The role of eddies for the deep water formation in the Labrador Sea

Previous ocean general circulation models of the North Atlantic tend to show large deficits in simulating observed characteristics of deep water formation in the Labrador Sea. It is shown that three key processes lead to significant improvements: 1) an adequate representation of the freshwater exchange with the Nordic Seas; 2) an efficient representation of eddy fluxes between the boundary currents and the interior of the Labrador Sea; 3) low (numerical) diapycnal mixing. Based on these results, a refined eddy resolving model of the North Atlantic is developed and analyzed. The model suggests two novel mechanisms of convection variability related to wind stress: 1) in case of enhanced wind stress a higher generation of well stratified Cape Desolation eddies leads to significantly lower Labrador Sea Water formation; 2) wind stresses parallel to the coast west of Greenland causes Ekman transports of relatively fresh and cold water off the coast towards the interior. This buoyant water at the surface stratifies the water column on the Greenland side of the Labrador Sea and suppresses deep convection

"Energy transfers in surface wave-averaged equations"

Ocean surface gravity waves play an important role for the air-sea momentum fluxes and the upper ocean mixing, and knowledge of the sea state leads in general circulation models to improved estimates of the ocean energy budget and allows to incorporate surface wave impacts, such as Langmuir turbulence. However, including the Stokes drift, in phase-averaged equations for the Eulerian mean motion leads to an Eulerian energy budget which is physically difficult to interpret. In this note, we show that a Lagrangian energy budget allows for a closed energy budget, in which all terms connecting the different energy compartments correspond to well known energy transfer terms. We show that the so-called Coriolis-Stokes force does not lead to an energy transfer between surface gravity waves and oceanic mean motions as previously suggested. In an energy budget for the Lagrangian mean kinetic energy, the work done by the Coriolis-Stokes force does not contribute, and should be used to estimate the kinetic energy balance in the wave affected surface mixed layer. The Lagrangian energy budget is used to discuss an energetically consistent framework, which can be used to couple a general circulation ocean model to a surface wave model.Comment: 33 pages, 7 figures, submitted to J. Phys. Oceanog

Seasonal variability of the Arabian Sea intermediate circulation and its impact on seasonal changes of the upper oxygen minimum zone

Oxygen minimum zones (OMZs) in the open ocean occur below the surface in regions of weak ventilation and high biological productivity with associated sinking organic matter. Very low levels of dissolved oxygen alter biogeochemical cycles and significantly affect marine life. One of the most intense though poorly understood OMZs in the world ocean is located in the Arabian Sea between 300 and 1000 m of depth. An improved understanding of the physical processes that have an impact on the OMZ in the Arabian Sea is expected to increase the reliability of assessments of its future development. This study uses reanalysis velocity fields from the ocean model HYCOM (Hybrid Coordinate Ocean Model), which are verified with observational data, to investigate advective pathways of Lagrangian particles into the Arabian Sea OMZ at intermediate depths between 200 and 800 m. In the eastern basin, the vertical expansion of the OMZ is strongest during the winter monsoon, revealing a core thickness 1000 m deep and oxygen values below 5 µmol kg−1. The minimum oxygen concentration might be favoured by a maximum water mass advection that follows the main advective pathway of Lagrangian particles along the perimeter of the basin into the eastern basin of the Arabian Sea during the winter monsoon. These water masses pass regions of high primary production and respiration, contributing to the transport of low-oxygenated water into the eastern part of the OMZ. The maximum oxygen concentration in the western basin of the Arabian Sea in May coincides with a maximum southward water mass advection in the western basin during the spring intermonsoon, supplying the western core of the OMZ with high-oxygenated water. The maximum oxygen concentration in the eastern basin of the Arabian Sea in May might be associated with the northward inflow of water across 10∘ N into the Arabian Sea, which is highest during the spring intermonsoon. The Red Sea outflow of advective particles into the western and eastern basin starts during the summer monsoon associated with the northeastward current during the summer monsoon. On the other hand, waters from the Persian Gulf are advected with little variation on seasonal timescales. As the weak seasonal cycle of oxygen concentration in the eastern and western basin can be explained by seasonally changing advection of water masses at intermediate depths into the Arabian Sea OMZ (ASOMZ), the simplified backward-trajectory approach seems to be a good method for prediction of the seasonality of advective pathways of Lagrangian particles into the ASOMZ

Global oxygen changes and oxygen variability in the eastern Pacific off Peru

Numerical model runs predict decreasing ocean oxygen with increasing CO2 emission scenarios. Oxygen measurements are generally sparse in the ocean, nonetheless at some key locations longer-term oxygen time series exist and trends for the global ocean can be estimated. In many regions especially the tropical oceans oxygen has decreased during the last 50 years, however especially in the subtropical ocean regions with increasing oxygen values exist. Typical oxygen trends range from -0.5 to +0.4 mol kg-1 yr-1 in the upper ocean for the last few decades, with a global mean oxygen trend of -0.066 mol kg-1yr-1 between 50°S and 50°N at 300 dbar for the period 1960 to 2010 [Stramma et al., 2012]. In a measurement to model comparison for the last 50 years the model reproduce the overall sign and to some extent magnitude of observed ocean deoxygenation, though with a mismatch in regional pattern. Further analysis of the processes that can explain the discrepancies between observed and modeled oxygen trends is required to better understand the climate sensitivity of oceanic oxygen fields and predict potential oxygen changes in the future. Further expansion of low oxygen regions in conjunction with overfishing may threaten the sustainability of pelagic fisheries and accelerate shifts in animal distributions and changes in ecosystem structure. In the eastern Pacific Ocean multidecadal variability (Pacific Decadal Oscillation) and also El Nino phases have a strong influence on long-term oxygen trends [e.g. Czeschel et al., 2012]. Historical data combined with new hydrographic measurements from two ship expeditions in the eastern tropical Pacific in 2009 and 2012 as well as oxygen sensor data from floats allow an enhanced view at the circulation, oxygen variability and trends in the oxygen minimum zone off Peru. Oxygen differences derived by comparison of ship sections show large variability in some locations. This local variability from eddies, seasonal and longer-term variability obscure trends in oceanic dissolved oxygen. Caution in interpretation of the data is necessary

Circulation, eddies, oxygen, and nutrient changes in the eastern tropical South Pacific Ocean

© The Author(s), 2015. This article is distributed under the terms of the Creative Commons Attribution License. The definitive version was published in Ocean Science 11 (2015): 455-470, doi:10.5194/os-11-455-2015.A large subsurface oxygen deficiency zone is located in the eastern tropical South Pacific Ocean (ETSP). The large-scale circulation in the eastern equatorial Pacific and off the coast of Peru in November/December 2012 shows the influence of the equatorial current system, the eastern boundary currents, and the northern reaches of the subtropical gyre. In November 2012 the equatorial undercurrent (EUC) is centered at 250 m depth, deeper than in earlier observations. In December 2012, the equatorial water is transported southeastward near the shelf in the Peru–Chile undercurrent (PCUC) with a mean transport of 1.4 Sv. In the oxygen minimum zone (OMZ), the flow is overlaid with strong eddy activity on the poleward side of the OMZ. Floats with parking depth at 400 m show fast westward flow in the mid-depth equatorial channel and sluggish flow in the OMZ. Floats with oxygen sensors clearly show the passage of eddies with oxygen anomalies. The long-term float observations in the upper ocean lead to a net community production estimate at about 18° S of up to 16.7 mmol C m−3 yr−1 extrapolated to an annual rate and 7.7 mmol C m−3 yr−1 for the time period below the mixed layer. Oxygen differences between repeated ship sections are influenced by the Interdecadal Pacific Oscillation (IPO), by the phase of El Niño, by seasonal changes, and by eddies, and hence have to be interpreted with care. At and south of the Equator the decrease in oxygen in the upper ocean since 1976 is related to an increase in nitrate, phosphate, and in part silicate.The Deutsche Forschungsgemeinschaft (DFG) provided support as part of the “Sonderforschungsbereich 754: Climate-Biogeochemistry Interactions in the Tropical Ocean, A5” (RC, LS). Additional support was provided through the German BMBF funded Project SOPRAN under FKZ 03F0662A (TF) and through the US NOAA Climate Program Office to the Woods Hole Oceanographic Institution (RAW)

Eastern Pacific oxygen time series from the Stratus mooring and from floats

In the tropical eastern South Pacific the Stratus Ocean Reference Station (~20°S, 85°W) is located in the transition zone between the oxygen minimum zone (OMZ) and the well oxygenated subtropical gyre. This region is also known for its high eddy frequency [Chaigneau et al., 2008]. From 6 April 2011 to 29 May 2012 oxygen was measured in the mooring from 9 oxygen optodes located between 45 m and 601 m depth at the southern boundary of the oxygen minimum zone. The oxygen time series describe the passage of several eddies, including a strong anticyclonic mode water eddy in February/March 2012 with oxygen decreasing by up to 200 mol/L and an available oxygen deficit of 10.5x1016 mol in comparison to its surrounding water. The eddy observed at the mooring was formed 11 months earlier off the coast of northern Chile. During its westward propagation one float was located for 3 months in this eddy and provided hydrographic and oxygen measurements along the path of the eddy. Several other floats were placed in eddies in the region, but did not stay continuously inside these eddies. The continuous oxygen measurements in the mooring and floats indicate high oxygen variability caused by eddies with enhanced oxygen in cyclonic eddies and reduced oxygen in anticyclonic eddies. Hence, oxygen trends determined from a few measurements might be biased by eddy processes. Finally, gliders with oxygen sensors may provide better eddy surveys than floats

Oscillatory sensitivity of Atlantic overturning to high-latitude forcing

The Atlantic Meridional Overturning Circulation (AMOC) carries warm upper waters into northern high-latitudes and returns cold deep waters southward. Under anthropogenic greenhouse gas forcing the AMOC is expected to weaken due to high-latitude warming and freshening. Here, we show that the sensitivity of the AMOC to an impulsive forcing at high latitudes is an oscillatory function of forcing lead time. This leads to the counter-intuitive result that a stronger AMOC can emerge as a result of, although some years after, anomalous warming at high latitudes. In our model study, there is no simple one-to-one correspondence between buoyancy forcing anomalies and AMOC variations, which retain memory of surface buoyancy fluxes in the subpolar gyre for 15-20 years. These results make it challenging to detect secular change from short observational time serie

Transport, properties, and life cycles of mesoscale eddies in the eastern tropical South Pacific

The influence of mesoscale eddies on the flow field and the water masses, especially the oxygen distribution of the eastern tropical South Pacific, is investigated from a mooring, float, and satellite data set. Two anticyclonic (ACE1/2), one mode-water (MWE), and one cyclonic eddy (CE) are identified and followed in detail with satellite data on their westward transition with velocities of 3.2 to 6.0cms−1 from their generation region, the shelf of the Peruvian and Chilean upwelling regime, across the Stratus Ocean Reference Station (ORS;  ∼ 20°S, 85°W) to their decaying region far west in the oligotrophic open ocean. The ORS is located in the transition zone between the oxygen minimum zone and the well oxygenated South Pacific subtropical gyre. Velocity, hydrographic, and oxygen measurements at the mooring show the impact of eddies on the weak flow region of the eastern tropical South Pacific. Strong anomalies are related to the passage of eddies and are not associated with a seasonal signal in the open ocean. The mass transport of the four observed eddies across 85°W is between 1.1 and 1.8Sv. The eddy type-dependent available heat, salt, and oxygen anomalies are 8.1×1018J (ACE2), 1.0×1018J (MWE), and −8.9×1018J (CE) for heat; 25.2×1010kg (ACE2), −3.1×1010kg (MWE), and −41.5×1010kg (CE) for salt; and −3.6×1016µmol (ACE2), −3.5×1016µmol (MWE), and −6.5×1016µmol (CE) for oxygen showing a strong imbalance between anticyclones and cyclones for salt transports probably due to seasonal variability in water mass properties in the formation region of the eddies. Heat, salt, and oxygen fluxes out of the coastal region across the ORS region in the oligotrophic open South Pacific are estimated based on these eddy anomalies and on eddy statistics (gained out of 23 years of satellite data). Furthermore, four profiling floats were trapped in the ACE2 during its westward propagation between the formation region and the open ocean, which allows for conclusions on lateral mixing of water mass properties with time between the core of the eddy and the surrounding water. The strongest lateral mixing was found between the seasonal thermocline and the eddy core during the first half of the eddy lifetime

Circulation, eddies, oxygen, and nutrient changes in the eastern tropical South Pacific Ocean

© The Author(s), 2015. This article is distributed under the terms of the Creative Commons Attribution License. The definitive version was published in Ocean Science 11 (2015): 455-470, doi:10.5194/os-11-455-2015.A large subsurface oxygen deficiency zone is located in the eastern tropical South Pacific Ocean (ETSP). The large-scale circulation in the eastern equatorial Pacific and off the coast of Peru in November/December 2012 shows the influence of the equatorial current system, the eastern boundary currents, and the northern reaches of the subtropical gyre. In November 2012 the equatorial undercurrent (EUC) is centered at 250 m depth, deeper than in earlier observations. In December 2012, the equatorial water is transported southeastward near the shelf in the Peru–Chile undercurrent (PCUC) with a mean transport of 1.4 Sv. In the oxygen minimum zone (OMZ), the flow is overlaid with strong eddy activity on the poleward side of the OMZ. Floats with parking depth at 400 m show fast westward flow in the mid-depth equatorial channel and sluggish flow in the OMZ. Floats with oxygen sensors clearly show the passage of eddies with oxygen anomalies. The long-term float observations in the upper ocean lead to a net community production estimate at about 18° S of up to 16.7 mmol C m−3 yr−1 extrapolated to an annual rate and 7.7 mmol C m−3 yr−1 for the time period below the mixed layer. Oxygen differences between repeated ship sections are influenced by the Interdecadal Pacific Oscillation (IPO), by the phase of El Niño, by seasonal changes, and by eddies, and hence have to be interpreted with care. At and south of the Equator the decrease in oxygen in the upper ocean since 1976 is related to an increase in nitrate, phosphate, and in part silicate.The Deutsche Forschungsgemeinschaft (DFG) provided support as part of the “Sonderforschungsbereich 754: Climate-Biogeochemistry Interactions in the Tropical Ocean, A5” (RC, LS). Additional support was provided through the German BMBF funded Project SOPRAN under FKZ 03F0662A (TF) and through the US NOAA Climate Program Office to the Woods Hole Oceanographic Institution (RAW)