Lateral diffusivity is computed from a tracer release experiment in the northeastern tropical Atlantic thermocline. The uncertainties of the estimates are inferred from a synthetic particle release ...using a high‐resolution ocean circulation model. The main method employed to compute zonal and meridional components of lateral diffusivity is the growth of the second moment of a cloud of tracer. The application of an areal comparison method for estimating tracer‐based diffusivity in the field experiments is also discussed. The best estimate of meridional eddy diffusivity in the Guinea Upwelling region at about 300 m depth is estimated to be
Ky=500 ± 200 m2 s−1. The zonal component of lateral diffusivity is estimated to be
Kx=1200 ± 600 m2 s−1, while areal comparison method yields areal equivalent zonal diffusivity component of
Kxe=1000 ± 500 m2 s−1. In comparison to Ky, Kx is about twice larger, resulting from the tracer patch stretching by zonal jets. Employed conceptual jet model indicates that zonal jet velocities of about
0.015 ± 0.005 m s−1 are required to explain the enhancement of the zonal eddy diffusivity component. Finally, different sampling strategies are tested on synthetic tracer release experiments. They indicate that the best sampling strategy is a sparse regular sampling grid covering most of the tracer patch.
Key Points
Meridional eddy diffusivity is found to be about 500 m2/s
Zonal eddy diffusivity is found to be about 1200 m2/s
Optimal tracer patch sampling strategies are reviewed
Abstract
The formation of a subsurface anticyclonic eddy in the Peru‐Chile Undercurrent (PCUC) in January and February 2013 is investigated using a multiplatform four‐dimensional observational ...approach. Research vessel, multiple glider, and mooring‐based measurements were conducted in the Peruvian upwelling regime near 12°30'S. The data set consists of >10,000 glider profiles and repeated vessel‐based hydrography and velocity transects. It allows a detailed description of the eddy formation and its impact on the near‐coastal salinity, oxygen, and nutrient distributions. In early January, a strong PCUC with maximum poleward velocities of ∼0.25 m/s at 100–200 m depth was observed. Starting on 20 January, a subsurface anticyclonic eddy developed in the PCUC downstream of a topographic bend, suggesting flow separation as the eddy formation mechanism. The eddy core waters exhibited oxygen concentration of <1 μmol/kg, an elevated nitrogen deficit of ∼17 μmol/L, and potential vorticity close to zero, which seemed to originate from the bottom boundary layer of the continental slope. The eddy‐induced across‐shelf velocities resulted in an elevated exchange of water masses between the upper continental slope and the open ocean. Small‐scale salinity and oxygen structures were formed by along‐isopycnal stirring, and indications of eddy‐driven oxygen ventilation of the upper oxygen minimum zone were observed. It is concluded that mesoscale stirring of solutes and the offshore transport of eddy core properties could provide an important coastal open ocean exchange mechanism with potentially large implications for nutrient budgets and biogeochemical cycling in the oxygen minimum zone off Peru.
Key Points:
Multiplatform observations of a subsurface anticyclone formation off Peru
Flow separation is suggested as the formation mechanism
The eddy provides an important coastal open ocean exchange mechanism for solutes
The deep water of the western Mediterranean Sea is known to have become warmer and saltier since about the 1950s. The causes of these changes have, however, not yet been sactisfactorily determined. ...Previous studies speculated on decreasing precipitation, greenhouse warming and/or anthropogenic reduction of the freshwater flux into the eastern Mediterranean. Here we report on results from a new oceanographic database of the western Mediterranean Sea together with determinations of longterm changes of the fresh water budget. We analyzed temperature and salinity data of the past 40 years to detect deviations from the longterm average. Certain areas and depth ranges are showing increases in temperature or salinity some of which have been found earlier and some which are new. From the regional and vertical distribution we conclude that the observed increases of temperature and salinity in the western Mediterranean Sea are caused both by changes in atmospheric conditions as described by the NAO‐index and by the regulation of Spanish rivers.
In a study, a general circulation ocean model was used to investigate the formation and propagation mechanisms of North Atlantic Oscillation (NAO)-generated temperature anomalies along the pathway of ...the North Atlantic Current (NAC). The NAO-like wind forcing generates temperature anomalies in the upper 440 m that propagate along the pathway of the NAC in general agreement with the observations.
The Mediterranean Sea has been investigated intensively since the early nineties, using modern techniques and collaborative approaches. This overview summarizes some of the resulting advances that ...were made concerning the physical oceanography of the western Mediterranean. The water mass formation processes are now much better understood and have been quantified to a large extent. The boundary conditions of the system in terms of surface fluxes and strait transports can be determined with improved accuracy, thus enabling future investigation of interannual variability. The dynamics of the surface and intermediate layers have revealed a variety of eddy and mesoscale processes that are important for the circulation and spreading of water masses. The deep circulation is being investigated with Lagrangian techniques (tracers and floats). First results show a large component of the deep water originating from the Tyrrhenian Sea and intense cyclonic and anticyclonic eddy flows.
Owing to its nearly enclosed nature, the Tyrrhenian Sea at first sight is expected to have a small impact on the distribution and characteristics of water masses in the other basins of the western ...Mediterranean. The first evidence that the Tyrrhenian Sea might, in fact, play an important role in the deep and intermediate water circulation of the entire western Mediterranean was put forward by Hopkins 1988. There, an outflow of water from the Tyrrhenian Sea into the Algero Provençal Basin was postulated in the depth range 700–1000 m, to compensate for an observed inflow of deeper water into the Tyrrhenian Sea. However, this outflow, the Tyrrhenian Deep Water (TDW), was undetectable since it would have hydrographic characteristics that could also be produced within the Algero‐Provençal Basin. A new data set of hydrographic, tracer, lowered Acoustic Doppler Current Profiler (LADCP), and deep float observations presented here allows us now to identify and track the TDW in the Algero‐Provençal Basin and to demonstrate the presence and huge extent of this water mass throughout the western Mediterranean. It extends from 600 m to 1600–1900 m depth and thus occupies much of the deep water regime. The outflow from the Tyrrhenian is estimated to be of the order of 0.4 Sv (Sv = 106 m3 s−1), based on the tracer balances. This transport has the same order of magnitude as the deep water formation rate in the Gulf of Lions. The Tyrrhenian Sea effectively removes convectively generated deep water (Western Mediterranean Deep Water (WMDW)) from the Algero‐Provençal Basin, mixes it with Levantine Intermediate water (LIW) above, and reinjects the product into the Algero‐Provençal Basin at a level between the WMDW and LIW, thus smoothing the temperature and salinity gradients between these water masses. The tracer characteristics of the TDW and the lowered ADCP and deep float observations document the expected but weak cyclonic circulation and larger flows in a vigorous eddy regime in the basin interior.
Historical winter sea ice concentration data are used to examine the relation between the Northern Annular Mode (NAM) and the sea ice concentration in the Nordic seas over the past 50 years. The well ...known basic response pattern of a seesaw between the Labrador Sea and the Greenland, Iceland and Barents seas is being reproduced. However, the response is not robust in the Greenland and Iceland seas. There the observed variability has a more complex relationship with surface temperatures and winds. We divide the sea ice response into three spectral bands: high (P < 5 year), band (5 < P < 15 year), and low pass (P > 15 year) filtered NAM indices. This division is motivated by the expected slow response of the ocean circulation which might play a significant role in the Greenland and Iceland seas. The response to the NAM is also examined separately for the periods before and after 1976 to identify variations due to the relocation of the northern centre of the North Atlantic Oscillation.
The spreading of Antarctic bottom water in the tropical Atlantic Rhein, Monika; Stramma, Lothar; Krahmann, Gerd
Deep-sea research. Part 1. Oceanographic research papers/Deep sea research. Part I, Oceanographic research papers,
04/1998, Letnik:
45, Številka:
4
Journal Article
Recenzirano
Odprti dostop
Hydrographic and tracer chlorofluorocarbon (CFC), component F11 data in the tropical Atlantic off Brazil taken in spring 1994 are used to describe the development of the water mass characteristics of ...Antarctic Bottom Water (AABW) between 10°S and 11°N. To compute the AABW transports, geostrophic computations and directly measured velocity fields are combined. Velocity profiles were measured with the Pegasus profiling system and an ADCP attached to the CTD.
The F11 increase from 10°S to 11°N, mainly in the upper part of the tracer-poor AABW, reveals the mixing of AABW along its path with the overlying North Atlantic Deep Water, which carries a significant F11 signal in the equatorial Atlantic. While propagating north of 5°S, the AABW shifts to higher salinities at a given temperature.
About one-third of the northward flowing AABW at 10°S (4.8
Sv) and at 5°S (4.7
Sv) west of about 31°30′W enters the Guiana Basin, mainly through the southern half of the Equatorial Channel at 35°W (1.5–1.8
Sv). The other part recirculates and some of it flows through the Romanche Fracture Zone into the eastern Atlantic. In the Guiana Basin, west of 40°W, the sloping topography and the strong, eastward flowing deep western boundary current might prevent the AABW from flowing west: thus it has to turn north at the eastern slope of the Ceara Rise (2.2
Sv). At 44°W, north of the Ceara Rise, AABW flows west in the interior of the basin in a main core near 7°15′N (1.9
Sv). A net return flow of about 0.5
Sv was found north of 8°43′N. A large fraction of the AABW (1.1
Sv) enters the eastern Atlantic through the Vema Fracture Zone, leaving only 0.3
Sv of AABW for the western Atlantic basins.
Historical winter sea ice concentration data are used to examine the relation between the Northern Annular Mode (NAM) and the sea ice concentration in the Nordic seas over the past 50 years. The well ...known basic response pattern of a seesaw between the Labrador Sea and the Greenland, Iceland and Barents seas is being reproduced. However, the response is not robust in the Greenland and Iceland seas. There the observed variability has a more complex relationship with surface temperatures and winds. We divide the sea ice response into three spectral bands: high (P< year), band (5<P<15 year), and low pass (P>15 year) filtered NAM indices. This division is motivated by the expected slow response of the ocean circulation which might play a significant role in the Greenland and Iceland seas. The response to the NAM is also examined separately for the periods before and after 1976 to identify variations due to the relocation of the northern centre of the North Atlantic Oscillation.
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