Water masses of the Southern Ocean: Their formation, circulation and global role

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

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

3. Role of the oceans • Oceans represent an enormous reservoir of heat: 2.5m of water has he same heat capacity as the entireair column • 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: • anthropogenic CO2 • oxygen, nutrients, etc.

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

5. Southern Ocean • The Southern Ocean is a unique component of the climate system: • No meridional boundaries • Very strong winds, fast oceanic currents • Connects Atlantic, Pacific and Indian oceans – acts as a giant “mixer” for several important water masses: Schmitz 1996

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

7. Water mass formation processes: • Surface fluxes of momentum (winds), heat and freshwater • Large scale advection (hundreds of km): • Subduction – movement along surfaces of constant density (isopycnals) • Upwelling/downwelling – vertical movement of water • Mixing by small-scale processes: • Waves (spatial scale of meters) – act across isopycnals • Eddies (spatial scale of 30-50 km) – mostly act along isopycnals

8. 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: WOCE Atlas: locations and errors of temperature measurements

9. Numerical Modeling • Advantages: • complete data coverage • ability to run experiments with various conditions and model changes in the system • Disadvantages: • insufficient spatial resolution • errors in representation of processes • Ocean General Circulation Models (OGCMs) used in these studies: • Based on Modular Ocean Model (MOM) of GFDL • Global realistic geometry and topography • Coarse 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 freshwater • Processes on spatial scales not explicitly resolved are parameterized

10. Mixed layers and SAMW • 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 • Affected by the winds and air-sea fluxes of heat/freshwater 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.) What is the time-mean response of the ocean to these storms? WOCE section SO3

11. 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 forcing • Main cause is the vertical mixing enhanced by storms

12. 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 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

13. 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 (Sørensen 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 ?

14. Eddies in the Southern Ocean Kamenkovich and Sarachik (2004) • In the Southern Ocean, eddies (spatial scale 30-50 km) act to flattenisopycnals (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)

15. 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: • Density of AAIWincreases⇒ densityat 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 Difference in density between a run with and without eddy “flattening effect” in the Southern Ocean

16. Resulting effects on the Atlantic circulation Run 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!

17. 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 water Increase in density of AAIW Higher density at low- and mid-latitudes Weaker meridional flow Maximum THC intensity decreases from 20x106 m3sec-1 to 15x106 m3sec-1

18. How does the surface warming of the Southern Ocean affect the global ocean? • GCM experiment: We impose anomalous surface warming over the Southern Ocean • Tropical Pacific warms within 20-50years; fast boundary-trapped Kelvin waves and AAIW play a central role • 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

19. AABW: global competition with the North Atlantic Deep Water (NADW) • 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 • AABW sinks to the bottom and spreads northward • In the Atlantic, it flows beneath the North Atlantic Deep Water (NADW): NADW AABW • 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

20. Role ofvertical mixing • 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 • In OGCMS, stronger vertical mixing has been shown to correspond to enhanced overturning of the NADW • How does mixing affect AABW?

21. Dependence of AABW on vertical mixingKamenkovich and Goodman (2000) Kv = 0.1 cm2 sec-1 • 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 cell • Stronger and thinner AABW cell Kv = 1.0 cm2 sec-1

22. Explanation: A conceptual model • Assume that a meridional flow is determined by the meridional pressure gradient • Consider a balance in the equation for density between advection and diffusion • Notations: Ta – volume transport of AABW, Tu – upwelling of AABW, kv – vertical mixing, Ha – thickness of AABW cell mixing

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

24. Summary and Conclusions • 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 • In particular: • Subantarctic 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 eddies • The 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: • AAIW strongly affects meridional overturning in the Atlantic as wells as stratification in the Tropics • AABW can change deep density and thermohaline circulation in the Atlantic

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