water masses of the southern ocean: their formation, circulation and global role igor v. kamenkovich...

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  • Water masses of the Southern Ocean: Their formation, circulation and global roleIgor V. KamenkovichUniversity of Washington, Seattle

  • OutlineBackgroundThermohaline circulation: role in climate, driving mechanisms, main branchesSouthern OceanWater masses of the Southern ocean from top to bottomUpper ocean: Subantarctic Mode WaterIntermediate depths: Antarctic Intermediate WaterVery deep ocean: Antarctic Bottom WaterSummary and Conclusions

  • Role of the oceansOceans represent an enormous reservoir of heat: 2.5m of water has he same heat capacity as the entire air columnDespite relatively slow oceanic currents, oceanic meridional heat transport is significant:Meridional heat transport: by the atmosphere (green), by the oceans (red), and the sum of the two (blue)Oceanic circulation redistributes important biochemical tracers: anthropogenic CO2oxygen, nutrients, etc.

  • Thermohaline circulationMassive movement of water massesThe simplest picture: Global conveyor belt

  • Southern OceanThe Southern Ocean is a unique component of the climate system:No meridional boundariesVery strong winds, fast oceanic currentsConnects Atlantic, Pacific and Indian oceans acts as a giant mixer for several important water masses:Schmitz 1996

  • Southern Ocean (contd.)Water masses that originate from the Southern Ocean:Subantarctic Mode Water (SAMW)Antarctic Intermediate Water (AAIW)Antarctic Bottom Water (AABW)What sets these water masses in motion?

  • Water mass formation processes:Surface fluxes of momentum (winds), heat and freshwaterLarge scale advection (hundreds of km): Subduction movement along surfaces of constant density (isopycnals)Upwelling/downwelling vertical movement of waterMixing by small-scale processes:Waves (spatial scale of meters) act across isopycnalsEddies (spatial scale of 30-50 km) mostly act along isopycnals

  • MethodologyThe goal is to understand the major underlying processes. The understanding comes around when observational data, numerical models and theory are combined to give a consistent pictureObservations in the Southern Ocean are sparse:WOCE Atlas: locations and errors of temperature measurements

  • Numerical Modeling Advantages:complete data coverageability to run experiments with various conditions and model changes in the systemDisadvantages: insufficient spatial resolutionerrors in representation of processes

    Ocean General Circulation Models (OGCMs) used in these studies:Based on Modular Ocean Model (MOM) of GFDLGlobal realistic geometry and topographyCoarse spatial resolution: 4 to 2 degrees in latitude and longitude; 25 vertical levelsOcean circulation is forced by surface winds and by fluxes of heat and freshwaterProcesses on spatial scales not explicitly resolved are parameterized

  • Mixed layers and SAMWWinds over the Southern Ocean are strong (5-7 msec-1); storms are frequent and powerful with wind speeds exceeding 15msec-1Observations: An isolated hurricane in the Northern Hemisphere Pacific causes episodic cooling of the surface and deepening of the mixed layer (Price 1981; Large et al. 1986; Price et al. 1994; Large and Crawford 1995, etc.)

    What is the time-mean response of the ocean to these storms?Subantarctic Mode Water (SAMW) is formed by convection during local winter at the northern edge of the Southern OceanCharacterized by uniform density and high concentration of oxygen Affected by the winds and air-sea fluxes of heat/freshwaterWOCE section SO3

  • Response of the mixed layer to storms (Kamenkovich 2005)This study is based on a comparison of two numerical simulations of the Southern Ocean: one with and one without wind storms

    Effects of storms on the mixed layer during the local summer the surface cools, subsurface ocean warms, the mixed layer deepens:Difference in the mixed-layer depth between a run with and without daily forcingMain cause is the vertical mixing enhanced by storms

  • Response of the mixed layer to storms Response during the local winter the mixed layer in the most of the Pacific sector is more shallow in the presence of storms:Difference in the mixed-layer depth between a run with and without daily forcingExplanation In the presence of storms: the mixed layer in summer/autumn is warmer density contrast with the ocean beneath the mixed layer is larger convection-driven deepening is slower

  • Antarctic Intermediate Water (AAIW)Cold and fresh AAIW is found in the southeast Pacific and southwest Atlantic (McCartney 1982; Talley 1996)Shows as a low-salinity tongue:AAIW formation is complicated and still a poorly understood process controlled by convection (McCartney, 1977), subduction (Srensen et al., 2001), mixing (Piola and Georgi, 1982)AAIW carries significant amount of heat into the Atlantic (e.g., Sloyan and Rintoul 2001)

    What is its role in global thermohaline circulation ?

  • Eddies in the Southern Ocean Kamenkovich and Sarachik (2004)In the Southern Ocean, eddies (spatial scale 30-50 km) act to flatten isopycnals (surfaces of constant density)OGCM In a numerical model (GCM) the eddies are not resolved but are parameterized expressed in terms of resolved, large-scale properties quantities

    Advantage: We can vary efficiency of eddy effects, and analyze changes in the global density and flow patternsSimulated density distribution in the Southern Ocean: OGCM runs with eddy flattening effect (red) and without (blue)

  • Resulting effects on density in the AtlanticChanges in the stratification of the Southern Ocean caused by eddy flattening effects spread into the entire Atlantic:Difference in density between a run with and without eddy flattening effect in the Southern Ocean Density of AAIW increases density at the low- and mid-latitudes increases meridional pressure gradient weakens meridional flow weakens

    Density of the deep ocean changes as a result of changes in the circulation

  • Resulting effects on the Atlantic circulationRun with no eddy flattening effect meridional overturning in the Atlantic is 19 Sv (106m3sec-1)Run with eddy flattening effect in the Southern Ocean overturning is 12 Sv (106m3sec-1)

    The only difference with the above case is in eddies in the Southern Ocean!Run with eddy flattening effect everywhere overturning is still 12 Sv (106m3sec-1)

    Eddies in the Southern Ocean play a dominant role!

  • Changes in AAIW density due to surface heating/coolingKamenkovich and Sarachik (2004, 2005)Changes in the surface density of the Southern Ocean affect North Atlantic through the intermediate waterIncrease in density of AAIWHigher density at low- and mid-latitudesWeaker meridionalflowMaximum THCintensity decreases from20x106 m3sec-1 to15x106 m3sec-1

  • How does the surface warming of the Southern Ocean affect the global ocean?GCM experiment: We impose anomalous surface warming over the Southern OceanTropical Pacific warms within 20-50 years; fast boundary-trapped Kelvin waves and AAIW play a central roleWarming at the Equator deepens the thermocline, affects ENSOResponse of the Atlantic ocean is much slower due to a different geometry of the basin

  • AABW: global competition with the North Atlantic Deep Water (NADW)Antarctic Bottom Water (AABW) is the deepest and densest water massIt forms at the Antarctic coast due to winter-time freezing and resulting brine rejectionAABW sinks to the bottom and spreads northwardIn the Atlantic, it flows beneath the North Atlantic Deep Water (NADW):At the Last Glacial Maximum (21,000 years ago) paleoclimate records suggest weaker and shallower NADW and enhanced AABW circulationHypothesis (Shin et al. 2003): these changes are caused by enhanced AABW formationNADWAABW

  • Role of vertical mixingVertical (diapycnal) mixing is primarily driven by breaking of internal waves

    Direct measurements (Polzin et al., 1997) suggest that mixing is the largest near the rough topography

    In OGCMS, stronger vertical mixing has been shown to correspond to enhanced overturning of the NADW

    How does mixing affect AABW?

  • Dependence of AABW on vertical mixingKamenkovich and Goodman (2000)OGCM study We vary vertical diffusivity intensity of the vertical mixing in the model and analyze changes in the Atlantic thermohaline circulation

    Increased vertical mixing leads to:Stronger and thicker NADW cellStronger and thinner AABW cell

    Kv = 0.1 cm2 sec-1Kv = 1.0 cm2 sec-1

  • Explanation: A conceptual modelAssume that a meridional flow is determined by the meridional pressure gradientConsider a balance in the equation for density between advection and diffusionNotations: Ta volume transport of AABW, Tu upwelling of AABW, kv vertical mixing, Ha thickness of AABW cellmixing

  • Results: AABW transport and thicknessResults from OGCM are shown by squares and circles; results from a conceptual model by linesNADW transport increases with increasing mixingAABW thickness decreases with mixingNADW thickness increases with mixingAABW transport increases with increasing mixingAgreement between OGCMS and a conceptual model is good !

  • Summary and ConclusionsThe results point to an important role of the Southern Ocean in global ocean circulationWater masses of the Southern Ocean are affected by several dynamical processes: surface winds, air-sea exchanges of heat and moisture, mixing by eddies and internal wavesIn particular:Subantarctic Mode Water (SAMW) is affected by storm-induced mixingAntarctic Intermediate Water (AAIW) is sensitive to air-sea exchanges of heat and by mixing by ocean eddiesThe transport of the Antarctic Bottom Water (AABW) is controlled by vertical mixingWe have demonstrated that AAIW and AABW are capable of affecting global thermohaline circulation:AAIW s