Paul G. Myers

Modeling the paleocirculation of the Mediterranean: The Last Glacial Maximum and the Holocene with emphasis on the formation of sapropel S 1

An ocean general circulation model is used to simulate the thermohaline circulation in the Mediterranean sea during the last glacial maximum and the Holocene, when the sapropel S1 was deposited. The model is forced by prescribed surface temperatures and salinities, where present-day values lead to very realistic surface buoyancy fluxes. Different paleoreconstructions for the surface salinity and temperature distributions during these periods are tested. In both periods, under all reconstructions, antiestuarine flow is maintained at Gibraltar and Sicily. The Holocene circulation has fresh intermediate water produced in the Adriatic and an upward salt flux from the old waters below help maintain its outflow at Sicily. The depth of ventilation around the basin is broadly consistent with the shallowest sapropel layers observed. Shoaling of the eastern pycnocline occurs in all experiments in both periods, possibly indicating enhanced productivity, although the reasons for this are different in each case.

Response of the Mediterranean Sea thermohaline circulation to observed changes in the winter wind stress field in the period 1980–1993

This paper seeks to model changes in deep water production in the eastern Mediterranean induced by changes in winter wind stress. An analysis of individual monthly wind stress fields over the Mediterranean for 1980–1993 from the SOC flux data set shows that an intensification of the winter mean (mainly January) wind stress over the Aegean Sea and Levantine basin occurred in the latter half of this period. A weakening of the Mistral occurred at the same time. Two monthly wind stress climatologies were created using the 1980–1987 and 1988–1993 periods, and these were used to force an ocean general circulation model of the Mediterranean, with climatological surface T, S relaxation. The Levantine intermediate water (LIW) dispersal path in the Ionian is altered in the 1988–1993 experiment with no pathway to the Adriatic and, consequently, greatly reduced exchange at Otranto and a collapse in Adriatic deep water formation. In contrast, there is an increased exchange of LIW at the Cretan arc straits and enhanced Aegean deep water production in the 1988–1993 experiment. Much more Aegean water exits into the Levantine and Ionian basins as is shown by an east-west cross section south of Crete, along a similar path to the Meteor cruise in 1995. Changes in air-sea fluxes are diagnosed from the model showing a small increase in wintertime cooling over the Aegean and reduced cooling over the Adriatic after 1987. While the changes in air-sea fluxes are probably underrepresented by this simulation, the large changes induced by the wind forcing suggest this could be a mechanism in the altered thermohaline state of the eastern Mediterranean since 1987.

Impact of the circulation on Sapropel formation in the eastern Mediterranean

The role of the thermohaline circulation in controlling export production, oxygenation of deep waters, and hence possible sapropel formation in the eastern Mediterranean is examined using a simple nutrient-cycling model. The model is driven by velocity fields from a general circulation model and receives fluxes of nutrient from river run-off and atmospheric deposition. The model is used to study three scenarios: a strong anti-estuarine circulation, a weakened anti-estuarine circulation, and a weak estuarine circulation. Nutrient transports, ventilation of oxygen, and deposition of organic matter are investigated in each case. With a present-day circulation the model provides reasonable agreement with observed phosphate and oxygen profiles and for export production. With the weakened anti-estuarine circulation, consistent with surface salinity reconstructions for the most recent sapropel S1, there is a modest increase in export production and reduced ventilation leading to anoxia in intermediate and deep waters. Sapropel formation is possible near the coastal margins, particularly if there is enhanced river run-off. With an estuarine circulation, there is significant increase in export production in addition to anoxia below a shallow winter mixed layer. While both the latter circulations allow sapropel formation in the model, the estuarine case is distinguished by higher organic carbon deposition and anoxia in the near-surface waters.

JEBAR, Bottom Pressure Torque, and Gulf Stream Separation

Abstract A diagnostic, finite element, barotropic ocean model has been used to simulate the mean circulation in the North Atlantic. With the inclusion of the joint effect of baroclinicity and relief (JEBAR), the Gulf Stream is found to separate at the correct latitude, ∼35°N, off Cape Hatteras. Results suggest that the JEBAR term in three key regions (offshore of the separation point in the path of the main jet, along the slope region of the North Atlantic Bight, and in the central Irminger Sea) is crucial in determining the separation point. The transport driven by the bottom pressure torque component of JEBAR dominates the solution, except in the subpolar gyre, and is also responsible for the separation of the Gulf Stream. Excluding high latitudes (in the deep-water formation regions) density variations in the upper 1000 m of the water column govern the generation of the necessary bottom pressure torque in our model. Examination of results from the World Ocean Circulation Experiment-Community Modelling ...

A Diagnostic Barotropic Finite-Element Ocean Circulation Model

Abstract The finite-element method possesses many advantages over more traditional numerical techniques used to solve systems of differential equations. These advantages include a number of conservation properties and a natural treatment of boundary conditions. The methods piecewise nature makes it useful when dealing with irregular domains and similarly when using variable horizontal resolution. To take advantage of these properties, a finite-element representation of the linearized, steady-state, barotropic potential vorticity equation is developed. The Stommel problem is used as an initial test for the model. A fourth-order eddy viscosity term is then added, and the resulting problem is solved in both simply and multiply connected domains under both slip and no-slip boundary conditions. The beta-plane assumption is then relaxed, and the model is reformulated in spherical coordinates. A realistic geography and topography version of this model is also used to examine the barotropic circulation in the No...

Recent increases in Arctic freshwater flux affects Labrador Sea convection and Atlantic overturning circulation.

The Atlantic Meridional Overturning Circulation (AMOC) is an important component of ocean thermohaline circulation. Melting of Greenlands ice sheet is freshening the North Atlantic; however, whether the augmented freshwater flux is disrupting the AMOC is unclear. Dense Labrador Sea Water (LSW), formed by winter cooling of saline North Atlantic water and subsequent convection, is a key component of the deep southward return flow of the AMOC. Although LSW formation recently decreased, it also reached historically high values in the mid-1990s, making the connection to the freshwater flux unclear. Here we derive a new estimate of the recent freshwater flux from Greenland using updated GRACE satellite data, present new flux estimates for heat and salt from the North Atlantic into the Labrador Sea and explain recent variations in LSW formation. We suggest that changes in LSW can be directly linked to recent freshening, and suggest a possible link to AMOC weakening.

Decline and partial rebound of the Labrador Current 1993-2004: Monitoring ocean currents from altimetric and conductivity-temperature-depth data

Monitoring and understanding of Labrador Current ariability is important because it is intimately linked to the meridional overturning circulation and the marine ecosystem off northeast North America. Nevertheless, knowledge of its decadal variability is inadequate because of scarcity of current meter data. By using a novel synthesis of satellite altimetry with conductivity-temperaturedepth (CTD) data we assess the Labrador Current variability north of the Hamilton Bank (56oN) over 1993-2004. Our analysis shows a decline of the surface-to-bottom transport of current by 6.3 ± 1.5 Sv (1 Sv =106 m3 s-1) in the 1990s (significant at the 99% confidence level) and a likely partial rebound of 3.2 ± 1.7 Sv in the early 2000s (significant at the 89% confidence level only). The inferred multiyear changes in the Labrador Current transport seem to be primarily barotropic and positively correlated (at the 99% level) with the North Atlantic Oscillation at zero lag implying a fast response of the regional circulation to the atmospheric forcing variability. The results compare favorably with direct current measurements and recent model-based findings on the multi-year variability of the subpolar gyre and its underlying mechanisms. The study demonstrates the feasibility of combining altimetry and CTD data for assessing the climatic variability of the boundary currents.

Interdecadal variability in an idealized model of the North Atlantic

A coarse resolution model is developed to study the thermohaline circulation of the North Atlantic. This model is driven by the annual mean Hellerman and Rosenstein wind stress field, Levitus sea surface restoring temperatures, and Schmitt, Bogden, and Dorman freshwater flux fields (mixed boundary conditions) together with various parameterizations of Arctic freshwater export into the North Atlantic. The model simulations indicate the existence of self-sustained, internal variability of the thermohaline circulation with a period of about 20 years. Associated with the variability is a large variation in the deep-water formation rate in the Labrador Sea and hence the poleward heat transport in the North Atlantic. It is shown that the variability is insensitive to the freshwater flux and wind forcing used and that the timescale for this thermally driven convective/advective oscillation is set by the cooling time of the Labrador Sea. The variability is robust to various parameterizations of Arctic freshwater export but may be suppressed if there is a strong freshwater flux through the Canadian Archipelago (or equivalently, large precipitation) into the Labrador Sea. The importance of topography, although poorly resolved in this coarse resolution study, is addressed and the results are compared with a coupled atmosphere-ocean simulation and observations taken over the North Atlantic.

On the importance of the choice of wind stress forcing to the modeling of the Mediterranean Sea circulation

A 1/4° degree ocean general circulation model is used to examine the role that four different wind stress climatologies play on the circulation of the Mediterranean. The wind stress climatologies examined are those derived from numerical weather prediction models (National Meteorological Center (NMC) and European Centre for Medium-Range Weather Forecasts (ECMWF)) and one based on observations (Southampton Oceanography Centre (SOC)). Significant differences exist between the wind climatologies over the Mediterranean and in the response of an ocean general circulation model forced by the different climatologies. Excessive coastal upwelling/downwelling is found to be associated with the extreme zonal nature of one of the climatologies. Surface circulation differences include the position and penetration of the Mid-Mediterranean Jet into the Levantine, the Ionian, and the Tyrrhenian circulations. Significant differences exist in the pathways for dispersal of Levantine Intermediate Water. Under the SOC forcing, there is a reduction in Eastern Mediterranean Deep Water formation in the Southern Adriatic, compensated by the production of intermediate or deep water in the Aegean. The ECMWF climatology is found to be associated with much more cyclonic doming in the Gulf of Lions, leading to better formation of Western Mediterranean Deep Water.

Overturning in the Subpolar North Atlantic Program: A New International Ocean Observing System

A new ocean observing system has been launched in the North Atlantic in order to understand the linkage between the meridional overturning circulation and deep water formation. For decades oceanographers have understood the Atlantic Meridional Overturning Circulation (AMOC) to be primarily driven by changes in the production of deep water formation in the subpolar and subarctic North Atlantic. Indeed, current IPCC projections of an AMOC slowdown in the 21st century based on climate models are attributed to the inhibition of deep convection in the North Atlantic. However, observational evidence for this linkage has been elusive: there has been no clear demonstration of AMOC variability in response to changes in deep water formation. The motivation for understanding this linkage is compelling since the overturning circulation has been shown to sequester heat and anthropogenic carbon in the deep ocean. Furthermore, AMOC variability is expected to impact this sequestration as well as have consequences for regional and global climates through its effect on the poleward transport of warm water. Motivated by the need for a mechanistic understanding of the AMOC, an international community has assembled an observing system, Overturning in the Subpolar North Atlantic (OSNAP), to provide a continuous record of the trans-basin fluxes of heat, mass and freshwater and to link that record to convective activity and water mass transformation at high latitudes. OSNAP, in conjunction with the RAPID/MOCHA array at 26°N and other observational elements, will provide a comprehensive measure of the three-dimensional AMOC and an understanding of what drives its variability. The OSNAP observing system was fully deployed in the summer of 2014 and the first OSNAP data products are expected in the fall of 2017.