Physical Oceanography (PO)

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Department members investigate the dynamics and thermodynamics of ocean circulation. They work globally from the Arctic to the Antarctic and from the Strait of Gibraltar to the Philippine shelf on the full range of oceanic processes, from mixing on centimeter scales to heat balance on the global scale.

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  • Article
    Mixing and air–sea buoyancy fluxes set the time-mean overturning circulation in the subpolar North Atlantic and Nordic Seas
    (European Geosciences Union, 2023-06-02) Evans, Dafydd Gwyn ; Holliday, N. Penny ; Bacon, Sheldon ; Le Bras, Isabela
    The overturning streamfunction as measured at the OSNAP (Overturning in the Subpolar North Atlantic Program) mooring array represents the transformation of warm, salty Atlantic Water into cold, fresh North Atlantic Deep Water (NADW). The magnitude of the overturning at the OSNAP array can therefore be linked to the transformation by air–sea buoyancy fluxes and mixing in the region north of the OSNAP array. Here, we estimate these water mass transformations using observational-based, reanalysis-based and model-based datasets. Our results highlight that air–sea fluxes alone cannot account for the time-mean magnitude of the overturning at OSNAP, and therefore a residual mixing-driven transformation is required to explain the difference. A cooling by air–sea heat fluxes and a mixing-driven freshening in the Nordic Seas, Iceland Basin and Irminger Sea precondition the warm, salty Atlantic Water, forming subpolar mode water classes in the subpolar North Atlantic. Mixing in the interior of the Nordic Seas, over the Greenland–Scotland Ridge and along the boundaries of the Irminger Sea and Iceland Basin drive a water mass transformation that leads to the convergence of volume in the water mass classes associated with NADW. Air–sea buoyancy fluxes and mixing therefore play key and complementary roles in setting the magnitude of the overturning within the subpolar North Atlantic and Nordic Seas. This study highlights that, for ocean and climate models to realistically simulate the overturning circulation in the North Atlantic, the small-scale processes that lead to the mixing-driven formation of NADW must be adequately represented within the model's parameterisation scheme.
  • Article
    Integrating fishers’ knowledge with oceanographic observations to understand changing ocean conditions in the Northeast United States
    (Frontiers Media, 2023-05-23) Olsen, Noelle A. ; Bahr, Frank ; Bethoney, N. David ; Mercer, Anna M. ; Gawarkiewicz, Glen
    Recent warming in the Northeast United States continental shelf ecosystem has raised several concerns about the impacts on the ecosystem and commercial fisheries. In 2014, researchers from the Commercial Fisheries Research Foundation and Woods Hole Oceanographic Institution founded the Shelf Research Fleet to involve fishers in monitoring the rapidly changing ocean environment and encourage sharing of ecological knowledge. The Shelf Research Fleet is a transdisciplinary, cooperative program that trains commercial fishers to collect oceanographic information by deploying conductivity, temperature, and depth (CTD) instruments while commercially fishing. A total of 806 CTD profiles have been collected by the Shelf Research Fleet through December 2022. Participating vessels can view the conductivity and temperature water column profiles they collect in real-time. These profiles help inform their fishing practices and give insights when unexpected species appear in their gear or if their catch composition changes from previous years. The data collected by the Shelf Research Fleet are shared with and processed by researchers from numerous partnering institutions. The Shelf Research Fleet data have been used by researchers to better understand oceanographic phenomena including marine heatwaves, shelf-break exchange processes, warm core rings, and salinity maximum intrusions onto the continental shelf. The scope of the Shelf Research Fleet has grown over time to include efforts to more directly link oceanographic results with biological observations to better understand how changing ocean conditions are affecting commercially important species. This article describes the approach, successes, challenges, and future directions of the Shelf Research Fleet and aims to outline a framework for a cost-effective research program that engages fishers in the collection of oceanographic data, strengthening partnerships between fishing industry members and the scientific community.
  • Article
    Forcing and impact of the Northern Hemisphere continental snow cover in 1979–2014
    (European Geosciences Union, 2023-05-23) Gastineau, Guillaume ; Frankignoul, Claude ; Gao, Yongqi ; Liang, Yu-Chiao ; Kwon, Young-Oh ; Cherchi, Annalisa ; Ghosh, Rohit ; Manzini, Elisa ; Matei, Daniela ; Mecking, Jennifer ; Suo, Lingling ; Tian, Tian ; Yang, Shuting ; Zhang, Ying
    The main drivers of the continental Northern Hemisphere snow cover are investigated in the 1979–2014 period. Four observational datasets are used as are two large multi-model ensembles of atmosphere-only simulations with prescribed sea surface temperature (SST) and sea ice concentration (SIC). A first ensemble uses observed interannually varying SST and SIC conditions for 1979–2014, while a second ensemble is identical except for SIC with a repeated climatological cycle used. SST and external forcing typically explain 10 % to 25 % of the snow cover variance in model simulations, with a dominant forcing from the tropical and North Pacific SST during this period. In terms of the climate influence of the snow cover anomalies, both observations and models show no robust links between the November and April snow cover variability and the atmospheric circulation 1 month later. On the other hand, the first mode of Eurasian snow cover variability in January, with more extended snow over western Eurasia, is found to precede an atmospheric circulation pattern by 1 month, similar to a negative Arctic oscillation (AO). A decomposition of the variability in the model simulations shows that this relationship is mainly due to internal climate variability. Detailed outputs from one of the models indicate that the western Eurasia snow cover anomalies are preceded by a negative AO phase accompanied by a Ural blocking pattern and a stratospheric polar vortex weakening. The link between the AO and the snow cover variability is strongly related to the concomitant role of the stratospheric polar vortex, with the Eurasian snow cover acting as a positive feedback for the AO variability in winter. No robust influence of the SIC variability is found, as the sea ice loss in these simulations only drives an insignificant fraction of the snow cover anomalies, with few agreements among models.
  • Article
    Seasonality of the Meridional Overturning Circulation in the subpolar North Atlantic
    (Nature Research, 2023-05-25) Fu, Yao ; Lozier, M Susan ; Biló, Tiago Carrilho ; Bower, Amy S. ; Cunningham, Stuart A. ; Cyr, Frédéric ; de Jong, M. Femke ; deYoung, Brad ; Drysdale, Lewis ; Fraser, Neil ; Fried, Nora ; Furey, Heather H. ; Han, Guoqi ; Handmann, Patricia ; Holliday, N. Penny ; Holte, James ; Inall, Mark E. ; Johns, William E. ; Jones, Sam ; Karstensen, Johannes ; Li, Feili ; Pacini, Astrid ; Pickart, Robert S. ; Rayner, Darren ; Straneo, Fiammetta ; Yashayaev, Igor
    Understanding the variability of the Atlantic Meridional Overturning Circulation is essential for better predictions of our changing climate. Here we present an updated time series (August 2014 to June 2020) from the Overturning in the Subpolar North Atlantic Program. The 6-year time series allows us to observe the seasonality of the subpolar overturning and meridional heat and freshwater transports. The overturning peaks in late spring and reaches a minimum in early winter, with a peak-to-trough range of 9.0 Sv. The overturning seasonal timing can be explained by winter transformation and the export of dense water, modulated by a seasonally varying Ekman transport. Furthermore, over 55% of the total meridional freshwater transport variability can be explained by its seasonality, largely owing to overturning dynamics. Our results provide the first observational analysis of seasonality in the subpolar North Atlantic overturning and highlight its important contribution to the total overturning variability observed to date.
  • Article
    A Possible hysteresis in the Arctic Ocean due to release of subsurface heat during sea ice retreat
    (American Meteorological Society, 2023-05-01) Beer, Emma ; Eisenman, Ian ; Wagner, Till J. W. ; Fine, Elizabeth C.
    The Arctic Ocean is characterized by an ice-covered layer of cold and relatively fresh water above layers of warmer and saltier water. It is estimated that enough heat is stored in these deeper layers to melt all the Arctic sea ice many times over, but they are isolated from the surface by a stable halocline. Current vertical mixing rates across the Arctic Ocean halocline are small, due in part to sea ice reducing wind–ocean momentum transfer and damping internal waves. However, recent observational studies have argued that sea ice retreat results in enhanced mixing. This could create a positive feedback whereby increased vertical mixing due to sea ice retreat causes the previously isolated subsurface heat to melt more sea ice. Here, we use an idealized climate model to investigate the impacts of such a feedback. We find that an abrupt “tipping point” can occur under global warming, with an associated hysteresis window bounded by saddle-node bifurcations. We show that the presence and magnitude of the hysteresis are sensitive to the choice of model parameters, and the hysteresis occurs for only a limited range of parameters. During the critical transition at the bifurcation point, we find that only a small percentage of the heat stored in the deep layer is released, although this is still enough to lead to substantial sea ice melt. Furthermore, no clear relationship is apparent between this change in heat storage and the level of hysteresis when the parameters are varied.
  • Article
    Satellite-observed strong subtropical ocean warming as an early signature of global warming
    (Nature Research, 2023-05-24) Yang, Hu ; Lohmann, Gerrit ; Stepanek, Christian ; Wang, Qiang ; Huang, Rui Xin ; Shi, Xiaoxu ; Liu, Jiping ; Chen, Dake ; Wang, Xulong ; Zhong, Yi ; Yang, Qinghua ; Bao, Ying ; Müller, Juliane
    Satellite observations covering the last four decades reveal an ocean warming pattern resembling the negative phase of the Pacific Decadal Oscillation. This pattern has therefore been widely interpreted as a manifestation of natural climate variability. Here, we re-examine the observed warming pattern and find that the predominant warming over the subtropical oceans, while mild warming or even cooling over the subpolar ocean, is dynamically consistent with the convergence and divergence of surface water. By comparison of observations, paleo-reconstructions, and model simulations, we propose that the observed warming pattern is likely a short-term transient response to the increased CO2 forcing, which only emerges during the early stage of anthropogenic warming. On centennial to millennial timescales, the subpolar ocean warming is expected to exceed the temporally dominant warming of the subtropical ocean. This delayed but amplified subpolar ocean warming has the potential to reshape the ocean-atmosphere circulation and threaten the stability of marine-terminating ice sheets.The observed sea surface temperature pattern of strong warming concentrated in the subtropical oceans is likely to be an early signature of climate change, according to observations and model simulations, whereas ocean warming at high latitudes is expected to become dominant in the future.
  • Article
    Langmuir turbulence controls on observed diurnal warm layer depths
    (American Geophysical Union, 2023-05-24) Wang, Xingchi ; Kukulka, Tobias ; Farrar, J. Thomas ; Plueddemann, Albert J. ; Zippel, Seth F.
    The turbulent ocean surface boundary layer (OSBL) shoals during daytime solar surface heating, developing a diurnal warm layer (DWL). The DWL significantly influences OSBL dynamics by trapping momentum and heat in a shallow near‐surface layer. Therefore, DWL depth is critical for understanding OSBL transport and ocean‐atmosphere coupling. A great challenge for determining DWL depth is considering wave‐driven Langmuir turbulence (LT), which increases vertical transport. This study investigates observations with moderate wind speeds (4–7 m/s at 10 m height) and swell waves for which breaking wave effects are less pronounced. By employing turbulence‐resolving large eddy simulation experiments that cover observed wind, wave, and heating conditions based on the wave‐averaged Craik‐Lebovich equation, we develop a DWL depth scaling unifying previous approaches. This scaling closely agrees with observed DWL depths from a year‐long mooring deployment in the subtropical North Atlantic, demonstrating the critical role of LT in determining DWL depth and OSBL dynamics.
  • Article
    The Atlantic Meridional Overturning Circulation at 35°N from deep moorings, floats, and satellite altimeter
    (American Geophysical Union, 2023-05-13) Le Bras, Isabela Alexander‐Astiz ; Willis, Josh ; Fenty, Ian
    From 2004 to 2014, the Line W moorings measured a 0.7 Sv yr−1 slowing of the deep western boundary current (DWBC) offshore of Cape Cod. Here, we combine these deep mooring observations with float and satellite altimeter data and find that this DWBC change corresponded to a slowing of the cross‐basin Atlantic Meridional Overturning Circulation (AMOC) of about 0.3 Sv yr−1. Our AMOC transport time series corresponds well with the Estimating the Circulation and Climate of the Ocean state estimate, particularly when the Line W mooring data influences our volume closure. We compare our 35°N time series with a similar time series at 41°N as well as the 26°N RAPID AMOC, and find AMOC declines across datasets from 2004 to 2014. However, when we extend our analysis to 2004–2019, there are no significant trends at any latitude. These observations suggest that AMOC decadal variability is meridionally coherent from 26°N to 41°N and that the DWBC may reflect this variability.Plain Language SummaryThe Atlantic ocean hosts an overturning circulation that is thought to be an important piece of our climate system. This circulation pattern spans the width of the basin, making it difficult and costly to measure, so direct observations of the overturning circulation are scarce. In this study we combine existing mooring, float, and satellite altimeter observations to estimate the overturning circulation at a new latitude (35°N), and compare it to existing estimates at 26°N and 41°N as well as the ECCO ocean state estimate. We find that the long term (about 10 year) AMOC variability is consistent across latitudes and data products. While we cannot rule out a decreasing AMOC trend during the 20th century, we find that natural variability is too large to detect a net AMOC decrease in direct observations since 2004.Key PointsWe compile an Atlantic Meridional Overturning Circulation (AMOC) time series at 35°N from deep moorings, floats, and altimeter that agrees with the Estimating the Circulation and Climate of the Ocean state estimateThe 2004 to 2014 slowing of the deep western boundary current corresponded to an AMOC decline at 35°NWe find no evidence of long‐term AMOC decline, but consistent decadal variability across 26°N, 35°N, and 41°N
  • Article
    Saildrone direct covariance wind stress in various wind and current regimes of the tropical Pacific
    (American Meteorological Society, 2023-04-01) Reeves Eyre, J. E. Jack ; Cronin, Meghan F. ; Zhang, Dongxiao ; Thompson, Elizabeth J. ; Fairall, Christopher W. ; Edson, James B.
    High-frequency wind measurements from Saildrone autonomous surface vehicles are used to calculate wind stress in the tropical east Pacific. Comparison between direct covariance (DC) and bulk wind stress estimates demonstrates very good agreement. Building on previous work that showed the bulk input data were reliable, our results lend credibility to the DC estimates. Wind flow distortion by Saildrones is comparable to or smaller than other platforms. Motion correction results in realistic wind spectra, albeit with signatures of swell-coherent wind fluctuations that may be unrealistically strong. Fractional differences between DC and bulk wind stress magnitude are largest at wind speeds below 4 m s −1 . The size of this effect, however, depends on choice of stress direction assumptions. Past work has shown the importance of using current-relative (instead of Earth-relative) winds to achieve accurate wind stress magnitude. We show that it is also important for wind stress direction. Significance Statement We use data from Saildrone uncrewed oceanographic research vehicles to investigate the horizontal forces applied to the surface of the ocean by the action of the wind. We compare two methods to calculate the forces: one uses several simplifying assumptions, and the other makes fewer assumptions but is error prone if the data are incorrectly processed. The two methods agree well, suggesting that Saildrone vehicles are suitable for both methods and that the data processing methods work. Our results show that it is important to consider ocean currents, as well as winds, in order to achieve accurate magnitude and direction of the surface forces.
  • Article
    Ocean temperature observations in Hurricane Dorian (2019)
    (American Meteorological Society, 2023-06-01) Densmore, Casey R. ; Sanabia, Elizabeth R. ; Jayne, Steven R.
    Upper-ocean temperatures from 72 airborne expendable bathythermographs (AXBTs) collected during U.S. Air Force Hurricane Hunter flights into Hurricane Dorian (2019) over a 72-h period are examined. Three transects collected behind the storm reveal increased cross-track sea surface temperature gradient magnitudes as Dorian intensified to a category-5 hurricane and slowed while approaching the Bahamas. The cold wake, evident in vertical and horizontal cross sections from in situ and satellite sensors, appears as an expected response to tropical cyclone passage. Atypical, however, is the 2°C surface cooling observed over 36 h in a pair of transects ahead of hurricane force winds in Dorian, likely due to changes in the tropical cyclone’s translation speed and direction and/or proximity to the Gulf Stream and continental shelf. Collocated AXBT pairs document a dynamical regime shift from mixing to upwelling as Dorian slows and turns. Relationships between time-integrated wind stress and sea surface temperature indicate track-relative differences varying with storm translation speed and heading changes, paralleling the shift in cooling dynamics. Significance Statement We studied in situ and satellite ocean temperature observations beneath Hurricane Dorian (2019) as the storm moved slowly, turned north, and weakened near Grand Bahama Island. We found a distinct change in the spatial distribution of cool upper-ocean temperatures beneath the storm, which indicated a shift in the primary cooling mechanism from ocean mixing to upwelling. This mechanism shift is important because hurricanes depend on warm ocean temperatures for energy, and upwelling roughly doubles the area of cooling beneath the storm. Our results highlight the effects of large heading changes on the upper-ocean response beneath tropical cyclones, especially in tandem with slow translation speeds.
  • Article
    The Southern Ocean mixed layer and its boundary fluxes: fine-scale observational progress and future research priorities
    (The Royal Society, 2023-06-26) Swart, Sebastiaan ; du Plessis, Marcel D. ; Nicholson, Sarah-Anne ; Monteiro, Pedro M. S. ; Dove, Lilian A. ; Thomalla, Sandy ; Thompson, Andrew F. ; Biddle, Louise C. ; Edholm, Johan M. ; Giddy, Isabelle ; Heywood, Karen J. ; Lee, Craig ; Mahadevan, Amala ; Shilling, Geoff ; de Souza, Ronald Buss
    Interactions between the upper ocean and air-ice-ocean fluxes in the Southern Ocean play a critical role in global climate by impacting the overturning circulation and oceanic heat and carbon uptake. Remote and challenging conditions have led to sparse observational coverage, while ongoing field programmes often fail to collect sufficient information in the right place or at the time-space scales required to constrain the variability occurring in the coupled ocean-atmosphere system. Only within the last 10 years have we been able to directly observe and assess the role of the fine-scale ocean and rapidly evolving atmospheric marine boundary layer on the upper limb of the Southern Ocean's overturning circulation. This review summarizes advances in mechanistic understanding, arising in part from observational programmes using autonomous platforms, of the fine-scale processes (1-100 km, hours-seasons) influencing the Southern Ocean mixed layer and its variability. We also review progress in observing the ocean interior connections and the coupled interactions between the ocean, atmosphere and cryosphere that moderate air-sea fluxes of heat and carbon. Most examples provided are for the ice-free Southern Ocean, while major challenges remain for observing the ice-covered ocean. We attempt to elucidate contemporary research gaps and ongoing/future efforts needed to address them. This article is part of a discussion meeting issue 'Heat and carbon uptake in the Southern Ocean: the state of the art and future priorities'.
  • Article
    Oceanic bottom mixed layer in the Clarion-Clipperton Zone: potential influence on deep-seabed mining plume dispersal
    (Springer, 2023-04-25) Chen, Si-Yuan Sean ; Ouillon, Raphael ; Muñoz-Royo, Carlos ; Peacock, Thomas
    The oceanic bottom mixed layer (BML) is a well mixed, weakly stratified, turbulent boundary layer. Adjacent to the seabed, the BML is of intrinsic importance for studying ocean mixing, energy dissipation, particle cycling and sediment-water interactions. While deep-seabed mining of polymetallic nodules is anticipated to commence in the Clarion-Clipperton Zone (CCZ) of the northeastern tropical Pacific Ocean, knowledge gaps regarding the form of the BML and its potentially key influence on the dispersal of sediment plumes generated by deep-seabed mining activities are yet to be addressed. Here, we report recent field observations from the German mining licence area in the CCZ that characterise the structure and variability of the BML locally. Quasi-uniform profiles of potential temperature extending from the seafloor reveal the presence of a spatially and temporally variable BML with an average local thickness of approximately 250 m. Deep horizontal currents in the region have a mean speed of 3.5 cm s-1 and a maximum speed of 12 cm s-1 at 18.63 ms above bottom over an 11 month record. The near-bottom currents initially have a net southeastward flow, followed by westward and southward flows with the development of complex, anticyclonic flow patterns. Theoretical predictions and historical data show broad consistency with mean BML thickness but cannot explain the observed heterogeneity of local BML thickness. We postulate that deep pressure anomalies induced by passing surface mesoscale eddies and abyssal thermal fronts could affect BML thickness, in addition to local topographic effects. A simplified transport model is then used to study the influence of the BML on the interplay between turbulent diffusion and sediment settling in the transport of deep-seabed mining induced sediment plumes. Over a range of realistic parameter values, the effects of BML on plume evolution can vary significantly, highlighting that resolving the BML will be a crucial step for accurate numerical modelling of plume dispersal.
  • Article
    Increased gulf stream warm core ring formations contributes to an observed increase in salinity maximum intrusions on the Northeast Shelf
    (Nature Research, 2023-05-09) Silver, Adrienne ; Gangopadhyay, Avijit ; Gawarkiewicz, Glen ; Fratantoni, Paula ; Clark, Jenifer
    We present observational evidence of a significant increase in Salinity Maximum intrusions in the Northeast US Shelf waters in the years following 2000. This increase is subsequent to and influenced by a previously observed regime-shift in the annual formation rate for Gulf Stream Warm Core Rings, which are relatively more saline than the shelf waters. Specifically, mid-depth salinity maximum intrusions, a cross-shelf exchange process, has shown a quadrupling in frequency on the shelf after the year 2000. This increase in intrusion frequency can be linked to a similar increase in Warm Core Ring occupancy footprint along the offshore edge of the shelf-break which has greatly increased the abundance of warm salty water within the Slope Sea. The increased ring occupancy footprint along the shelf follows from the near doubling in annual Warm Core Ring formation rate from the Gulf Stream. The increased occurrence of intrusions is likely driven by a combination of a larger number of rings in the slope sea and the northward shift in the GS position which may lead to more interactions between rings and the shelf topography. These results have significant implications for interpreting temporal changes in the shelf ecosystem from the standpoint of both larval recruitment as well as habitability for various important commercial species.
  • Article
    Exceptional stratospheric contribution to human fingerprints on atmospheric temperature
    (National Academy of Sciences, 2023-05-16) Santer, Benjamin D. ; Po-Chedley, Stephen ; Zhao, Lilong ; Zou, Cheng-Zhi ; Fu, Qiang ; Solomon, Susan ; Thompson, David W. J. ; Mears, Carl ; Taylor, Karl E.
    In 1967, scientists used a simple climate model to predict that human-caused increases in atmospheric CO2 should warm Earth’s troposphere and cool the stratosphere. This important signature of anthropogenic climate change has been documented in weather balloon and satellite temperature measurements extending from near-surface to the lower stratosphere. Stratospheric cooling has also been confirmed in the mid to upper stratosphere, a layer extending from roughly 25 to 50 km above the Earth’s surface (S25 − 50). To date, however, S25 − 50 temperatures have not been used in pattern-based attribution studies of anthropogenic climate change. Here, we perform such a “fingerprint” study with satellite-derived patterns of temperature change that extend from the lower troposphere to the upper stratosphere. Including S25 − 50 information increases signal-to-noise ratios by a factor of five, markedly enhancing fingerprint detectability. Key features of this global-scale human fingerprint include stratospheric cooling and tropospheric warming at all latitudes, with stratospheric cooling amplifying with height. In contrast, the dominant modes of internal variability in S25 − 50 have smaller-scale temperature changes and lack uniform sign. These pronounced spatial differences between S25 − 50 signal and noise patterns are accompanied by large cooling of S25 − 50 (1 to 2C over 1986 to 2022) and low S25 − 50 noise levels. Our results explain why extending “vertical fingerprinting” to the mid to upper stratosphere yields incontrovertible evidence of human effects on the thermal structure of Earth’s atmosphere.
  • Article
    Mixing and overturning across the Brazil‐Malvinas Confluence
    (American Geophysical Union, 2023-04-23) Orúe‐Echevarría, Dorleta ; Polzin, Kurt L. ; Naveira Garabato, Alberto C. ; Forryan, Alexander ; Pelegrí, Josep L.
    The rates of isopycnal stirring and water mass transport by mesoscale eddies, and of diapycnal mixing by small‐scale turbulence, across the Brazil‐Malvinas Confluence (BMC) are assessed from a set of microstructure and hydrographic measurements in the Argentine Basin. This assessment is founded on a theoretical framework that applies a triple decomposition to the temperature variance equation and assumes eddies to transfer potential vorticity downgradient. The BMC is found to host widespread intense isopycnal stirring at rates of O(103–104 m2 s−1), and generally weak diapycnal mixing at rates of O(10−6–10−5 m2 s−1). Despite such disparity, both diapycnal mixing and isopycnal stirring play roles of comparable importance in determining regional water mass properties within surface and mode waters. In deeper layers, isopycnal stirring prevails. Eddies are further diagnosed to effect an important cross‐BMC transport, at rates of O(1 m2 s−1). When scaled by the along‐stream extent of the BMC, these rates integrate to volume transports that may be as large as O(10 Sverdrups). This suggests that cross‐BMC transfers of waters are substantially effected by eddy‐induced flows.
  • Article
    Acoustic travel-time variability observed on a 150-km radius tomographic array in the Canada Basin during 2016–2017
    (Acoustical Society of America, 2023-05-02) Worcester, Peter F. ; Dzieciuch, Matthew A. ; Vazquez, Heriberto J. ; Cornuelle, Bruce D. ; Colosi, John A. ; Krishfield, Richard A. ; Kemp, John N.
    The Arctic Ocean is undergoing dramatic changes in response to increasing atmospheric concentrations of greenhouse gases. The 2016–2017 Canada Basin Acoustic Propagation Experiment was conducted to assess the effects of the changes in the sea ice and ocean structure in the Beaufort Gyre on low-frequency underwater acoustic propagation and ambient sound. An ocean acoustic tomography array with a radius of 150 km that consisted of six acoustic transceivers and a long vertical receiving array measured the impulse responses of the ocean at a variety of ranges every four hours using broadband signals centered at about 250 Hz. The peak-to-peak low-frequency travel-time variability of the early, resolved ray arrivals that turn deep in the ocean was only a few tens of milliseconds, roughly an order of magnitude smaller than observed in previous tomographic experiments at similar ranges, reflecting the small spatial scale and relative sparseness of mesoscale eddies in the Canada Basin. The high-frequency travel-time fluctuations were approximately 2 ms root-mean-square, roughly comparable to the expected measurement uncertainty, reflecting the low internal-wave energy level. The travel-time spectra show increasing energy at lower frequencies and enhanced semidiurnal variability, presumably due to some combination of the semidiurnal tides and inertial variability.
  • Article
    Heat stored in the Earth system 1960–2020: where does the energy go?
    (Copernicus Publications, 2023-04-17) von Schuckmann, Karina ; Minière, Audrey ; Gues, Flora ; Cuesta-Valero, Francisco José ; Kirchengast, Gottfried ; Adusumilli, Susheel ; Straneo, Fiammetta ; Ablain, Michaël ; Allan, Richard P ; Barker, Paul M ; Beltrami, Hugo ; Blazquez, Alejandro ; Boyer, Tim ; Cheng, Lijing ; Church, John ; Desbruyeres, Damien ; Dolman, Han ; Domingues, Catia M ; García-García, Almudena ; Giglio, Donata ; Gilson, John E ; Gorfer, Maximilian ; Haimberger, Leopold ; Hakuba, Maria Z ; Hendricks, Stefan ; Hosoda, Shigeki ; Johnson, Gregory C ; Killick, Rachel ; King, Brian ; Kolodziejczyk, Nicolas ; Zemp, Michael
    The Earth climate system is out of energy balance, and heat has accumulated continuously over the past decades, warming the ocean, the land, the cryosphere, and the atmosphere. According to the Sixth Assessment Report by Working Group I of the Intergovernmental Panel on Climate Change, this planetary warming over multiple decades is human-driven and results in unprecedented and committed changes to the Earth system, with adverse impacts for ecosystems and human systems. The Earth heat inventory provides a measure of the Earth energy imbalance (EEI) and allows for quantifying how much heat has accumulated in the Earth system, as well as where the heat is stored. Here we show that the Earth system has continued to accumulate heat, with 381±61 ZJ accumulated from 1971 to 2020. This is equivalent to a heating rate (i.e., the EEI) of 0.48±0.1 W m−2. The majority, about 89 %, of this heat is stored in the ocean, followed by about 6 % on land, 1 % in the atmosphere, and about 4 % available for melting the cryosphere. Over the most recent period (2006–2020), the EEI amounts to 0.76±0.2 W m−2. The Earth energy imbalance is the most fundamental global climate indicator that the scientific community and the public can use as the measure of how well the world is doing in the task of bringing anthropogenic climate change under control. Moreover, this indicator is highly complementary to other established ones like global mean surface temperature as it represents a robust measure of the rate of climate change and its future commitment. We call for an implementation of the Earth energy imbalance into the Paris Agreement's Global Stocktake based on best available science. The Earth heat inventory in this study, updated from von Schuckmann et al. (2020), is underpinned by worldwide multidisciplinary collaboration and demonstrates the critical importance of concerted international efforts for climate change monitoring and community-based recommendations and we also call for urgently needed actions for enabling continuity, archiving, rescuing, and calibrating efforts to assure improved and long-term monitoring capacity of the global climate observing system. The data for the Earth heat inventory are publicly available, and more details are provided in Table 4.
  • Article
    Shelf break exchange processes influence the availability of the northern shortfin squid, Illex illecebrosus, in the Northwest Atlantic
    (Wiley, 2023-04-14) Salois, Sarah L. ; Hyde, Kimberly J. W. ; Silver, Adrienne ; Lowman, Brooke A. ; Gangopadhyay, Avijit ; Gawarkiewicz, Glen ; Mercer, Anna J. M. ; Manderson, John P. ; Gaichas, Sarah K. ; Hocking, Daniel J. ; Galuardi, Benjamin ; Jones, Andrew W. ; Kaelin, Jeff ; DiDomenico, Greg ; Almeida, Katie ; Bright, Bill ; Lapp, Meghan
    The United States Northern Shortfin squid fishery is known for its large fluctuations in catch at annual scales. In the last 5 years, this fishery has experienced increased availability of Illex illecebrosus along the Northeast US continental shelf (NES), resulting in high catch per unit effort (CPUE) and early fishery closures due to quota exceedance. The fishery occurs within the Northwest Atlantic, whose complex dynamics are set up by the interplay between the large‐scale Gulf Stream, mesoscale eddies, Shelfbreak Jet, and shelf‐slope exchange processes. Our ability to understand and quantify this regional variability is requisite for understanding the availability patterns of Illex, which are largely influenced by oceanographic conditions. In an effort to advance our current understanding of the seasonal and interannual variability in this species' relative abundance on the NES, we used generalized additive models to examine the relationships between the physical environment and hotspots of productivity to changes in CPUE of I. illecebrosus in the Southern stock component, which comprises the US fishery. Specifically, we derived oceanographic indicators by pairing high‐resolution remote sensing data and global ocean reanalysis physical data to high‐resolution fishery catch data. We identified a suite of environmental covariates that were strongly related to instances of higher catch rates. In particular, bottom temperature, warm core rings, subsurface features, and frontal dynamics together serve as indicators of habitat condition and primary productivity hotspots, providing great utility for understanding the distribution of Illex with the potential for forecasting seasonal and interannual availability.
  • Article
    Southern Ocean warming and its climatic impacts
    (Elsevier, 2023-05-12) Cai, Wenju ; Gao, Libao ; Luo, Yiyong ; Li, Xichen ; Zheng, Xiaotong ; Zhang, Xuebin ; Cheng, Xuhua ; Jia, Fan ; Purich, Ariaan ; Santoso, Agus ; Du, Yan ; Holland, David M. ; Shi, Jia-Rui ; Xiang, Baoqiang ; Xie, Shang-Ping
    The Southern Ocean has warmed substantially, and up to early 21st century, Antarctic stratospheric ozone depletion and increasing atmospheric CO2 have conspired to intensify Southern Ocean warming. Despite a projected ozone recovery, fluxes to the Southern Ocean of radiative heat and freshwater from enhanced precipitation and melting sea ice, ice shelves, and ice sheets are expected to increase, as is a Southern Ocean westerly poleward intensification. The warming has far-reaching climatic implications for melt of Antarctic ice shelf and ice sheet, sea level rise, and remote circulations such as the intertropical convergence zone and tropical ocean-atmosphere circulations, which affect extreme weathers, agriculture, and ecosystems. The surface warm and freshwater anomalies are advected northward by the mean circulation and deposited into the ocean interior with a zonal-mean maximum at ∼45°S. The increased momentum and buoyancy fluxes enhance the Southern Ocean circulation and water mass transformation, further increasing the heat uptake. Complex processes that operate but poorly understood include interactive ice shelves and ice sheets, oceanic eddies, tropical-polar interactions, and impact of the Southern Ocean response on the climate change forcing itself; in particular, limited observations and low resolution of climate models hinder rapid progress. Thus, projection of Southern Ocean warming will likely remain uncertain, but recent community effort has laid a solid foundation for substantial progress.
  • Article
    Increasing deep-water overflow from the Pacific into the South China Sea revealed by mooring observations
    (Nature Research, 2023-04-10) Zhou, Chun ; Xiao, Xin ; Zhao, Wei ; Yang, Jiayan ; Huang, Xiaodong ; Guan, Shoude ; Zhang, Zhiwei ; Tian, Jiwei
    Cold and dense water from the North Pacific Ocean that spills through the Luzon Strait, the only deep conduit between the South China Sea (SCS) and the Pacific Ocean, renews deep-water mass, modulates hydrographic and biogeochemical cycles, and drives abyssal and overturning circulations in the SCS. The variability of this key oceanic process, however, has been poorly studied, mainly due to a lack of sustained observations. A comprehensive observational program that started in 2009 has provided 12 years of continuous time series of velocity and volume transport within the Luzon Strait. Here we show the observation-based assessment of decadal trends of deep-water transport through this vital passage. With the estimated 12-year mean volume transport of the deep-water overflow into the SCS of 0.84 ± 0.39 Sv (1 Sv = 10 ms), a significant linear upward trend of 9% is revealed during this period. This is consistent with long-term changes in satellite-observed ocean bottom pressure. The results of this study may have broad implications for the overturning circulations and biogeochemical processes, including carbon cycles in this region.