The warm-water “Halo” around Maud Rise: Properties, dynamics and Impact

• Published in: Deep-sea research. Part I, Oceanographic research papers Vol. 54; no. 6; pp. 871 - 896
• Main Authors: de Steur, L., Holland, D.M., Muench, R.D., McPhee, M.G.
• Format: Journal Article
• Language: English
• Published: Oxford Elsevier Ltd 01-06-2007; Elsevier; Pergamon Press Inc
• Subjects: Dynamics of the ocean (upper and deep oceans); Earth, ocean, space; Eddies; Exact sciences and technology; External geophysics; Flow–topography interactions; Hydrography; Marine; Maud Rise seamount; Ocean-atmosphere interaction; Oceanography; Oceans; Physics of the oceans; Upper-ocean processes; Weddell Sea; Winter; Antarctica; Weddell Sea; PSW, Antarctic Ocean, Maud Rise; PSW, Weddell Sea; Maud Rise seamount; Weddell Sea; Upper-ocean processes; Eddies; Flow–topography interactions; models; Vortex shedding; Anticyclonic eddy; topography; Measurement campaign; Deep water; Flow-topography interactions; Annular configuration; Ocean bottom; Polynya; Cyclonic eddy; Marine hydrography; Current measurement; sea ice; Antarctic Ocean; polar regions; Ventilation; Polar region; Seamount
• ISSN: 0967-0637; 1879-0119
• DOI: 10.1016/j.dsr.2007.03.009

Regional hydrographic and current observations from the 2005 MaudNESS winter field campaign in the Maud Rise seamount region of the eastern Weddell Sea show that an annular Halo consisting largely of Warm Deep Water (WDW) encircled the Rise at depths just below the mixed layer. The Halo was associated with elevated isopycnals and, on the northern flank of the Rise, strong subsurface velocities up to 20
cm
s
−1. Intercomparison of these observations with winter 1986 and 1994 conditions confirms the presence of the Halo and suggests that it, and associated warm pools west of the Rise, are at least semipermanent features of the region. These observational results compare well with the output from an isopycnic ocean model for a variety of parameters including shape of the seamount, inflow conditions and vertical stratification. The model captures processes associated with a steady westward flow impinging on the isolated seamount and shows (1) that the dynamics of the warm-water Halo with a shallow mixed layer are related to the formation of a jet surrounding the Rise and the overlying Taylor column and (2) that eddies of alternating sign (cyclones and anticyclones) are formed from instability of the jet-like flow structure, and are subsequently shed from the western flanks of the Rise. The eddies closest to the rise are dominated by cyclones which tend to adhere to the flanks more strongly than anticyclones. The formation and passage of approximately 3–5 eddies per year is seen in the sea-surface-height anomalies over a 12-year period. Despite apparent spatial and temporal variability in the dynamics of the Halo and shedding of eddies, the time-mean picture is such that significantly elevated isopycnals with WDW below the mixed layer are always present on the flanks of Maud Rise. This mechanism likely contributes annually to earlier seasonal ice loss in the eastern Weddell Sea than farther west. For unusually strong inflow conditions, possibly due to large-scale interannual variability, the Halo becomes more intense and overlies a much larger part of Maud Rise, potentially preconditioning the area for deep ocean ventilation and a subsequent polynya event such as observed in the 1970s.