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

  • 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

  • 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

  • Massive movement of water masses

  • The simplest picture: Global “conveyor belt”


Southern ocean

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


Southern ocean contd

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?


Water mass formation processes

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


Methodology

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


Numerical modeling

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


Mixed layers and samw

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


Response of the mixed layer to storms kamenkovich 2005

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


Response of the mixed layer to 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 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)

  • 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 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)


Resulting effects on density in the atlantic

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


Resulting effects on the atlantic circulation

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!


Changes in aaiw density due to surface heating cooling kamenkovich and sarachik 2004 2005

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


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 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?


Dependence of aabw on vertical mixing kamenkovich and goodman 2000

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


Explanation a conceptual model

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


Results aabw transport and thickness

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


Summary and conclusions

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


Future directions

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


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