Dengler, Marcus

Greatbatch, Richard John

Kanzow, Torsten

Krahmann, Gerd

Thomsen, Sören

OceanRep

Near-inertial waves interacting with a coherent anticyclone off PeruThomsen, S.1, Greatbatch R. J.1, Dengler M.1, Kanzow T.2, Krahmann G.1  (1) GEOMAR Helmholtz Centre for Ocean Research Kiel, Germany (2) AWI Helmholtz Centre for Polar and Marine Research, Bremerhaven, GermanyMotivationExperimentEddy formationNear-inertial wavesThe interaction between near-inertial wave propagation and geostrophic flow was already investiga-ted by Kunze (1985). Anticyclones can trap and enhance downward propagation of near-inertial wave energy. A critical-layer can be formed below these eddies where the associated vorticity ano-maly vanishes. Several recent model studies point out the importance of this eddy near-inertial wave interaction for the downward transport of near-inertial energy into the deeper ocean. There it could provide an energy source for small scale dissipation. However, observations of critical layer trapping are rare.A multi-platform observational study based on several gliders, moorings and shipboard measure-ments was carried out off Peru in January / February 2013 to investigate the interaction between mesoscale eddies and near-inertial waves.A coherent anticyclone formed in the study area allowing detailed investigation of its impact on the near-inertial energy distribution.Figure 1:  Mean sea surface temperature in Jan/Feb 2013 off Peru from MODIS in color (left). On the right: water depth (grey con-tours, 200 m interval), glider tracks (colored lines), CTD stations (black circles) and the two mooring positions (blue squares).Figure 2: Left column: depth-averaged horizontal circulation for eight periods based on vmADCP (blue), moored ADCP (red) and glider drift-inferred velocities (black). The middle and right column show the temporal evolution of the along- and cross-shore velocity components (vmADCP) respec-tively along the grey transects (left column). Isopycnals (25.6, 26.2 and 26.4) in grey. 13°S Jan 8 - 11 0.3 m/sJan 28 - Feb 3  78°WJan 19 - 22 12.5°Jan 22 - 27Feb  7 - 11Feb  17 - 22Feb 23 - 27 13°S 12.5° 13°S 12.5° 13°S 12.5° 13°S 12.5° 13°S 12.5° 13°S 12.5°  77°glidershipmooringdgj100300−0.200.2100300100300100300100300100300100300 150   100   50   150  100   50m/sa cbe fh ik lm onp rqs t u  distance to coast / km[m] Alongshore velocity Cross-shore velocity Jan 8-11   Jan 19-22   Jan 22-27   Jan    28-Feb 3   Feb 9   Feb 19-22   Feb 22-23Peru-Chile UndercurrentEddy formationEddyEddy moves offshoreFlow separationUndercurrent recoversEddy generation mechanismMarshall and Tansley [2001] propose that the separation of a barotropic boundary current at a verti-cal sidewall takes place when r < L = (U/β)^1/2. Using a modified condition for flow separation of a boundary current accounting for topographic beta, it is shown that the conditions for flow separation is indeed fulfilled.Discussion400 meddypath78°W 77°13°S12°Sradius0.02 0.04 0.06 0.0802.57.5x1012°S 13°6 810121416bendradius[km]Leffective betaalong 400 m isobath[s  ]topographic slope(b)(a)5Figure 3: Left: bottom slope in color and depth (grey). The radius of the topographic curvature and the 400 m isobath are shown in black. The three black crosses indicate the position of the eddy centre at three different time periods (Jan. 22 - 27, Jan. 28 - Feb. 3 and Feb. 7 - 11). The right panel shows the effective beta (β eff) (black) and the resulting length scale L =(U/β_eff)^1/2 (red) along the 400 m isobath.50100150200250300Cross-shore velocity component (6 day lowpassed)−0.2−0.100.10.250100150200250300−0.1−0.0500.050.10.15−0.100.113 20 27 03 10 17 24 03−0.100.1Cross-shore velocity component (NIW bandpassed)Cross-shore velocity component in 407m (NIW bandpassed)Cross-shore velocity component in 703m (NIW bandpassed)[m][m]m/sm/sm/sm/sFigure 5 - Cross-shore velocity component from moored ADCP measurements bet-ween 50 - 300m (a)  low passed (6 days), (b) NIW bandpassed (1.5-3 days) and RCM measurements at (c) 407 m and (d) 703m both NIW bandpassed.Figure 4: Along-shore velocity (upper panel) and cross-shore velocities (lower panel) through the eddy observed by vessel mounted ADCPs. Te middle panel show the position of the tran-sects and the depth averaged near-surface velocity.Enhanced near-inertial energy (NIE) is found at the eddy base possiibly due to downward propagati-on of NIE within the anticyclone and NIW accumulation at a critical layer below.What are the sinks of NIE at the critical depth ? Kunze et al. (1995) suggest three possible pathways: into (1) mean flow, (2) untrapped waves or (3) dissipationReferences:Eric Kunze, 1985: Near-Inertial Wave Propagation In Geostrophic Shear. J. Phys. Oceanogr., 15, 544–565.Eric Kunze, Raymond W. Schmitt, and John M. Toole, 1995: The Energy Balance in a Warm-Core Ring's Near-Inertial Critical Layer. J. Phys. Oceanogr., 25, 942–957.Marshall, D. P., and C. E. Tansley (2001), An implicit formula for boundary current separation, Journal of Physical Oceanography, 31(6), 1633–1638, doi:10.1175/1520- 0485(2001)031¡1633:AIFFBC¿2.0.CO;2.distance to coast in kmdepth in malong−shore velocity 09−Feb−2013 12:09 − 09−Feb−2013 22:01−160 −140 −120 −100 −80 −60 −40 −20 0−500−400−300−200−1000 cm/s−0.3−0.2−0.100.10.20.3distance to coast in kmdepth in mcross−shore velocity 09−Feb−2013 12:09 − 09−Feb−2013 22:01−160 −140 −120 −100 −80 −60 −40 −20 0−500−400−300−200−1000 cm/s−0.3−0.2−0.100.10.20.3distance to coast in kmdepth in malong−shore velocity 08−Feb−2013 02:17 − 08−Feb−2013 08:03−160 −140 −120 −100 −80 −60 −40 −20 0−500−400−300−200−1000 cm/s−0.3−0.2−0.100.10.20.3distance to coast in kmdepth in mcross−shore velocity 08−Feb−2013 02:17 − 08−Feb−2013 08:03−160 −140 −120 −100 −80 −60 −40 −20 0−500−400−300−200−1000 cm/s−0.3−0.2−0.100.10.20.3   78oW  30’   77oW  13oS 30’   12oS0.5 m/sm93 transects eddy − mean vel. (57−305m)transect1transect2transect3transect4Transect 3Transect 1Kunze 1985 Kunze et al 1995

Near-inertial waves interacting with a coherent anticyclone off Peru

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