Water masses of the southern ocean their formation circulation and global role
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Water masses of the Southern Ocean: Their formation, circulation and global role. Igor V. Kamenkovich University of Washington, Seattle. Outline. Background Thermohaline circulation : role in climate, driving mechanisms, main branches Southern Ocean

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Water masses of the southern ocean their formation circulation and global role

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

Igor V. Kamenkovich

University of Washington, Seattle


Outline
Outline circulation and global role

  • 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


Role of the oceans
Role of the oceans circulation and global role

  • 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.


Thermohaline circulation
Thermohaline circulation circulation and global role

  • Massive movement of water masses

  • The simplest picture: Global “conveyor belt”


Southern ocean
Southern Ocean circulation and global role

  • 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


Southern ocean contd
Southern Ocean (contd.) circulation and global role

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?


Water mass formation processes
Water mass formation processes: circulation and global role

  • 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


Methodology
Methodology circulation and global role

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


Numerical modeling
Numerical Modeling circulation and global role

  • 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


Mixed layers and samw
Mixed layers and SAMW circulation and global role

  • 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


Response of the mixed layer to storms kamenkovich 2005
Response of the mixed layer to storms ( circulation and global roleKamenkovich 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


Response of the mixed layer to storms
Response of the mixed layer to storms circulation and global role

  • 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


Antarctic intermediate water aaiw
Antarctic Intermediate Water (AAIW) circulation and global role

  • 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 ?


Eddies in the southern ocean kamenkovich and sarachik 2004
Eddies in the Southern Ocean circulation and global roleKamenkovich 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)


Resulting effects on density in the atlantic
Resulting effects on density in the Atlantic circulation and global role

  • 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


Resulting effects on the atlantic circulation
Resulting effects on the Atlantic circulation circulation and global role

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!


Changes in aaiw density due to surface heating cooling kamenkovich and sarachik 2004 2005
Changes in AAIW density due to surface heating/cooling circulation and global roleKamenkovich 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


How does the surface warming of the southern ocean affect the global ocean
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


Aabw global competition with the north atlantic deep water nadw
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


Role of vertical mixing
Role of (NADW)vertical 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?


Dependence of aabw on vertical mixing kamenkovich and goodman 2000
Dependence of AABW on vertical mixing (NADW)Kamenkovich 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


Explanation a conceptual model
Explanation: A conceptual model (NADW)

  • 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


Results aabw transport and thickness
Results: AABW transport and thickness (NADW)

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


Summary and conclusions
Summary and Conclusions (NADW)

  • 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


Future directions
Future directions (NADW)

  • 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


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