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Deep Convective Plumes in the Ocean

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Deep Convective Plumes in the Ocean FEATURE DEEP CONVECTIVE PLUMES IN THE OCEAN By Terri Paluszkiewicz, Roland W. Garwood and Donald W. Denbo DEEP CONVECTION is a process in which recent advances in the study of deep open-ocean the mesoscale ocean circulation and atmospheric convective plumes. forcing work together to weaken the ambient Deep convection occurs primarily in the Green- This process plays stratification and to cause surface waters to sink land, Labrador, Weddell, and Mediterranean Seas, to great depths with large vertical velocities. and the water masses formed in these locations an important role in This process plays an important role in bottom contribute to the deep circulation. Estimates of the bottom and and intermediate water formation and ultimately amount of deep water formed by open-ocean deep in the large-scale thermohaline circulation, convection range from 5 × 106 to 10 × 106 m 3 intermediate water which in turn plays a central part in determining s-~; this is of the same order of magnitude as the global climate. Until now, our understanding of estimates of deep water produced near ocean formation and deep convection was based on observations and boundaries (7 × 106 to 10 × 106 m 3 s -~) (Sankey, ultimately in the models of the mesoscale effects of deep convec- 1973; Killworth, 1979, 1983; Carmack, 1990). tion. With recent advances in numerical model- During the preconditioning phase of deep convec- large-scale ing techniques and instrumentation, we have tion, a background of low static stability is cre- thermohaline begun to study the role of the mixing elements: ated. This is followed by a mixing phase that oc- the deep, penetrative convective plumes shown curs where preconditioning has been active and is circulation . in Figure I. These plumes are turbulent, have a characterized by intense and rapid cooling, evapo- large vertical scale and have attributes that are ration and/or freezing. Surface waters then become unique for turbulent plumes: they are affected denser than deeper waters; as a consequence, by the nonlinear relationship between tempera- parcels of water descend with entrainment and ture and pressure in the density of sea water and vigorous mixing at vertical velocities of 2-10 cm by rotation. Numerical model results, such as s -~. In the sinking and spreading phase, the newly those shown here, clearly demonstrate that tur- formed dense water equilibrates with the surround- bulent mixing affects climate-related processes; ings and spreads horizontally. that is, deep convective plumes are evidence Deep convective plumes (Fig. 1) transport and that small-scale features affect the large-scale mix water over many hundreds of meters during a circulation. time period of hours to days (Aagaard and Car- Two types of convection have been linked with mack, 1989). Where deep convection occurs, the deep water formation: near-boundary sinking and across-isopycnal advection produces intermediate deep penetrative sinking in the open ocean and bottom water and controls the vertical struc- (Solomon, 1974; Carmack, 1990). The proper rep- ture of deep water masses. Ultimately, these water resentation of these processes in numerical models masses influence the deep circulation patterns that becomes important in simulating the thermohaline in turn affect the meridional heat transport within circulation. In coarse resolution models, open- the ocean. Manabe and Stouffer's (1993) climate ocean deep convection is not resolved, and re- simulations have shown that the thermal and dy- search is underway to understand the process and namical structure of the oceans could change in re- to develop a parameterization of this process for sponse to model climates with increased CO2. In ocean circulation models. The near-boundary sink- their simulations, the addition of low-salinity sur- ing is also of interest, but here we focus on some face water at high latitudes prevents the ventilation of the deep ocean, thus reducing, or in some cases nearly extinguishing, the thermohaline circulation. T. Paluszkiewicz, D.W. Denbo, Pacific Northwest Labora- The dynamics of the oceanic uptake of CO~ and tory, Battelle/Marine Sciences Laboratory, 1529 W. Sequim tracers is therefore strongly determined by the rate Bay Rd., Sequim, WA 98382, USA. R.W. Garwood, Naval Postgraduate School, Monterey, CA 93940, USA. of downward transport of surface waters to depth. OCEANOGRAPHY°VoI.7, NO. 2"1994 37 Active convective elements with horizontal scales of <1 km have been observed in the Green- land Sea (Schott et al., 1993a) and Mediterranean Sea (Schott et al., 1993b; The THETIS Group, 500 1994). Surface evidence of convective plumes was T found in synthetic aperture radar (SAR) imagery by Carsey and Garwood (1993) (Fig. 2a). The SAR imagery was able to distinguish plumes be- i000 cause they appeared to be ice covered and the con- vective return areas were open water. The sizes of the plumes (200-400 m in diameter) agreed with model experiments and with the scaling arguments 1500 of Jones and Marshall (1993). The surface expres- sion of plumes and the association with ice indi- cate that the brine from ice growth might act as a trigger of convection. The compensating upwelling flow and consequent heat flux may retard ice 500~" growth. Scott and Killworth (1991) found evi- dence of vertical plumes that were not the result of surface instabilities. These vertically well-mixed If . ;. areas may have been 3 km wide and extended lOOO downward from the base of the mixed layer (Fig. 3a.). These observations indicate that there may be , L two different modes of instability, parcel instabili- ties and layer instabilities. 1500 T Theories and Hypotheses for Deep Convective 2000 2500 2000 2500 2000 2500 2000 Plumes east (rn) 2500 e~t (m) east, (ra) Gravity-driven oceanic convection is both •" 0.04 m/s nonlinear and nonhydrostatic, with three-dimen- -1.6 -1.5 -1.4 -1.3 -1.2 -1.1 sional interactions among turbulent eddies result- ('~ ing in a cascade of turbulent kinetic energy Fig. l: The simulated potential temperature evolution from an LES experi- (TKE) from the largest plume scale to the mi- ment of open-ocean deep convective plume formation (Courtesy D. Denbo). croscale. The following two processes, normally negligible in turbulent boundary-layer dynamics, become important in the deep convective plumes: Observations of Deep Convective Plumes (i) thermobaricity, a consequence of the nonlinear There are several scales of organization that are relationship between temperature and pressure in believed to be important in deep convection in the density of sea water and (ii) the Coriolis which plumes represent the smallest-scale process. force. Gascard (1991) notes that on the mesoscale, chim- Gill (1973) first showed that sinking cold and neys and eddies play an important role in the saline Weddell Sea shelf water formed by freez- water mass formation process. ing conditions would experience additional down- Plumes are convectively driven vertical mo- ward acceleration with increased pressure as the tions. Whereas shallow convection, or mixed-layer dense water flowed down the sloping bottom. Mc- deepening, has scales of hundreds of meters (Kill- Dougall (1984) called this nonlinear effect in the Plumesare convec- worth, 1983; Guest and Davidson, 1991), deep equation of state thermobaricio,. Thermobaricity convective plumes have vertical scales of 1 to 2 influences deep convection significantly when sea tively driven vertical km (Gascard and Clarke, 1983), horizontal scales water is colder than 0°C. In an analysis of the motions. on the order of 1 km and associated vertical veloc- TKE budget, thermobaricity has been shown to ities of 2-10 cm s ~ (Rudels et al., 1989; Schott enhance the buoyant production of mixed-layer and Leaman, 1991; Carsey and Garwood, 1993; turbulence, resulting in a probable increase in en- Jones and Marshall, 1993; Roach et al., 1993; trainment and a possible mid-depth maximum in Schott et al., 1993a: Denbo and Skyllingstad, TKE for the deepest turbulent mixing layers in 1994; Garwood et al., 1994). Time scales associ- the polar seas (Garwood, 1991). The use of the ated with convective plumes are reported to be full nonlinear equation of state for sea water is several hours to several days (Rudels et al., 1989; necessary in order to retain the thermobaric ef- Roach et al., 1993). The convective plumes appear fect on the energetics of deep convection (Mc- as isolated regions of vertically coherent, down- Dougall, 1987; Aagaard and Carmack, 1989; ward vertical velocity when measured by acoustic Denbo and Skyllingstad, 1992; Garwood et al., doppler current profilers (ADCPs). 1994; Paluszkiewicz and Romea, 1994). 38 OCEANOGRAPHY'VoI. 7. No. 2o1994 ERS-1 SAR BLOWUP has been stored seasonally. Both of these instabili- FROM: 3016-2133 73.6°N, 11.1°W ties are dependent on the likelihood that a water FEBRUARY 12, 1992 parcel or water column that was hydrostatically stable near the surface could become hydrostati- cally unstable if displaced downward with com- mensurate pressure increase. A parcel instability occurs when a mixed layer .!. ~ X parcel penetrates below a critical depth and, having become more dense than the ambient water, contin- ues to sink, thereby escaping the well-mixed surface } :-. i) layer and penetrating the pycnocline. This process is analogous to the similarly named process in the atmospheric boundary layer that is associated with cumulus formation (Garwood, 1992a). Penetrative convection is when a plume overshoots the depth of static stability due to the momentum in the plume. When penetrative convection occurs, it causes addi- tional mixing at the mixed-layer base beyond that predicted by hydrostatic adjustment. Additional penetration or super-penetration may be possible 6.4 km when the thermobaric instability provides additional COPYRIGHT ESA 1992 (a) downward momentum to the parcel. A related layer instability may occur if the whole mixed layer is downwelled in response to large-scale convergence associated with wind forc- ing (Ekman pumping) or by preconditioning asso- ciated with larger-scale dynamics.
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