Large-scale Ocean Research Articles

Intraseasonal Sea Level Variability in the Persian Gulf

AbstractSatellite observations are used to establish the dominant magnitudes, scales, and mechanisms of intraseasonal variability in ocean dynamic sea level (ζ) in the Persian Gulf over 2002–15. Empirical orthogonal function (EOF) analysis applied to altimetry data reveals a basinwide, single-signed intraseasonal fluctuation that contributes importantly to ζ variance in the Persian Gulf at monthly to decadal time scales. An EOF analysis of Gravity Recovery and Climate Experiment (GRACE) observations over the same period returns a similar large-scale mode of intraseasonal variability, suggesting that the basinwide intraseasonal ζ variation has a predominantly barotropic nature. A linear barotropic theory is developed to interpret the data. The theory represents Persian Gulf average ζ () in terms of local freshwater flux, barometric pressure, and wind stress forcing, as well as ζ at the boundary in the Gulf of Oman. The theory is tested using a multiple linear regression with these freshwater flux, barometric pressure, wind stress, and boundary ζ quantities as input and as output. The regression explains 70% ± 9% (95% confidence interval) of the intraseasonal variance. Numerical values of regression coefficients computed empirically from the data are consistent with theoretical expectations from first principles. Results point to a substantial nonisostatic response to surface loading. The Gulf of Oman ζ boundary condition shows lagged correlation with ζ upstream along the Indian subcontinent, Maritime Continent, and equatorial Indian Ocean, suggesting a large-scale Indian Ocean influence on intraseasonal variation mediated by coastal and equatorial waves and hinting at potential predictability. This study highlights the value of GRACE for understanding sea level in an understudied marginal sea.

Strong Solvability of a Variational Data Assimilation Problem for the Primitive Equations of Large-Scale Atmosphere and Ocean Dynamics

For the primitive equations of large-scale atmosphere and ocean dynamics, we study the problem of determining by means of a variational data assimilation algorithm initial conditions that generate strong solutions which minimize the distance to a given set of time-distributed observations. We suggest a modification of the adjoint algorithm whose novel elements is to use norms in the variational cost functional that reflects the H^1-regularity of strong solutions of the primitive equations. For such a cost functional, we prove the existence of minima and a first-order adjoint condition for strong solutions that provides the basis for computing these minima. We prove the local convergence of a gradient-based descent algorithm to optimal initial conditions using the second-order adjoint primitive equations. The algorithmic modifications due to the H^1-norms are straightforwardly to implement into a variational algorithm that employs the standard L^2-metrics.

Evaluating the impact of Mediterranean overflow on the large-scale Atlantic Ocean circulation using neodymium isotopic composition

Palaeo-oceanographical archives in the Mediterranean Sea indicate the occurrence of Sapropel depositions, which are characteristic of anoxic bottom conditions and of a stratified sea. The impact of such drastic changes in the thermohaline circulation of the Mediterranean Sea on the large-scale circulations in the North Atlantic basin is not fully understood. Here we evaluate the impact of a direct perturbation of the Mediterranean Sea circulation through different idealized simulations with freshwater release rates of 20, 50, 100, and 200 mSv (1 mSv = 103 m3/s), using a the IPSL-CM5A-LR global coupled atmosphere-ocean model, with simulations up to 1000 years long. The simulations are compared to available data for the Mediterranean Sea and the North Atlantic. The Neodymium isotopic composition (εNd) anomalies in the North Atlantic are clearly dependent on the rate of fresh water release in the Mediterranean Sea, and subsequent modification of Atlantic meridional overturning circulation (AMOC) intensity and subpolar gyre (SPG) extension. Based on comparison with seawater εNd records from Rockall Trough, we suggest that a limited release of freshwater around 50 mSv may be more representative of hydrological conditions of the Mediterranean Sea during the Sapropel S1 deposition. Our results indicate that a decrease in Mediterranean overflow was clearly a factor in the intensification of upper AMOC following the Early Holocene period, when a large eastward expansion of the subpolar gyre is simulated. Also, eastward expansion of the subpolar gyre due to freshwater release in the Mediterranean (e.g. Sapropel-like events) and its impact on the εNd signature highlight the fact that the modification of ocean circulation in the North Atlantic basin might be more complex than a straightforward change in the AMOC, since there may also be a large impact on barotropic circulation. The simulations also produce a significant decrease in the εNd values of the Iceland-Scotland Overflow Water (between −3.5 and − 1.3 εNd unit), which is consistent with changes observed during the S1 event in the reconstructions. Thus the 3-dimensional changes in baroclinic and barotropic circulations of the ocean, as reconstructed in some εNd records, are broadly consistent with the sensitivity simulations performed, and might thus be partly related to changes in Mediterranean overflow, which would have contributed to modification of the Atlantic Ocean circulation following a complex 3-dimensional pattern that is detailed in the paper.

CAFE60v1: A 60-year large ensemble climate reanalysis. Part II: Evaluation

AbstractThe CSIRO Climate retrospective Analysis and Forecast Ensemble system: version 1 (CAFE60v1) provides a large (96 member) ensemble retrospective analysis of the global climate system from 1960 to present with sufficiently many realizations and at spatio-temporal resolutions suitable to enable probabilistic climate studies. Using a variant of the ensemble Kalman filter, 96 climate state estimates are generated over the most recent six decades. These state estimates are constrained by monthly mean ocean, atmosphere and sea ice observations such that their trajectories track the observed state while enabling estimation of the uncertainties in the approximations to the retrospective mean climate over recent decades. For the atmosphere, we evaluate CAFE60v1 in comparison to empirical indices of the major climate teleconnections and blocking with various reanalysis products. Estimates of the large scale ocean structure, transports and biogeochemistry are compared to those derived from gridded observational products and climate model projections (CMIP). Sea ice (extent, concentration and variability) and land surface (precipitation and surface air temperatures) are also compared to a variety of model and observational products. Our results show that CAFE60v1 is a useful, comprehensive and unique data resource for studying internal climate variability and predictability, including the recent climate response to anthropogenic forcing on multi-year to decadal time scales.

Contribution of Ocean Physics and Dynamics at Different Scales to Heat Uptake in Low-Resolution AOGCMs

AbstractUsing an ensemble of atmosphere–ocean general circulation models (AOGCMs) in an idealized climate change experiment, this study quantifies the contributions to ocean heat uptake (OHU) from ocean physical parameterizations and resolved dynamical processes operating at different scales. Analysis of heat budget diagnostics reveals a leading-order global heat balance in the subsurface upper ocean in a steady state between the large-scale circulation warming it and mesoscale processes cooling it, and shows that there are positive contributions from processes on all scales to the subsurface OHU during climate change. There is better agreement among the AOGCMs in the net OHU than in the individual scales/processes contributing to it. In the upper ocean and at high latitudes, OHU is dominated by small-scale diapycnal processes. Below 400 m, OHU is dominated by the superresidual transport, representing large-scale ocean dynamics combined with all parameterized mesoscale and submesoscale eddy effects. Weakening of the AMOC leads to less heat convergence in the subpolar North Atlantic and less heat divergence at lower latitudes, with a small overall effect on the net Atlantic heat content. At low latitudes, the dominance of advective heat redistribution is contrary to the diffusive OHU mechanism assumed by the commonly used upwelling-diffusion model. Using a density water-mass framework, it is found that most of the OHU occurs along isopycnal directions. This feature of OHU is used to accurately reconstruct the global vertical ocean warming profile from the surface heat flux anomalies, supporting advective (rather than diffusive) models of OHU and sea level rise.

RETRACTED ARTICLE: Ocean dynamic equations with the real gravity

Two different treatments in ocean dynamics are found between the gravity and pressure gradient force. Vertical component is 5–6 orders of magnitude larger than horizontal components for the pressure gradient force in large-scale motion, and for the gravity in any scale motion. The horizontal pressure gradient force is considered as a dominant force in oceanic motion from planetary to small scales. However, the horizontal gravity is omitted in oceanography completely. A non-dimensional C number (ratio between the horizontal gravity and the Coriolis force) is used to identify the importance of horizontal gravity in the ocean dynamics. Unexpectedly large C number with the global mean around 24 is obtained using the community datasets of the marine geoid height and ocean surface currents. New large-scale ocean dynamic equations with the real gravity are presented such as hydrostatic balance, geostrophic equilibrium, thermal wind, equipotential coordinate system, and vorticity equation.

Dynamics of the three-dimensional primitive equations of large-scale atmosphere

The main objective of this paper is to study the long-time behavior of weak solutions for the three-dimensional primitive equations of large-scale ocean. Due to the low regularity of the triple nonlinearity term, it is very difficult to obtain the uniqueness of weak solutions such that we can not study the long-time behavior of weak solutions by using the classical theory of infinite-dimensional dynamical systems. Inspired by the idea of ℓ-trajectory in [1], we introduce a new phase space (see Section 3.1 for the definition of ) on which the translation semigroup can be imposed naturally. We first establish the finite-dimensional global attractor in by using the classical theory of infinite-dimensional dynamical systems, via a Lipschitz continuous mapping from into H(see Section 2 for the definition of H), we obtain the corresponding finite-dimensional global attractor in H.

Reduced Order Models for the Quasi-Geostrophic Equations: A Brief Survey

Reduced order models (ROMs) are computational models whose dimension is significantly lower than those obtained through classical numerical discretizations (e.g., finite element, finite difference, finite volume, or spectral methods). Thus, ROMs have been used to accelerate numerical simulations of many query problems, e.g., uncertainty quantification, control, and shape optimization. Projection-based ROMs have been particularly successful in the numerical simulation of fluid flows. In this brief survey, we summarize some recent ROM developments for the quasi-geostrophic equations (QGE) (also known as the barotropic vorticity equations), which are a simplified model for geophysical flows in which rotation plays a central role, such as wind-driven ocean circulation in mid-latitude ocean basins. Since the QGE represent a practical compromise between efficient numerical simulations of ocean flows and accurate representations of large scale ocean dynamics, these equations have often been used in the testing of new numerical methods for ocean flows. ROMs have also been tested on the QGE for various settings in order to understand their potential in efficient numerical simulations of ocean flows. In this paper, we survey the ROMs developed for the QGE in order to understand their potential in efficient numerical simulations of more complex ocean flows: We explain how classical numerical methods for the QGE are used to generate the ROM basis functions, we outline the main steps in the construction of projection-based ROMs (with a particular focus on the under-resolved regime, when the closure problem needs to be addressed), we illustrate the ROMs in the numerical simulation of the QGE for various settings, and we present several potential future research avenues in the ROM exploration of the QGE and more complex models of geophysical flows.

Optimal distributed control of the three dimensional primitive equations of large-scale ocean and atmosphere dynamics

<p style='text-indent:20px;'>The main objective of this paper is to study the optimal distributed control of the three dimensional non-autonomous primitive equations of large-scale ocean and atmosphere dynamics. We apply the well-posedness and regularity results of solutions for this system as well as some abstract results from the nonlinear functional analysis to establish the existence of optimal controls as well as the first-order necessary optimality condition for an associated optimal control problem in which a distributed control is applied to the temperature.</p>

243 years of reconstructed streamflow volume and identification of extreme hydroclimatic events in the Conchos River Basin, Chihuahua, Mexico

The extreme hydroclimatic events in the Conchos River Basin, are strongly associated with ocean–atmospheric phenomena such as the El Nino Southern Oscillation, Pacific Decadal Oscillation, and Atlantic Multidecadal Oscillation, the streamflow reconstruction showed the occurrence of up to 10-year droughts. Dendrochronology is a modern science used to study hydroclimatic events through the measurement and standardization of tree-rings. Long-term dendrochronological series contribute to the improvement of our knowledge of past climatic events and provide information for wise management of water resources. The objective of this study was to generate an actualized streamflow volume reconstruction with a regional chronology and identify extreme hydroclimatic events in the Conchos River Basin. We analyzed the hydroclimatic behavior of the streamflow volume through a dendrochronological network in the Basin. The regional earlywood chronology was analyzed by single correlation and principal component analysis, which allowed obtaining the transfer model to determine extreme hydroclimatic events and defined the influence of large-scale ocean–atmospheric phenomena. Nine earlywood series showed a common variance and responded significantly (r = 0.68, n = 67, p < 0.05) to the water volume accumulated in the November–June period measured the water inflow to the Boquilla Dam. From that correlation, a 243-year (1775–2017) streamflow reconstruction was made. The hydrological analysis of the reconstruction showed the occurrence of up to 10-year long drought—from 1948 to 1957—and a period with maximum moisture between 1983 and 1994 with a probability of occurrence of 4.16% (24 year frequency). We found significant correlations between the reconstructed streamflow volumes and the Southern Oscillation Index (r = 0.52, p < 0.05, n = 203); Multivariate El Nino Southern Oscillation Index (r = 0.32, p < 0.05, n = 68), the Pacific Decadal Oscillation (r = 0.39, p < 0.05, n = 118), and the Atlantic Multidecadal Oscillation (0.32, p < 0.05, n = 70). The observed correlations with the drought indices such as the Palmer Drought Severity Index and Standardized Precipitation Evapotranspiration Index were also significant (r = 0.75, r = 0.77, p < 0.05, respectively). Our historical reconstruction of streamflow in the Conchos River basin contributes to a better understanding of the frequency of extreme hydroclimatic events affecting water availability in this binational basin.

Right Place, Right Time: An Informal Memoir.

My career spanned the revolution in understanding of the large-scale fluid ocean, as modern electronics produced vast new capabilities. I started in the days of almost purely mechanical instruments operated by seagoing scientists, ones not so different from those used more than a century earlier. Elegant theories existed of hypothetical steady-state oceans. Today, we understand that the ocean is a highly turbulent fluid, interacting over global scales, and it is now studied by large teams using spacecraft and diverse sets of self-contained in situ instrumentation. Mine was an accidental career: I was lucky to be in the right place at the right time.

Life history of an anticyclonic eddy in the Algerian basin from altimetry data, tracking algorithm and in situ observations.

Abstract Frequently-forming long-life mesoscale eddies are observed in the Algerian Basin that influence the circulation of the wider Western Mediterranean Sea. Most of these structures store and transport water masses including associated physical and biological properties throughout the entire basin. In order to study the evolution of a long-life anticyclonic eddy, we use a multiplatform approach based on remote sensing data analysis and in situ measurements. We present a case study of an anticyclonic eddy that persisted for 17 months within the basin. The feature was identified through a hybrid method of eddy detection and tracking applied to altimetry data, and sampled twice during two different oceanographic cruises in autumn 2004 and late spring 2005. Transect observations of potential temperature, salinity, density and the dissolved oxygen concentration were utilised to infer water mass properties and eddy characteristics. In situ data show that water of Atlantic origin, initially trapped by the eddy during its formation, was modified during the eddy lifetime. The time evolution of radius, kinetic energy and vorticity suggests dividing the eddy lifetime into three phases: the eddy formation, an intermediate period of high variability and a final, lower energy phase. The track followed by the eddy confirms the hypothesis of an interaction with the North Balearic Front and a consequent change of the eddy's physical properties. Decomposing the eddy's kinetic energy into mean and fluctuating terms allows us to describe its interaction with the mean circulation of the basin. In particular during formation, the southern part of the eddy absorbs energy from the mean circulation, while it provides energy to the mean flow in its northern part. In the second phase, when the eddy is far from the coast, it receives energy from the mean flow. Combining in situ data analysis with the results of the satellite-imagery-based detection and tracking method has proven to be a very useful method in assessing the evolution of a mesoscale structure in the Algerian Basin and its interaction with the large scale ocean dynamics of the Western Mediterranean Sea.

Projected slow down of South Indian Ocean circulation

Using an ensemble of 28 climate models, we examine hindcasts and ‘business as usual’ future changes to large-scale South Indian Ocean dynamics. We compare model ensemble seasonal-to-annual volume transports to observations and explore drivers of past and future circulation variability and change. Off the west coast of Australia, models consistently project a weakening of the Leeuwin Current and Undercurrent due to reduced onshore flow and downwelling. The reduced onshore flow is related to changes in the alongshore pressure gradient. While the alongshore pressure gradient change is consistent with the Indonesian Throughflow projected weakening, we found no inter-model relationship between these changes. In the south-western Indian Ocean, the models project a robust weakening of the North East and South East Madagascar Currents, Agulhas Current and transport through the Mozambique Channel. This reduced Indian Ocean western boundary flow is partly associated with a weaker Indonesian Throughflow and overturning circulation, where the latter is related to a decrease in the convergence of deep Southern Ocean waters into the Indian Ocean. In contrast to the weakening of other features, the westward flowing Agulhas Current extension south of Africa is projected to strengthen, which is consistent with an intensification of the Antarctic Circumpolar Current.

Ergodicity of two-dimensional primitive equations of large scale ocean in geophysics driven by degenerate noise

The current paper is devoted to ergodicity of the stochastic two-dimensional primitive equations of large scale ocean in geophysics driven by degenerate noise. The central challenge is to prove the asymptotically strong Feller property of the probability transition semigroups.

Influence of Track Changes on the Poleward Shift of LMI Location of Western North Pacific Tropical Cyclones

AbstractThe annual mean latitude at which tropical cyclones (TCs) reach their lifetime maximum intensity (LMI) over the western North Pacific Ocean basin has shifted northward since the early 1980s, and it is suggested that the shift is due to the northward migration of the mean TC formation location. In this study, the TC intensity is simulated with an intensity model to assess the historical records of TC intensity. During the period 1980–2015, the simulated poleward trend in the mean latitude of LMI is 0.44° (10 yr)−1, which agrees well with the one [0.48° (10 yr)−1] derived from the Joint Typhoon Warning Center (JTWC) dataset. This suggests that the observed poleward trend in the mean latitude of LMI is physically consistent with changes in the large-scale ocean–atmosphere environment and TC track. This study also demonstrates that the temporal change in the environmental parameters (sea surface temperature, outflow temperature, vertical wind shear, and ocean mixed layer depth) has little influence on the observed shift of the mean LMI latitude. The poleward migration of the mean LMI latitude is mainly due to the TC track shift, which results primarily from the change in the large-scale steering flow.

Guest Editorial: Relationships Between Coastal Sea Level and Large-Scale Ocean Circulation

Guest Editorial: Relationships Between Coastal Sea Level and Large-Scale Ocean Circulation

Approximation of stationary statistical properties of the three-dimensional primitive equations of large-scale ocean and atmosphere dynamics

The main objective of this paper is to study the temporal approximation scheme of the three-dimensional primitive equations of large-scale ocean and atmosphere dynamics. We first prove the existence of a global attractor in V by using the Sobolev compactness embedding theorem, and then we prove its global attractor and stationary statistical properties of the temporal approximation scheme convergence to those of the three-dimensional primitive equations of large-scale ocean and atmosphere dynamics as the time step goes to zero.

Synchronous Rendezvous for Networks of Marine Robots in Large Scale Ocean Monitoring

We develop a synchronous rendezvous strategy for a network of minimally actuated mobile sensors or active drifters to monitor a set of Lagrangian Coherent Structure (LCS) bounded regions, each exhibiting gyre-like flows. This paper examines the conditions under which a pair of neighboring agents achieves synchronous rendezvous relying solely on the inherent flow dynamics within each LCS bounded region. The objective is to enable drifters in adjacent LCS bounded regions to rendezvous in a periodic fashion to exchange and fuse sensor data. We propose an agent-level control strategy to regulate the drifter speed in each monitoring region as well as to maximize the time the drifters are connected and able to communicate at every rendezvous. The strategy utilizes minimal actuation to ensure synchronization between neighboring pairs of drifters to ensure periodic rendezvous. The intermittent synchronization policy enables a locally connected network of minimally actuated mobile sensors to converge to a common orbit frequency. Robustness analysis against possible disturbance in practice and simulations are provided to illustrate the results.

Tracking the volatile and magmatic history of Vesta from chromium stable isotope variations in eucrite and diogenite meteorites

Although Solar System bodies exhibit large variations in their volatile element abundances, the mechanisms and conditions that lead to these variations remain ambiguous. The howardite-eucrite-diogenite (HED) meteorites that likely sample the asteroid 4 Vesta, provide evidence for extensive volatile depletion on their parent body. Isotopic variations in moderately volatile elements, such as Zn, have been used to track the origin of such volatile loss. Although not nominally volatile, Cr is useful because it has several oxidized gas species that render it volatile under the oxidizing conditions that characterize planetary accretion. As such, volatile loss of Cr has the potential to produce an isotopically light evaporation residue under an equilibrium regime. This contrasts with other moderately volatile elements that show heavy isotope enrichments in the residue following both kinetic or equilibrium fractionation. Here, we report the Cr stable isotope composition of 11 eucrites and four diogenites. The eucrites possess systematically lighter Cr isotope compositions than diogenites, which is onset by the accumulation of isotopically heavy Cr3+-rich orthopyroxene and spinel in diogenites during their magmatic evolution. We estimate for the primary eucrite melt with Mg# ≈ 50, a δ53Cr (53Cr/52Cr deviation relative to NIST SRM 979 in per mile) of −0.22 ± 0.03‰ (2SD), lighter than any chondritic meteorite group by ∼0.1‰. This deficit may result from either partial melting with residual Cr3+-bearing phases (e.g. chromite) that retain heavy isotopes, or from vapor loss that occurred at equilibrium with a magma ocean on Vesta. Isotopic fractionation during partial melting would necessitate implausibly high Cr contents in the Vestan mantle, and oxygen fugacities high enough to stabilize chromite in the mantle source. Isotopic fractionation during evaporation would require an oxidized vapor and a reduced residue, as predicted by thermodynamic constraints on the composition of the vapor phase above a silicate magma ocean. Therefore, this Cr isotopic deficit between Vesta and chondrites may be caused by Cr loss at relatively high oxygen fugacity in a gas phase at equilibrium with the liquid from which it evolved. Temperatures of volatile loss are estimated to be lower than 2300 K, consistent with loss from a large-scale magma ocean model for formation of Vesta, which may be a common evolutionary stage in accreting planetesimals.

Equilibrium simulations of Marine Isotope Stage 3 climate

Abstract. An equilibrium simulation of Marine Isotope Stage 3 (MIS3) climate with boundary conditions characteristic of Greenland Interstadial 8 (GI-8; 38 kyr BP) is carried out with the Norwegian Earth System Model (NorESM). A computationally efficient configuration of the model enables long integrations at relatively high resolution, with the simulations reaching a quasi-equilibrium state after 2500 years. We assess the characteristics of the simulated large-scale atmosphere and ocean circulation, precipitation, ocean hydrography, sea ice distribution, and internal variability. The simulated MIS3 interstadial near-surface air temperature is 2.9 ∘C cooler than the pre-industrial (PI). The Atlantic meridional overturning circulation (AMOC) is deeper and intensified by ∼13 %. There is a decrease in the volume of Antarctic Bottom Water (AABW) reaching the Atlantic. At the same time, there is an increase in ventilation of the Southern Ocean, associated with a significant expansion of Antarctic sea ice and concomitant intensified brine rejection, invigorating ocean convection. In the central Arctic, sea ice is ∼2 m thicker, with an expansion of sea ice in the Nordic Seas during winter. Attempts at triggering a non-linear transition to a cold stadial climate state, by varying atmospheric CO2 concentrations and Laurentide Ice Sheet height, suggest that the simulated MIS3 interstadial state in the NorESM is relatively stable, thus underscoring the role of model dependency, and questioning the existence of unforced abrupt transitions in Greenland climate in the absence of interactive ice sheet–meltwater dynamics.