Responses of the Southern Ocean mixed layer depth to the eastern and central Pacific El Niño events during austral winter
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Abstract: Based on the Ocean Reanalysis System version 5 (ORAS5) and the fifth-generation reanalysis datasets derived from European Centre for Medium-Range Weather Forecasts (ERA5), we investigate the different impacts of the central Pacific (CP) El Niño and the eastern Pacific (EP) El Niño on the Southern Ocean (SO) mixed layer depth (MLD) during austral winter. The MLD response to the EP El Niño shows a dipole pattern in the South Pacific, namely the MLD dipole, which is the leading El Niño-induced MLD variability in the SO. The tropical Pacific warm sea surface temperature anomaly (SSTA) signal associated with the EP El Niño excites a Rossby wave train propagating southeastward and then enhances the Amundsen Sea low (ASL). This results in an anomalous cyclone over the Amundsen Sea. As a result, the anomalous southerly wind to the west of this anomalous cyclone advects colder and drier air into the southeast of New Zealand, leading to surface cooling through less total surface heat flux, especially surface sensible heat (SH) flux and latent heat (LH) flux, and thus contributing to the mix layer (ML) deepening. The east of the anomalous cyclone brings warmer and wetter air to the southwest of Chile, but the total heat flux anomaly shows no significant change. The warm air promotes the sea ice melting and maintains fresh water, which strengthens stratification. This results in a shallower MLD. During the CP El Niño, the response of MLD shows a separate negative MLD anomaly center in the central South Pacific. The Rossby wave train triggered by the warm SSTA in the central Pacific Ocean spreads to the Amundsen Sea, which weakens the ASL. Therefore, the anomalous anticyclone dominates the Amundsen Sea. Consequently, the anomalous northerly wind to the west of anomalous anticyclone advects warmer and wetter air into the central and southern Pacific, causing surface warming through increased SH, LH, and longwave radiation flux, and thus contributing to the ML shoaling. However, to the east of the anomalous anticyclone, there is no statistically significant impact on the MLD.
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Figure 1. Time series of Niño 3 index (blue line) from NOAA and Nep index (red line) based on Eq. (5) (a), and Niño 4 index (blue line) from NOAA and Ncp index (red line) based on Eq. (5) (b). Grey dashed lines denote one standard deviation of Nep and Ncp indices. Cor indicates correlations between two curves in each panel.
Figure 2. Composite images of MLD anomaly derived from the ORAS5 in austral, spring (SON) (a, e), summer (DJF) (b, f), autumn (MAM) (c, g), and winter (JJA) (d, h) for EP El Niño (a–d) and CP El Niño (e–h) events. The black stippled areas represent that the results are statistically significant at the 95% confidence level. Box 1 (red; 40°–60°S, 160°E–180°–145°W) and Box 2 (blue; 45°–65°S, 140°–70°W) in d and the Box 3 (blue; 42°–62°S, 165°–100°W) in h indicate the focus areas. SON: September, October, and November; DJF: December, January, and February; MAM: March, April, and May; JJA: June, July, and August.
Figure 3. Composite images of SSTA observed from the ERA5 for EP (a) and CP (b) El Niño events in austral winter (JJA, June, July, and August). The black stippled areas represent that the results are statistically significant at the 95% confidence level. Box 1 (red; 40°–60°S, 160°E–180°–145°W) and Box 2 (blue; 45°–65°S, 140°–70°W) in a and Box 3 (blue; 42°–62°S, 165°–100°W) in b indicate the focus areas, same as Boxes 1, 2, and 3 in Figs 2d and h.
Figure 4. Composite images of 300 hPa (a and b), 700 hPa (c and d), and 850 hPa (e and f) geopotential height anomaly (unit: m) in austral winter (JJA) derived from the ERA5 for EP El Niño (a, c, and e) and CP El Niño (b, d, and f) events. The gray shadings represent the results statistically significant at the 95% confidence level. Red solid, black solid, and blue dashed lines for geopotential height anomaly indicate the positive, zero, and negative anomalies, respectively. Black arrow indicates the path of Rossby wave propagation. Box 1 (red; 40°–60°S, 160°E–180°–145°W) and Box 2 (blue; 45°–65°S, 140°–70°W) and Box 3 (blue; 42°–62°S, 165°–100°W) in b, d, and f indicate the focus areas, same as Boxes 1, 2, and 3 in Figs 2 d and h.
Figure 5. Composite images of austral winter (JJA, June, July, and August) 300 hPa zonal wind speed anomaly derived from the ERA5 for EP El Niño (a) and CP El Niño (b) events and potential vorticity (PV) anomaly derived from the ERA5 for EP El Niño (c) and CP El Niño (d) events. The black stippled areas represent the results statistically significant at the 95% confidence level. Box 1 (red; 40°–60°S, 160°E–180°–145°W) and Box 2 (blue; 45°–65°S, 140°–70°W) and Box 3 (blue; 42°–62°S, 165°–100°W) indicate the focus areas, same as Boxes 1, 2, and 3 in Figs 2d and h.
Figure 6. Composite images of SLP anomaly (a and b, unit: hPa) (contours), and anomalous 10 m wind (c and d) (vectors) as well as wind speed (color) anomaly (unit: m/s) derived from the ERA5 for EP El Niño (a and c) and CP El Niño (b and d) events. Red solid, black solid, and blue dashed lines for SLP anomaly indicate the positive, zero, and negative anomalies, respectively. The gray shadings and black stippled areas represent the results statistically significant at the 95% confidence level. The red letters H and A represent the High-pressure anomaly center and anticyclone, respectively. The blue letters L and C represent the Low-pressure anomaly center and cyclone, respectively. Box 1 (red; 40°–60°S, 160°E–180°–145°W) and Box 2 (blue; 45°–65°S, 140°–70°W) in a and c and Box 3 (blue; 42°–62°S, 165°–100°W) in b and d indicate the focus areas, same as Boxes 1, 2, and 3 in Figs 2d and h.
Figure 7. Composite images of air temperature anomaly at 2 m (a and b), and
1000 hPa specific humidity anomaly (c and d) derived from the ERA5 for EP El Niño (a and c) and CP El Niño (b and d) events. The black stippled areas represent the results statistically significant at the 95% confidence level. Box 1 (red; 40°–60°S, 160°E–180°–145°W) and Box 2 (blue; 45°–65°S, 140°–70°W) in a and c and Box 3 (blue; 42°–62°S, 165°–100°W) in b and d indicate the focus areas, same as Boxes 1, 2, and 3 in Figs 2d and h.
Figure 8. Composite images of surface sensible heat flux (SH) anomaly (a and b), latent heat flux (LH) anomaly (c and d), net longwave radiation flux (LW) anomaly (e and f), net shortwave radiation flux (SW) anomaly (g and h), and total surface heat flux anomaly (i and j) derived from the ERA5 for EP El Niño (a, c, e, g, and i) and CP El Niño (b, d, f, h, and j) events. The positive anomaly represents downward. The black stippled areas represent the results statistically significant at the 95% confidence level. Box 1 (red; 40°–60°S, 160°E–180°–145°W) and Box 2 (blue; 45°–65°S, 140°–70°W) in a, c, e, g, and i and Box 3 (blue; 42°–62°S, 165°–100°W) in b, d, f, h, and j indicate the focus areas, same as Boxes 1, 2, and 3 in Figs 2d and h.
Figure 11. Composite pictures of SIC anomaly derived from the ORAS5 during EP El Niño events (a) and CP El Niño events (b). The black stippled areas represent the results statistically significant at the 95% confidence level. Box 1 (red; 40°–60°S, 160°E–180°–145°W) and Box 2 (blue; 45°–65°S, 140°–70°W) in a and Box 3 (blue; 42°–62°S, 165°–100°W) in b indicate the focus areas, same as Boxes 1, 2, and 3 in Figs 2d and h.
Figure 12. Schematic diagram illustrating the differences of critical elements during EP and CP El Niño events. Red box (40°–60°S, 160°E–180°–145°W) and blue box (45°–65°S, 140°–70°W) in a and blue box (42°–62°S, 165°–100°W) in b indicate the focus areas, same as Boxes 1, 2, and 3 in Figs 2d and h. Black arrow: Rossby wave train path; pink upward arrow: upward motion; pink and blue curved arrows: wind; H: high-pressure anomaly center; L: low-pressure anomaly center; Q: total surface heat flux; +: increase; −: decrease.
Table 1. Category of EP El Niño and CP El Niño events from 1980 to 2021 determined by Nep and Ncp indices
Category Year EP El Niño 1982/1983, 1991/1992, 1997/1998, 2006/2007, 2015/2016 CP El Niño 1990/1991, 1994/1995, 2002/2003, 2004/2005, 2009/2010, 2014/2015, 2018/2019, 2019/2020 
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