scholarly journals Causes of Interannual–Decadal Variability in the Meridional Overturning Circulation of the Midlatitude North Atlantic Ocean

2008 ◽  
Vol 21 (24) ◽  
pp. 6599-6615 ◽  
Author(s):  
Arne Biastoch ◽  
Claus W. Böning ◽  
Julia Getzlaff ◽  
Jean-Marc Molines ◽  
Gurvan Madec
Keyword(s):  
North Atlantic ◽  
Decadal Variability ◽  
North Atlantic Ocean ◽  
Meridional Overturning Circulation ◽  
Global Ocean ◽  
Overturning Circulation ◽  
Linear Character ◽  
The North ◽  
Basin Scale ◽  
Meridional Overturning

Abstract The causes and characteristics of interannual–decadal variability of the meridional overturning circulation (MOC) in the North Atlantic are investigated with a suite of basin-scale ocean models [the Family of Linked Atlantic Model Experiments (FLAME)] and global ocean–ice models (ORCA), varying in resolution from medium to eddy resolving (½°–1/12°), using various forcing configurations built on bulk formulations invoking atmospheric reanalysis products. Comparison of the model hindcasts indicates similar MOC variability characteristics on time scales up to a decade; both model architectures also simulate an upward trend in MOC strength between the early 1970s and mid-1990s. The causes of the MOC changes are examined by perturbation experiments aimed selectively at the response to individual forcing components. The solutions emphasize an inherently linear character of the midlatitude MOC variability by demonstrating that the anomalies of a (non–eddy resolving) hindcast simulation can be understood as a superposition of decadal and longer-term signals originating from thermohaline forcing variability, and a higher-frequency wind-driven variability. The thermohaline MOC signal is linked to the variability in subarctic deep-water formation, and rapidly progressing to the tropical Atlantic. However, throughout the subtropical and midlatitude North Atlantic, this signal is effectively masked by stronger MOC variability related to wind forcing and, especially north of 30°–35°N, by internally induced (eddy) fluctuations.

scholarly journals Does Net E − P Set a Preference for North Atlantic Sinking?

2012 ◽  
Vol 42 (11) ◽  
pp. 1781-1792 ◽  
Author(s):  
Selma E. Huisman ◽  
Henk A. Dijkstra ◽  
A. S. von der Heydt ◽  
W. P. M. de Ruijter
Keyword(s):  
North Atlantic ◽  
Deep Water ◽  
North Pacific ◽  
Multiple Equilibria ◽  
Meridional Overturning Circulation ◽  
Global Ocean ◽  
Overturning Circulation ◽  
The North ◽  
Meridional Overturning ◽  
The North Pacific

Abstract The present-day global meridional overturning circulation (MOC) with formation of North Atlantic Deep Water (NADW) and the absence of a deep-water formation in the North Pacific is often considered to be caused by the fact that the North Pacific basin is a net precipitative, while the North Atlantic is a net evaporative basin. In this paper, the authors study the effect of asymmetries in continent geometry and freshwater fluxes on the MOC both in an idealized two-dimensional model and in a global ocean model. This study approaches the problem from a multiple equilibria perspective, where asymmetries in external factors constrain the existence of steady MOC patterns. Both this multiple equilibria perspective and the fact that a realistic global geometry is used add new aspects to the problem. In the global model, it is shown that the Atlantic forced by net precipitation can have a meridional overturning circulation with northern sinking and a sea surface salinity that resembles the present-day salinity field. The model results are suggestive of the importance of factors other than the freshwater flux asymmetries, in particular continental asymmetries, in producing the meridional overturning asymmetry.

scholarly journals Impact of North Atlantic Freshwater Forcing on the Pacific Meridional Overturning Circulation under Glacial and Interglacial Conditions

2019 ◽  
Vol 32 (15) ◽  
pp. 4641-4659
Author(s):  
Hyo-Jeong Kim ◽  
Soon-Il An
Keyword(s):  
North Atlantic ◽  
North Pacific ◽  
Meridional Overturning Circulation ◽  
Global Ocean ◽  
Overturning Circulation ◽  
The North ◽  
Freshwater Forcing ◽  
The Pacific ◽  
Meridional Overturning ◽  
The North Pacific

Abstract The Pacific meridional overturning circulation (PMOC) is not well known compared to the Atlantic meridional overturning circulation (AMOC), due to its absence today. However, considering PMOC development under different climate conditions shown by proxy and modeling studies, a better understanding of PMOC is appropriate to properly assess the past and future climate change associated with global ocean circulation. Here, the PMOC response to freshwater forcing in the North Atlantic (NA) is investigated using an Earth system model of intermediate complexity under glacial (i.e., Last Glacial Maximum) and interglacial [i.e., preindustrial with/without inflow through Bering Strait (BS)] conditions. The water hosing over NA led to the shutdown of the AMOC, which accompanied an active PMOC except for the preindustrial condition with the opening BS, indicating that the emergence of the PMOC is constrained by the freshwater inflow through the BS, which hinders its destabilization through enhancing ocean stratification. However, the closure of the BS itself could not explain how the sinking motion is maintained in the North Pacific. Here we found that various atmospheric and oceanic processes are involved to sustain the active PMOC. First, an atmospheric teleconnection associated with the collapsed AMOC encouraged the evaporation in the sinking region, causing buoyancy loss at the surface of the North Pacific. Second, the strengthened subpolar gyre transported saltier water northward, enhancing dense water formation. Finally, the vigorous upwelling in the Southern Ocean enabled a consistent mass supply to the sinking region, with the aid of enhanced westerlies.

scholarly journals On the Loss of Wind-Induced Near-Inertial Energy to Turbulent Mixing in the Upper Ocean

2009 ◽  
Vol 39 (11) ◽  
pp. 3040-3045 ◽  
Author(s):  
Xiaoming Zhai ◽  
Richard J. Greatbatch ◽  
Carsten Eden ◽  
Toshiyuki Hibiya
Keyword(s):  
Turbulent Mixing ◽  
North Atlantic Ocean ◽  
Realistic Model ◽  
Meridional Overturning Circulation ◽  
Global Ocean ◽  
Ocean Mixing ◽  
Overturning Circulation ◽  
The North ◽  
Meridional Overturning ◽  
Inertial Energy

Abstract Wind-induced near-inertial energy has been believed to be an important source for generating the ocean mixing required to maintain the global meridional overturning circulation. In the present study, the near-inertial energy budget in a realistic model of the North Atlantic Ocean driven by synoptically varying wind forcing is examined. The authors find that nearly 70% of the wind-induced near-inertial energy at the sea surface is lost to turbulent mixing within the top 200 m and, hence, is not available to generate diapycnal mixing at greater depth. Assuming this result can be extended to the global ocean, it is estimated that the wind-induced near-inertial energy available for ocean mixing at depth is, at most, 0.1 TW. This confirms a recent suggestion that the role of wind-induced near-inertial energy in sustaining the global overturning circulation might have been overemphasized.

scholarly journals Observed Basin-Scale Response of the North Atlantic Meridional Overturning Circulation to Wind Stress Forcing

2017 ◽  
Vol 30 (6) ◽  
pp. 2029-2054 ◽  
Author(s):  
Shane Elipot ◽  
Eleanor Frajka-Williams ◽  
Chris W. Hughes ◽  
Sofia Olhede ◽  
Matthias Lankhorst
Keyword(s):  
Time Series ◽  
Wind Stress ◽  
North Atlantic ◽  
Atlantic Meridional Overturning Circulation ◽  
Meridional Overturning Circulation ◽  
Overturning Circulation ◽  
The North ◽  
Basin Scale ◽  
Meridional Overturning ◽  
The North Atlantic

Abstract The response of the North Atlantic meridional overturning circulation (MOC) to wind stress forcing is investigated from an observational standpoint, using four time series of overturning transports below and relative to 1000 m, overlapping by 3.6 yr. These time series are derived from four mooring arrays located on the western boundary of the North Atlantic: the RAPID Western Atlantic Variability Experiment (WAVE) array (42.5°N), the Woods Hole Oceanographic Institution Line W array (39°N), RAPID–MOC/MOCHA (26.5°N), and the Meridional Overturning Variability Experiment (MOVE) array (16°N). Using modal decompositions of the analytic cross-correlation between transports and wind stress, the basin-scale wind stress is shown to significantly drive the MOC coherently at four latitudes, on the time scales available for this study. The dominant mode of covariance is interpreted as rapid barotropic oceanic adjustments to wind stress forcing, eventually forming two counterrotating Ekman overturning cells centered on the tropics and subtropical gyre. A second mode of covariance appears related to patterns of wind stress and wind stress curl associated with the North Atlantic Oscillation, spinning anomalous horizontal circulations that likely interact with topography to form overturning cells.

scholarly journals Impact of Agulhas Leakage on the Atlantic Overturning Circulation in the CCSM4

2014 ◽  
Vol 27 (1) ◽  
pp. 101-110 ◽  
Author(s):  
Wilbert Weijer ◽  
Erik van Sebille
Keyword(s):  
North Atlantic ◽  
South Atlantic ◽  
Meridional Overturning Circulation ◽  
Overturning Circulation ◽  
Climate System Model ◽  
The North ◽  
Salinity Variability ◽  
Meridional Overturning ◽  
The North Atlantic ◽  
The Impact

Abstract The impact of Agulhas leakage variability on the strength of the Atlantic meridional overturning circulation (AMOC) in the Community Climate System Model, version 4 (CCSM4) is investigated. In this model an advective connection exists that transports salinity anomalies from the Agulhas region into the North Atlantic on decadal (30–40 yr) time scales. However, there is no identifiable impact of Agulhas leakage on the strength of the AMOC, suggesting that the salinity variations are too weak to significantly modify the stratification in the North Atlantic. It is argued that this study is inconclusive with respect to an impact of Agulhas leakage on the AMOC. Salinity biases leave the South Atlantic and Indian Oceans too homogeneous, in particular erasing the observed salinity front in the Agulhas retroflection region. Consequently, salinity variability in the southeastern South Atlantic is found to be much weaker than observed.

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