Geochemical proxies of ocean circulation and weathering inputs: Radiogenic isotopes of Nd, Pb, Sr, Hf, and Os

Marine records of the radiogenic isotope composition of the elements neodymium (Nd), lead (Pb), hafnium (Hf), strontium (Sr), and osmium (Os) allow the reconstruction of past ontinental weathering inputs on different time scales as a function of their respective oceanic residence times. Sr and Os have oceanic residence times significantly longer than the global mixing time of the ocean and are efficiently mixed on a global scale. Their isotope composition changes on long time scales as a function of plate tectonics and major orogenies, which allows their use as precise stratigraphic tools for the entire Phanerozoic. In contrast, Hf, Pb, and in particular Nd, have residence times on the order of or shorter than the global mixing time of the ocean, which results in distinct isotopic signatures of water masses and allows the reconstruction of past water mass mixing and weathering inputs on both long and short time scales. Here applications of these isotopes systems with a focus on the shorter residence time tracers are reviewed (without claiming to be comprehensive) and problems and potential solutions are discussed. Keywords: Radiogenic isotopes, paleo-oceanography, ocean circulation, water mass mixing, continental weatherin

Wind Forced Variability in Eddy Formation, Eddy Shedding, and the Separation of the East Australian Current

The East Australian Current (EAC), like many other subtropical western boundary currents, is believed to be penetrating further poleward in recent decades. Previous observational and model studies have used steady state dynamics to relate changes in the westerly winds to changes in the separation behavior of the EAC. As yet, little work has been undertaken on the impact of forcing variability on the EAC and Tasman Sea circulation. Here using an eddy‐permitting regional ocean model, we present a suite of simulations forced by the same time‐mean fields, but with different atmospheric and remote ocean variability. These eddy‐permitting results demonstrate the nonlinear response of the EAC to variable, nonstationary inhomogeneous forcing. These simulations show an EAC with high intrinsic variability and stochastic eddy shedding. We show that wind stress variability on time scales shorter than 56 days leads to increases in eddy shedding rates and southward eddy propagation, producing an increased transport and southward reach of the mean EAC extension. We adopt an energetics framework that shows the EAC extension changes to be coincident with an increase in offshore, upstream eddy variance (via increased barotropic instability) and increase in subsurface mean kinetic energy along the length of the EAC. The response of EAC separation to regional variable wind stress has important implications for both past and future climate change studies

Surface salinity of the North Atlantic : can we reconstruct its fluctuations over the last one hundred years ?

Surface samples have been collected in the North Atlantic in the past one hundred years for determining the ocean salinity and its temperature. A large share of the data we have used were collected by merchant vessels of weather ships of European countries and to a large extent are listed in reports, in particular in the "Bulletin Hydrographique". We investigate whether these data are relevant for determining low frequency fluctuations of the sea surface salinity. We find many crossing in the 1920s for which salinity is anomalously high compared with the climatology or with other crossings collected on the same ship line. These anomalies are indicative of a contamination of the sample. By examining hydrographic data, reports and recent experience in collectionand storage in sea water, we can attribute these large errors to unclean buckets where salt crystals dissolve into the sample and to breathing of the samples during the storage. Each of these stages contributes in estimating a too large salinity and adds to the scatter of the measurements. (D'après résumé d'auteur

Model simulations on the long-term dispersal of 137Cs released into the Pacific Ocean off Fukushima

A sequence of global ocean circulation models, with horizontal mesh sizes of 0.5°, 0.25° and 0.1°, are used to estimate the long-term dispersion by ocean currents and mesoscale eddies of a slowly decaying tracer (half-life of 30 years, comparable to that of 137Cs) from the local waters off the Fukushima Dai-ichi Nuclear Power Plants. The tracer was continuously injected into the coastal waters over some weeks; its subsequent spreading and dilution in the Pacific Ocean was then simulated for 10 years. The simulations do not include any data assimilation, and thus, do not account for the actual state of the local ocean currents during the release of highly contaminated water from the damaged plants in March–April 2011. An ensemble differing in initial current distributions illustrates their importance for the tracer patterns evolving during the first months, but suggests a minor relevance for the large-scale tracer distributions after 2–3 years. By then the tracer cloud has penetrated to depths of more than 400 m, spanning the western and central North Pacific between 25°N and 55°N, leading to a rapid dilution of concentrations. The rate of dilution declines in the following years, while the main tracer patch propagates eastward across the Pacific Ocean, reaching the coastal waters of North America after about 5–6 years. Tentatively assuming a value of 10 PBq for the net 137Cs input during the first weeks after the Fukushima incident, the simulation suggests a rapid dilution of peak radioactivity values to about 10 Bq m−3 during the first two years, followed by a gradual decline to 1–2 Bq m−3 over the next 4–7 years. The total peak radioactivity levels would then still be about twice the pre-Fukushima values

Marine biogeochemical responses to the North Atlantic Oscillation in a coupled climate model

In this study a coupled ocean-atmosphere model containing interactive marine biogeochemistry is used to analyze interannual, lagged, and decadal marine biogeochemical responses to the North Atlantic Oscillation (NAO), the dominant mode of North Atlantic atmospheric variability. The coupled model adequately reproduces present-day climatologies and NAO atmospheric variability. It is shown that marine biogeochemical responses to the NAO are governed by different mechanisms according to the time scale considered. On interannual time scales, local changes in vertical mixing, caused by modifications in air-sea heat, freshwater, and momentum fluxes, are most relevant in influencing phytoplankton growth through light and nutrient limitation mechanisms. At subpolar latitudes, deeper mixing occurring during positive NAO winters causes a slight decrease in late winter chlorophyll concentration due to light limitation and a 10%–20% increase in spring chlorophyll concentration due to higher nutrient availability. The lagged response of physical and biogeochemical properties to a high NAO winter shows some memory in the following 2 years. In particular, subsurface nutrient anomalies generated by local changes in mixing near the American coast are advected along the North Atlantic Current, where they are suggested to affect downstream chlorophyll concentration with 1 year lag. On decadal time scales, local and remote mechanisms act contemporaneously in shaping the decadal biogeochemical response to the NAO. The slow circulation adjustment, in response to NAO wind stress curl anomalies, causes a basin redistribution of heat, freshwater, and biogeochemical properties which, in turn, modifies the spatial structure of the subpolar chlorophyll bloom

The role of the basic state in the ENSO-monsoon relationship and implications for predictability

The impact of systematic model errors on a coupled simulation of the Asian Summer monsoon and its interannual variability is studied. Although the mean monsoon climate is reasonably well captured, systematic errors in the equatorial Pacific mean that the monsoon-ENSO teleconnection is rather poorly represented in the GCM. A system of ocean-surface heat flux adjustments is implemented in the tropical Pacific and Indian Oceans in order to reduce the systematic biases. In this version of the GCM, the monsoon-ENSO teleconnection is better simulated, particularly the lag-lead relationships in which weak monsoons precede the peak of El Nino. In part this is related to changes in the characteristics of El Nino, which has a more realistic evolution in its developing phase. A stronger ENSO amplitude in the new model version also feeds back to further strengthen the teleconnection. These results have important implications for the use of coupled models for seasonal prediction of systems such as the monsoon, and suggest that some form of flux correction may have significant benefits where model systematic error compromises important teleconnections and modes of interannual variability

Variability of coastal and ocean water temperature in the upper 700 m along the western Iberian Peninsula from 1975 to 2006

Temperature is observed to have different trends at coastal and ocean locations along the western Iberian Peninsula from 1975 to 2006, which corresponds to the last warming period in the area under study. The analysis was carried out by means of the Simple Ocean Data Assimilation (SODA). Reanalysis data are available at monthly scale with a horizontal resolution of 0.5° × 0.5° and a vertical resolution of 40 levels, which allows obtaining information beneath the sea surface. Only the first 21 vertical levels (from 5.0 m to 729.35 m) were considered here, since the most important changes in heat content observed for the world ocean during the last decades, correspond to the upper 700 m. Warming was observed to be considerably higher at ocean locations than at coastal ones. Ocean warming ranged from values on the order of 0.3 °C dec(-1) near surface to less than 0.1 °C dec(-1) at 500 m, while coastal warming showed values close to 0.2 °C dec(-1) near surface, decreasing rapidly below 0.1 °C dec(-1) for depths on the order of 50 m. The heat content anomaly for the upper 700 m, showed a sharp increase from coast (0.46 Wm(-2)) to ocean (1.59 Wm(-2)). The difference between coastal and ocean values was related to the presence of coastal upwelling, which partially inhibits the warming from surface of near shore water.publishe

Reduced Antarctic meridional overturning circulation reaches the North Atlantic Ocean

Author Posting. © American Geophysical Union, 2008. This article is posted here by permission of American Geophysical Union for personal use, not for redistribution. The definitive version was published in Geophysical Research Letters 35 (2008): L22601, doi:10.1029/2008GL035619.We analyze abyssal temperature data in the western North Atlantic Ocean from the 1980s–2000s, showing that reductions in Antarctic Bottom Water (AABW) signatures have reached even that basin. Trans-basin oceanographic sections occupied along 52°W from 1983–2003 and 66°W from 1985–2003 quantify abyssal warming resulting from deepening of the strong thermal boundary between AABW and North Atlantic Deep Water (NADW), hence a local AABW volume reduction. Repeat section data taken from 1981–2004 along 24°N also show a reduced zonal gradient in abyssal temperatures, consistent with decreased northward transport of AABW. The reduction in the Antarctic limb of the MOC within the North Atlantic highlights the global reach of climate variability originating around Antarctica.NOAA and NSF supported the 2003 U.S. CLIVAR/CO2 Repeat Hydrography Program reoccupations of the 52 W and 66 W sections, led by Chief Scientists John Toole and Terrence Joyce, respectively. The U.K. National Environment Research Council supported the 2004 reoccupation of the 24 N section, led by Chief Scientist Stuart Cunningham. The hard work of all contributing to the collection and processing of data analyzed here is gratefully acknowledged. The NOAA Office of Oceanic and Atmospheric Research and the NOAA Climate Program Office supported the analysis