TY - JOUR
T1 - Seasonal-to-interannual response of Southern Ocean mixed layer depth to the southern annular mode from a global 1/10° ocean model
AU - Li, Qian
AU - Lee, Sukyoung
AU - England, Matthew H.
AU - McClean, Julie L.
N1 - Funding Information:
Acknowledgments. We thank Lynne Talley for helpful discussions about the relationship between the SAM and mode water formation in the Southern Ocean. QL and SL are supported by the National Science Foundation under Grant AGS-1455577. MHE is supported by the Australian Research Council (ARC), including the ARC Centre of Excellence for Climate Extremes, and the Centre for Southern Hemisphere Oceans Research (CSHOR). JLM was supported by DOE U.S. Office of Science Grant DE-SC0014440 for this study. High-performance computing resource Grant TG-OCE110013 (JLM) was used to run the POP simulation at NSF’s Extreme Science and Engineering Discovery Environment (XSEDE), while OCE-0850463 supported Elena Yuleva (UCSD) and JLM to carry out the simulation. The POP output is available from XSEDE.
Funding Information:
We thank Lynne Talley for helpful discussions about the relationship between the SAM and mode water formation in the Southern Ocean. QL and SL are supported by the National Science Foundation under Grant AGS-1455577. MHE is supported by the Australian Research Council (ARC), including the ARC Centre of Excellence for Climate Extremes, and the Centre for Southern Hemisphere Oceans Research (CSHOR). JLM was supported by DOE U.S. Office of Science Grant DE-SC0014440 for this study. High-performance computing resource Grant TG-OCE110013 (JLM) was used to run the POP simulation at NSF?s Extreme Science and Engineering Discovery Environment (XSEDE), while OCE-0850463 supported Elena Yuleva (UCSD) and JLM to carry out the simulation. The POP output is available from XSEDE.
Publisher Copyright:
© 2019 American Meteorological Society.
PY - 2019/9/1
Y1 - 2019/9/1
N2 - The relationship between the southern annular mode (SAM) and Southern Ocean mixed layer depth (MLD) is investigated using a global 0.18 resolution ocean model. The SAM index is defined as the principal component time series of the leading empirical orthogonal function of extratropical sea level pressure from September to December, when the zonally symmetric SAM feature is most prominent. Following positive phases of the SAM, anomalous deep mixed layers occur in the subsequent fall season, starting in May, particularly in the southeast Pacific. Composite analyses reveal that for positive SAM phases enhanced surface cooling caused by anomalously strong westerlies weakens the stratification of the water column, leading to deeper mixed layers during spring when the SAM signal is at its strongest. During the subsequent summer, the surface warms and the mixed layer shoals. However, beneath the warm surface layer, anomalously weak stratification persists throughout the summer and into fall. When the surface cools again during fall, the mixed layer readily deepens due to this weak interior stratification, a legacy from the previous springtime conditions. Therefore, the spring SAM–fall MLD relationship is interpreted here as a manifestation of reemergence of interior water mass anomalies. The opposite occurs after negative phases of the SAM, with anomalously shallow mixed layers resulting. Additional analyses reveal that for the MLD region in the southeast Pacific, the effects of salinity variations and Ekman heat advection are negligible, although Ekman heat transport may play an important role in other regions where mode water is formed, such as south of Australia and in the Indian Ocean.
AB - The relationship between the southern annular mode (SAM) and Southern Ocean mixed layer depth (MLD) is investigated using a global 0.18 resolution ocean model. The SAM index is defined as the principal component time series of the leading empirical orthogonal function of extratropical sea level pressure from September to December, when the zonally symmetric SAM feature is most prominent. Following positive phases of the SAM, anomalous deep mixed layers occur in the subsequent fall season, starting in May, particularly in the southeast Pacific. Composite analyses reveal that for positive SAM phases enhanced surface cooling caused by anomalously strong westerlies weakens the stratification of the water column, leading to deeper mixed layers during spring when the SAM signal is at its strongest. During the subsequent summer, the surface warms and the mixed layer shoals. However, beneath the warm surface layer, anomalously weak stratification persists throughout the summer and into fall. When the surface cools again during fall, the mixed layer readily deepens due to this weak interior stratification, a legacy from the previous springtime conditions. Therefore, the spring SAM–fall MLD relationship is interpreted here as a manifestation of reemergence of interior water mass anomalies. The opposite occurs after negative phases of the SAM, with anomalously shallow mixed layers resulting. Additional analyses reveal that for the MLD region in the southeast Pacific, the effects of salinity variations and Ekman heat advection are negligible, although Ekman heat transport may play an important role in other regions where mode water is formed, such as south of Australia and in the Indian Ocean.
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U2 - 10.1175/JCLI-D-19-0159.1
DO - 10.1175/JCLI-D-19-0159.1
M3 - Article
AN - SCOPUS:85074774915
SN - 0894-8755
VL - 32
SP - 6177
EP - 6195
JO - Journal of Climate
JF - Journal of Climate
IS - 18
ER -