Large-scale Ocean Circulation Research Articles

Observations of Seasonal Upwelling and Downwelling in the Beaufort Sea Mediated by Sea Ice

AbstractWe present observational estimates of Ekman pumping in the Beaufort Gyre region. Averaged over the Canada Basin, the results show a 2003–14 average of 2.3 m yr−1 downward with strong seasonal and interannual variability superimposed: monthly and yearly means range from 30 m yr−1 downward to 10 m yr−1 upward. A clear, seasonal cycle is evident with intense downwelling in autumn and upwelling during the winter months, despite the wind forcing being downwelling favorable year-round. Wintertime upwelling is associated with friction between the large-scale Beaufort Gyre ocean circulation and the surface ice pack and contrasts with previous estimates of yearlong downwelling; as a consequence, the yearly cumulative Ekman pumping over the gyre is significantly reduced. The spatial distribution of Ekman pumping is also modified, with the Beaufort Gyre region showing alternating, moderate upwelling and downwelling, while a more intense, yearlong downwelling averaging 18 m yr−1 is identified in the northern Chukchi Sea region. Implications of the results for understanding Arctic Ocean dynamics and change are discussed.

Marine heatwaves off eastern Tasmania: Trends, interannual variability, and predictability

Surface waters off eastern Tasmania are a global warming hotspot. Here, mean temperatures have been rising over several decades at nearly four times the global average rate, with concomitant changes in extreme temperatures – marine heatwaves. These changes have recently caused the marine biodiversity, fisheries and aquaculture industries off Tasmania’s east coast to come under stress. In this study we quantify the long-term trends, variability and predictability of marine heatwaves off eastern Tasmania. We use a high-resolution ocean model for Tasmania’s eastern continental shelf. The ocean state over the 1993–2015 period is hindcast, providing daily estimates of the three-dimensional temperature and circulation fields. Marine heatwaves are identified at the surface and subsurface from ocean temperature time series using a consistent definition. Trends in marine heatwave frequency are positive nearly everywhere and annual marine heatwave days and penetration depths indicate significant positive changes, particularly off southeastern Tasmania. A decomposition into modes of variability indicates that the East Australian Current is the dominant driver of marine heatwaves across the domain. Self-organising maps are used to identify 12 marine heatwave types, each with its own regionality, seasonality, and associated large-scale oceanic and atmospheric circulation patterns. The implications of this work for marine ecosystems and their management were revealed through review of past impacts and stakeholder discussions regarding use of these data.

Trends in the consecutive days of temperature and precipitation extremes in China during 1961–2015

Background and aimsConsecutive climatic extremes have more intense impacts on natural ecosystems and human activities than occasional events. There were many studies about the frequency or intensity of extreme weather events, but few focused on the consecutiveness or continuousness of climatic extremes. We analyzed the temporal and spatial distributions and tendencies in the consecutive temperature and precipitation extremes in China during 1961–2015. MethodsDaily temperature and precipitation data at 1867 meteorological stations over China was used and four consecutive indices of climate extremes, i.e. cold spell duration indicator (CSDI), warm spell duration indicator (WSDI), consecutive dry days (CDD) and consecutive wet days (CWD), were calculated by RClimDex 1.0. Linear trends in the time series of consecutive days of temperature and precipitation extremes were examined and their statistical significance was evaluated using Mann–Kendall test. Results and discussionThere were obvious differences in the spatial distributions of consecutive days of climate extremes in China. During 1961–2015, CSDI and CWD decreased significantly at rates of 0.9 and 0.1 days per decade respectively, while WSDI increased significantly at rate of 0.8 days per decade in China. Spatially, CSDI decreased at rates of 0–3.0 days per decade in almost all parts of China, and WSDI increased at rates of 0–2.0 days per decade in most parts of China. The spatial trends of CDD and CWD were significant only in several regions of China. CSDI and WSDI had higher percent changes than those of CDD and CWD. Changes in the CSDI and WSDI were associated with large-scale oceanic and atmospheric circulation oscillations, such as Atlantic Multidecadal Oscillation (AMO), El Niño/Southern Oscillation (ENSO) and Pacific Decadal Oscillation (PDO). With global warming, there will be fewer cold extremes, more frequent hot extremes and precipitation extremes. ConclusionsGiven the increases in the frequency and intensity of some consecutive climatic extremes and an increasing physical exposure and socio-economic vulnerability to such extremes in China, more strategies and capacities of mitigation and adaptation to consecutive climatic extremes are essential for the local government and climate-sensitive sectors.

Role of overturns in optimal mixing in stratified mixing layers

Turbulent mixing plays a major role in enabling the large-scale ocean circulation. The accuracy of mixing rates estimated from observations depends on our understanding of basic fluid mechanical processes underlying the nature of turbulence in a stratified fluid. Several of the key assumptions made in conventional mixing parameterizations have been increasingly scrutinized in recent years, primarily on the basis of adequately high resolution numerical simulations. We add to this evidence by compiling results from a suite of numerical simulations of the turbulence generated through stratified shear instability processes. We study the inherently intermittent and time-dependent nature of wave-induced turbulent life cycles and more specifically the tight coupling between inherently anisotropic scales upon which small-scale isotropic turbulence grows. The anisotropic scales stir and stretch fluid filaments enhancing irreversible diffusive mixing at smaller scales. We show that the characteristics of turbulent mixing depend on the relative time evolution of the Ozmidov length scale$L_{O}$compared to the so-called Thorpe overturning scale$L_{T}$which represents the scale containing available potential energy upon which turbulence feeds and grows. We find that when$L_{T}\sim L_{O}$, the mixing is most active and efficient since stirring by the largest overturns becomes ‘optimal’ in the sense that it is not suppressed by ambient stratification. We argue that the high mixing efficiency associated with this phase, along with observations of$L_{O}/L_{T}\sim 1$in oceanic turbulent patches, together point to the potential for systematically underestimating mixing in the ocean if the role of overturns is neglected. This neglect, arising through the assumption of a clear separation of scales between the background mean flow and small-scale quasi-isotropic turbulence, leads to the exclusion of an highly efficient mixing phase from conventional parameterizations of the vertical transport of density. Such an exclusion may well be significant if the mechanism of shear-induced turbulence is assumed to be representative of at least some turbulent events in the ocean. While our results are based upon simulations of shear instability, we show that they are potentially more generic by making direct comparisons with$L_{T}-L_{O}$data from ocean and lake observations which represent a much wider range of turbulence-inducing physical processes.

The bottom water exchange between the Singapore Strait and the West Johor Strait

As a part of the border between Singapore and Malaysia, the West Johor Strait (WJS) suffered newly from harmful algal blooms. There is no previous study showing the source of the nutrients in the WJS. This paper is investigating the possible water exchange between the water in the WJS and the bottom water in Singapore Strait.This paper adopts a two-level nesting atmosphere-ocean coupled models to downscale the global atmosphere-ocean model into the Singapore coastal water, keeping the large-scale and long-term ocean and climate circulation signals and the advantages of the high-resolution. Based on the high-resolution ocean circulation fields, a Lagrangian particle tracking model is used to trace the Singapore Strait's bottom water movement and the water mixing in the WJS.The results showed that the numerical models well resolved the Singapore coastal water regional circulation. There is a small but significant bottom water (1.25%) transport from the Singapore Strait to the WJS, which occurs from the southwest coastline of Singapore. The bottom water in the Singapore Strait prefers to enter the WJS during the spring tide and the flood period, and stay in Johor Strait for 6.4 days. The spring tide is the first-order factor for the water vertical mixing in the WJS, the wind is also very important for the vertical mixing especially in neap tide condition. An overall very important factor is the light perturbation. With the strongest vertical mixing of nutrients and bottom sediments due to the spring tide, the latter ones may inhibit the light penetration during the spring tide and reduce the algal bloom. The light penetration otherwise is greater during the neap tide, when the winds are the most important factor and hence favor the algal bloom. With the strongest wind in February and the longest permanence time in June and the sufficient nutrient supply in February and June, the most serious algal blooms may happen in February and June in the WJS.

Stochastic Subgrid-Scale Ocean Mixing: Impacts on Low-Frequency Variability

In global ocean models, the representation of small-scale, high-frequency processes considerably influences the large-scale oceanic circulation and its low-frequency variability. This study investigates the impact of stochastic perturbation schemes based on three different subgrid-scale parameterizations in multidecadal ocean-only simulations with the ocean model NEMO at 1° resolution. The three parameterizations are an enhanced vertical diffusion scheme for unstable stratification, the Gent–McWilliams (GM) scheme, and a turbulent kinetic energy mixing scheme, all commonly used in state-of-the-art ocean models. The focus here is on changes in interannual variability caused by the comparatively high-frequency stochastic perturbations with subseasonal decorrelation time scales. These perturbations lead to significant improvements in the representation of low-frequency variability in the ocean, with the stochastic GM scheme showing the strongest impact. Interannual variability of the Southern Ocean eddy and Eulerian streamfunctions is increased by an order of magnitude and by 20%, respectively. Interannual sea surface height variability is increased by about 20%–25% as well, especially in the Southern Ocean and in the Kuroshio region, consistent with a strong underestimation of interannual variability in the model when compared to reanalysis and altimetry observations. These results suggest that enhancing subgrid-scale variability in ocean models can improve model variability and potentially its response to forcing on much longer time scales, while also providing an estimate of model uncertainty.

Non-Rayleigh control of upper-ocean Cd isotope fractionation in the western South Atlantic

We present seawater Cd isotopic compositions in five depth profiles and a continuous surface water transect, from 50°S to the Equator, in the western South Atlantic, sampled during GEOTRACES cruise 74JC057 (GA02 section, Leg 3), and investigate the mechanisms governing Cd isotope cycling in the upper and deep ocean.The depth profiles generally display high ε112/110Cd at the surface and decrease with increasing depth toward values typical of Antarctic Bottom Water (AABW). However, at stations north of the Subantarctic Front, the decrease in ε112/110Cd is interrupted by a shift to values intermediate between those of surface and bottom waters, which occurs at depths occupied by North Atlantic Deep Water (NADW). This pattern is associated with variations in Cd concentration from low surface values to a maximum at mid-depths and is attributed to preferential utilization of light Cd by phytoplankton in the surface ocean. Our new results show that in this region Cd-deficient waters do not display the extreme, highly fractionated ε112/110Cd reported in some earlier studies from other oceanic regions. Instead, in the surface and subsurface southwest (SW) Atlantic, when [Cd] drops below 0.1 nmol kg−1, ε112/110Cd are relatively homogeneous and cluster around a value of +3.7, in agreement with the mean value of 3.8±3.3 (2SD, n=164) obtained from a statistical evaluation of the global ocean Cd isotope dataset. We suggest that Cd-deficient surface waters may acquire their Cd isotope signature via sorption of Cd onto organic ligands, colloids or bacterial/picoplankton extracellular functional groups. Alternatively, we show that an open system, steady-state model is in good accord with the observed Cd isotope systematics in the upper ocean north of the Southern Ocean. The distribution of ε112/110Cd in intermediate and deep waters is consistent with the water mass distribution, with the north–south variations reflecting changes in the mixing proportion of NADW and either AABW or AAIW depending on the depth. Overall, the SW Atlantic Cd isotope dataset demonstrates that the large-scale ocean circulation exerts the primary control on ε112/110Cd cycling in the global deep ocean.

An explicit asymptotic preserving low Froude scheme for the multilayer shallow water model with density stratification

We present an explicit scheme for a two-dimensional multilayer shallow water model with density stratification, for general meshes and collocated variables. The proposed strategy is based on a regularized model where the transport velocity in the advective fluxes is shifted proportionally to the pressure potential gradient. Using a similar strategy for the potential forces, we show the stability of the method in the sense of a discrete dissipation of the mechanical energy, in general multilayer and non-linear frames. These results are obtained at first-order in space and time and extended using a second-order MUSCL extension in space and a Heun's method in time. With the objective of minimizing the diffusive losses in realistic contexts, sufficient conditions are exhibited on the regularizing terms to ensure the scheme's linear stability at first and second-order in time and space. The other main result stands in the consistency with respect to the asymptotics reached at small and large time scales in low Froude regimes, which governs large-scale oceanic circulation. Additionally, robustness and well-balanced results for motionless steady states are also ensured. These stability properties tend to provide a very robust and efficient approach, easy to implement and particularly well suited for large-scale simulations. Some numerical experiments are proposed to highlight the scheme efficiency: an experiment of fast gravitational modes, a smooth surface wave propagation, an initial propagating surface water elevation jump considering a non-trivial topography, and a last experiment of slow Rossby modes simulating the displacement of a baroclinic vortex subject to the Coriolis force.

Characteristics of meltwater export from Jakobshavn Isbræ and Ilulissat Icefjord

ABSTRACT Jakobshavn Isbræ, which terminates in Ilulissat Icefjord, has undergone rapid retreat and is currently the largest contributor to ice-sheet mass loss among Greenland's marine terminating glaciers. Accelerating mass loss is increasing fresh water discharge to the ocean, which can feed back on ice melt, impact marine ecosystems and potentially modify regional and larger scale ocean circulation. Here we present hydrographic observations, including inert geochemical tracers, that allow the first quantitative description of the glacially-modified waters exported from the Jakobshavn/Icefjord system. Observations within the fjord suggest a deep-reaching overturning cell driven by glacial buoyancy forcing. Modified waters containing submarine meltwater (up to 2.5 ± 0.12%), subglacial discharge (up to 6 ± 0.37%) and large portions of entrained ocean waters are seen to exit the fjord and flow north. The exported meltwaters form a buoyant coastal gravity current reaching to 100 m depth and extending 10 km offshore.

Barium isotopes reveal role of ocean circulation on barium cycling in the Atlantic

We diagnose the relative influences of local-scale biogeochemical cycling and regional-scale ocean circulation on Atlantic barium cycling by analysing four new depth profiles of dissolved Ba concentrations and isotope compositions from the South and tropical North Atlantic. These new profiles exhibit systematic vertical, zonal and meridional variations that reflect the influence of both local-scale barite cycling and large-scale ocean circulation. Epipelagic decoupling of dissolved Ba and Si reported previously in the tropics is also found to be associated with significant Ba isotope heterogeneity. As such, we contend that this decoupling originates from the depth segregation of opal and barite formation but is exacerbated by weak vertical mixing. Zonal influence from isotopically-‘heavy’ water masses in the western North Atlantic evidence the advective inflow of Ba-depleted Upper Labrador Sea Water, which is not seen in the eastern basin or the South Atlantic. Meridional variations in Atlantic Ba isotope systematics below 2000m appear entirely controlled by conservative mixing. Using an inverse isotopic mixing model, we calculate the Ba isotope composition of the Ba-poor northern end-member as +0.45‰ and the Ba-rich southern end-member +0.26‰, relative to NIST SRM 3104a. The near-conservative behaviour of Ba below 2000m indicates that Ba isotopes can serve as an independent tracer of the provenance of northern- versus southern-sourced water masses in the deep Atlantic Ocean. This finding may prove useful in palaeoceanographic studies, should appropriate sedimentary archives be identified, and offers new insights into the processes that cycle Ba in seawater.

Spatial variability of the Arctic Ocean's double-diffusive staircase

The Arctic Ocean thermohaline stratification frequently exhibits a staircase structure overlying the Atlantic Water Layer that can be attributed to the diffusive form of double-diffusive convection. The staircase consists of multiple layers of O(1) m in thickness separated by sharp interfaces, across which temperature and salinity change abruptly. Through a detailed analysis of Ice-Tethered Profiler measurements from 2004-2013, the double-diffusive staircase structure is characterized across the entire Arctic Ocean. We demonstrate how the large-scale Arctic Ocean circulation influences the small-scale staircase properties. These staircase properties (layer thicknesses and temperature and salinity jumps across interfaces) are examined in relation to a bulk vertical density ratio spanning the staircase stratification. We show that the Lomonosov Ridge serves as an approximate boundary between regions of low density ratio (approximately 3 to 4) on the Eurasian side and higher density ratio (approximately 6 to 7) on the Canadian side. We find that the Eurasian Basin staircase is characterized by fewer, thinner layers than that in the Canadian Basin, although the margins of all basins are characterized by relatively thin layers and the absence of a well-defined staircase. A double-diffusive 4/3-flux law parametrization is used to estimate vertical heat fluxes in the Canadian Basin to be O(0.1) Wm-2. It is shown that the 4/3-flux law may not be an appropriate representation of heat fluxes through the Eurasian Basin staircase. Here, molecular heat fluxes are estimated to be between O(0.01) Wm-2 and O(0.1) Wm-2. However, many uncertainties remain about the exact nature of these fluxes. This article is protected by copyright. All rights reserved.

Random attractors for the three dimensional stochastical planetary geostrophic equations of large-scale ocean circulation

The objection of this paper is to study the asymptotical behavior of solutions for the three dimensional planetary geostrophic equations of large-scale ocean circulation with small additive noise. In this paper, we prove the existence of random attractors for the three dimensional planetary geostrophic equations of large-scale ocean circulation with small additive noise. Furthermore, we prove the random attractor of the three dimensional planetary geostrophic equations of large-scale ocean circulation with small additive noise will be close to the global attractor of the three dimensional planetary geostrophic equations of large-scale ocean circulation for any when the parameter of the perturbation goes to zero.

Greenland Melt and the Atlantic Meridional Overturning Circulation

More than a decade of observations of the meridional overturning circulation in the subtropical North Atlantic show it to be highly variable on time scales of days to years and with an overall trend toward slowing down. Over the same time period, melting from Greenland (and elsewhere in the Arctic, including from sea ice) has been increasing, resulting in greater freshwater input to the northern North Atlantic. In this article, we examine evidence for the impact, if any, of this influx of freshwater on the large-scale ocean circulation and for potential changes.

Physical characteristics of the Gulf Stream as an indicator of the quality of large-scale circulation modeling

The general idea of this work is to show that the efficiency of modeling boundary currents (compared to the results of observations) can serve as an indicator of correctness for the modeling of the entire large-scale ocean circulation. The results of calculation of the mean surface currents in the Gulf Stream area based on direct measurements from drifters are presented together with the results of numerical modeling of variability of the Gulf Stream transport at 33°N over the period 2005–2014 based on data from Argo profiling buoys.

Brief communication: Impacts of a developing polynya off Commonwealth Bay, East Antarctica, triggered by grounding of iceberg B09B

Abstract. The dramatic calving of the Mertz Glacier tongue in 2010, precipitated by the movement of iceberg B09B, reshaped the oceanographic regime across the Mertz Polynya and Commonwealth Bay, regions where high-salinity shelf water (HSSW) – the precursor to Antarctic bottom water (AABW) – is formed. Here we present post-calving observations that suggest that this reconfiguration and subsequent grounding of B09B have driven the development of a new polynya and associated HSSW production off Commonwealth Bay. Supported by satellite observations and modelling, our findings demonstrate how local icescape changes may impact the formation of HSSW, with potential implications for large-scale ocean circulation.

The Effect of the Kerguelen Plateau on the Ocean Circulation

AbstractThe Kerguelen Plateau is a major topographic feature in the Southern Ocean. Located in the Indian sector and spanning nearly 2000 km in the meridional direction from the polar to the subantarctic region, it deflects the eastward-flowing Antarctic Circumpolar Current and influences the physical circulation and biogeochemistry of the Southern Ocean. The Kerguelen Plateau is known to govern the local dynamics, but its impact on the large-scale ocean circulation has not been explored. By comparing global ocean numerical simulations with and without the Kerguelen Plateau, this study identifies two major Kerguelen Plateau effects: 1) The plateau supports a local pressure field that pushes the Antarctic Circumpolar Current northward. This process reduces the warm-water transport from the Indian to the Atlantic Ocean. 2) The plateau-generated pressure field shields the Weddell Gyre from the influence of the warmer subantarctic and subtropical waters. The first effect influences the strength of the Antarctic Circumpolar Current and the Agulhas leakage, both of which are important elements in the global thermohaline circulation. The second effect results in a zonally asymmetric response of the subpolar gyres to Southern Hemisphere wind forcing.

Interannual and Decadal Sea Surface Height Variability Over the Northwest Atlantic Slope

The Northwest Atlantic continental slope features strong interactions among the western boundary currents of the subpolar and subtropical gyres, and, thus, the sea-level variability over the slope may be an indicator for the large-scale ocean circulation. In this study, temporal and spatial sea-level variability in the Northwest Atlantic continental slope has been investigated based on a satellite altimetry dataset and a temperature and salinity dataset. The altimetric results from 1993 to 2012 are compared with steric height anomalies relative to 1500 m, calculated from the temperature and salinity dataset. This study shows significant interannual and decadal sea-level variability, with prominent regional differences and varying linkages to large-scale atmospheric and oceanic variability in the North Atlantic. Both the altimeric and steric height anomalies in the western Labrador Sea are negatively correlated with the winter North Atlantic Oscillation (NAO) index primarily via wintertime deep convection. The altimetric height anomalies in the Laurentian Fan have a weak (insignificant at the 95% confidence level) positive correlation with those in the Labrador Sea, while the steric height anomalies have a negative correlation. The thermosteric (halosteric) height anomalies in the Labrador Sea are negatively (positively) correlated with the winter NAO index, while those in the Laurentian Fan are not correlated with the winter NAO index. The along-slope differences in the interannual and decadal variations of the sea surface height anomalies imply that there is an interior pathway of the southward-flowing Labrador Sea intermediate water toward the central North Atlantic basin before reaching 55 ˆN.

NHWAVE: Consistent boundary conditions and turbulence modeling

Large-scale σ-coordinate ocean circulation models neglect the horizontal variation of σ in the calculation of stress terms and boundary conditions. Following this practice, the effects of surface and bottom slopes in the dynamic surface and bottom boundary conditions have been usually neglected in the available non-hydrostatic wave-resolving models using a terrain-following grid. In this paper, we derive consistent surface and bottom boundary conditions for the normal and tangential stress fields as well as a Neumann-type boundary condition for scalar fluxes. Further, we examine the role of surface slopes in the predicted near-surface velocity and turbulence fields in surface gravity waves. By comparing the predicted velocity field in a deep-water standing wave in a closed basin, we show that the consistent boundary conditions do not generate unphysical vorticity at the free surface, in contrast to commonly used, simplified stress boundary conditions developed by ignoring all contributions except vertical shear in the transformation of stress terms. In addition, it is shown that the consistent boundary conditions significantly improve predicted wave shape, velocity and turbulence fields in regular surf zone breaking waves, compared with the simplified case. A more extensive model-data comparison of various breaking wave properties in different types of surface breaking waves is presented in companion papers (Derakhti et al., 2016a,b).

Modeling the water circulation in the North Atlantic in the scope of the CORE-II experiment

A numerical experiment on the reproduction of the variability in the state of North Atlantic water in 1948–2007 with a spatial resolution of 0.25° has been performed using the global ocean model developed at Institute of Numerical Mathematics, Russian Academy of Sciences (INM RAS), and the Shirshov Institute of Oceanology (IO RAS) (the INM–IO model). The data on the state of the atmosphere, radiation fluxes, and bulk formulas of the CORE-II protocol are used as boundary conditions. Five successive 60-year calculation cycles have been performed in order to obtain the quasi-equilibrium state of a model ocean. For the last 20 years, the main elements of large-scale ocean circulation have been analyzed and compared with the WOA09 atlas data and the results of other models.

Improvements in Land-Use Classification for Estimating Daytime Surface Temperatures and Sea-Breeze Flows in Southern California

Abstract Understanding the interactions between large-scale atmospheric and oceanic circulation patterns and changes in land cover and land use (LCLU) due to urbanization is a relevant subject in many coastal climates. Recent studies by Lebassi et al. found that the average maximum air temperatures during the summer in two populated California coastal areas decreased at low-elevation areas open to marine air penetration during the period of 1970–2005. This coastal cooling was attributed to an increase in sea-breeze activity. The aims of this work are to better understand the coastal flow patterns and sea–land thermal gradient by improving the land-cover classification scheme in the region using updated airborne remote sensing data and to assess the suitability of the updated regional atmospheric modeling system for representing maritime flows in this region. This study uses high-resolution airborne data from the NASA Hyperspectral Infrared Imager (HyspIRI) mission preparatory flight campaign over Southern California and surface ground stations to compare observations against model estimations. Five new urban land classes were created using broadband albedo derived from the Airborne Visible and Infrared Imaging Spectrometer (AVIRIS) sensor and then assimilated into the Weather Research and Forecasting (WRF) Model. The updated model captures the diurnal spatial and temporal sea-breeze patterns in the region. Results show notable improvements of simulated daytime surface temperature and coastal winds using the HyspIRI-derived products in the model against the default land classification, reaffirming the importance of accounting for heterogeneity of urban surface properties.