Water masses of the Southern Ocean: Their formation, circulation and global role Igor V. Kamenkovich University of Washington, Seattle

Outline 1. Background Thermohaline circulation: role in climate, driving mechanisms, main branchesThermohaline circulation: role in climate, driving mechanisms, main branches Southern OceanSouthern Ocean 2. Water masses of the Southern ocean from top to bottom Upper ocean: Subantarctic Mode WaterUpper ocean: Subantarctic Mode Water Intermediate depths: Antarctic Intermediate WaterIntermediate depths: Antarctic Intermediate Water Very deep ocean: Antarctic Bottom WaterVery deep ocean: Antarctic Bottom Water 3. Summary and Conclusions

Role of the oceans Oceans represent an enormous reservoir of heat: 2.5m of water has he same heat capacity as the entire air column Oceans represent an enormous reservoir of heat: 2.5m of water has he same heat capacity as the entire air column Despite relatively slow oceanic currents, oceanic meridional heat transport is significant: Despite 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: Oceanic circulation redistributes important biochemical tracers: anthropogenic CO 2anthropogenic CO 2 oxygen, nutrients, etc.oxygen, nutrients, etc.

Thermohaline circulation Massive movement of water masses Massive movement of water masses The simplest picture: Global “conveyor belt” The simplest picture: Global “conveyor belt”

Southern Ocean The Southern Ocean is a unique component of the climate system: The Southern Ocean is a unique component of the climate system: No meridional boundariesNo meridional boundaries Very strong winds, fast oceanic currentsVery strong winds, fast oceanic currents Connects Atlantic, Pacific and Indian oceans – acts as a giant “mixer” for several important water masses:Connects 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: 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?

Surface fluxes of momentum (winds), heat and freshwater Surface fluxes of momentum (winds), heat and freshwater Large scale advection (hundreds of km): Large scale advection (hundreds of km): Subduction – movement along surfaces of constant density (isopycnals)Subduction – movement along surfaces of constant density (isopycnals) Upwelling/downwelling – vertical movement of waterUpwelling/downwelling – vertical movement of water Mixing by small-scale processes: Mixing by small-scale processes: Waves (spatial scale of meters) – act across isopycnalsWaves (spatial scale of meters) – act across isopycnals Eddies (spatial scale of km) – mostly act along isopycnalsEddies (spatial scale of km) – mostly act along isopycnals Water mass formation processes:

Methodology The 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 picture Observations in the Southern Ocean are sparse: Observations in the Southern Ocean are sparse: WOCE Atlas: locations and errors of temperature measurements

Numerical Modeling Advantages: Advantages: complete data coveragecomplete data coverage ability to run experiments with various conditions and model changes in the systemability to run experiments with various conditions and model changes in the system Disadvantages: Disadvantages: insufficient spatial resolutioninsufficient spatial resolution errors in representation of processeserrors in representation of processes Ocean General Circulation Models (OGCMs) used in these studies: Ocean General Circulation Models (OGCMs) used in these studies: Based on Modular Ocean Model (MOM) of GFDLBased on Modular Ocean Model (MOM) of GFDL Global realistic geometry and topographyGlobal realistic geometry and topography Coarse spatial resolution: 4 to 2 degrees in latitude and longitude; 25 vertical levelsCoarse spatial resolution: 4 to 2 degrees in latitude and longitude; 25 vertical levels Ocean circulation is forced by surface winds and by fluxes of heat and freshwaterOcean circulation is forced by surface winds and by fluxes of heat and freshwater Processes on spatial scales not explicitly resolved are parameterizedProcesses on spatial scales not explicitly resolved are parameterized

Mixed layers and SAMW Winds over the Southern Ocean are strong (5-7 msec -1 ); storms are frequent and powerful with wind speeds exceeding 15msec -1 Observations: 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.) Observations: 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 Ocean Subantarctic Mode Water (SAMW) is formed by convection during local winter at the northern edge of the Southern Ocean Characterized by uniform density and high concentration of oxygen Characterized by uniform density and high concentration of oxygen Affected by the winds and air-sea fluxes of heat/freshwater Affected by the winds and air-sea fluxes of heat/freshwater WOCE 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 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: 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 forcing Main cause is the vertical mixing enhanced by storms Main 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: 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 forcing Explanation 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 Explanation 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 ) Cold and fresh AAIW is found in the southeast Pacific and southwest Atlantic ( McCartney 1982; Talley 1996 ) Shows as a low-salinity tongue: Shows as a low-salinity tongue: AAIW formation is complicated and still a poorly understood process controlled by convection (McCartney, 1977), subduction (Sørensen et al., 2001), mixing (Piola and Georgi, 1982) AAIW formation is complicated and still a poorly understood process controlled by convection (McCartney, 1977), subduction (Sørensen et al., 2001), mixing (Piola and Georgi, 1982) AAIW carries significant amount of heat into the Atlantic (e.g., Sloyan and Rintoul 2001) 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 km) act to flatten isopycnals (surfaces of constant density) In the Southern Ocean, eddies (spatial scale 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 patterns Simulated density distribution in the Southern Ocean: OGCM runs with eddy “flattening effect” (red) and without (blue)

Resulting effects on density in the Atlantic Changes in the stratification of the Southern Ocean caused by eddy “flattening effects” spread into the entire Atlantic: Changes 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 circulation Run with no “eddy flattening” effect – meridional overturning in the Atlantic is 19 Sv (10 6 m 3 sec -1 ) Run with eddy “flattening effect” in the Southern Ocean – overturning is 12 Sv (10 6 m 3 sec -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 (10 6 m 3 sec -1 ) Eddies in the Southern Ocean play a dominant role!

Changes in AAIW density due to surface heating/cooling Kamenkovich and Sarachik (2004, 2005) Changes in the surface density of the Southern Ocean affect North Atlantic through the intermediate water Changes in the surface density of the Southern Ocean affect North Atlantic through the intermediate water Increase in density of AAIW Higher density at low- and mid-latitudes Weaker meridional flow Maximum THC intensity decreases from 20x10 6 m 3 sec -1 to 15x10 6 m 3 sec -1

How does the surface warming of the Southern Ocean affect the global ocean? GCM experiment: We impose anomalous surface warming over the Southern Ocean GCM experiment: We impose anomalous surface warming over the Southern Ocean Tropical Pacific warms within years; fast boundary- trapped Kelvin waves and AAIW play a central role Tropical Pacific warms within years; fast boundary- trapped Kelvin waves and AAIW play a central role Warming at the Equator deepens the thermocline, affects ENSO Warming at the Equator deepens the thermocline, affects ENSO Response of the Atlantic ocean is much slower due to a different geometry of the basin Response 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 mass Antarctic Bottom Water (AABW) is the deepest and densest water mass It forms at the Antarctic coast due to winter-time freezing and resulting brine rejection It forms at the Antarctic coast due to winter-time freezing and resulting brine rejection AABW sinks to the bottom and spreads northward AABW sinks to the bottom and spreads northward In the Atlantic, it flows beneath the North Atlantic Deep Water (NADW): In 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 circulation At the Last Glacial Maximum (21,000 years ago) paleoclimate records suggest weaker and shallower NADW and enhanced AABW circulation Hypothesis (Shin et al. 2003): these changes are caused by enhanced AABW formation Hypothesis (Shin et al. 2003): these changes are caused by enhanced AABW formation NADW AABW

Role of vertical mixing Vertical (diapycnal) mixing is primarily driven by breaking of internal waves Vertical (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 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 In OGCMS, stronger vertical mixing has been shown to correspond to enhanced overturning of the NADW How does mixing affect AABW? How does mixing affect AABW?

Dependence of AABW on vertical mixing Kamenkovich 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 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: Increased vertical mixing leads to: Stronger and thicker NADW cellStronger and thicker NADW cell Stronger and thinner AABW cellStronger and thinner AABW cell K v = 0.1 cm 2 sec -1 K v = 1.0 cm 2 sec -1

Explanation: A conceptual model Assume that a meridional flow is determined by the meridional pressure gradient Assume that a meridional flow is determined by the meridional pressure gradient Consider a balance in the equation for density between advection and diffusion Consider a balance in the equation for density between advection and diffusion Notations: T a – volume transport of AABW, T u – upwelling of AABW, k v – vertical mixing, H a – thickness of AABW cell Notations: T a – volume transport of AABW, T u – upwelling of AABW, k v – vertical mixing, H a – thickness of AABW cell mixing

Results: AABW transport and thickness Results from OGCM are shown by squares and circles; results from a conceptual model – by lines NADW transport increases with increasing mixing AABW thickness decreases with mixing NADW thickness increases with mixing AABW transport increases with increasing mixing Agreement between OGCMS and a conceptual model is good !

Summary and Conclusions The results point to an important role of the Southern Ocean in global ocean circulation The results point to an important role of the Southern Ocean in global ocean circulation Water 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 waves Water 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 waves In particular: In particular: Subantarctic Mode Water (SAMW) is affected by storm-induced mixingSubantarctic Mode Water (SAMW) is affected by storm-induced mixing Antarctic Intermediate Water (AAIW) is sensitive to air-sea exchanges of heat and by mixing by ocean eddiesAntarctic Intermediate Water (AAIW) is sensitive to air-sea exchanges of heat and by mixing by ocean eddies The transport of the Antarctic Bottom Water (AABW) is controlled by vertical mixingThe transport of the Antarctic Bottom Water (AABW) is controlled by vertical mixing We have demonstrated that AAIW and AABW are capable of affecting global thermohaline circulation: We have demonstrated that AAIW and AABW are capable of affecting global thermohaline circulation: AAIW strongly affects meridional overturning in the Atlantic as wells as stratification in the TropicsAAIW strongly affects meridional overturning in the Atlantic as wells as stratification in the Tropics AABW can change deep density and thermohaline circulation in the AtlanticAABW can change deep density and thermohaline circulation in the Atlantic

Future directions Scenarios of past and future climate reorganizations: Scenarios of past and future climate reorganizations: past abrupt climate changes (etc., transitions from glacial periods, Dansgaard- Oeschger oscillations)past abrupt climate changes (etc., transitions from glacial periods, Dansgaard- Oeschger oscillations) future climate change due to emission of anthropogenic ‘greenhouse gasses”future climate change due to emission of anthropogenic ‘greenhouse gasses” Better understanding of the physics of interactions between small and large scales: Better understanding of the physics of interactions between small and large scales: Role of eddies: eddy-resolving models can help!Role of eddies: eddy-resolving models can help! Topography-intensified mixingTopography-intensified mixing