An official website of the United States government
Abstract The severity of marine heatwaves (MHWs) that are increasingly impacting ocean ecosystems, including vulnerable coral reefs, has primarily been assessed using remotely sensed sea-surface temperatures (SSTs), without information relevant to heating across ecosystem depths. Here, using a rare combination of SST, high-resolution in-situ temperatures, and sea level anomalies observed over 15 years near Moorea, French Polynesia, we document subsurface MHWs that have been paradoxical in comparison to SST metrics and associated with unexpected coral bleaching across depths. Variations in the depth range and severity of MHWs was driven by mesoscale (10s to 100s of km) eddies that altered sea levels and thermocline depths and decreased (2007, 2017 and 2019) or increased (2012, 2015, 2016) internal-wave cooling. Pronounced eddy-induced reductions in internal waves during early 2019 contributed to a prolonged subsurface MHW and unexpectedly severe coral bleaching, with subsequent mortality offsetting almost a decade of coral recovery. Variability in mesoscale eddy fields, and thus thermocline depths, is expected to increase with climate change, which, along with strengthening and deepening stratification, could increase the occurrence of subsurface MHWs over ecosystems historically insulated from surface ocean heating by the cooling effects of internal waves.
more »
« less
Vertical structure of subsurface marine heatwaves in a shallow nearshore upwelling system
Abstract Marine heatwaves (MHWs) are increasing in frequency and intensity globally and are among the greatest threats to marine ecosystems. However, limited studies have characterized subsurface MHWs, particularly in shallow waters. We utilized nearly two decades of full water-column (~ 10 m) observations from a unique automated profiler in central California to characterize, for the first time, the vertical structure of MHWs in a shallow nearshore upwelling system. We found MHWs have similar average durations and intensities across all depths, but there were ~ 17% more bottom MHW days than surface MHW days. Nearly one third of bottom MHWs occurred independently of surface MHWs, indicating that satellites miss a significant fraction of events. MHWs showed distinct seasonality with more frequent and intense events during the fall/winter when weak stratification allowed for MHWs to occupy a larger portion of the water column and persist longer. During summer, strong stratification limited the vertical extent of MHWs, leading to surface- and bottom-trapped events with shorter durations and intensities. Additionally, MHW initiation and termination across depths was consistently linked to anomalously low and high coastal upwelling, respectively. This study highlights the need for expansion of subsurface monitoring of MHWs globally amid a warming planet.
more »
« less
- PAR ID:
- 10572943
- Publisher / Repository:
- Nature Publishing Group
- Date Published:
- Journal Name:
- Scientific Reports
- Volume:
- 15
- Issue:
- 1
- ISSN:
- 2045-2322
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
More Like this
-
-
Planktonic foraminiferal assemblages reflect warming during two recent mid-latitude marine heatwavesUnder future climate scenarios, ocean temperatures that are presently extreme and qualify as marine heatwaves (MHW) are forecasted to increase in frequency and intensity, but little is known about the impact of these events on one of the most common paleoproxies, planktonic foraminifera. Planktonic foraminifera are globally ubiquitous, shelled marine protists. Their abundances and geochemistry vary with ocean conditions and fossil specimens are commonly used to reconstruct ancient ocean conditions. Planktonic foraminiferal assemblages are known to vary globally with sea surface temperature, primary productivity, and other hydrographic conditions, but have not been studied in the context of mid-latitude MHWs. For this study, the community composition and abundance of planktonic foraminifera were quantified for 2010-2019 along the Newport Hydrographic Line, a long-term monitoring transect at 44.6°N in the Northern California Current (NCC). Samples were obtained from archived plankton tows spanning 46 to 370 km offshore during annual autumn (August – October) cruises. Two MHWs impacted the region during this timeframe: the first during 2014-2016 and a second, shorter duration MHW in 2019. During the 2014-2016 MHW, warm water subtropical and tropical foraminifera species were more prevalent than the typical polar, subpolar, and transitional species common to this region. Cold water species were abundant again after the first MHW dissipated in late 2016. During the second, shorter-duration MHW in 2019, the assemblage consisted of a warm water assemblage but did not include tropical species. The foraminiferal assemblage variability correlated with changes in temperature and salinity in the upper 100 meters and was not correlated with distance offshore or upwelling. These results suggest that fossil foraminiferal assemblages from deep sea sediment cores may provide insight into the magnitude and frequency of past MHWs.more » « less
-
Abstract The reappearance of a northeast Pacific marine heatwave (MHW) sounded alarms in late summer 2019 for a warming event on par with the 2013–2016 MHW known as The Blob. Despite these two events having similar magnitudes in surface warming, differences in seasonality and salinity distinguish their evolutions. We compare and contrast the ocean's role in the evolution and persistence of the 2013–2016 and 2019–2020 MHWs using mapped temperature and salinity data from Argo floats. An unusual near‐surface freshwater anomaly in the Gulf of Alaska during 2019 increased the stability of the water column, preventing the MHW from penetrating deep as strongly as the 2013–2016 event. This freshwater anomaly likely contributed to the intensification of the MHW by increasing the near‐surface buoyancy. The gradual buildup of subsurface heat content throughout 2020 in the region suggests the potential for persistent ecological impacts.more » « less
-
Coastal marine heatwaves have destructive and lasting impacts on foundational species 13 and are increasing in frequency, duration, and magnitude. High atmospheric temperatures are 14 often associated with marine heatwaves (MHW) which are defined as 5-days of water 15 temperatures above a seasonally varying 90th percentile threshold. In this study we consider the 16 prevalence of MHW propagation into surficial sediments to cause sediment heatwaves (SHW). 17 Within a shallow, subtidal seagrass meadow in Virginia, USA, sediment temperature was 18 measured at hourly intervals at a depth of 5 cm between June 2020-October 2022 at the meadow 19 edge and central meadow interior. The observed sediment temperature, along with a 29-year 20 record of water temperature and water level was used to develop a sediment temperature model 21 for each location. Modeled sediment temperatures were used to identify sediment heatwaves that 22 may thermally stress belowground seagrass. At both meadow locations, sediment heatwave 23 frequency increased at a rate twice that of MHWs in the average global open ocean, coinciding 24 with a 172% increase in the annual number of SHW days, from 11 to 30 days year-1 between 25 1994-2022. Sediment heatwaves at both meadow locations co-occurred with a MHW 79-81% of 26 the time, with nearly all SHWs having a zero day lag. The top 10% most extreme MHWs and 27 SHWs occurred between November and April when thermal stress to seagrass was unlikely. In 28 June 2015 a SHW co-occurred with an anomalously long duration MHW that was associated 29 with a 90% decline in seagrass from this system, suggesting that SHWs may have contributed to 30 the observed seagrass loss. These results document heatwave propagation across the pelagic-31 sediment interface which likely occur broadly in shallow systems with impacts to critical coastal 32 ecosystem processes and species dynamics.more » « less
-
ABSTRACT Marine heatwaves (MHWs) caused by multiple phenomena with days to months duration are increasingly common disturbances in ocean ecosystems. We investigated the impacts of MHWs on pelagic communities using spatially resolved time‐series of multiple trophic levels from the Southern California Current Ecosystem. Indices of phytoplankton biomass mostly declined during MHWs because of reduced nutrient supply (exceptingProchlorococcus) and were generally more sensitive to marine heatwave intensity than duration. By contrast, mesozooplankton (as estimated by zooplankton displacement volume) were somewhat more strongly correlated with MHW duration than intensity. Zooplankton anomalies were also positively correlated with fucoxanthin (diatom) anomalies, highlighting possible bottom‐up influences during MHWs. Mobile consumers (forage fish) showed more complex responses, with fish egg abundance declining during MHWs but not correlating with any MHW characteristics. Our findings provide partial evidence of how MHW characteristics can shape variable ecological responses due to the differing life spans and behaviours of different trophic levels.more » « less
