Subtropical mode water variability in a climatologically forced model in the northwestern Pacific Ocean

Author Posting. © American Meteorological Society, 2012. This article is posted here by permission of American Meteorological Society for personal use, not for redistribution. The definitive version was published in Journal of Physical Oceanography 42 (2012): 126–140, doi:10.1175/2011JPO4513.1.A climatologically forced high-resolution model is used to examine variability of subtropical mode water (STMW) in the northwestern Pacific Ocean. Despite the use of annually repeating atmospheric forcing, significant interannual to decadal variability is evident in the volume, temperature, and age of STMW formed in the region. This long time-scale variability is intrinsic to the ocean. The formation and characteristics of STMW are comparable to those observed in nature. STMW is found to be cooler, denser, and shallower in the east than in the west, but time variations in these properties are generally correlated across the full water mass. Formation is found to occur south of the Kuroshio Extension, and after formation STMW is advected westward, as shown by the transport streamfunction. The ideal age and chlorofluorocarbon tracers are used to analyze the life cycle of STMW. Over the full model run, the average age of STMW is found to be 4.1 yr, but there is strong geographical variation in this, from an average age of 3.0 yr in the east to 4.9 yr in the west. This is further evidence that STMW is formed in the east and travels to the west. This is qualitatively confirmed through simulated dye experiments known as transit-time distributions. Changes in STMW formation are correlated with a large meander in the path of the Kuroshio south of Japan. In the model, the large meander inhibits STMW formation just south of Japan, but the export of water with low potential vorticity leads to formation of STMW in the east and an overall increase in volume. This is correlated with an increase in the outcrop area of STMW. Mixed layer depth, on the other hand, is found to be uncorrelated with the volume of STMW.E.M.D. acknowledges support of the Doherty Foundation and National Science Foundation (OCE-0849808). S.R.J was sponsored by the National Science Foundation (OCE-0849808). Participation of S.P. and F.B. was supported by the National Science Foundation by its sponsorship of the National Center for Atmospheric Research.2012-07-0

On the possible long-term fate of oil released in the Deepwater Horizon incident, estimated using ensembles of dye release simulations

We have conducted an ensemble of 20 simulations using a high resolution global ocean model in which dye was continuously injected at the site of the Deepwater Horizon drilling rig for two months. We then extended these simulations for another four months to track the dispersal of the dye in the model. We have also performed five simulations in which dye was continuously injected at the site of the spill for four months and then run them out to one year from the initial spill date. The experiments can elucidate the approximate timescales and space scales of dispersal of polluted waters and also give a quantitative estimate of the dilution rate. Given the uncertainty in rates of chemical or biological degradation for oil or an oil–dispersant mixture, we do not include a decay term for the dye. Thus, these results should be considered an absolute upper bound on the possible spatial extent of the dispersal of oil or oil–dispersant mixture. The model results indicate that it is likely that oil-polluted waters from the Deepwater Horizon incident will, at some time over the six months following the initial spill date, be transported at relatively low concentrations over a significant part of the North-West Atlantic Ocean. However, this does not imply that oil will reach the eastern shores of North America, or that it will even be detectable. We present probabilities for the transport timescales and estimates of ensemble mean arrival times, and we briefly discuss the likely dispersion timescales and pathways of dye released in the subsurface ocean

Impact of eddy–wind interaction on eddy demographics and phytoplankton community structure in a model of the North Atlantic Ocean

Author Posting. © The Author(s), 2010. This is the author's version of the work. It is posted here by permission of Elsevier B.V. for personal use, not for redistribution. The definitive version was published in Dynamics of Atmospheres and Oceans 52 (2011): 80-94, doi:10.1016/j.dynatmoce.2011.01.003.Two eddy-resolving (0.1-degree) physical-biological simulations of the North Atlantic Ocean are compared, one with the surface momentum flux computed only from wind velocities and the other using the difference between air and ocean velocity vectors. This difference in forcing has a significant impact on the intensities and relative number of different types of mesoscale eddies in the Sargasso Sea. Eddy/wind interaction significantly reduces eddy intensities and increases the number of mode-water eddies and “thinnies” relative to regular cyclones and anticyclones; it also modifies upward isopycnal displacements at the base of the euphotic zone, increasing them in the centers of mode water eddies and at the edges of cyclones, and decreasing them in the centers of cyclones. These physical changes increase phytoplankton growth rates and biomass in mode-water eddies, bringing the biological simulation into better agreement with field data. These results indicate the importance of including the eddy/wind interaction in simulations of the physics and biology of eddies in the subtropical North Atlantic. However, eddy intensities in the simulation with eddy/wind interaction are lower than observed, which suggests a decrease in horizontal viscosity or an increase in horizontal grid resolution will be necessary to regain the observed level of eddy activity.LAA and DJM gratefully acknowledge the support of NASA grant 07-CARBON07-17. SCD and IDL gratefully acknowledge support from the NSF Center for Microbial Oceanography, Research and Education (C-MORE; NSF EF-0424599)

The DOE E3SM Coupled Model Version 1: Overview and Evaluation at Standard Resolution

This work documents the first version of the U.S. Department of Energy (DOE) new Energy Exascale Earth System Model (E3SMv1). We focus on the standard resolution of the fully coupled physical model designed to address DOE mission-relevant water cycle questions. Its components include atmosphere and land (110-km grid spacing), ocean and sea ice (60 km in the midlatitudes and 30 km at the equator and poles), and river transport (55 km) models. This base configuration will also serve as a foundation for additional configurations exploring higher horizontal resolution as well as augmented capabilities in the form of biogeochemistry and cryosphere configurations. The performance of E3SMv1 is evaluated by means of a standard set of Coupled Model Intercomparison Project Phase 6 (CMIP6) Diagnosis, Evaluation, and Characterization of Klima simulations consisting of a long preindustrial control, historical simulations (ensembles of fully coupled and prescribed SSTs) as well as idealized CO2 forcing simulations. The model performs well overall with biases typical of other CMIP-class models, although the simulated Atlantic Meridional Overturning Circulation is weaker than many CMIP-class models. While the E3SMv1 historical ensemble captures the bulk of the observed warming between preindustrial (1850) and present day, the trajectory of the warming diverges from observations in the second half of the twentieth century with a period of delayed warming followed by an excessive warming trend. Using a two-layer energy balance model, we attribute this divergence to the model’s strong aerosol-related effective radiative forcing (ERFari+aci = -1.65 W/m2) and high equilibrium climate sensitivity (ECS = 5.3 K).Plain Language SummaryThe U.S. Department of Energy funded the development of a new state-of-the-art Earth system model for research and applications relevant to its mission. The Energy Exascale Earth System Model version 1 (E3SMv1) consists of five interacting components for the global atmosphere, land surface, ocean, sea ice, and rivers. Three of these components (ocean, sea ice, and river) are new and have not been coupled into an Earth system model previously. The atmosphere and land surface components were created by extending existing components part of the Community Earth System Model, Version 1. E3SMv1’s capabilities are demonstrated by performing a set of standardized simulation experiments described by the Coupled Model Intercomparison Project Phase 6 (CMIP6) Diagnosis, Evaluation, and Characterization of Klima protocol at standard horizontal spatial resolution of approximately 1° latitude and longitude. The model reproduces global and regional climate features well compared to observations. Simulated warming between 1850 and 2015 matches observations, but the model is too cold by about 0.5 °C between 1960 and 1990 and later warms at a rate greater than observed. A thermodynamic analysis of the model’s response to greenhouse gas and aerosol radiative affects may explain the reasons for the discrepancy.Key PointsThis work documents E3SMv1, the first version of the U.S. DOE Energy Exascale Earth System ModelThe performance of E3SMv1 is documented with a set of standard CMIP6 DECK and historical simulations comprising nearly 3,000 yearsE3SMv1 has a high equilibrium climate sensitivity (5.3 K) and strong aerosol-related effective radiative forcing (-1.65 W/m2)Peer Reviewedhttps://deepblue.lib.umich.edu/bitstream/2027.42/151288/1/jame20860_am.pdfhttps://deepblue.lib.umich.edu/bitstream/2027.42/151288/2/jame20860.pd

Lagrangian flow in the California Undercurrent, an observation and model comparison

During the period 1992-1998, 38 isobaric RAFOS floats wre deployed to sample the subsurface flow of the California Undercurrent. The deployments, released over the California contintenal slope west of San Francisco, have sampled robust year-round poleward subsurface associated with the Undercurrent most seasons and the combined inshore current and Undercurrent in winter. Two other types of flow have been seen: a region of weak flow with little net displacement just west of the California Undercurrent, and an active westward propagating eddy field. This eddy field appears to be the primary mechanism for moving floats ffrom the Undercurrent into the ocean interior. The observations and statistics from the RAFOS floats are compared with Lagrangian estimates of particles tracked in a global high resolution ocean simulation in order to evaluate the fidelity of the model along an eastern boundary. The results show that the model reproduces the general character of the flow reasonably well, but underestimates both the mean and eddy energies by a substantial amount

A new look at ocean ventilation time scales and their uncertainties

A suite of eddy‐resolving ocean transient tracer model simulations are first compared to observations. Observational and model pCFC‐11 ages agree quite well, with the eddy‐resolving model adding detail. The CFC ages show that the thermocline is a barrier to interior ocean exchange with the atmosphere on time scales of 45 years, the measureable CFC transient, although there are exceptions. Next, model simulations are used to quantify effects on tracer ages of the spatial dependence of internal ocean tracer variability due to stirring from eddies and biases from nonstationarity of the atmospheric transient when there is mixing. These add to tracer age uncertainties and biases, which are large in frontal boundary regions, and small in subtropical gyre interiors. These uncertainties and biases are used to reinterpret observed temporal trends in tracer‐derived ventilation time scales taken from observations more than a decade apart, and to assess whether interpretations of changes in tracer ages being due to changes in ocean ventilation hold water. For the southern hemisphere subtropical gyres, we infer that the rate of ocean ventilation 26–27.2 σθ increased between the mid‐1990s and the decade of the 2000s. However, between the mid‐1990s and the decade of the 2010s, there is no significant trend—perhaps except for South Atlantic. Observed age/AOU/ventilation changes are linked to a combination of natural cycles and climate change, and there is regional variability. Thus, for the future it is not clear how strong or steady in space and time ocean ventilation changes will be. Key Points Eddy‐resolving simulations quantifying ocean variability and biases from tracers are used to reinterpret temporal trends in ventilation Ocean ventilation increased in southern subtropical gyres between mid‐1990s and 2000s, while between mid‐1990s and 2010s there was no trend Observed age/ventilation changes are linked to a combination of natural cycles and climate change and there is regional variabilit

Global eddy-resolving ocean simulations driven by 1985-1995 atmospheric winds

Results are presented from a high‐resolution global ocean model that is driven through three decadal cycles of increasingly realistic prescribed atmospheric forcing from the period 1985–1995. The model used (the Parallel Ocean Program) is a z level primitive equation model with active thermohaline dynamics based on the formulation of Bryan [1969] rewritten for massively parallel computers. Improvements to the model include an implicit free‐surface formulation of the barotropic mode [Dukowicz and Smith, 1994] and the use of pressure averaging for increasing the numerical time step. This study extends earlier 0.5° simulations of Semtner and Chervin [1992] to higher horizontal resolution with improved treatments of ocean geometry and surface forcing. The computational grid is a Mercator projection covering the global ocean from 77°N to 77°S and has 20 vertical levels. Three successive simulations have been performed on the CM‐5 Connection Machine system at Los Alamos using forcing fields from the European Centre for Medium‐Range Weather Forecasts (ECMWF). The first run uses monthly wind stresses for 1985–1995 and restoring of surface temperature and salinity to the Levitus [1982] seasonal climatology. The second run is the same but with 3 day‐averaged rather than monthly averaged wind stress fields, and the third is the same as the second but uses the monthly climatological ECMWF heat fluxes of Barnier et al. [1995] instead of restoring to climatological sea surface temperatures. Many features of the wind‐driven circulation are well represented in the model solutions, such as the overall current patterns, the numerous regions of hydrodynamic instability which correspond to those observed by satellite altimetry, and the filamented structure of the Antarctic Circumpolar Current. However, some features such as the separation points of the Gulf Stream and Kuroshio and the transport through narrow passages such as the Florida Straits are clearly inaccurate and indicate that still higher resolution may be required to correct these deficiencies. Water mass properties and some aspects of the thermohaline circulation are also not always well reproduced, which is partly due to the relatively short length of the integrations. The use of the ECMWF heat fluxes, rather than restoring to climatological surface temperatures, leads to stronger and more realistic surface and deep western boundary currents (primarily in the Atlantic) as well as more realistic meridional heat transport; this is primarily because the equilibrium meridional heat transport implied by the ECMWF surface fluxes is quite large. The ECMWF heat fluxes also produce improved seasonal cycles of sea surface temperature and height in both the northern and southern hemispheres. The 3‐day wind forcing gives rise to modes of model variability that are clearly seen in synoptic observations, such as the large‐scale 20–100‐day oscillations seen in the TOPEX/POSEIDON data, which are barotropic oscillations induced by the high‐frequency wind forcing. Additional studies on other aspects of the simulations described here are being conducted by a variety of investigators, and some of these are briefly described

Simulated Lagrangian pathways between the Leeuwin Current System and the upper-ocean circulation of the southeast Indian Ocean

The Leeuwin Current System, along the west Australian coast (22°S–34°S), forms a unique but poorly understood eastern boundary regime in which tropical waters flow poleward. Here we depict the three-dimensional paths connecting this eastern boundary system with the upper-ocean large-scale circulation around Australia based on selected trajectories from an online numerical particle tracking performed during the 1993/1997 integration of the 0.28° Los Alamos National Laboratory Parallel Ocean Program model. The simulated trajectories reveal a wealth of details about the regional circulation that are difficult to understand from observed and model Eulerian data alone. They reveal links between the Leeuwin Current, Leeuwin Undercurrent, Eastern Gyral Current, and zonal flows within the Subtropical Gyre. New findings include: a remote tropical source of the Leeuwin Current in the equatorial Indian Ocean, via the South Java Current; inshore (along the southern part of the North West Shelf) and offshore routes in the Indo-Australian Basin feeding the Leeuwin Current; strong exchange between the Leeuwin Undercurrent and adjacent Subtropical Gyre through a series of near surface eastward jets and deeper westward jets; and the tropical origin of the Eastern Gyral Current as a recirculation of the South Equatorial Current. We propose a current schematic summarising the links between the meridional boundary flows off Western Australia and the larger-scale circulation