The oceanographic and climate communities are putting significant effort into reaching a consensus on a common definition for Marine Heatwaves (MHW). The absence of such a unified definition poses a substantial obstacle when it comes to making retrospective comparisons between various MHW studies. This hindrance is critical for achieving a mechanistic understanding of the role of MHWs in marine ecosystems.

However, why is it so challenging to characterize and define MHWs? The answer is straightforward: there isn't a single, distinct dynamical mechanism responsible for the persistence of heat anomalies in the ocean, which we refer to as MHWs. Unlike variability associated with phenomena such as large oceanic eddies, oceanic fronts, upwelling systems, tropical cyclones, or climate modes, prolonged heat anomalies do not exhibit characteristic time or spatial scales. As a result, common MHW definitions group together prolonged temperature anomalies lasting from days to years and spanning from a few kilometers to thousands of kilometers in scale.

Analyzing sea surface temperature anomalies through power spectra reveals a "red" power spectrum with no discernible time scales. A similar analysis in spatial dimensions similarly shows a lack of any specific scale. Given this absence of emergent scales, we suggest adopting a process-based definition for MHWs. Such an approach would classify all events into a smaller number of categories, each linked to a specific driver or dynamical process operating on certain spatiotemporal scales. This shift could significantly reduce the subjectivity involved in selecting the temporal and spatial scales required for current MHW definitions, ultimately advancing our understanding of these events.

How to cite: Liguori, G.: The need to adopt process-based or impact-based definitions for marine heatwaves, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-17696, https://doi.org/10.5194/egusphere-egu24-17696, 2024.

A marine heatwave (MHW) is typically defined as an anomalous warm event in the surface ocean, with wide-ranging impacts on marine and socio-economic systems. The surface warming associated with MHWs can penetrate into the deep ocean; however, the vertical structure of MHWs is poorly known in the global ocean. Here, we identify four main types of MHWs with different vertical structures using Argo profiles: shallow, subsurface-reversed, subsurface-intensified, and deep MHWs. These MHW types are characterized by different spatial distributions with hotspots of subsurface-reversed and subsurface-intensified MHWs at low latitudes and shallow and deep MHWs at middle-high latitudes. These vertical structures are influenced by ocean dynamical processes, including oceanic planetary waves, boundary currents, eddies, and mixing. The area and depth of all types of MHWs exhibit significant increasing trends over the past two decades. These results contribute to a better understanding of the physical drivers and ecological impacts of MHWs in a warming climate. 

How to cite: Zhang, Y., Du, Y., Feng, M., and Hobday, A. J.: Vertical structures of global marine heatwaves, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-16606, https://doi.org/10.5194/egusphere-egu24-16606, 2024.

The Mediterranean Sea (MS) has been experiencing progressively intensified Marine heatwave (MHW) conditions over the past decades, associated with severe environmental and socioeconomic impacts. Building upon prior research on physical mechanisms underlying the occurrence of MHWs, here we assess the relative role of air-sea heat exchange in driving the onset and decline phases of surface MHWs in the basin, utilizing remote sensing and reanalysis data for the period 1993-2022. Although contributing positively to the SST evolution during most MHWs, surface heat flux is identified as the primary driver in less than half of the onset/decline MHW phases. This finding suggests that oceanic processes play a crucial role in driving SST anomalies during MHWs in the basin. The role of surface heat flux becomes more pronounced during onset periods and warmer seasons, with the latent heat being the most significant heat flux component in modulating SST anomalies during both MHW phases and across all seasons. Heat flux emerges as the major driver of most onset phases in the Adriatic and the Aegean Seas. Onset/decline phases shorter than 5 days exhibit a weaker heat flux contribution compared to longer phases. Moreover, an inverse relationship between event severity and heat flux contribution is observed. At the subsurface, mixed layer shoaling is observed over the entire duration of most events, particularly for those of shorter duration. Therefore, the surface cooling right after the peak intensity day is likely not associated with vertical mixing in such cases. After the MHW end day, a significant mixed layer deepening in most cases suggests that further dissipation of heat is commonly driven by vertical mixing. This study emphasizes the need for considering subsurface information for MHW studies and accounting for limitations associated with the definitions employed for MHW phases. Clearly articulating such choices, tailored to the specific contexts of individual studies, is vital for precise interpretation and meaningful comparisons across different studies on MHW drivers.

How to cite: Denaxa, D., Korres, G., Bonino, G., Masina, S., and Hatzaki, M.: Investigating the role of air-sea heat flux for marine heatwaves in the Mediterranean Sea, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-10707, https://doi.org/10.5194/egusphere-egu24-10707, 2024.

The year 2023 was characterized by record-breaking global surface air and sea surface temperatures (SSTs), the latter reaching a record 21° C in April (excluding the polar regions; Copernicus 2023). As June to October were the warmest on record globally (WMO 2023), extreme and long-lasting marine heatwave (MHW) events were observed, especially in the North Atlantic and the Mediterranean Sea. In general, the occurrence of MHWs in the subtropics and western boundary current regions is predominantly driven by atmospherically induced processes such as the net ocean heat uptake from the atmosphere, associated with a reduction in latent heat loss and increased shortwave radiation (Schlegel et al. 2021; Vogt et al. 2022). The atmospheric circulation with persistent high‑pressure systems and anomalously weak wind speeds associated with increased insolation is the dominant driver of the above processes. We focus on the state of the atmosphere at the surface and in the mid-troposphere during 2023 and identify specific atmospheric patterns and SST anomaly structures. To detect MHWs and calculate their characteristics we use the daily gridded NOAA OI SST version 2.1 dataset (Huang et al. 2021, updated), derived from the AVHRR satellite, in-situ ship and buoy SST data. For the atmospheric component, we used the mean sea level pressure (SLP), the horizontal wind at 10 m, the geopotential height at 500 hPa (zg₅₀₀) and the 2 m maximum temperature (T_max) from the ECMWF ERA5 reanalysis (Hersbach et al. 2020, updated). Atmospheric and ocean datasets are provided globally with a high resolution (0.25°). We use daily anomalies with 1983 to 2012 as the reference period (as recommended by Hobday et al. 2018). The evaluation of MHW metrics such as frequency, duration, mean and cumulative intensity in different subregions of the North Atlantic and Mediterranean revealed that the most frequent MHWs were observed in the western Mediterranean (WMED), the longest MHWs in the central northeast Atlantic and the cumulatively most intense MHWs in the northwest Atlantic and central northeast Atlantic. The most intense MHWs are found in the WMED and off Newfoundland. During summer we detect asynchronous, above normal SLP, zg₅₀₀ and T_max over the northwest Atlantic, the WMED and the Black Sea, representing a type of blocking condition. A weakened Azores High, associated with reduced wind speed, mixing and upwelling, allows SSTs to rise substantially in the central northeast Atlantic during summer (Copernicus 2023). The first Empirical Orthogonal Function shows an antiphase dipole of SST and zg500 anomalies (explained variances of 43.9 % and 34.3 %, respectively) between the Mediterranean and West of the British Isles as well as monopol SST and zg500 anomalies (explained variances of 57.7 % and 41.9 %, respectively) over the northwestern Atlantic and the Labrador Sea.

How to cite: Behr, L., Xoplaki, E., Luther, N., Tragou, E., Luterbacher, J., and Zervakis, V.: The 2023 marine heatwave in the North Atlantic and the Mediterranean Sea: ocean response to atmospheric circulation, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-7671, https://doi.org/10.5194/egusphere-egu24-7671, 2024.

The Northeast Pacific Ocean has experienced episodes of intense and persistent warm conditions, also known as marine heatwaves, with devastating ecological impacts. Being able to predict these extreme events a few seasons in advance is therefore very important, but has proven elusive in many cases. While the intensity of Northeast Pacific marine heatwaves has been related to local stochastic atmospheric forcing with limited predictability, their evolution and persistence may be controlled by large-scale climate influences. Here we use a multi-variate statistical approach to identify these large-scale drivers, as well as the initial states that optimally develop into a marine heatwave at a later time in this region. Results indicate that a decadal mode of variability related to the Pacific Decadal Oscillation plays a key role in creating conditions favorable to the development of Northeast Pacific marine heatwaves. This mode is also implicated in the development of Central Pacific El Niño events, which may contribute to the persistence of the Northeast Pacific warm anomalies. In addition, this mode of variability appears to be responsible for the increased Northeast Pacific sea surface temperature variance in recent decades, suggesting that changes in internal climate variability may be responsible for the enhanced MHW activity in this region during this recent period.

How to cite: Capotondi, A., Newman, M., Xu, T., and Di Lorenzo, E.: Large-scale drivers of Northeast Pacific MHWs in a changing climate, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-6542, https://doi.org/10.5194/egusphere-egu24-6542, 2024.

Marine heatwaves are expected to become more frequent, intense, and longer-lasting in a warming world. However, it remains unclear whether feedback processes could amplify or dampen extreme ocean temperatures. Here we impose the observed atmospheric flow in coupled climate simulations to determine how the record-breaking 2019 Northeast Pacific marine heatwave would have unfolded in preindustrial times, and how it could unravel in a +4°C warmer world compared to present-day conditions. We find that air-sea interactions, involving reductions in clouds and ocean mixed-layer depth and air advection from fast-warming subpolar regions, modulate warming rates within the marine heatwave. In a +4°C warmer climate, global oceans are +1.9°C warmer than present levels, and regional mean warming in the Northeast Pacific can reach +2.3–2.7 ± 0.25°C. Our identified feedback processes are projected to further amplify the intensity and spatial extent of analogous Northeast Pacific summer marine heatwaves beyond those thresholds, with a warming reaching +2.9 ± 0.15°C above present levels. Such an event-specific amplification would place even greater stress on marine ecosystems and fisheries.

How to cite: Athanase, M., Sánchez-Benítez, A., Goessling, H., Pithan, F., and Jung, T.: Projected amplification of summer marine heatwaves in a warming Northeast Pacific Ocean, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-4034, https://doi.org/10.5194/egusphere-egu24-4034, 2024.

The increasing frequency and intensity of heatwave events have led to a significant rise in heat-related threads on land and in the ocean during recent years. A classic example of a marine heat wave (MHW) is the 2014 – 2016 warm event that spread across the northeastern Pacific (NEP) Ocean—an event that researchers coined “the blob”. Here we use an adjoint sensitivity approach to shed new light on potential causes for reoccurring NEP marine heatwaves events in the region of the NEP. The study is based on the Massachusetts Institute of Technology general circulation model (MITgcm) and its adjoint, for which the mean sea surface temperature (SST) of different target regions (region 1: 145°~ 160°W, 48°~ 56°N; region 2: 130°~ 145°W, 40°~ 48°N) and different target years (e.g. year 2014) was set as objective function. The adjoint sensitivities show that during the year of emergence, air-sea turbulent surface heat flux is the dominant atmospheric driver. The horizontal temperature advection, i.e., the impact of the basin-wide ocean circulation, is found to be less important, but might act as a preconditioning of MHW through climate oscillations (e.g. NPGO). Because atmospheric forcing anomalies occurring within the 18 months prior to the MHW event play a particularly critical role in driving the overall response locally through air-sea interactions, the leading 18 month atmospheric conditions in the central North Pacific can be considered as predictive signals for later marine heatwave events. Based on our preliminary findings, it can be concluded that 2024 may not be a heatwave year for NEP region. 

How to cite: Wang, X., Köhl, A., and Stammer, D.: Mechanism and Forecast Potential of North Pacific Marine Heatwaves inferred from Adjoint Sensitivities, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-8423, https://doi.org/10.5194/egusphere-egu24-8423, 2024.

As marine heatwaves (MHWs) become a growing concern for marine ecosystems, an effective ecosystem management necessitates precise monitoring of such periods with exceptionally high water temperatures. As satellite-based temperature measurements do not reach beyond the sea surface, identifying subsurface MHWs has so far relied on lower-resolution data obtained from (autonomous) in-situ measurements. In this study, we assess to which extent subsurface MHWs, defined statically by a seasonally varying 90^th percentile, can be deduced from surface properties that can be remotely-sensed at a high spatio-temporal resolution. To this end, we build a Random Forest (RF) classification model with daily data from a high-resolution numerical hindcast simulation focused on the Eastern Pacific (1979-2019). The RF is trained to distinguish between extreme and non-extreme temperatures at the depth of the climatologically maximum mixed layer depth (MLD), i.e. a depth that is decoupled from the sea surface throughout most parts of the year. We train the RF on the first 80% of the hindcast simulation data (i.e., 1979-2011) and use a range of predictor variables, such as anomalies of sea surface temperature (SST), height (SSH) and salinity (SSS) as well as derivatives of these physical variables. Testing the model on the last 20% of the hindcast simulation (2012-2019), the RF correctly identifies more than two thirds of all subsurface extreme states, leaving only about 30% of subsurface extremes unidentified. Yet, of all RF-based subsurface extreme classifications, about 40% of subsurface temperatures are false positives. Nevertheless, the RF model outperforms a simple SST based extrapolation of extreme states into the ocean interior. The RF-based classification is mostly guided by SSH and SST anomalies (together reduce impurity by about 50%), followed by climate indices like the Oceanic Niño Index (ONI) and the Pacific Decadal Oscillation (combined impurity reduction by 20%). This simulation-based study emphasizes the potential of exploring remote sensing data, particularly SST and SSH, to extend the monitoring of MHWs beneath the sea surface. Integrating this high-resolution statistical estimate with lower-resolution in-situ hydrographic information has the potential to make subsurface MHW monitoring a feasible and valuable tool for marine ecosystem management.

How to cite: Köhn, E. E., Münnich, M., Vogt, M., and Gruber, N.: Towards monitoring subsurface marine heatwaves based on sea surface properties in the Eastern Pacific, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-18946, https://doi.org/10.5194/egusphere-egu24-18946, 2024.

Heading into a potential El Niño in 2023/24, concern was high amongst Australian marine stakeholders regarding potential marine heatwave impacts on marine industries and systems in the coming summer. Targeted climate outlook briefings for the Great Barrier Reef and Western Australian coral reefs have been provided prior to and throughout the summer months by the Australian Bureau of Meteorology for the past 10-15 years, however in 2023 these were requested much earlier than usual. Also in 2023, national level seafood-focused briefings were requested by the fisheries sector for the first time, with various state and regional level meetings and information requests also occurring.

Subseasonal to seasonal forecast information played a critical role in these briefings, providing both the big picture in terms of climate drivers impacting Australian waters as well as regional information regarding sea surface temperatures around Australia. These forecast products are operationally produced by the Australian Bureau of Meteorology using the seasonal prediction system ACCESS-S. Clear communication of forecast probabilities and model skill was essential. New prototype marine heatwave forecasts were also presented to marine stakeholders, indicating where there was a high likelihood of marine heatwaves occurring in the upcoming season, together with likely severity. Demand for this new information on temperatures extremes was high and provided impetus for setting up coordinated briefings and response plans across sectors.

Forecasts can provide a 'preparation window' for marine stakeholders to implement proactive management strategies prior to high-risk conditions, noting however that not all industries have the same level of agility to respond. Subseasonal to seasonal forecast tools, that are useful, usable and used, provide valuable information to assist marine stakeholders in managing climate risk and vulnerability in a warming climate.

How to cite: Spillman, C., Hobday, A., Smith, G., and Hartog, J.: Building industry resilience through seasonal forecast briefings to Australian marine stakeholders heading into the 2023/24 summer, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-1558, https://doi.org/10.5194/egusphere-egu24-1558, 2024.

Marine heatwaves (MHW) can have large negative impacts on life in the ocean, such as kelp forest and corals. These environments are vital for protecting a range of different species in the ocean. With global warming, the occurrence and intensity of MHW are expected to increase, also in the polar regions. The Barents Sea has experienced large climate changes, becoming less influenced by sea ice during the last decades. Being able to predict the likelihood of MHW to occur in the Barents Sea could be highly beneficial to fisheries, aquaculture, and other relevant stakeholders. Such information could be useful in long-term risk assessment. In this study, we assess for the first time the skill of the Norwegian Climate Prediction Model (NorCPM) in predicting the likelihood of MHW. For this analysis, we focus on intense MHW in July 2016 taking place in the Barents Sea, and previously documented by satellite data. We find promising results in the seasonal predictions from NorCPM, where the predictions show increased probability for MHW to occur in July 2016 compared to July 2015 (when the MHW activity was lower than in 2016). The increased probability was already seen six months prior to the event. Furthermore, we downscale the results from the global NorCPM to a more refined grid with a horizontal resolution of 10km. This test case shows that downscaling can provide valuable information on the subsurface signature of MHW. We found the event in July 2016 to be shallow (down to about 50m) compared to another MHW event in July 2013, where warm anomalies occupied the whole water column. These results suggest that the event in July 2016 was atmospheric-driven, consistent with a previous study, whereas the event in 2013 is more likely to be ocean-driven. The results from this case study are promising for future seasonal prediction of MHW using NorCPM, and more in-depth studies are needed to quantify the predictive skill for different cases and different regions.

How to cite: Langehaug, H. R., Sandø, A. B., Hordoir, R., Counillon, F., Chiu, P.-G., and Raj, R.: Marine heatwaves: Can we predict them in the Barents Sea?, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-5667, https://doi.org/10.5194/egusphere-egu24-5667, 2024.

Marine Protected Areas (MPAs) are designated areas aimed at preserving marine ecosystems. However, they encounter the persistent obstacle of increasing ocean temperature. The occurrence of extreme warming events, known as Marine Heatwaves (MHWs), poses a significant threat to the delicate balance of marine ecosystems within MPAs. To understand the future changes in marine heatwaves (MHWs) in these regions, it is crucial to utilize advanced climate modeling capable of accurately capturing regional bathymetric features in MPAs, like coastlines, continental shelves, or islands. In this study, we utilized the SSP585 greenhouse warming simulations conducted with the OpenIFS-FESOM2 coupled model (AWI-CM3, 31 km atmosphere resolution, 4-15 km ocean resolution) to explore future changes in MHWs in the epipelagic to the upper mesopelagic zones (0-500m depth) of the global MPAs. In the current climate, MHWs in the MPAs exhibit greater maximum intensity and higher frequency than the global averages. However, MHWs in MPAs have shorter durations, leading to a lower cumulative intensity. The average warming rate within the MPAs is similar to or slightly lower than the average warming rate of the global ocean. Nevertheless, the MPAs are expected to see a 20% greater increase in the cumulative intensities of MHWs compared to the global ocean, from the past to the future. The findings suggest that marine protected areas (MPAs) are more susceptible to extreme temperature events compared to open ocean zones. Our findings underscore the significance of addressing anthropogenic warming to safeguard MPAs, emphasizing the need for prompt measures to mitigate these impacts and protect these vital marine ecosystems. 

How to cite: Cho, E. B., Kwon, E. Y., and Timmermann, A.: Future Intensification of Marine Heatwaves in Marine Protected Areas, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-7402, https://doi.org/10.5194/egusphere-egu24-7402, 2024.

Under global warming, the frequency and intensity of marine heatwaves are increasing. However, the inhibition of atmospheric forcing marine heatwaves (AMHW) on upwelling and its impact on marine ecosystems remain poorly understood. To address this issue, the satellite sea surface temperature and reanalysis data during 1998-2021 were analyzed in three distinct upwelling systems, northwestern South China Sea. The results showed that the coastal tide-induced upwelling in the west (W) of Hainan Island is primarily suppressed by enhanced stratification during the AMHW events, since the coastal tide-induced upwelling is insensitive to wind weakening. Contrarily, the wind-driven upwelling in the east (E) and northeast (NE) of Hainan Island are jointly regulated by wind and stratification during the AMHW. Specifically, the AMHW events have a stronger inhibitory effect in the upwelling and phytoplankton growth in the NE than that in the E. The causes could be the followings: (1) the background upwelling in the NE region is stronger than in the E, thus the NE region has a higher susceptibility to the wind weakening; (2) the wind-driven upwelling begins to be suppressed by AMHW when the high-pressure system is aligned with the coastline of the upwelling. In the NE region, the location of the high-pressure center during the occurrence of AMHW is positioned in closer proximity to the upwelling area. Moreover, the inhibitory effect of wind weakening and stratification enhancing on upwelling changes with the development of the AMHW. Before and during the mature phase of AMHW, stratification and wind jointly inhibit upwelling and phytoplankton growth, while it shifts to stratification dominated (>85%) during the decline phase. This study suggests that MHW has a great impact on the upwelling ecosystem, especially the wind-driven upwelling, which should be given high attention under global warming (with increasing MHW events in the future).

How to cite: Liu, S., Lao, Q., and Chen, F.: Impacts of Marine Heatwave Events on Three Distinct Upwelling Systems and its Implication for Marine Ecosystems in the Northern South China Sea, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-1942, https://doi.org/10.5194/egusphere-egu24-1942, 2024.

When modelling any climatic system, it is important to carefully consider the relation between the many timescales that govern its evolution, since a certain change in their interplay can drastically affect the likelihood of observing critical transitions to distinct environmental regimes. In this study, we present how the onset of marine heatwaves - that are responsible for inducing prolonged periods of positive temperature fluctuations - can weaken state-based resilience leading to noise-induced shifts between species’ concentration levels in plankton communities. This is shown in a modified Truscott-Brindley model, a stochastically forced fast-slow system that encapsulates the interaction between phytoplankton and zooplankton species during red tide events in marine environments. Deterministically, the system can be bistable, possessing stable states with high and low phytoplankton biomass, or in an excitable monostable regime, where phytoplankton blooms form following perturbations. Environmental perturbations to the (temperature-dependent) species’ growth rates are modelled using multiplicative noise terms, namely Ornstein-Uhlenbeck processes with a correlation time parameter τ. During marine heatwaves, the correlation time τ of the external perturbations will increase. With ensemble Monte Carlo simulations of phytoplankton collapses, we demonstrate how mean first-exit times from the domain of attraction scale as the noise intensity weakens, across different prescribed values for the correlation time τ. These results yield numerical approximations for the systems’ quasipotential barrier heights - a concept from Freidlin and Wentzell’s theory of large deviations that quantifies resistance to noise-induced escape from a given domain - which elucidates a non-monotonic relation between the system vulnerability to critical transitions and the correlation time τ of the external perturbations. Indeed, initially there is a notable drop in system resilience as the correlation time τ grows from zero, although as τ increases further beyond a critical value, the system resilience begins to then increase. This non-monotonic relation is also reflected in the action values of most probable transition paths for escaping the domain of attraction, found using an augmented Lagrangian method to overcome the degenerate noise present in the system. These findings are compared and contrasted with results from other studies exploring how climate tipping points, or stochastic escapes from a domain of attraction, depend on the correlation time of the external perturbations. Finally, we consider candidate time-series for correlation times constructed from temperature records for the North Sea across periods including anomalously high values, and discuss whether - subject to these - varying system vulnerability to critical transitions is more sensitive to the rate of emergence or duration of the marine heatwaves.

How to cite: Deeley, R., Grafke, T., and Feudel, U.: The increased likelihood of plankton community changes following marine heatwaves, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-12034, https://doi.org/10.5194/egusphere-egu24-12034, 2024.

Marine heatwave (MHW), a prolonged period of anomalously warm seawater, has a catastrophic repercussion on marine ecosystems. With global warming, MHWs have become increasingly frequent, intense, and prolonged. To avoid irreversible damages from such extreme events, net-zero human-caused carbon emissions by 2050s, called carbon neutrality, were proposed. Here, we evaluate the impact of carbon neutrality on MHWs in the late 21^st century using multi-model projections from the Coupled Model Intercomparison Project Phase 6 (CMIP6) Shared Socioeconomic Pathway (SSP)1-1.9 and SSP3-7.0 scenarios. It is found that if the current “regional rivalry” over carbon emissions policy continues into this century (i.e., SSP3-7.0), the MHWs in the late 21^st century will become stronger over 1°C and longer lasting over 365 days than historical ones, especially in the western boundary current and equatorial current regions. Approximately 68% of the global ocean will be exposed to permanent MHWs, regionally 93% in the Indian Ocean, 76% in the Pacific Ocean, 68% in the Atlantic Ocean, 65% in the Coastal Ocean, and 48% in the Southern Ocean. Such extreme MHWs can be significantly reduced by achieving carbon neutrality (i.e., SSP1-1.9). In particular, the proportion of exposure to permanent MHWs can be reduced to as low as 0.02 to 0.07%, depending on the region. This result underscores the critical importance of ongoing efforts to achieve net-zero carbon emissions to reduce the potential ecological risks induced by extreme MHW exposure.

How to cite: Oh, S.-G., Son, S.-W., Jeong, S., and Cho, Y.-K.: Significant reduction of potential exposure to extreme marine heatwaves by achieving carbon neutrality, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-2720, https://doi.org/10.5194/egusphere-egu24-2720, 2024.

An extreme event, Marine heatwave, has become a pressing concern in recent years. It is defined as a discrete event where the sea surface temperature remains above a specific threshold value of climatology for several consecutive days, and the intervals between two consecutive abnormal high-temperature events are less than two days. Due to climate change, there is an increasing trend in both the intensity and duration of marine heat waves. Marine heatwaves may not directly affect human society; however, they can pose significant threats to marine ecosystems, coastal communities, and the ocean carbon sink, thereby impacting human well-being. The ocean carbon sink is the most significant carbon sink among the world's three major carbon sinks. It absorbs around 25% of anthropogenic carbon dioxide emissions annually. Dissolved inorganic carbon within the ocean carbon sink relies on the carbon sequestration of biological pumps such as coral, seagrasses, and kelps to store it in the deep water. Influenced by the El Niño-Southern Oscillation and currents, the northeastern Pacific Ocean is a hotspot for marine heatwaves, typically beginning from the North Pacific offshore regions in the spring and impacting the U.S. West Coast in the fall. Consequently, the coastal area of California is selected as the study area and divided into three regions.

Previous studies have shown that the escalating severity of marine heatwaves may result in these biological pumps losing their functions or habitats. However, regarding ocean carbon sequestration, whether the incapacities of these biological pumps due to marine heatwaves will have a short-term impact on the carbon sequestration capacity in the ocean remains to be verified. This study aims to analyze the time series of marine heatwaves and ocean carbon sink capacity with the time series analysis and determine the impacts on ocean carbon sink. We categorize marine heatwave extreme events in California into three indicators and the ocean carbon sequestration capacity into physical and biological indicators. Improved Complete Ensemble Empirical Mode Decomposition with Adaptive Noise (ICEEMDAN) is employed to extract the trends and interannual variations. Meanwhile, to identify the correlations between the marine heatwave and the ocean carbon sink at different time points and different time scales, we apply Time-Dependent Intrinsic Correlation (TDIC). Due to the longer temporal scales in changes in the ocean, the impact of marine heatwaves on the ocean carbon sink may have a potential delay. Therefore, we employ Time-Dependent Intrinsic Cross-Correlation (TDICC), a method based on TDIC that could be utilized to analyze the time-lag effects in the interaction between marine heatwaves and the ocean carbon sink.

How to cite: Fu, C.-H. and Tsai, C. W.: Impact of Marine Heatwaves on Ocean Carbon Sink: A Case Study of Coastal Areas in California, USA, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-16628, https://doi.org/10.5194/egusphere-egu24-16628, 2024.

Marine Heatwaves (MHWs) are prolonged events of anomalously warm sea water temperature, and have major detrimental effects to marine ecosystems and the world's economy. Thanks to satellite remote sensing of sea surface temperature, significant advances have been made regarding the characterization and impact of MHWs on global scales, however, these data are typically inadequate to resolve most estuarine environments with complex shorelines and reduced spatial scales. In our work we analyzed a novel data set with over three decades of in situ surface and subsurface temperature records to investigate MHWs in the largest estuary in the US: the Chesapeake Bay. Our major findings will be presented in detail, including MHW characteristics in the Bay, their trends, subsurface structure and impact on Bay hypoxia. Projections of trends found in our work suggest that by the end of the century the Chesapeake Bay will reach a semi-permanent MHW state, when extreme temperatures will be present over half of the year, and thus could have devastating impacts to the bay ecosystem and regional economy. Improving our basic understanding of MHWs, their trends and impact on hypoxia in the Chesapeake Bay is necessary to guide management decisions in this valuable environment.

How to cite: Mazzini, P., Shunk, N., Pianca, C., and Walter, R.: Marine Heatwaves in the Chesapeake Bay: Characteristics, Subsurface Structure and Impact on Hypoxia, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-20695, https://doi.org/10.5194/egusphere-egu24-20695, 2024.

The Arctic is warming faster than any other regions, a phenomenon known as Arctic amplification, which has far-reaching effects for global climate. Modelled historical simulations show a significant underestimation of the amplification and the future projection exhibits non-negligible model spread. Here we show that in a future warming scenario, the warming in the Arctic is generally larger when comparing high-resolution climate models with low-resolution versions. We attribute the different extent of Arctic warming to Arctic marine heatwaves (MHWs), known as episodes of extreme ocean surface warming. The resolution of the MHWs, which are stronger and more realistic in the high-resolution model versions, increases the melting of sea ice and thus the absorption of solar radiation by the ocean in the short term, thereby reinforcing the long-term trend of Arctic warming. We point out that the amplification of Arctic warming is underestimated by the current generation of climate models, which generally have low resolution, thereby underestimating Arctic marine heat waves. In addition, Arctic heatwaves cause extreme temperature fluctuations associated with increased stratification. This poses major challenges to Arctic ecosystems and has a negative impact through direct physiological temperature effects and indirectly through nutrient supply and taxonomic shifts. We conclude that the eddy- and storm-resolving models provide a new perspective on how the Earth system responds to past and future climate and environmental extremes.

How to cite: Gou, R., Deng, Y., Wolf, K., Cui, Y., Hoppe, C., Wu, L., Shu, Q., and Lohmann, G.: Underestimated Arctic warming and potential ecosystem impact due to unresolved marine heatwaves, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-2617, https://doi.org/10.5194/egusphere-egu24-2617, 2024.

How to cite: Colombi, N., Kropf, C. M., Burger, F. A., Bresch, D. N., and Frölicher, T. L.: Modelling marine heatwaves impact on shallow and upper mesophotic tropical coral reefs , EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-15499, https://doi.org/10.5194/egusphere-egu24-15499, 2024.