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  • Author or Editor: Denis L. Volkov x
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Denis L. Volkov

Abstract

The distribution of surface eddy kinetic energy (EKE) depicts main oceanic surface circulation features. The interannual variability of EKE and associated geostrophic velocity anomalies in the North Atlantic Ocean were analyzed to describe the variations in oceanic currents between 1993 and 2002. The sea level anomaly maps of the combined TOPEX/Poseidon + ERS-1/2 and TOPEX/Poseidon-alone satellite altimetry data were used to derive EKE. The study focused on the areas of the Gulf Stream extension (GS), North Atlantic Current (NAC), Azores Current (AC), and the northeastern (Rockall Channel and Iceland Basin) and northwestern (Irminger Basin and Labrador Sea) North Atlantic. The interannual variability of the altimetry-derived EKE field in the GS extension area reflected the meridional displacements of the GS core described in earlier studies. The interannual change of EKE in the AC was characterized by high values in 1993–95 followed by lower EKE in subsequent years. The interannual variability of EKE in the NAC area west of the Mid-Atlantic Ridge exhibited an out-of-phase change between the band centered at ∼47°N and two bands on either side centered at ∼43° and ∼50°N. In the Rockall Channel the geostrophic velocity anomalies indicated an intensified northeastward flow in 1993–95 followed by a relaxation in 1996–2000. The EKE band associated with the NAC branch in the Iceland Basin was found to be extended farther west after 1996, possibly following the North Atlantic Oscillation (NAO)-induced shift of the subpolar front. A rise of EKE was observed in the Irminger Basin from 1995 to 1999. This rise may have been associated with large anticyclonic geostrophic velocity anomalies, which indicated significant weakening of the cyclonic circulation in the Irminger Basin after 1996, and/or with possibly intensified eddy generation mechanisms due to the NAO-induced approach of the subpolar front. The interannual change of EKE in the Labrador Sea did not appear to always follow the atmospheric forcing expressed by NAO. Therefore other eddy generation mechanisms in the Labrador Sea can be important.

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Denis L. Volkov and Lee-Lueng Fu

Abstract

Recent studies have shown that the formation of the well-defined, zonally oriented Azores Current may be the result of water mass transformation associated with the Mediterranean outflow in the Gulf of Cadiz. As the denser Mediterranean water descends down the continental slope, it entrains overlying North Atlantic Central Water. It is believed that the Azores Current then forms as part of the horizontal recirculating gyre generated through the β-plume mechanism. In this study, the authors further explore this hypothesis by performing a series of numerical experiments. These experiments are based on a high-resolution general circulation model that includes the Mediterranean Sea and that realistically simulates the water mass exchange through the Strait of Gibraltar and the transport and variability of the Azores Current. The authors show that the divergence of the relative vorticity flux and the planetary vorticity flux, associated with planetary waves, are the main factors determining the variability of the Azores Current. It is shown experimentally that the closure of the Strait of Gibraltar leads to a complete disappearance of the Azores Current. On the other hand, with the open Strait of Gibraltar, the Azores Current persists even when the wind forcing over the region is turned off. The atmospheric forcing is thus not responsible for the formation of the Azores Current, but it affects the variability of the current with a minor effect on its magnitude. Numerical experiments suggest that the strength and the variability of the Azores Current depend on the magnitude of the water mass exchange through the Strait of Gibraltar but not on its seasonal variability.

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Denis L. Volkov, Molly Baringer, David Smeed, William Johns, and Felix W. Landerer

Abstract

The Mediterranean Sea can be viewed as a “barometer” of the North Atlantic Ocean, because its sea level responds to oceanic-gyre-scale changes in atmospheric pressure and wind forcing, related to the North Atlantic Oscillation (NAO). The climate of the North Atlantic is influenced by the Atlantic meridional overturning circulation (AMOC) as it transports heat from the South Atlantic toward the subpolar North Atlantic. This study reports on a teleconnection between the AMOC transport measured at 26.5°N and the Mediterranean Sea level during 2004–17: a reduced/increased AMOC transport is associated with a higher/lower sea level in the Mediterranean. Processes responsible for this teleconnection are analyzed in detail using available satellite and in situ observations and an atmospheric reanalysis. First, it is shown that on monthly to interannual time scales the AMOC and sea level are both driven by similar NAO-like atmospheric circulation patterns. During a positive/negative NAO state, stronger/weaker trade winds (i) drive northward/southward anomalies of Ekman transport across 26.5°N that directly affect the AMOC and (ii) are associated with westward/eastward winds over the Strait of Gibraltar that force water to flow out of/into the Mediterranean Sea and thus change its average sea level. Second, it is demonstrated that interannual changes in the AMOC transport can lead to thermosteric sea level anomalies near the North Atlantic eastern boundary. These anomalies can (i) reach the Strait of Gibraltar and cause sea level changes in the Mediterranean Sea and (ii) represent a mechanism for negative feedback on the AMOC.

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Molly Baringer, Mariana B. Bif, Tim Boyer, Seth M. Bushinsky, Brendan R. Carter, Ivona Cetinić, Don P. Chambers, Lijing Cheng, Sanai Chiba, Minhan Dai, Catia M. Domingues, Shenfu Dong, Andrea J. Fassbender, Richard A. Feely, Eleanor Frajka-Williams, Bryan A. Franz, John Gilson, Gustavo Goni, Benjamin D. Hamlington, Zeng-Zhen Hu, Boyin Huang, Masayoshi Ishii, Svetlana Jevrejeva, William E. Johns, Gregory C. Johnson, Kenneth S. Johnson, John Kennedy, Marion Kersalé, Rachel E. Killick, Peter Landschützer, Matthias Lankhorst, Tong Lee, Eric Leuliette, Feili Li, Eric Lindstrom, Ricardo Locarnini, Susan Lozier, John M. Lyman, John J. Marra, Christopher S. Meinen, Mark A. Merrifield, Gary T. Mitchum, Ben Moat, Didier Monselesan, R. Steven Nerem, Renellys C. Perez, Sarah G. Purkey, Darren Rayner, James Reagan, Nicholas Rome, Alejandra Sanchez-Franks, Claudia Schmid, Joel P. Scott, Uwe Send, David A. Siegel, David A. Smeed, Sabrina Speich, Paul W. Stackhouse Jr., William Sweet, Yuichiro Takeshita, Philip R. Thompson, Joaquin A. Triñanes, Martin Visbeck, Denis L. Volkov, Rik Wanninkhof, Robert A. Weller, Toby K. Westberry, Matthew J. Widlansky, Susan E. Wijffels, Anne C. Wilber, Lisan Yu, Weidong Yu, and Huai-Min Zhang
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