Wind-forced interannual variability of the Atlantic Meridional Overturning Circulation at 26.5°N

Zhao, Jianrong; Johns, William E.

doi:10.1002/2013jc009407

Cited by 118 publications

(121 citation statements)

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“…It is becoming clear that changes in the geostrophic contribution to the AMOC often involve a complex oceanic adjustment to the wind forcing. Zhao and Johns () show that most of the observed interannual variability in overturning and its components at 26°N can be reproduced in a linear, two‐layer model forced only by winds (Figure ).…”

Section: Monitoring and Understanding Observed Amoc Variability At 26°nmentioning

confidence: 99%

Recent Contributions of Theory to Our Understanding of the Atlantic Meridional Overturning Circulation

et al. 2019

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Revolutionary observational arrays, together with a new generation of ocean and climate models, have provided new and intriguing insights into the Atlantic Meridional Overturning Circulation (AMOC) over the last two decades. Theoretical models have also changed our view of the AMOC, providing a dynamical framework for understanding the new observations and the results of complex models. In this paper we review recent advances in conceptual understanding of the processes maintaining the AMOC. We discuss recent theoretical models that address issues such as the interplay between surface buoyancy and wind forcing, the extent to which the AMOC is adiabatic, the importance of mesoscale eddies, the interaction between the middepth North Atlantic Deep Water cell and the abyssal Antarctic Bottom Water cell, the role of basin geometry and bathymetry, and the importance of a three‐dimensional multiple‐basin perspective. We review new paradigms for deep water formation in the high‐latitude North Atlantic and the impact of diapycnal mixing on vertical motion in the ocean interior. And we discuss advances in our understanding of the AMOC's stability and its scaling with large‐scale meridional density gradients. Along with reviewing theories for the mean AMOC, we consider models of AMOC variability and discuss what we have learned from theory about the detection and meridional propagation of AMOC anomalies. Simple theoretical models remain a vital and powerful tool for articulating our understanding of the AMOC and identifying the processes that are most critical to represent accurately in the next generation of numerical ocean and climate models.

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Section: Monitoring and Understanding Observed Amoc Variability At 26°nmentioning

confidence: 99%

Recent Contributions of Theory to Our Understanding of the Atlantic Meridional Overturning Circulation

et al. 2019

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“…Huthnance (2004) indicated that coastal tide gauges can be especially effective monitors for the largest-scale oceanic signals. Zhao and Johns (2014) shows that wind-driven variations over the sub-tropical gyre can result in coherent variations along the GS path over thousands of kilometers, so that the open oceanshelf transmitting mechanism discussed by Huthnance seems possible for those GS signals. There are also some differences in the CTW's characteristics between waves with periods shorter or longer than ∼10 days, whereas in the higher frequency range, near-resonance CTW can amplify the coastal response (Huthnance 2004).…”

Section: Introductionmentioning

confidence: 99%

“…The motivation for this study is twofold: First, the exact mechanism of the GS-driven CSL has not been completely explained or separated from other drivers of CSL. For example, coherent variations of CSL variations along the coast can also result from wind-driven sea level (Woodworth et al 2014;Zhao and Johns 2014) vertical divergence of large-scale ocean currents (Thompson and Mitchum 2014), from the impact of atmospheric pressure (Piecuch and Ponte 2015) or changes in the southward flowing Slope Current (Rossby et al 2010); changes in the latter may relate to climatic change in sub-polar regions (Hakkinen and Rhines 2004) or in the flow of coastal Labrador waters into the region (Xu and Oey 2011). Second, even if the GS contributes to variations in CSL, it is not clear on what time scales this driver may be valid.…”

Section: Introductionmentioning

confidence: 99%

Can the Gulf Stream induce coherent short-term fluctuations in sea level along the US East Coast? A modeling study

Ezer

2016

Ocean Dynamics

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Much attention has been given in recent years to observations and models that show that variations in the transport of the Atlantic Meridional Overturning Circulation (AMOC) and in the Gulf Stream (GS) can contribute to interannual, decadal, and multi-decadal variations in coastal sea level (CSL) along the US East Coast. However, less is known about the impact of short-term (time scales of days to weeks) fluctuations in the GS and their impact on CSL anomalies. Some observations suggest that these anomalies can cause unpredictable minor tidal flooding in low-lying areas when the GS suddenly weakens. Can these short-term CSL variations be attributed to changes in the transport of the GS? An idealized numerical model of the GS has been set up to test this proposition. The regional model uses a 1/12°grid with a simplified coastline to eliminate impacts from estuaries and small-scale coastal features and thus isolate the GS impact. The GS in the model is driven by inflows/outflows, representing the Florida Current (FC), the Slope Current (SC), and the Sargasso Sea (SS) flows. Forcing the model with an oscillatory FC transport with a period of 2, 5, and 10 days produced coherent CSL variations from Florida to the Gulf of Maine with similar periods. However, when imposing variations in the transports of the SC or the SS, they induce CSL variations only north of Cape Hatteras. The suggested mechanism is that variations in GS transport produce variations in sea level gradient across the entire GS length and this large-scale signal is then transmitted into the shelf by the generation of coastal-trapped waves (CTW). In this idealized model, the CSL variations induced by variations of ∼10 Sv in the transport of the GS are found to resemble CSL variations induced by ∼5 m s −1 zonal wind fluctuations, though the mechanisms of wind-driven and GS-driven sea level are quite different. Better understanding of the relation between variations in offshore currents and CSL will help to improve the prediction of both short-term water level anomalies that cause flooding, as well as spatial variations in long-term sea level variability and coastal sea level rise.

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“…Numerical investigations into the sources of variability to the Atlantic MOC interannual variability suggest that much of the variability may be attributable to winds (Cabanes et al, 2008;Roberts et al, 2013;Zhao and Johns, 2014;Yang, 2015;Pillar et al, 2016). Buoyancy forcing instead affects decadal variations (Polo et al, 2014;Yeager, 2015).…”

Section: Introductionmentioning

confidence: 99%

Compensation between meridional flow components of the Atlantic MOC at 26° N

et al. 2016

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Abstract. From ten years of observations of the Atlantic meridional overturning circulation (MOC) at 26 • N (2004-2014), we revisit the question of flow compensation between components of the circulation. Contrasting with early results from the observations, transport variations of the Florida Current (FC) and upper mid-ocean (UMO) transports (top 1000 m east of the Bahamas) are now found to compensate on sub-annual timescales. The observed compensation between the FC and UMO transports is associated with horizontal circulation and means that this part of the correlated variability does not project onto the MOC. A deep baroclinic response to wind-forcing (Ekman transport) is also found in the lower North Atlantic Deep Water (LNADW; 3000-5000 m) transport. In contrast, co-variability between Ekman and the LNADW transports does contribute to overturning. On longer timescales, the southward UMO transport has continued to strengthen, resulting in a continued decline of the MOC. Most of this interannual variability of the MOC can be traced to changes in isopycnal displacements on the western boundary, within the top 1000 m and below 2000 m. Substantial trends are observed in isopycnal displacements in the deep ocean, underscoring the importance of deep boundary measurements to capture the variability of the Atlantic MOC.

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Wind-forced interannual variability of the Atlantic Meridional Overturning Circulation at 26.5°N

Cited by 118 publications

References 52 publications

Recent Contributions of Theory to Our Understanding of the Atlantic Meridional Overturning Circulation

Recent Contributions of Theory to Our Understanding of the Atlantic Meridional Overturning Circulation

Can the Gulf Stream induce coherent short-term fluctuations in sea level along the US East Coast? A modeling study

Compensation between meridional flow components of the Atlantic MOC at 26° N

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Wind-forced interannual variability of the Atlantic Meridional Overturning Circulation at 26.5°N

Cited by 118 publications

References 52 publications

Recent Contributions of Theory to Our Understanding of the Atlantic Meridional Overturning Circulation

Recent Contributions of Theory to Our Understanding of the Atlantic Meridional Overturning Circulation

Can the Gulf Stream induce coherent short-term fluctuations in sea level along the US East Coast? A modeling study

Compensation between meridional flow components of the Atlantic MOC at 26° N

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Compensation between meridional flow components of the Atlantic MOC at 26° N