Marine Heatwaves/Cold‐Spells Associated With Mixed Layer Depth Variation Globally

• Published in: Geophysical research letters 2024-12, Vol.51 (24), p.n/a
• Main Authors: Sun, Wenjin, Wang, Yindi, Yang, Yifei, Yang, Jingsong, Ji, Jinlin, Dong, Changming
• Format: Article
• Language: English
• Subjects: Cold; Confidence intervals; Coral reef ecosystems; Coral reefs; Current rings; Ecosystems; Eddies; Heat waves; marine cold‐spells; Marine ecosystems; Marine heatwaves; Marine invertebrates; Mesoscale eddies; Mesoscale phenomena; Mixed layer; Mixed layer depth; mixed layer depth variation; Ocean circulation; Sea surface temperature; Statistical analysis; Surface temperature; Variation; Vortices
• ISSN: 0094-8276; 1944-8007
• DOI: 10.1029/2024GL112325

Marine heatwaves (MHWs) and cold‐spells (MCSs) are extreme sea surface temperature events with significant impacts on marine ecosystems. However, the connection between these events and mixed layer depth (MLD) variations, as well as how their intensity relates to MLD changes, remains unclear. Integrating OISST V2.1 data with Argo profiles, this analysis finds that during MHWs, MLD decreases by 8.10% globally, while during MCSs, it increases by 8.13%. In 5° × 5° bins, 80.46% of ocean regions show MLD shallowing during MHWs, while 67.69% show deepening during MCSs. A significant global correlation between the intensity of MHWs/MCSs and MLD changes, with coefficients of −0.85 and −0.86, respectively. MHWs are more common in mesoscale anticyclonic eddies (AEs) (19.45%) than in cyclonic eddies (CEs) (10.11%). For MCSs, the pattern reverses, with 8.57% in AEs and 20.82% in CEs. Restratification and mesoscale eddies are two important factors driving MLD changes during these events. Plain Language Summary Marine heatwaves (MHWs) involve prolonged periods of sea surface temperatures (SSTs) above the 90th percentile of the climatological threshold, while marine cold‐spells (MCSs) involve SSTs below the 10th percentile. MHWs and MCSs both significantly impact marine ecosystems, particularly fragile coral reef ecosystems. A substantial amount of literature currently examines the characteristics of MHWs/MCSs, such as their frequency, duration, and cumulative days. However, the relationship between MHWs/MCSs and internal oceanic factors like mixed layer depth (MLD) variation is not fully understood. By combining OISST V2.1 data with Argo profiles, this study finds that MHWs are linked to significant MLD shallowing compared to background values. There is also a strong, statistically significant correlation between MHW intensity and the degree of MLD shallowing, at the 99% confidence level. In contrast, during MCSs, the MLD typically deepens relative to the climatological background. However, the degree of this deepening varies regionally with MCS intensity. Interestingly, MLD shallowing is observed during MCSs when the intensity exceeds −2.4°C. Further analysis indicates that mesoscale eddies and restratification are two mechanisms driving the variation in MLD during MHWs and MCSs. Key Points During marine heatwaves (MHWs), the mixed layer depth (MLD) shallows by 8.10% on average globally, while during MCSs, it deepens by 8.13% A significant correlation between the inten