Heatwaves in the ocean can rapidly disrupt marine ecosystems and the economies that depend on them. A global analysis of these events casts light on their causes and sets the stage for revealing how ...they might change in the future.
The rapid pace of environmental change in the Anthropocene necessitates the development of a new suite of tools for measuring ecosystem dynamics. Sentinel species can provide insight into ecosystem ...function, identify hidden risks to human health, and predict future change. As sentinels, marine apex (top) predators offer a unique perspective into ocean processes, given that they can move across ocean basins and amplify trophic information across multiple spatiotemporal scales. Because use of the terms “ecosystem sentinel” and “climate sentinel” has proliferated in the scientific literature, there is a need to identify the properties that make marine predators effective sentinels. We provide a clear definition of the term “sentinel”, review the attributes of species identified as sentinels, and describe how a suite of such sentinels could strengthen our understanding and management of marine ecosystems. We contend that the use of marine predators as ecosystem sentinels will enable rapid response and adaptation to ecosystem variability and change.
Marine heatwaves (MHWs)-periods of exceptionally warm ocean temperature lasting weeks to years-are now widely recognized for their capacity to disrupt marine ecosystems
. The substantial ecological ...and socioeconomic impacts of these extreme events present significant challenges to marine resource managers
, who would benefit from forewarning of MHWs to facilitate proactive decision-making
. However, despite extensive research into the physical drivers of MHWs
, there has been no comprehensive global assessment of our ability to predict these events. Here we use a large multimodel ensemble of global climate forecasts
to develop and assess MHW forecasts that cover the world's oceans with lead times of up to a year. Using 30 years of retrospective forecasts, we show that the onset, intensity and duration of MHWs are often predictable, with skilful forecasts possible from 1 to 12 months in advance depending on region, season and the state of large-scale climate modes, such as the El Niño/Southern Oscillation. We discuss considerations for setting decision thresholds based on the probability that a MHW will occur, empowering stakeholders to take appropriate actions based on their risk profile. These results highlight the potential for operational MHW forecasts, analogous to forecasts of extreme weather phenomena, to promote climate resilience in global marine ecosystems.
The 2015–2016 El Niño is by some measures one of the strongest on record, comparable to the 1982–1983 and 1997–1998 events that triggered widespread ecosystem change in the northeast Pacific. Here we ...describe impacts of the 2015–2016 El Niño on the California Current System (CCS) and place them in historical context using a regional ocean model and underwater glider observations. Impacts on the physical state of the CCS are weaker than expected based on tropical sea surface temperature anomalies; temperature and density fields reflect persistence of multiyear anomalies more than El Niño. While we anticipate El Niño‐related impacts on spring/summer 2016 productivity to be similarly weak, their combination with preexisting anomalous conditions likely means continued low phytoplankton biomass. This study highlights the need for regional metrics of El Niño's effects and demonstrates the potential to assess these effects before the upwelling season, when altered ecosystem functioning is most apparent.
Key Points
Impacts of the 2015‐2016 El Nino on the California Current System's physical state are evaluated using model, satellite, and glider data
Local temperature and density anomalies are much weaker than expected based on tropical sea surface temperature anomalies
Relatively weak El Nino imprint occurs on backdrop of large multiyear anomalies that may continue to dominate the biological response
The California Current System represents a confluence of different water masses originating in the subarctic, subtropical, and tropical eastern Pacific. Variations in their relative influence can ...alter regional biogeochemistry and ecosystem structure. We perform an optimum multiparameter analysis on historical hydrographic data to quantify the spatiotemporal variability of water mass contributions to the California Current. Within the pycnocline, a strong cross‐shore gradient in the primary water mass source reflects the dominant advective pathways within the California Current and California Undercurrent. The El Niño Southern Oscillation imparts variability in the relative contributions and depth structure of source waters, allowing stronger upwelling during La Niña to more effectively tap nutrient‐rich, oxygen‐poor waters originating in the eastern tropical North Pacific. This regional water mass history provides context for understanding the drivers and pathways of biogeochemical variability in the California Current and demonstrates that oceanic changes occurring far afield can have regionally heterogeneous impacts.
Plain Language Summary
Waters found in the California Current come from different parts of the ocean: the subarctic, subtropical, and tropical eastern Pacific. Each of these source waters has its own characteristic combination of properties like temperature, saltiness, and nutrient and oxygen levels. Variations in the relative contributions of these source waters can impact local conditions, including oxygen and nutrient content and the properties of upwelled waters. Here we explore long time series of hydrographic data from an oceanic region off southern California to quantify the relative contributions of different source water masses, and their spatial and interannual variability. We describe a spatially heterogeneous water mass structure which is significantly impacted by the El Niño–Southern Oscillation, with important implications for regional biogeochemistry and ecosystem structure. The analysis demonstrates that regional variability in the California Current can be driven by oceanic changes occurring far afield.
Key Points
Optimum multiparameter analysis is used to quantify source water contributions and spatiotemporal variability of water mass structure in the California Current
Interannual variability in source water mass distributions is associated with regional biogeochemical variability
ENSO cycle impacts relative contributions and depth structure of source waters, with implications for ecosystem structure
Thermal displacement by marine heatwaves Jacox, Michael G; Alexander, Michael A; Bograd, Steven J ...
Nature (London),
08/2020, Letnik:
584, Številka:
7819
Journal Article
Recenzirano
Marine heatwaves (MHWs)-discrete but prolonged periods of anomalously warm ocean temperatures-can drastically alter ocean ecosystems, with profound ecological and socioeconomic impacts
. Considerable ...effort has been directed at understanding the patterns, drivers and trends of MHWs globally
. Typically, MHWs are characterized on the basis of their intensity and persistence at a given location-an approach that is particularly relevant for corals and other sessile organisms that must endure increased temperatures. However, many ecologically and commercially important marine species respond to environmental disruptions by relocating to favourable habitats, and dramatic range shifts of mobile marine species are among the conspicuous impacts of MHWs
. Whereas spatial temperature shifts have been studied extensively in the context of long-term warming trends
, they are unaccounted for in existing global MHW analyses. Here we introduce thermal displacement as a metric that characterizes MHWs by the spatial shifts of surface temperature contours, instead of by local temperature anomalies, and use an observation-based global sea surface temperature dataset to calculate thermal displacements for all MHWs from 1982 to 2019. We show that thermal displacements during MHWs vary from tens to thousands of kilometres across the world's oceans and do not correlate spatially with MHW intensity. Furthermore, short-term thermal displacements during MHWs are of comparable magnitude to century-scale shifts inferred from warming trends
, although their global spatial patterns are very different. These results expand our understanding of MHWs and their potential impacts on marine species, revealing which regions are most susceptible to thermal displacement, and how such shifts may change under projected ocean warming. The findings also highlight the need for marine resource management to account for MHW-driven spatial shifts, which are of comparable scale to those associated with long-term climate change and are already happening.
In terrestrial systems, the green wave hypothesis posits that migrating animals can enhance foraging opportunities by tracking phenological variation in high-quality forage across space (i.e., ...“resource waves”). To track resource waves, animals may rely on proximate cues and/or memory of long-term average phenologies. Although there is growing evidence of resource tracking in terrestrial migrants, such drivers remain unevaluated in migratory marine megafauna. Here we present a test of the green wave hypothesis in a marine system. We compare 10 years of blue whale movement data with the timing of the spring phytoplankton bloom resulting in increased prey availability in the California Current Ecosystem, allowing us to investigate resource tracking both contemporaneously (response to proximate cues) and based on climatological conditions (memory) during migrations. Blue whales closely tracked the long-term average phenology of the spring bloom, but did not track contemporaneous green-up. In addition, blue whale foraging locations were characterized by low long-term habitat variability and high long-term productivity compared with contemporaneous measurements. Results indicate that memory of long-term average conditions may have a previously underappreciated role in driving migratory movements of long-lived species in marine systems, and suggest that these animals may struggle to respond to rapid deviations from historical mean environmental conditions. Results further highlight that an ecological theory of migration is conserved across marine and terrestrial systems. Understanding the drivers of animal migration is critical for assessing how environmental changes will affect highly mobile fauna at a global scale.
The California Current System (CCS) is a biologically productive Eastern Boundary Upwelling System that experiences considerable environmental variability on seasonal and interannual timescales. ...Given that this variability drives changes in ecologically and economically important living marine resources, predictive skill for regional oceanographic conditions is highly desirable. Here, we assess the skill of seasonal sea surface temperature (SST) forecasts in the CCS using output from Global Climate Forecast Systems in the North American Multi-Model Ensemble (NMME), and describe mechanisms that underlie SST predictability. A simple persistence forecast provides considerable skill for lead times up to ~4 months, while skill above persistence is mostly confined to forecasts of late winter/spring and derives primarily from predictable evolution of ENSO-related variability. Specifically, anomalously weak (strong) equatorward winds are skillfully forecast during El Niño (La Niña) events, and drive negative (positive) upwelling anomalies and consequently warm (cold) temperature anomalies. This mechanism prevails during moderate to strong ENSO events, while years of ENSO-neutral conditions are not associated with significant forecast skill in the wind or significant skill above persistence in SST. We find also a strong latitudinal gradient in predictability within the CCS; SST forecast skill is highest off the Washington/Oregon coast and lowest off southern California, consistent with variable wind forcing being the dominant driver of SST predictability. These findings have direct implications for regional downscaling of seasonal forecasts and for short-term management of living marine resources.
Recently, there has been substantial effort to understand the fundamental characteristics of warm ocean temperature extremes-known as marine heatwaves (MHWs). However, MHW research has primarily ...focused on the surface signature of these events. While surface MHWs (SMHW) can have dramatic impacts on marine ecosystems, extreme warming along the seafloor can also have significant biological outcomes. In this study, we use a high-resolution (~8 km) ocean reanalysis to broadly assess bottom marine heatwaves (BMHW) along the continental shelves of North America. We find that BMHW intensity and duration varies strongly with bottom depth, with typical intensities ranging from ~0.5 °C-3 °C. Further, BMHWs can be more intense and persist longer than SMHWs. While BMHWs and SMHWs often co-occur, BMHWs can also exist without a SMHW. Deeper regions in which the mixed layer does not typically reach the seafloor exhibit less synchronicity between BMHWs and SMHWs.
Coastal upwelling is responsible for thriving marine ecosystems and fisheries that are disproportionately productive relative to their surface area, particularly in the world's major eastern boundary ...upwelling systems. Along oceanic eastern boundaries, equatorward wind stress and the Earth's rotation combine to drive a near‐surface layer of water offshore, a process called Ekman transport. Similarly, positive wind stress curl drives divergence in the surface Ekman layer and consequently upwelling from below, a process known as Ekman suction. In both cases, displaced water is replaced by upwelling of relatively nutrient‐rich water from below, which stimulates the growth of microscopic phytoplankton that form the base of the marine food web. Ekman theory is foundational and underlies the calculation of upwelling indices such as the “Bakun Index” that are ubiquitous in eastern boundary upwelling system studies. While generally valuable first‐order descriptions, these indices and their underlying theory provide an incomplete picture of coastal upwelling. Here we review the relevant dynamics and limitations of classical upwelling indices, particularly related to representation of the surface wind stress, the influence of geostrophic currents, and the properties of upwelled water. To address these shortcomings, we present two new upwelling indices for the U.S. West Coast (31–47°N), which are available from 1988 to present. The Coastal Upwelling Transport Index and the Biologically Effective Upwelling Transport Index provide improved estimates of vertical transport and vertical nitrate flux, respectively, by leveraging technological and scientific advances realized since the introduction of the Bakun Index nearly a half century ago.
Plain Language Summary
The California Current System, running along the North American West Coast, hosts a rich and diverse marine ecosystem that provides considerable socioeconomic benefit. The process underlying this exceptional biological productivity is wind‐driven coastal upwelling, which delivers deep, nutrient‐rich water to the sunlit surface layer and stimulates growth of phytoplankton that form the base of the marine food web. Given the ecological importance of upwelling, indices designed to monitor its intensity (e.g., the “Bakun Index”) were introduced nearly 50 years ago. While these indices have proved extremely useful, they have a number of limitations as they are derived from relatively coarse resolution atmospheric pressure fields. In particular, uncertainties arise in the estimation of wind stress and from the omission of the influence of ocean circulation. Furthermore, historical indices estimate only the amount of water upwelled, not the nutrient content of that water. Here we present new indices that leverage ocean models, satellite data, and in situ observations to more accurately estimate upwelling strength as well as the amount of nitrate being upwelled. The new indices are publicly available, extend from 1988 to present, and will be valuable for monitoring upwelling in near real time and for understanding its impacts on the marine ecosystem.
Key Points
New upwelling indices are presented for the U.S. West Coast (31–47°N) to address shortcomings in historical indices
The Coastal Upwelling Transport Index (CUTI) estimates vertical volume transport (i.e., upwelling/downwelling)
The Biologically Effective Upwelling Transport Index (BEUTI) estimates vertical nitrate flux
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