Why the Mixed Layer Depth Matters When Diagnosing Marine Heatwave Drivers Using a Heat Budget Approach

Abstract

Marine heatwaves (MHWs) are extreme warming events that can result in significant damage to marine ecosystems and local economies. The primary drivers of these events have been frequently studied using an upper ocean heat budget. However, various surface mixed layer (SML) depths have been used with little attention paid to the impact of the depth chosen on heat budget term estimates. We analyse MHW drivers in two dynamically contrasting regions off the east coast of Australia (East Australian Current extension) and the west coast of New Zealand over a 30-year period (1985–2014, inclusive). We compare the magnitude of the air-sea heatflux and advection terms in a volume-averaged heat budget using three different SML depth estimates. We show that the SML depth over which the heat budget is calculated has direct consequences on the identification of MHW dominant drivers. The air-sea heatflux term is amplified when the SML depth is underestimated and dampened when overestimated. The variation in the magnitude of the advection term is dependent on the barotropic or baroclinic structure of the currents. We, also, show that the impact on MHW driver classification is both temporally and regionally dependent. Generally, a deep SML estimate results in more MHWs being classified as advection and less classified as air-sea heatflux-driven. However, during the cool months, a shallow estimate produces the opposite pattern and to a varying degree of intensity depending on the region's dynamics. Use of daily and spatially variable SML depth in a heat budget calculation allows the comparison between regions with different dynamics influencing the mixed layer depth. These results show that when using a heat budget approach to explore marine heatwaves over extended time and space (e.g., regions and seasons), it is imperative to consider the temporal and spatial variability in the SML depth.

Weakened Seasonality of the Ocean Surface Mixed Layer Depth in the Southern Indian Ocean During 1980–2019

“…The seasonality changes in the SIO MLD may be important for understanding the long‐term changes in regional climate (Behera & Yamagata, 2001; Sharma et al., 2023; Zhao et al., 2023) and marine heatwaves (Amaya et al., 2021; Elzahaby et al., 2022; Saranya et al., 2022; J. Shi et al., 2022) in recent decades. For example, Yamagami and Tozuka (2015) show that the interdecadal MLD shoaling in the south Indian Ocean can amplify the warming effect from shortwave radiation, and thus even a weak atmospheric forcing may trigger the south subtropical Indian Ocean dipole (SIOD), which is an important climate mode that substantially influences the South African and Australian climate.…”

“…The seasonality changes in the SIO MLD may be important for understanding the long-term changes in regional climate (Behera & Yamagata, 2001;Sharma et al, 2023;Zhao et al, 2023) and marine heatwaves (Amaya et al, 2021;Elzahaby et al, 2022;Saranya et al, 2022;J. Shi et al, 2022) in recent decades.…”

Complex phylogenetic origin and geographic isolation drive reef fishes response to environmental variability in oceanic islands of the southwestern Atlantic

“…However, we found detectable effects of marine heat waves (MHW) on abundances of reef fishes, occurring most frequently in winter (seasonal variation shown in the Supporting information). These transient extreme SSTA events are linked to the impact of global warming on the heat budget of the upper ocean layer (Frolicher et al 2018, Elzahaby et al 2022). Physiological and energetic impacts on reef fish communities can result from severe MHW events, as in the summer of 2013/14 (not considered in our analyses) that caused a two standard deviation reduction below average of surface chlorophyll‐a concentration in the South Atlantic (Rodrigues et al 2019).…”

Marine Heatwaves in the East Asian Marginal Seas Facilitated by Boreal Summer Intraseasonal Oscillations

“…(2023) pointed out that the recent long‐lasting MHW during the summer of 2022 was initiated by the shallow mixed layer resulting from the large river discharge from the Yangtze–Huaihe River floods. While it is well‐known that the factors responsible for land heatwaves often trigger marine heatwaves, the significance of oceanic factors, including reduced mixed layer depth (MLD), has only recently gained attention (Amaya et al., 2021; Capotondi et al., 2022; Elzahaby et al., 2022; Jacox et al., 2022; Oh et al., 2023; Sallée et al., 2021; Yan et al., 2020). These studies addressed that the reduced mixed layer depth often play the dominant role in triggering MHW.…”

“…Nevertheless, the relative importance of these two factors still needs to be explored. Although previous studies have considered the variation in mid‐latitude MLD as one of the contributing factor to the genesis of MHWs in the NECS‐SYS, they have treated the mixed layer in this region as a slowly changing component at seasonal to interannual timescales (Amaya et al., 2021; Elzahaby et al., 2022; Lee et al., 2023). However, this characterization may not accurately reflect the dynamic nature of the mixed layer in mid‐latitude regions.…”