Synte Peacock

The CCSM4 Ocean Component

AbstractThe ocean component of the Community Climate System Model version 4 (CCSM4) is described, and its solutions from the twentieth-century (20C) simulations are documented in comparison with observations and those of CCSM3. The improvements to the ocean model physical processes include new parameterizations to represent previously missing physics and modifications of existing parameterizations to incorporate recent new developments. In comparison with CCSM3, the new solutions show some significant improvements that can be attributed to these model changes. These include a better equatorial current structure, a sharper thermocline, and elimination of the cold bias of the equatorial cold tongue all in the Pacific Ocean; reduced sea surface temperature (SST) and salinity biases along the North Atlantic Current path; and much smaller potential temperature and salinity biases in the near-surface Pacific Ocean. Other improvements include a global-mean SST that is more consistent with the present-day observa...

Twentieth-century oceanic carbon uptake and storage in CESM1(BGC)

AbstractOcean carbon uptake and storage simulated by the Community Earth System Model, version 1–Biogeochemistry [CESM1(BGC)], is described and compared to observations. Fully coupled and ocean-ice configurations are examined; both capture many aspects of the spatial structure and seasonality of surface carbon fields. Nearly ubiquitous negative biases in surface alkalinity result from the prescribed carbonate dissolution profile. The modeled sea–air CO2 fluxes match observationally based estimates over much of the ocean; significant deviations appear in the Southern Ocean. Surface ocean pCO2 is biased high in the subantarctic and low in the sea ice zone. Formation of the water masses dominating anthropogenic CO2 (Cant) uptake in the Southern Hemisphere is weak in the model, leading to significant negative biases in Cant and chlorofluorocarbon (CFC) storage at intermediate depths. Column inventories of Cant appear too high, by contrast, in the North Atlantic. In spite of the positive bias, this marks an im...

Transit-Time Distributions in a Global Ocean Model

Abstract Results from a simulation of the ocean “transit-time distribution” (“TTD”) for global and regional ocean surface boundary conditions are presented based on a 5000-yr integration using the Parallel Ocean Program ocean general circulation model. The TTD describes the probability that water at a given interior point in the ocean was at some point on the ocean surface a given amount of time ago. It is shown that the spatial distribution of ocean TTDs can be understood in terms of conventional wisdom regarding time scales and pathways of the ventilated thermocline and the thermohaline circulation–driven deep-ocean circulation. The true mean age from the model (the first moment of the TTD) is demonstrated to be very large everywhere, because of very long-tailed distributions. Regional TTD distributions are presented for distinct surface boundary subregions, and it is shown how these can help in the interpretation of the global TTD. The spatial structure of each regional TTD is shown to become essential...

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.

Projected Twenty-First-Century Changes in Temperature, Precipitation, and Snow Cover over North America in CCSM4

AbstractResults from a suite of ensembles of twenty-first-century climate projections made using the Community Climate System Model, version 4 (CCSM4) are analyzed to document model bias and to explore possible future changes in air temperature, precipitation, and snow cover over North America. Large biases still exist in all analyzed fields in this version of the model, and the necessary assumption in future climate projections is therefore that the bias persists into the future, such that the differences in a field between two time periods are meaningful indications of potential changes. Projected temperature increases show strong regional patterns with spatial similarities for all the emissions scenarios considered, although there are considerable differences in the magnitude of the projected change. Projections indicate an increase in total precipitation over much of North America for all emissions scenarios, with the exception of the Southwest United States. All of North America except parts of north...

Recent changes in the air-sea gas exchange of methyl chloroform

Atmospheric measurements of methyl chloroform provide important constraints on the rate of oxidation of hydrocarbons in Earths atmosphere. Estimates of the loss of methyl chloroform to the oceans play a small but important role in these calculations. Here, we examine the ocean-atmosphere interaction of methyl chloroform in a global ocean model. Contrary to previous assumptions, these simulations suggest that the high-latitude oceans are currently a source of this chemical to the atmosphere. If confirmed, this finding alters estimates of the change in the atmospheric oxidation rate of hydrocarbons. We highlight the potential usefulness of methyl chloroform as a tracer of ocean circulation.

Kuroshio pathways in a climatologically forced model

A high-resolution ocean model forced with an annually repeating atmosphere is used to examine variability of the Kuroshio, the western boundary current in the North Pacific Ocean. A large meander (LM) in the path of the Kuroshio south of Japan develops and disappears in a highly bimodal fashion on decadal timescales. The modeled meander is comparable in timing and spatial extent to an observed feature in the region. Various characteristics of the LM are examined, including relative vorticity, transport, and velocity shear. The many similarities between the model and observations indicate that the meander results from intrinsic oceanic variability, which is represented in this climatologically forced model. Each LM is preceded by a smaller “trigger” meander that originates at the south end of Kyushu, moves up the coast, and develops into the LM. However, there are also many meanders very similar in character to the trigger meander that do not develop into LMs. Formation of an LM only occurs when a deep anticyclone associated with the trigger meander forms near Koshu Seamount. Furthermore, the major axis of that deep anticyclone must be oriented away from the coast, rather than alongshore. In the specific case of interaction of a trigger meander with a deep anticyclone with major axis oriented away from the coastline, LM formation occurs.

A dynamic-flow carbon-cycle box model and high-latitude sensitivity

Most of the hypotheses put forward to explain glacial–interglacial cycles in atmospheric pCO2 are centred on Southern-Ocean-based mechanisms. This is in large part because: (1) timing constraints rule out changes in the North Atlantic as the trigger; (2) the concept of “high-latitude sensitivity” eliminates changes in the non-polar oceans as likely contenders. Many of the Southern-Ocean-based mechanisms for changing atmospheric pCO2 on glacial–interglacial time-scales are based on results from highly simplified box models with prescribed flow fields and fixed particulate flux. It has been argued that box models are significantly more “high-latitude sensitive” than General Circulation Models. In light of this, it is important to understand whether this high-latitude sensitivity is a feature common to all box models, and whether the apparent degree of sensitivity changes for different tracers and parameters. We introduce a new metric for assessing how “high-latitude sensitive” a particular solution is to perturbations. With this metric, we demonstrate that a given model may be high-latitude sensitive to certain parameters but not to others. We find that the incorporation of a dynamic-based flow field and a Michaelis–Menten type nutrient feedback can have a significant impact on the apparent sensitivity of the model to perturbations. The implications of this for current box-model-based estimates of atmospheric pCO2 drawdown are discussed.

Subtropical Mode Water Variability in a Climatologically Forced Model in the Northwestern Pacific Ocean

AbstractA 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, ...

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.