Meridional Overturning Cell Research Articles

Time-Dependent Adjustment in a Simple Model of the Mid-Depth Meridional Overturning Cell

Abstract A recently proposed reduced-gravity model of the warm-water branch of the middepth meridional overturning circulation in a rectangular basin with a circumpolar connection is extended to include time dependence. The model describes the balance between gain of warm water through northward Ekman advection across the circumpolar current, loss of warm water through eddy fluxes southward across the current, net gain or loss of warm water through diabatic processes north of the current, and changes in the thickness of the warm-water layer. The steady solutions are the same as those found previously, when the previous parameterization of diabatic fluxes is used. Time-dependent solutions are considered for the approach of the solution to a new equilibrium when the forcing or parameters are abruptly changed and then held fixed. An initial adjustment occurs through a combination of boundary and equatorial adjustment, followed by planetary wave propagation. The longer-term adjustment to equilibrium consists primarily of the slow change in eastern boundary thickness of the warm layer, which controls the mean depth of the entire layer. An approximate analytical solution of the time-dependent equations yields an explicit expression for the intrinsic time scale of the long-term adjustment, which depends on the eddy and diabatic flux parameters and on the equilibrium solution toward which the time-dependent solution adjusts. Numerical solutions are also considered with a second, advective–diffusive diabatic flux parameterization. These solutions differ quantitatively but not qualitatively from those with the original parameterization. For the range of parameter values considered, the adjustment time scale has dimensional values of several decades to more than a century, but the meridional flux of warm water may respond to changes in external parameters or forcing much more rapidly than this time scale for equilibration of the eastern boundary thickness and thermocline structure.

Agulhas ring injection into the South Atlantic during glacials and interglacials

Abstract. Recent proxies suggest that, at the end of the last glacial, there was a significant increase in the injection of Agulhas rings into the South Atlantic (SA). This brought about a dramatic increase in the salt-influx (from the Indian Ocean) into the SA helping re-start the then-collapsed meridional overturning cell (MOC), leading to the termination of the Younger Dryas (YD). Here, we propose a mechanism through which large variations in ring production take place. Using nonlinear analytical solutions for eddy shedding, we show that there are restricted possibilities for ring detachment when the coast is oriented in the north-south direction. We define a critical coastline angle below which there is rings shedding and above which there is almost no shedding. In the case of the Agulhas region, the particular shape of the African continent implies that rings can be produced only when the retroflection occurs beyond a specific latitude where the angle is critical. During glaciation, the wind stress curl (WSC) vanished at a latitude lower than that of the critical angle, which prohibited the retroflection from producing rings. When the latitude at which the WSC vanishes migrated poleward towards its present day position, the corresponding coastline angle decreased below the critical angle and allowed for a vigorous production of rings. Simple process-oriented numerical simulations (using the Bleck and Boudra model) are in satisfactory agreement with our results and enable us to affirm that, during the glacials, the behavior of the Agulhas Current (AC) was similar to that of the modern East Australian Current (EAC), for which the coastline slant is supercritical.

The Collapse of the Bering Strait Ice Dam and the Abrupt Temperature Rise in the Beginning of the Holocene

Abstract This paper focuses on the abrupt increase in the oceanic and atmospheric temperature in the Northern Hemisphere at the beginning of the Holocene, approximately 11 000 yr before the present. De Boer and Nof hypothesized that, at that time, the Bering Strait (BS) opened up abruptly because of the breakup of an ice dam (by rising sea levels). It is proposed further here that this sudden opening caused an abrupt increase in the mean temperature of the Northern Hemisphere. An analytical, coupled ocean–atmosphere model is applied to the North Atlantic in an attempt to quantify the temperature change resulting from the opening of the BS. Heat, salt, and mass are all conserved within a box in the North Atlantic. A convection condition allows water to enter the deep layer and the island rule relates the wind field to the mass fluxes. The conventional approach that the meridional overturning cell (MOC) was not operating during the Younger Dryas because of an overwhelming freshwater flux is adopted here. Opening the BS in the early Holocene allowed these freshwater anomalies to be flushed out into the Pacific, reviving convection and the transport of heat northward. Restarting convection with an open BS increases mean oceanic and atmospheric temperature by 3° and 23°C, respectively. These values are comparable to those found in both the Centre Européen de Recherche et d’Enseignement des Géosciences de l’Environnement (CEREGE) alkenone and Greenland Ice Sheet Project 2 (GISP 2) ice core records. Of course, restarting convection increases the temperature even with a closed BS, but in the closed BS case the oceanic increase is slightly higher (4°C instead of 3°C), whereas the atmospheric is much lower (17.5°C instead of 23°C). This is because, by requiring a continuous sea level around the Americas, an open BS allows the wind field to limit the amount of Southern Ocean water that enters the South Atlantic. This controlled volume flux (8 Sv) is considerably smaller than that allowed into the Atlantic in the no-wind control closed BS case (17 Sv).

A New Analytical Model for Heinrich Events and Climate Instability

Abstract The authors focus on Heinrich events and the question of whether the arrest and restart of convection can explain the associated sudden changes in oceanic and atmospheric temperature. For this purpose, a new (mixed) dynamical-box model is developed in which the ocean and atmosphere communicate via both Ekman layers and convection. The conservation of heat, salt, volume flux, and a “convection condition” yields a system of algebraic equations that are solved analytically. As expected, it is found that as the freshwater flux increases, the convective ocean temperature decreases. The heat flux from the ocean to the atmosphere, the transport of the oceanic meridional overturning cell (MOC), and the corresponding atmospheric flow generated by the heat flux from the ocean all decrease. However, the outgoing air temperature increases with increasing freshwater flux. This counterintuitive increase is because a decreased latent and sensible heat flux (to a humid atmosphere) means a reduced temperature difference between the warmer ocean and the cooler atmosphere, implying a cooler ocean and warmer atmosphere. For each wind speed, there is a critical freshwater flux beyond which the convection collapses and the temperatures of both the ocean and the air plunge because equatorial water is no longer flowing northward to replace the frigid northern waters. The above points to a potentially new instability process that was probably active during glaciation periods—when ice and snow are abundant, even the smallest amount of freshwater flux will cause local warming which, in turn, will cause increased melting, resulting in an ever-increased freshwater flux until the critical flux is reached and the MOC collapses. The model suggests that switching convection on and off changed the glacial ocean temperature by 4°C and the glacial air temperature by 12.5°C, both consistent with the Greenland Ice Sheet Project (GISP II) ice core record and the Centre Européen de Recherche et d’Enseignement des Géosciences de l’Environnement (CEREGE) alkenone record.

Does the Atlantic meridional overturning cell really have more than one stable steady state?

The prevailing view regarding the oceanic meridional overturning cell (MOC) in the Atlantic is that, for a given North Atlantic freshwater flux, it has at least two stable states, one with a large surface (northward) mass flux and the other with a small flux. It has been argued that some abrupt paleoclimatic changes which occurred in the North Atlantic and the regions surrounding it might be related to a shift between these two distinctly different states. Here, we argue that, although the Atlantic MOC can indeed collapse due to a large freshwater flux, the actual ocean does not have multiple states for the same freshwater flux. The two state scenarios has its origin in the analytical box model of Stommel [1961. Thermohaline convection with two stable regimes of flow. Tellus 2, 244–230] and in a series of numerical models starting with that of Bryan [1986. High latitude salinity effects and interhemispheric thermohaline circulations. Nature 303, 301–304]. Using hybrid global analytical models involving both wind and density variations we demonstrate here that the application of Stommel's model to the North Atlantic yields multiple solutions because it considers the origin of the MOC upper limb to be a box whose export of water depends on its temperature and salinity which are not known in advance. When this origination box is replaced by a (observationally supported) Southern Ocean box whose surface water export depends solely on the wind, and when, together with this choice, the diapycnal diffusivities and eddy viscosities are taken to be as small as the usually observed values, the multi-solution scenario disappears and one gets only a single solution. Using the Uvic climate model, we re-confirm earlier results and argue that numerical models have multiple stable states and a resulting hysteresis because of the spuriously high eddy diffusivity that is typically used explicitly or implicitly. This is so because the diffusivity artificially introduces dense-to-light water conversion analogous to Stommel's origination box. Since we used a level model rather than a layered or isopynic model, the small vertical diffusivity limit still retains significant cross-isopycnal mixing due to the horizontal diffusivity, which is not supported by observations. Consequently, while our runs shows a tendency to no-hysteresis in the limit of small cross-isopycnal flow, we cannot actually reach that limit.

Transports across the 2002 Greenland‐Portugal Ovide section and comparison with 1997

The first Ovide cruise occurred in June–July 2002 on R/V Thalassa between Greenland and Portugal. The absolute transports across the Ovide line are estimated using a box inverse model constrained by direct acoustic Doppler current profiler velocity measurements and by an overall mass balance (±3 Sv, where 1 Sv = 106 m3 s−1) across the section. Main currents are studied and compared to the results of the similar Fourex section performed in August 1997 and revisited here. The meridional overturning cell (MOC) is estimated in two different ways, both leading to a significantly lower value in June 2002 than in August 1997, consistent with the relative strength of the main components of the MOC (North Atlantic Current and deep western boundary current). It has been found that the MOC calculated on density levels is more robust and meaningful than when calculated on depth levels, and it is found to be 16.9 ± 1.0 Sv in 2002 versus 19.2 ± 0.9 Sv in 1997. The 2002 heat transport of 0.44 ± 0.04 × 1015 W is also significantly different from the 0.66 ± 0.05 × 1015 W found in 1997, but it is consistent with the much weaker integrated warm water transport across the section than in 1997.

Mechanisms of decadal variability in the shallow subtropical‐tropical circulation of the Atlantic Ocean: A model study

A suite of basin‐scale models of the thermohaline and wind‐driven circulation in the Atlantic Ocean is used to study the mechanisms of decadal variability in the shallow subtropical‐tropical cells (STCs). The emphasis is on the spatial patterns of the transport anomalies in the tropical thermocline, particularly their manifestation in the equatorial current system and on the relative role of changes in the deep meridional overturning cell (MOC) associated with variations in the formation of Labrador Sea Water (LSW) in the subpolar North Atlantic. Using wind stress and heat flux variations based on NCEP/NCAR‐reanalysis products, the variability of the zonally integrated STC transports is similar to that obtained in a recent regional model study, corroborating the role of both the southern and northern STC in supporting wind‐driven transport anomalies of O(2 Sv) near the equator. Sensitivity experiments indicate that changes in subarctic MOC transports associated with the strong variability in LSW formation during the last decades contributed a signal of O(0.3 Sv) to the upper‐layer equatorial transports. Whereas the local wind‐driven variability clearly dominates on interannual‐decadal timescales and is confined to depths down to 150 m, the weak MOC‐related signal is primarily reflected in an interdecadal modulation of the STC transports. While a strong part in the STC's transport anomalies is associated with the western boundary current (NBC), there is an important contribution also by weaker, interior ocean flow anomalies which tend to counteract the variability of the NBC.

The Bering Strait's grip on the northern hemisphere climate

The last 10,000 years have been characterized by distinctly stable climates. For the earlier glacial period (up to 125,000 years ago), climate records show long-lasting large-amplitude oscillations, generally known as Dansgaard–Oeschger (D/O) or Heinrich events. These fluctuations are believed to be a result of freshwater anomalies in the North Atlantic which dramatically reduce the transport of the meridional overturning cell (MOC). They are followed by a recovery of the MOC.Here, we propose that such long lasting instabilities in the meridional circulation are only possible during glacial periods when the Bering Strait (BS) is closed. An analytical ocean model which includes both wind and thermohaline processes) is used to show that, during interglacial periods (when the BS is open) perturbations in North Atlantic Deep Water (NADW) formation are quickly damped out. This new mechanism involves the strong winds in the Southern Ocean (SO) which, with an open BS, quickly [O(1–10 years)] flush any large low salinity anomalies out of the Atlantic and into the Pacific Ocean. During glacial periods, the stabilizing effect is prevented by the closure of the BS which traps the anomalies within the Atlantic, causing long lasting perturbations. We also show that no continuous fresh-water flux is needed in order to keep the collapsed MOC from a recovery in the closed basin case, but a relatively large continuous flux of 0.19Sv is required in order to keep the collapsed state in the open BS case. With a smaller freshwater transport, the open MOC quickly recovers. This also indicates that an open BS is more stable than the closed BS case.Process-oriented numerical simulations using idealized geometry in a two-layer ocean support our analytical solutions and show that the flushing mechanism is active even when the shallow BS sill is included.

Did an Open Panama Isthmus Correspond to an Invasion of Pacific Water into the Atlantic?

Abstract Recent general circulation simulations suggest that, prior to the closure of the Panama Isthmus (the narrow strip of land connecting North and South America), low-salinity Pacific Ocean water invaded the Atlantic Ocean via the gap between North and South America. According to this scenario, the invasion decreased the Atlantic Ocean salinity to the point at which North Atlantic Deep Water (NADW) formation was impossible and, consequently, there was probably no “conveyer belt.” In line with this scenario, it has been suggested that the closure of the isthmus led to an increased salinity in the Atlantic that, in turn, led to the present-day NADW formation and the familiar meridional overturning cell (MOC). Using simple dynamical principles, analytical modeling, process-oriented numerical experiments, and modern-day wind stress, it is shown that, in the absence of NADW formation, one would expect a westward flow from the Atlantic to the Pacific Ocean through an open Panama Isthmus. This contradicts t...

Low-Latitude Circulation and Mass Transport Pathways in a Model of the Tropical Atlantic Ocean*

An eddy-resolving numerical ocean circulation model is used to investigate the pathways of low-latitude intergyre mass transport associated with the upper limb of the Atlantic meridional overturning cell (MOC). Numerical experiments with and without applied wind stress and an imposed MOC exhibit significant differences in intergyre transport, western boundary current intensity, and mesoscale ring production. The character of interaction between low-latitude wind- and overturning-driven circulation systems is found to be predominantly a linear superposition in the annual mean, even though nonlinearity in the form of diapycnal transport is essential to some segments of the mean pathway. Within a mesoscale band of 10‐100 day period, significant nonlinear enhancement of near-surface variability is observed. In a realistically forced model experiment, a 14 Sv upperocean MOC return flow is partitioned among three pathways connecting the equatorial and tropical wind-driven gyres. A frictional western boundary current with both surface and intermediate depth components is the dominant pathway and accounts for 6.8 Sv of intergyre transport. A diapycnal pathway involving wind-forced equatorial upwelling and interior Ekman transport is responsible for 4.2 Sv. Translating North Brazil Current rings contribute approximately 3.0 Sv of intergyre transport.

Variability of the simulated meridional heat transport in the North Atlantic for the period 1951–1993

A 43‐year ocean model simulation for the period 1951–1993 is analyzed, with a focus on the meridional heat transport (MHT) as a proxy for the strength of meridional overturning cell (MOC) at 25°N. The surface heat flux forcing associated with the North Atlantic Oscillation (NAO) pattern is related to the variability in MHT both on interannual and longer timescales. The manifestation of the interannual and decadal oceanic response to NAO is nonlocal, as evidenced by the concentrated heat content and sea level variability at the Gulf Stream and North Atlantic Current regions and shown by the comparison of the model sea level variability to the altimeter data between low MHT years in mid‐1980s and high MHT years in early 1990s. The same area is singled out by empirical orthogonal function analysis in the leading mode of the sea level. MHT and time series of the leading sea level mode are highly correlated, which reflects the importance of the MOC in determining the variability of the heat content of the whole basin. Also, the model suggests that the MHT/MOC has entered a very strong phase since the mid‐1980s and this trend has continued up to the end of 1993; this behavior follows the general trend in the NAO index.

On the relationship between dense water formation and the “Meridional Overturning Cell” in the North Atlantic Ocean

We analyze the three-dimensional structure of the Meridional Overturning Cell (MOC) in a numerical model of the North Atlantic and Arctic Oceans. The MOC is found to consist of several downward branches: some associated with dense water formation driven by surface buoyancy forcing (Labrador Sea, northern Norwegian Sea and Barents Sea), and some associated with interior mixing processes in the model (most notably entrainment of thermocline waters into the dense overflows at the Greenland–Scotland Ridge). Two experiments with altered surface buoyancy conditions are performed, both resulting in a change in the dense water formation rate in the Labrador Sea. A change of the same sign is observed in the strength of the MOC (and thus in the meridional heat flux); however, the change is less (20–50%) than would be expected from the change in the dense water formation rate. The MOC and the rate of dense water formation at higher latitudes seem not to be tightly linked on the decadal time scale. Instead, significant changes in the volume of water masses may take place. Taking this phenomenon into account may improve the interpretation of the chain of events that constitutes a climate oscillation, whether it be observed in models or in nature.