201 research outputs found
Abyssal Atlantic circulation during the Last Glacial Maximum: Constraining the ratio between transport and vertical mixing
The oceanâs role in regulating atmospheric carbon dioxide on glacialâinterglacial timescales remains an
unresolved issue in paleoclimatology. Reduced mixing between deep water masses may have aided oceanic
storage of atmospheric CO_2 during the Last Glacial Maximum (LGM), but data supporting this idea have
remained elusive. The ÎŽ^(13)C of benthic foraminifera indicate the Atlantic Ocean was more chemically
stratified during the LGM, but the nonconservative nature of ÎŽ^(13)C complicates interpretation of the LGM
signal. Here we use benthic foraminiferal ÎŽ^(18)O as a conservative tracer to constrain the ratio of meridional
transport to vertical diffusivity in the deep Atlantic. Our calculations suggest that the ratio was at least twice
as large at the LGM. We speculate that the primary cause was reduced mixing between northern and
southern component waters, associated with movement of this water mass boundary away from the zone of
intense mixing near the seafloor. The shallower water mass boundary yields an order of magnitude increase
in the volume of southern component water, suggesting its residence time may have increased substantially.
Our analysis supports the idea that an expanded volume of Antarctic Bottom Water and limited vertical
mixing enhanced the abyssal oceanâs ability to trap carbon during glacial times
The Southwest Pacific Ocean circulation and climate experiment (SPICE) : report to CLIVAR SSG
The Southwest Pacific Ocean Circulation and Climate Experiment (SPICE) is an international research program under the auspices of CLIVAR. The key objectives are to understand the Southwest Pacific Ocean circulation and the South Pacific Convergence Zone (SPCZ) dynamics, as well as their influence on regional and basin-scale climate patterns. South Pacific thermocline waters are transported in the westward flowing South Equatorial Current (SEC) toward Australia and Papua-New Guinea. On its way, the SEC encounters the numerous islands and straits of the Southwest Pacific and forms boundary currents and jets that eventually redistribute water to the equator and high latitudes. The transit in the Coral, Solomon, and Tasman Seas is of great importance to the climate system because changes in either the temperature or the amount of water arriving at the equator have the capability to modulate the El Nino-Southern Oscillation, while the southward transports influence the climate and biodiversity in the Tasman Sea. After 7 years of substantial in situ oceanic observational and modeling efforts, our understanding of the region has much improved. We have a refined description of the SPCZ behavior, boundary currents, pathways, and water mass transformation, including the previously undocumented Solomon Sea. The transports are large and vary substantially in a counter-intuitive way, with asymmetries and gating effects that depend on time scales. This paper provides a review of recent advancements and discusses our current knowledge gaps and important emerging research directions
Effects of salt compensation on the climate model response in simulations of large changes of the Atlantic meridional overturning circulation
© 2007 American Meteorological Society (https://www.ametsoc.org/ams/index.cfm/publications/ethical-guidelines-and-ams-policies/ams-copyright-policy/
Potential climatic transitions with profound impact on Europe
We discuss potential transitions of six climatic subsystems with large-scale impact on Europe, sometimes denoted as tipping elements. These are the ice sheets on Greenland and West Antarctica, the Atlantic thermohaline circulation, Arctic sea ice, Alpine glaciers and northern hemisphere stratospheric ozone. Each system is represented by co-authors actively publishing in the corresponding field. For each subsystem we summarize the mechanism of a potential transition in a warmer climate along with its impact on Europe and assess the likelihood for such a transition based on published scientific literature. As a summary, the âtippingâ potential for each system is provided as a function of global mean temperature increase which required some subjective interpretation of scientific facts by the authors and should be considered as a snapshot of our current understanding. <br/
Environmental and biological controls on Mg and Li in deep-sea scleractinian corals
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 Earth and Planetary Science Letters 300 (2010): 215-225, doi:10.1016/j.epsl.2010.09.029.Deep-sea scleractinian corals precipitate aragonite skeletons that provide valuable
archives of past ocean conditions. During calcification biological mediation causes variability in
trace metal incorporation and isotopic ratios of the aragonite such that signals caused by
environmental controls can be overwhelmed. This complicates the interpretation of geochemical
proxies used for paleo-reconstructions. In this study we examine the environmental controls on
the Mg/Li ratio of 34 individuals from seven genera of deep-sea scleractinian corals:
Desmophyllum, Balanophyllia, Caryophyllia, Enallopsammia, Flabellum, Trochocyanthus, and
Lophelia. In addition we examine the distributions of Mg and Li in Desmophyllum and
Balanophyllia using laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS).
Both Mg/Ca and Li/Ca ratios increased by more than a factor of 2 in the center of
calcification regions compared to the outer, fibrous regions of the coral skeleton. As a result,
replicate ~10 mg subsamples of coral show less variability in the Mg/Li ratio than Mg/Ca.
Microscale Mg and Li results are consistent with Rayleigh-type incorporation of trace metals
with additional processes dominating composition within centers of calcification. Comparison of
Mg/Li to seawater properties near the site of collection shows that the ratio is not controlled by
either carbonate ion or salinity. It appears that temperature is the major control on the Mg/Li
ratio. For all 34 samples the temperature correlation (R2=0.62) is significantly better than for
Mg/Ca (R2=0.06). For corals of the family Caryophyllidae the R2 value increases to 0.82 with
the exclusion of one sample that was observed to have an altered, chalky texture. Despite this
excellent correlation the scatter in the data suggests that the Mg/Li ratio of deep-sea corals
cannot be used to reconstruct temperature to better than approximately ±1.6°C without better
temperature control and additional calibration points on modern coral samples.Financial Support was provided by the USGS WHOI Co-operative agreement, NSF-ANT grant numbers 0636787 and 80295700 and the WHOI Ocean Life Institute. David Case was supported by the WHOI Summer Student
Fellowship
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Advances in reconstructing the AMOC using sea surface observations of salinity
The Atlantic meridional overturning circulation (AMOC) is one of the main drivers of climate variability at decadal and longer time scales. As there are no direct multi-decadal observations of this key circulation, the reconstruction of past AMOC variations is essential. This work presents a step forward in reconstructing the AMOC using climate models and time-varying surface nudging of salinity and temperature data, for which independent multi-decadal observed series are available. A number of nudging protocols are explored in a perfect model framework to best reproduce the AMOC variability accommodating to the characteristics of SST and SSS available products. As reference SST products with sufficient space and time coverage are available, we here choose to focus on the limitations associated to SSS products with the goal of providing protocols using independent salinity products. We consider a global gridded dataset and, additionally, a coarser SSS dataset restricted to the Atlantic and with a quite low spatial resolution (order of 10 degrees vs. 2 for the model grid). We show how, using the latter, we can improve the efficiency of the nudging on the AMOC reconstruction by adding a high-resolution annual cycle to the coarse resolution SSS product as well as a spatial downscaling to account for SSS gradient. The final protocol retained for the coarse SSS data is able to reconstruct a 100-year long AMOC period (average of 10.18 Sv and a standard deviation of 1.39 Sv), with a correlation of 0.76 to the target and a RMSE of 0.99 Sv. These values can be respectively compared to 0.85 and 0.75 Sv when using the global salinity surface observations. This work provides a first step towards understanding the limitations and prospects of historical AMOC reconstructions using different sea surface salinity datasets for the surface nudging
A modeling study of the seasonal oxygen budget of the global ocean
Author Posting. © American Geophysical Union, 2007. This article is posted here by permission of American Geophysical Union for personal use, not for redistribution. The definitive version was published in Journal of Geophysical Research 112 (2007): C05017, doi:10.1029/2006JC003731.An ecosystem model embedded in a global ocean general circulation model is used to quantify roles of biological and physical processes on seasonal oxygen variations. We find that the thermally induced seasonal net outgassing (SNO) of oxygen is overestimated by about 30% if gas phase equilibrium is assumed, and we find that seasonal variations in thermocline oxygen due to biology are approximated well using the oxygen anomaly. Outside the tropics and the north Indian Ocean, biological SNO is, on average, 56% of net community production (defined as net oxygen production above 76 m) during the outgassing period and 35% of annual net community production. In the same region the seasonal drawdown of the oxygen anomaly within the upper thermocline (76â500 m) is 76% of the remineralization during the drawdown and 48% of annual remineralization. Applying model-derived relationships to observed O2 climatologies and using independent estimates for tropical and monsoonal systems, we estimate global net community production to be 14.9 ± 2.5 Pg C yrâ1.R.N., X.J., and F.L. were supported from the
following grants: NOAA NA16GP2987, NASA NAG5-6451, and NSF
OCE-9711937
Linking mixing processes and climate variability to the heat content distribution of the Eastern Mediterranean abyss
The heat contained in the ocean (OHC) dominates the Earthâs energy budget and hence represents a fundamental parameter for understanding climate changes. However, paucity of observational data hampers our knowledge on OHC variability, particularly in abyssal areas. Here, we analyze water characteristics, observed during the last three decades in the abyssal Ionian Sea (Eastern Mediterranean), where two competing convective sources of bottom water exist. We find a heat storage of ~1.6 W/m2â twice that assessed globally in the same period â exceptionally well-spread throughout the local abyssal layers. Such an OHC accumulation stems from progressive warming and salinification of the Eastern Mediterranean, producing warmer near-bottom waters. We analyze a new process that involves convectively-generated waters reaching the abyss as well as the triggering of a diapycnal mixing due to rough bathymetry, which brings to a warming and thickening of the bottom layer, also influencing water-column potential vorticity. This may affect the prevailing circulation, altering the local cyclonic/anticyclonic long-term variability and hence precondition future water-masses formation and the redistribution of heat along the entire water-column
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An ensemble of eddy-permitting global ocean reanalyses from the MyOcean project
A set of four eddy-permitting global ocean reanalyses produced in the framework of the MyOcean project have been compared over the altimetry period 1993â2011. The main differences among the reanalyses used here come from the data assimilation scheme implemented to control the ocean state by inserting reprocessed observations of sea surface temperature (SST), in situ temperature and salinity profiles, sea level anomaly and sea-ice concentration. A first objective of this work includes assessing the interannual variability and trends for a series of parameters, usually considered in the community as essential ocean variables: SST, sea surface salinity, temperature and salinity averaged over meaningful layers of the water column, sea level, transports across pre-defined sections, and sea ice parameters. The eddy-permitting nature of the global reanalyses allows also to estimate eddy kinetic energy. The results show that in general there is a good consistency between the different reanalyses. An intercomparison against experiments without data assimilation was done during the MyOcean project and we conclude that data assimilation is crucial for correctly simulating some quantities such as regional trends of sea level as well as the eddy kinetic energy. A second objective is to show that the ensemble mean of reanalyses can be evaluated as one single system regarding its reliability in reproducing the climate signals, where both variability and uncertainties are assessed through the ensemble spread and signal-to-noise ratio. The main advantage of having access to several reanalyses differing in the way data assimilation is performed is that it becomes possible to assess part of the total uncertainty. Given the fact that we use very similar ocean models and atmospheric forcing, we can conclude that the spread of the ensemble of reanalyses is mainly representative of our ability to gauge uncertainty in the assimilation methods. This uncertainty changes a lot from one ocean parameter to another, especially in global indices. However, despite several caveats in the design of the multi-system ensemble, the main conclusion from this study is that an eddy-permitting multi-system ensemble approach has become mature and our results provide a first step towards a systematic comparison of eddy-permitting global ocean reanalyses aimed at providing robust conclusions on the recent evolution of the oceanic state
The role of salinity in the decadal variability of the North Atlantic meridional overturning circulation
Author Posting. © The Author(s), 2009. This is the author's version of the work. It is posted here by permission of Springer for personal use, not for redistribution. The definitive version was published in Climate Dynamics 33 (2009): 777-793, doi:10.1007/s00382-008-0523-2.An OGCM hindcast is used to investigate the linkages between North Atlantic Ocean
salinity and circulation changes during 1963â2003. The focus is on the eastern subpolar
region consisting of the Irminger Sea and the eastern North Atlantic where a careful
assessment shows that the simulated interannual to decadal salinity changes in the upper
1500 m reproduce well those derived from the available record of hydrographic
measurements. In the model, the variability of the Atlantic meridional overturning
circulation (MOC) is primarily driven by changes in deep water formation taking place in
the Irminger Sea and, to a lesser extent, the Labrador Sea. Both are strongly influenced by
the North Atlantic Oscillation (NAO). The modeled interannual to decadal salinity changes
in the subpolar basins are mostly controlled by circulation-driven anomalies of freshwater
flux convergence, although surface salinity restoring to climatology and other boundary
fluxes each account for approximately 25% of the variance. The NAO plays an important
role: a positive NAO phase is associated with increased precipitation, reduced northward
salt transport by the wind-driven intergyre gyre, and increased southward flows of
freshwater across the Greenland-Scotland ridge. Since the NAO largely controlled deep
convection in the subpolar gyre, fresher waters are found near the sinking region during
convective events. This markedly differs from the active influence on the MOC that salinity
exerts at decadal and longer timescales in most coupled models. The intensification of the
MOC that follows a positive NAO phase by about 2 years does not lead to an increase in
the northward salt transport into the subpolar domain at low frequencies because it is
cancelled by the concomitant intensification of the subpolar gyre which shifts the subpolar
front eastward and reduces the northward salt transport by the North Atlantic Current
waters. This differs again from most coupled models, where the gyre intensification
precedes that of the MOC by several years.Support from NSF Grant
82677800 with the Woods Hole Oceanographic Institution, and (to CF) from the Institut
universitaire de France and European FP6 project DYNAMITE (contract 003903-GOCE)
and (to JD) from the NOAA Office of Hydrologic Development through a scientific
appointment administered by UCAR is gratefully acknowledged
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