted. Vectors less than 0.05 m2 s–2 are not shown in (a–b). Black vectors in (c–d) are significant at the 95% confidence level.
Because snow cover is a diagnostic variable in the CAM5, it is not easy to simulate the solo impact of the snow cover using this model. To explore the impact of the snow cover on the March−April EAT, we perform one experiment with the Linear Barotropic Model (LBM) (Watanabe and Jin, 2002). The dry LBM is often used to explore the impact of the snow via the diabatic heat forcing experiment (e.g., Jia et al., 2018; Ma et al., 2021). A horizontal resolution of T42 (equivalent to approximately 2.8°) with 20 vertical sigma levels is used in the LBM simulation. The LBM simulation is integrated for 20 days with a steady forcing. Since previous studies have reported that the atmospheric response to the prescribed mid-latitude forcing will be more stable and representative after 15 days (e.g., Jia et al., 2018; Ma et al., 2021; Yu et al., 2023), we analyze the averaged simulations over days 16–20.
To mimic the impact of the resultant regional cold source due to the more Northeast Asian snow cover, we design an LBM experiment with a diabatic heat forcing over Northeast Asia (42.5°–55°N, 110°–135°E), referred to as LBM_NEASC. The horizontal distributions and the vertical profiles of the heat forcing in LBM_NEASC are shown in Fig. S4 in the ESM. According to Figs. 9a and 9d, the heat forcing has a minimum of –1.0 K d–1 at a height of σ = 0.830 (in σ-coordinate). In response to the persistent cooling over Northeast Asia, the northern part of the EAT strengthens in March and April (Figs. 14e, f).
Summarizing the above sensitivity experiment results, we can conclude that the two SST factors and one snow cover factor could contribute to the persistent EAT anomaly in March and April. Results in this subsection offer further support to the observation analysis and CESM-LENS simulation.
Corr. Coeff. | NASSTI | TIOSSTI | NEASCI | ||
March / April | March / April | March / April | |||
PC1 | 0.35* / 0.32* | 0.48* / 0.45* | 0.55* / 0.50* |