Person: Wunsch, Carl
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Wunsch
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Carl
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Wunsch, Carl
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Publication Exploratory Description of Low Frequency Ocean Temperature Variability and Its Global Structure(2019) Wunsch, CarlAn exploratory description is made of nearly global potential temperature variability from to months to 20 years using the field derived from the ECCOv4 state estimate. Relative to higher frequencies, longer periods do not exhibit simplification in the space-time structures. Frequency spectra at individual points are usefully described by a reduced form of a previously proposed analytic model. In contrast, the vertical structure of the variability at low frequencies--periods beyond 1 year---has a very complex form, with only a few global generalizations apparent. Meridional wavenumber spectra, reflecting the dominant zonality of oceanic low frequencies, are spatially comparatively simple, while the zonal wavenumber spectra are spatially complex and not very meaningful. The emergence of strong spatial structures at longer periods is consistent with the presence of complex time-mean (0 frequency) structures in bottom topography, sidewalls, and meteorological forcing.Publication Towards determining uncertainties in global oceanic mean values of heat, salt, and surface elevation(Stockholm University Press, 2018-01-01) Wunsch, CarlLower-bounds on uncertainties in oceanic data and a model are calculated for the 20-year time means and their temporal evolution for oceanic temperature, salinity, and sea surface height, during the data-dense interval 1994-2013. The essential step of separating stochastic from systematic or deterministic elements of the fields is explored by suppressing the globally correlated components of the fields. Justification lies in the physics and the brevity of a 20- year estimate relative to the full oceanic adjustment time, and the inferred near-linearity of response on short time intervals. Lower-bound uncertainties reflecting theonly stochastic elements of the state estimate are then calculated from bootstrap estimates. Trends are estimated as 2.2±0.2mm/y in elevation, 0.0011±00001◦C/y, and (-2.825±017) × 10−5y for surface elevation, temperature and salt, with formal 2-standard deviation uncertainties. The 2 temperature change correspnds to a 20-year average ocean heating rate of 048 ± 01 W/m of which 0.1W/m2 arises from the geothermal forcing. Systematic errors must be determined separately.Publication Dynamically and Kinematically Consistent Global Ocean Circulation and Ice State Estimates(Elsevier BV, 2013) Wunsch, Carl; Heimbach, PatrickThe World Ocean Circulation Experiment drove the development of estimates of the decadal scale time evolving general circulation that are dynamically and kinematically consistent. A long timescale, and a goal of estimation rather than prediction, preclude the use of meteorological methods called “data assimilation (DA).” Instead, “state estimation” methods are reviewed here and distinguished from DA. Results from the dynamically consistent family of solutions from the project Estimating the Circulation and Climate of the Ocean based upon least-squares Lagrange multipliers (adjoints) are used to discuss the determination of the dominant elements of the circulation in the period since 1992—which marked the beginning of the satellite altimetric record. Significant changes documented in the Arctic in recent decades now mandate consideration of the coupled ocean-cryospheric state.Publication Paleophysical Oceanography with an Emphasis on Transport Rates(Annual Reviews, 2010) Huybers, Peter; Wunsch, CarlPaleophysical oceanography is the study of the behavior of the fluid ocean of the past, with a specific emphasis on its climate implications, leading to a focus on the general circulation. Even if the circulation is not of primary concern, heavy reliance on deep-sea cores for past climate information means that knowledge of the oceanic state when the sediments were laid down is a necessity. Like the modern problem, paleoceanography depends heavily on observations, and central difficulties lie with the very limited data types and coverage that are, and perhaps ever will be, available. An approximate separation can be made into static descriptors of the circulation (e.g., its water-mass properties and volumes) and the more difficult problem of determining transport rates of mass and other properties. Determination of the circulation of the Last Glacial Maximum is used to outline some of the main challenges to progress. Apart from sampling issues, major difficulties lie with physical interpretation of the proxies, transferring core depths to an accurate timescale (the “age-model problem”), and understanding the accuracy of time-stepping oceanic or coupled-climate models when run unconstrained by observations. Despite the existence of many plausible explanatory scenarios, few features of the paleocirculation in any period are yet known with certainty.Publication Time Series Analysis. A Heuristic Primer(2010) Wunsch, CarlPublication Bidecadal Thermal Changes in the Abyssal Ocean(American Meteorological Society, 2014) Wunsch, Carl; Heimbach, PatrickA dynamically consistent state estimate is used for the period 1992–2011 to describe the changes in oceanic temperatures and heat content, with an emphasis on determining the noise background in the abyssal (below 2000 m) depths. Interpretation requires close attention to the long memory of the deep ocean, implying that meteorological forcing of decades to thousands of years ago should still be producing trendlike changes in abyssal heat content. Much of the deep-ocean volume remained unobserved. At the present time, warming is seen in the deep western Atlantic and Southern Oceans, roughly consistent with those regions of the ocean expected to display the earliest responses to surface disturbances. Parts of the deeper ocean, below 3600 m, show cooling. Most of the variation in the abyssal Pacific Ocean is comparatively featureless, consistent with the slow, diffusive approach to a steady state expected there. In the global average, changes in heat content below 2000 m are roughly 10% of those inferred for the upper ocean over the 20-yr period. A useful global observing strategy for detecting future change has to be designed to account for the different time and spatial scales manifested in the observed changes. If the precision estimates of heat content change are independent of systematic errors, determining oceanic heat uptake values equivalent to 0.1 W m−2 is possibly attainable over future bidecadal periods.Publication Ocean Mixing(Oxford University Press, 2017) Wunsch, CarlOceanic mixing is one of the major determinants of the ocean circulation and its climatological influences. Existing distributions of mixing properties determine the rates of storage and redistribution within the climate system of fundamental scalar tracers including heat, fresh water, oxygen, carbon, and others. Observations have overturned earlier concepts that mixing rates might be approximately uniform throughout the ocean volume, with profound implications for determining the circulation and its properties. Inferences about past and potential future oceanic circulations and the resulting climate influence require determination of changed energy inputs and the expected consequent adjustment of mixing processes and their influence.Publication Pore Fluids and the LGM Ocean Salinity–Reconsidered(Elsevier, 2015) Wunsch, CarlPore fluid chlorinity/salinity data from deep-sea cores related to the salinity maximum of the last glacial maximum (LGM) are analyzed using estimation methods deriving from linear control theory. With conventional diffusion coefficient values and no vertical advection, results show a very strong dependence upon initial conditions at -100 ky. Earlier inferences that the abyssal Southern Ocean was strongly salt-stratified in the LGM with a relatively fresh North Atlantic Ocean are found to be consistent within uncertainties of the salinity determination, which remain of order ±1 g/kg. However, an LGM Southern Ocean abyss with an important relative excess of salt is an assumption, one not required by existing core data. None of the present results show statistically significant abyssal salinity values above the global average, and results remain consistent, apart from a general increase owing to diminished sea level, with a more conventional salinity distribution having deep values lower than the global mean. The Southern Ocean core does show a higher salinity than the North Atlantic one on the Bermuda Rise at different water depths. Although much more sophisticated models of the pore-fluid salinity can be used, they will only increase the resulting uncertainties, unless considerably more data can be obtained. Results are consistent with complex regional variations in abyssal salinity during deglaciation, but none are statistically significant.Publication Last Glacial Maximum and deglacial abyssal seawater oxygen isotopic ratios(Copernicus GmbH, 2016) Wunsch, CarlAn earlier analysis of pore-water salinity (chlorinity) in two deep-sea cores, using terminal constraint methods of control theory, concluded that although a salinity amplification in the abyss was possible during the LGM, it was not required by the data. Here the same methodology is applied to δ18Ow in the upper 100 m of four deep-sea cores. An ice volume amplification to the isotopic ratio is, again, consistent with the data but not required by it. In particular, results are very sensitive, with conventional diffusion values, to the assumed initial conditions at −100 ky and a long list of noise (uncertainty) assumptions. If the calcite values of δ18O are fully reliable, then published enriched values of the ratio in seawater are necessary to preclude sub-freezing temperatures, but the seawater δ18O in pore fluids does not independently require the conclusion.Publication Climate Change as an Intergenerational Problem(National Academy of Sciences, 2013) Wunsch, Carl; Schmitt, Raymond W.; Baker, D. James