Previous works show that wind forcing is the main source of circulation seasonal variability in the Rı ´o de la Plata estuary, located on the southeastern coast of South America. Wind forcing exceeds by far the role of fresh water discharges. However, due to a lack of enough observations, the features and causes of surface wind variability are not well understood yet. Therefore this paper presents a comprehensive study of surface wind variability over the Rı ´o de la Plata estuary using the National Center for Environmental Prediction-National Center for Atmospheric Research (NCEP/NCAR) reanalysis data between 1948 and 1997. It is expected that this study contributes to better understand, model and thus predict the estuary circulation.An onshore to offshore rotation characterizes the seasonal variations of the surface winds from summer to winter. A linear trend analysis shows a displacement of the summer-winter seasonal features to earlier months. On interannual time-scales, the first leading pattern describes east-west changes of surface winds that seems to be forced by the quasibiennial tropospheric oscillation excited in the western tropical Pacific and previously identified by many authors. The conditions over the South Atlantic and in particular the Rı ´o de la Plata are influenced by such oscillation through an atmospheric Rossby wave train propagating out of the tropics. This result is very important for its implication on the predictability levels in the region. The second leading mode is associated with anticyclonic/cyclonic wind rotations off the estuary on interannual times scales which are related with changes in both atmospheric and oceanic surface conditions at Southern Hemispher high-latitudes.
Existing studies on recent global warming are almost exclusively based on environmental data from the Earth's surface. Seasonal information on the effects of climate change on subsurface settings of mid to high latitudes is extremely scarce. Here, we present the first temperature proxy record from bottom (c. 50 m) water settings of the North Sea employing the oxygen isotope composition of ocean quahog shells. Results indicate that δ 18 O aragonite measured across shells of Arctica islandica can provide reliable estimates (±0.25 to ±0.4°C) of the ambient bottom water temperatures. Over the period AD 1880-2001, warming trends in bottom waters are of the order of 0.042 to 0.138°C/decade. Apparently, the annual maximum-temperature trend shows a twofold increase over the past four decades (0.236°C/decade) while the minimum-temperature trend has remained relatively stable (0.042°C/decade). During the same time interval, however, annual maximum temperatures at the sea surface quadrupled. Shell oxygen-isotope-derived winter temperatures also provide a proxy for the winter North Atlantic oscillation index (WNAO). Some 28 to 50% of the variability in minimum temperatures below the thermocline can be explained by changes of the WNAO. Our new tool enables testing and verification of climate models prior to the 20th century greenhouse forcing.
[1] The impact of assimilating global ocean bottom pressure (OBP) information from the Gravity Recovery and Climate Experiment (GRACE) gravity anomalies on the circulation estimate was investigated. For this an estimate of the ocean circulation is being inferred by extending the 50-year-long German part of the Estimating the Circulation and Climate of the Ocean (GECCO) ocean synthesis into recent years. The assimilation system is an improved version of the previous GECCO optimization, which now includes a sea ice model, has enhanced resolution on a truly global domain including the Arctic Ocean. By analyzing differences to a synthesis that additionally assimilated OBP, the GRACE data was found to provide complementary information to standard ocean data sets including satellite altimetry when assimilated. Although in principle standard ocean data sets include the OBP information, the reason why this cannot be extracted is the much larger prior errors for hydrographic and altimeter data in comparison to OBP data owing to the fact that only the former two need to include the unresolved eddy signal. The largest impact of gravity data is found to be on the barotropic circulation, particularly in the subtropical gyres and the polar latitudes. Remaining differences between the simulated and observed OBP information are associated with meridional stripes in the GRACE gravity maps and with the leakage of terrestrial hydrological information into the ocean. Additional differences close to the continental boundaries are related to the self-attraction and loading, processes that are not included in the models.
In the Nordic seas, we combine a computation of absolute surface current flow derived from geodetic data with in situ historical hydrographic data to estimate the absolute volume, heat, and salt transports as a function of depth. Our mean dynamic topography (MDT) is calculated from marine, airborne and satellite gravimetry, combined with satellite altimetry, using a new algorithm called the iterative combination method (ICM). Residual noise in the gravimetric geoid is the limit on MDT resolution and is suppressed using a Gaussian filter with a width at half‐peak amplitude of 59 km. Detailed and coherent flow paths for surface geostrophic currents are clearly identified. ICM MDT was used as fixed boundary condition to transform historical hydrography into absolute estimates of volume, heat, and salt transport, replacing the assumption of an isobaric surface at a predetermined depth. For the inflow of Atlantic Water (potential temperature Θ > 6°C) through the Faroe‐Shetland Channel into the Nordic seas, we obtain time‐averaged fluxes between 1993 and 1996 of 3.5 Sv (volume), 121 TW (heat), and 124 × 106 kg s−1 (salt), very close to reported observations from acoustic Doppler current profiler moorings and conductivity‐temperature‐depth data. For the Svinøy section, we obtain a northward transport of Atlantic Water (S > 35.0, T > 5.0°C) of 3.9 Sv in the eastern branch of the Norwegian Atlantic Current comparable with reported measurements of 4.2 Sv. Similarly good agreement is found for the Hornbanki and Iceland‐Faroe Ridge sections and for monitoring Atlantic Water outflow across the Barents Sea Opening to the Arctic shelf.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.
hi@scite.ai
10624 S. Eastern Ave., Ste. A-614
Henderson, NV 89052, USA
Copyright © 2024 scite LLC. All rights reserved.
Made with 💙 for researchers
Part of the Research Solutions Family.