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Open Ocean Deep Convection Building a Thermocline Convection in A Open ocean deep convection cool, suck Convection ‘takes away’ stratification cold subduction PV 0 eddies Eddies flux PV in from the ‘outside’ warm, pump Building a thermocline Warming and pumping eddies ‘creates’ stratification thermocline PV 0 warm subduction Eddies flux PV ‘away’ warm, suck warm, pump cool, suck Convection in a global context Role of convection in the general circulation GFD experiments in climate Pole Eq Pole I. Open Ocean Deep Convection John Marshall, MIT 1 Why is convection important? Laboratory analogue 2 Why is convection interesting? • Modified by rotation • Interplay between convection and baroclinic instability 3 Parameterization Not just of deep convection but lateral processes in mixed layers in general Melting of an ice disk floating on a rotating homogeneous fluid 1 Why is ocean convection important? 1 km Wintertime mixed layer depth (m) Lecture III • Associated (in an as yet unclear way) with the sinking branch of thermohaline circulation • Primary agency of water- mass transformation Salinity in Atlantic Phases of open-ocean deep convection preconditioning cool, suck It’s a swallowing process - not a ‘chimney’ mixing Think of a snake swallowing an egg spreading 2 Why is Ocean convection interesting? Involves the interplay of two dynamical processes • upright convection modified by rotation, ‘plumes’ - mix stratification away to create a ‘mixed patch’ • baroclinic instability, ‘eddies’ - control lateral exchange between the mixed patch and ambient fluid Studied in laboratory, numerical and field experiments Ultimate, ultra-non-hard…. 2 3 buoyancy flux Scaling ideas v.v.very high Ra Bom s 23 Initially, for small times Ra 10 Scales can be expressed in terms of ‘l ’ Deardorff, 1985 1 Bo w u Bol 3 (i) Scale constrained by depth of ocean (ii) Scale constrained by earth’s rotation 1 Fernando et al, 1991 Non-dimensional l rot 1 B 2 R Jones and Marshall, 1993 number o H H f3 Maxworthy and Narimousa, 1994 Dynamical ideas • Extract buoyancy from surface of homogeneous, rotating ocean Jones and Marshall, 1993 Helfrich 1994 H Numbers f 104s1 Ocean - rotation important B 107m2s3 H 1km Natural Rossby number R 0. 1 1 1 lrot 1km o R l rot 1 B 2 o H H f3 Atmosphere - rotation not important Radius of deformation on convective scale l B 102m2s3 H 10km R H o lrot 100km Ro 10 50 Role of lateral inhomogeneities Spatially homogeneous Draws buoyancy 2Bt vertically h N Lateral homogeneities are a Spatially inhomogeneous ubiquitous feature of mixed layers B Deep convection is just an extreme example Draws buoyancy laterally Slantwise convection PV fbz uzby No zonal pressure gradients 1 B Slantwise convection in 2 2 d model Du Dt fv 0 D Dy Dt u fy 0 v Dt z y thermal wind m surface b surface angular momentum buoyancy m u fy b Convection sets PV 0 not N2 Haine and Marshall, 1997 Interplay between convection and baroclinic instability cool Vertical velocity w Relax the angular momentum constraint Upright convection Jack Whitehead Convection Baroclinic instability reminiscent of metrological flows Baroclinic instability Eddies flux buoyancy vertically to offset loss from the surface Mixed patches Melting ice disc floating on rotating fluid Helen Jones Marshall and Schott, 1999 R 102 o Baroclinic instability is a prime agency Reminds one of meteorology of vertical buoyancy transport Restratification and spreading Helen Jones John Lazier March October Hydrographic section Depth across Labrador Sea Visbeck, Marshall and Jones, JPO, 1996 Scaling for patches Jones and Marshall, JPO, 1997 Fox-Kemperer, Ferrari, JPO, 2007 v b Kb y H where K v l L H ; v u f b y l L L wb b y H s L H vb b z s s time to hence restratify H2 always v b c f b z b y down-gradient and for c=0.027 2 2 H always upward w b c f b y Prediction from scaling 3 Parameterization (impossible) • typically plumes are represented as a mixing process setting large- scale PV field to zero. - adjustment - enhanced vertical mixing - entrainment at base • challenge lies in representation of geostrophic eddy scales Eddies are a (the?) primary agency of vertical as well as lateral heat transport Take a residual-mean perspective Review: Eulerian/Residual mean cool, suck Eulerian-mean buoyancy budget vb z v b w b vb wb B y z y z z y Decompose eddy flux (Held and Schneider, 1999) v b ,w b =+w b /s,w b v b w b /s,0 isopycnal horizontal b y Must recognize that this where s is the isopycnal slope can be represented as b z an advective flux. Gent and McWilliams, 1990 Residual-mean buoyancy budget isopycnal w b /s,w b v b y w b z 0 where w b b y leftover 1 w b 1 v b w b /s ,0 1 v b where s y v b v b B R z R R where v , and R z y R Bs v David Marshall, JMR,1997 vres v v Examples Mixed layer restratification in an idealized numerical experiment MITgcm Fox-Kemper, Ferrari et al, 2007 3-d model Parameterized, 2d model w b b y 2 2 H w b c f b y •1.5 days, 5-6 Aug 2006 •Mixed layer restratifies under weakening wind AESOP Observations of Rapid forcing Restratification near Monterey Bay •Characterized mixed layer evolution in Lagrangian (float- following) frame. After one day 30 kt wind 10 kt wind Courtesy E. D’Asaro Ubiquitous feature of mixed-layer evolution Baylor Fox-Kemperer, MIT Summary • Ocean is stiffened by rotation on the convective scale • Plumes are mixing agents set PV to zero • Geostrophic eddies are a primary agency of vertical as well as lateral heat transport profound implications for parameterization Some aspects are of general interest • Importance of natural Rossby number • Little scale separation between convective and geostrophic eddy scales • Spatial inhomogeneity Upright convection baroclinic instability Reviews OPEN-OCEAN CONVECTION: OBSERVATIONS,THEORY AND MODELS Marshall, J and F. Schott Reviews of Geophysics, 37, 1, 1999 General comments A O Atmosphere • dynamics of mixed patches Baroclinic instability as an agency of vertical buoyancy transport • modern eddy parameterizations GM TEM Ocean • lateral inhomogeneities completely change the Rayleigh problem 1 2 • importance of R l rot 1 B o H H f3 Does the l ro t scale appear in the atmosphere?.
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