Revisiting the Meteor 1925-1927 hydrographic dataset reveals centennial full-depth changes in the Atlantic Ocean

The hydrographic data set of the German Atlantic Expedition (GAE) 1925-1927 is compared with the contemporary profiling float and ship-based hydrography to reveal full-depth changes in the Atlantic Ocean between 19°N and 64°S. The volume-mean warming over the last 80 years amounts to 0.119 ± 0.067°C, accompanied by an increase in salinity of 0.014 ± 0.010. A clear vertical structure of these changes is observed: on average, the ocean has warmed by 0.272 ± 0.093°C and became saltier by 0.030 ± 0.014 down to about 2000 m, but cooled and freshened slightly in the deeper layers. These changes can be traced throughout the whole hydrographic survey, indicating the basin-wide character of the observed changes on a centennial timescale. The observed warming is consistent with climate model simulations over the 20th century, suggesting an attribution to anthropogenic forcing. Comparison with the pre-GAE cruises reveals no discernible warming between the 1870s and 1906/1911. © 2013 American Geophysical Union. All Rights Reserved

World Ocean Circulation Experiment – Argo Global Hydrographic Climatology

The paper describes the new gridded World Ocean Circulation Experiment-Argo Global Hydrographic Climatology (WAGHC). The climatology has a 1∕4° spatial resolution resolving the annual cycle of temperature and salinity on a monthly basis. Two versions of the climatology were produced and differ with respect to whether the spatial interpolation was performed on isobaric or isopycnal surfaces, respectively. The WAGHC climatology is based on the quality controlled temperature and salinity profiles obtained before January 2016, and the average climatological year is in the range from 2008 to 2012.To avoid biases due to the significant step-like decrease of the data below 2&thinsp;km, the profile extrapolation procedure is implemented. We compare the WAGHC climatology to the 1∕4° resolution isobarically averaged WOA13 climatology, produced by the NOAA Ocean Climate Laboratory (Locarnini et al., 2013) and diagnose a generally good agreement between these two gridded products. The differences between the two climatologies are basically attributed to the interpolation method and the considerably extended data basis. Specifically, the WAGHC climatology improved the representation of the thermohaline structure, in both the data poor polar regions and several data abundant regions like the Baltic Sea, the Caspian sea, the Gulf of California, the Caribbean Sea, and the Weddell Sea. Further, the dependence of the ocean heat content anomaly (OHCA) time series on the baseline climatology was tested. Since the 1950s, both of the baseline climatologies produce almost identical OHCA time series. The gridded dataset can be found at https://doi.org/10.1594/WDCC/WAGHC_V1.0 (Gouretski, 2018).</p

Data and Services at the Integrated Climate Data Center (ICDC) at the University of Hamburg

KlimawandelEarth observation data obtained from remote sensing sensors and in-situ data archives are fundamental for our current understanding of the Earth’s climate system. Such data are an important pre-requisite for Earth System research and should be easy to access and easy to use. In addition such data should be quality assessed and attached with information about uncertainties and long-term stability. If these data sets are stored in a self-explanatory, easy-to-use format, their usefulness and scientific value increase. This is the guideline for the Integrated Climate Data Center (ICDC) at the Center for Earth System Research and Sustainability (CEN), University of Hamburg. ICDC offers a reliable, quick and easy data access along with expert support for users and data providers. The ICDC provides several types of worldwide accessible in situ and satellite Earth observation data of the atmosphere, ocean, land surface, and cryosphere via the web portal http://icdc.zmaw.de. Recently, data from socio-economic sciences have been integrated into ICDC’s data base to enhance interdisciplinary collaboration. On ICDC’s web portal, each data set has its own page. It contains the data access points, a short data description, information about spatiotemporal coverage and resolution, data quality, important reference documents and contacts, and about how to cite the data set. The data are converted into netCDF or ASCII format. Consistency and quality checks are carried out – often in the framework of international collaborations. Literature studies are conducted to learn about potential limitations or preferred application areas of the data offered. The data sets can be accessed through the web page via FTP, HTTP or OPeNDAP. Using the Live Access Server, users can visualize data as maps, along transects and profiles, zoom into key regions, and create time series. In both fields, visualization and data access, ICDC tries to provide fast response times and high reliability

An indicator of the multiple equilibria regime of the Atlantic meridional overturning circulation

Recent model results have suggested that there may be a scalar indicator ? monitoring whether the Atlantic meridional overturning circulation (MOC) is in a multiple equilibrium regime. The quantity ? is based on the net freshwater transport by the MOC into the Atlantic basin. It changes sign as soon as the steady Atlantic MOC enters the multiple equilibrium regime because of an increased freshwater input in the northern North Atlantic. This paper addresses the issue of why the sign of ? is such a good indicator for the multiple equilibrium regime. Changes in the Atlantic freshwater budget over a complete bifurcation diagram and in finite amplitude perturbation experiments are analyzed in a global ocean circulation model. The authors show that the net anomalous freshwater transport into or out of the Atlantic, resulting from the interactions of the velocity perturbations and salinity background field, is coupled to the background (steady state) state freshwater budget and hence to ?. The sign of ? precisely shows whether this net anomalous freshwater transport is stabilizing or destabilizing the MOC. Therefore, it can indicate whether the MOC is in a single or multiple equilibrium regime.<br/

An additional deep-water mass in Drake Passage as revealed by 3He data

We present 3He data froma repeat section across Drake Passage, fromthree sections off the South American continent in the Pacific, at 28?S, 35?S, and 43?S, and fromthree sections in the Atlantic, eastward of the Malvinas, close to 35?W, and near the Greenwich Meridian. In Drake Passage, a distinct high-3He signal is observed that is centered just above the boundary of the Lower and the Upper Circumpolar Deep Water (LCDW, UCDW), and is concentrated towards the northern continental slope. 3He concentrations in the Antarctic Circumpolar Current (ACC) upstream of Drake Passage (World Ocean Circulation Experiment section P19 at 88?W) are markedly lower than those found in Drake Passage, and a regional source of primordial helium in the path of the ACC that might cause the high-3He feature can be ruled out. We explain the feature by addition of high-3He waters present at the 43?S Pacific section. This supports a previous, similar interpretation of a low-salinity anomaly in Drake Passage (Naveira Garabato et al., Deep- Sea Research I 49 (2002) 681), that is strongly related to the high-3He feature. Employing multiparameter water mass analysis (including 3He as a parameter), we find that deep waters as met at the 43?S Pacific section, flowing south along the South American continental slope, contribute substantially to the ACC waters in Drake Passage (fractions exceed 50% locally). Lesser, but laterally more extended contributions are found east of the Malvinas, and still smaller ones are present at 35?W and at the Greenwich Meridian. Using velocity measurements from one of the two Drake Passage sections, we estimate the volume transport of these waters to be 7.071.2 Sv, but the average transport may be somewhat lower as the other realization had a less pronounced signal. The enhanced 3He signature in Drake Passage is essentially confined north of the Polar Front. Further downstreamthe signature crosses this front, to the extent that at 35?W the contributions south and north of it are of similar magnitude. At the same time, the 3He levels north of the front are reduced due to a substantial admixture of low-3He North Atlantic Deep Water, such that 3He becomes highest south of the front. The flow of Southeast Pacific deep slope waters entering the ACC constitutes the predominant exit pathway of the primordial helium released in the deep Pacific, and represents a considerable fraction of the deep water return flow fromthe Pacific into the ACC. Therefore and also because the density range of the added deep slope waters is intermediate between those of UCDW and LCDW, they must be considered a distinct water mass. r 2003 Elsevier Ltd. All rights reserved

Abyssal Atlantic circulation during the Last Glacial Maximum: Constraining the ratio between transport and vertical mixing

The ocean’s role in regulating atmospheric carbon dioxide on glacial‐interglacial 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 δ^(13)C of benthic foraminifera indicate the Atlantic Ocean was more chemically stratified during the LGM, but the nonconservative nature of δ^(13)C complicates interpretation of the LGM signal. Here we use benthic foraminiferal δ^(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 ocean’s ability to trap carbon during glacial times

Deoxygenation in the oxygen minimum zone of the eastern tropical North Atlantic

Observations and model results both indicate increasing oxygen minimum zones (OMZ) in the tropical oceans. Here we report on record low dissolved oxygen minimum concentrations in the eastern tropical North Atlantic in fall of 2008, with less than 40 mu mol kg(-1) in the core of the OMZ. There we find a deoxygenation rate of similar to 0.5 mu mol kg(-1) a(-1) during the last decades on two repeat sections at 7.5 and 11 degrees N. The potential temperature and salinity in the surface and central water layers increased on both sections compared to previous observations. However, in contrast to the oxygen decrease in the core of the OMZ, increasing oxygen concentrations were observed in the central water layer above the OMZ. The observed deoxygenation was thus restricted to the core of the oxygen minimum layer. It remains unclear whether the vertical expansion of the oxygen minimum represents a long time trend or decadal variation