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

Igor V. Kamenkovich

University of Washington, Seattle

OutlineOutline

1.1. BackgroundBackground

• Thermohaline circulationThermohaline circulation: role in climate, driving : role in climate, driving

mechanisms, main branchesmechanisms, main branches

• Southern OceanSouthern Ocean

2.2. Water masses of the Southern ocean from top to bottomWater masses of the Southern ocean from top to bottom

• Upper ocean: Upper ocean: Subantarctic Mode WaterSubantarctic Mode Water

• Intermediate depths: Intermediate depths: Antarctic Intermediate WaterAntarctic Intermediate Water

• Very deep ocean: Very deep ocean: Antarctic Bottom WaterAntarctic Bottom Water

3.3. Summary and ConclusionsSummary and Conclusions

Role of the oceansRole of the oceans Oceans represent an enormous reservoir of heat: Oceans represent an enormous reservoir of heat: 2.5m of water 2.5m of water has he has he

same heat capacity as the same heat capacity as the entireentire air columnair column

Despite relatively slow oceanic currents, oceanic Despite relatively slow oceanic currents, oceanic meridional heat meridional heat

transporttransport is significant: 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 COanthropogenic CO22

• oxygen, nutrients, etc.oxygen, nutrients, etc.

Thermohaline circulationThermohaline circulation Massive movement of water massesMassive movement of water masses

The simplest picture: Global “conveyor belt”The simplest picture: Global “conveyor belt”

Southern OceanSouthern 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 Connects Atlantic, Pacific and Indian oceans – acts as a giant “mixer” for several

important water masses:important water masses:

Schmitz 1996

Southern Ocean (contd.)Southern Ocean (contd.)

Water masses that originate from the Southern Ocean:Water masses that originate from the Southern Ocean:Subantarctic Mode Water Subantarctic Mode Water

((SAMWSAMW))Antarctic Intermediate Antarctic Intermediate Water (Water (AAIWAAIW))

Antarctic Bottom Antarctic Bottom Water (Water (AABWAABW))

What sets these water masses in motion?What sets these water masses in motion?

Surface fluxes Surface fluxes of momentum (winds), heat and freshwaterof momentum (winds), heat and freshwater

Large scale advection Large scale advection (hundreds of km): (hundreds of km):

• SubductionSubduction – movement along surfaces of constant density ( – movement along surfaces of constant density (isopycnalsisopycnals))

• Upwelling/downwelling Upwelling/downwelling – vertical movement of water– vertical movement of water

MixingMixing by small-scale processes: by small-scale processes:

• WavesWaves (spatial scale of meters) – act across isopycnals (spatial scale of meters) – act across isopycnals

• EddiesEddies (spatial scale of 30-50 km) – mostly act along isopycnals (spatial scale of 30-50 km) – mostly act along isopycnals

Water mass formation processes:Water mass formation processes:

MethodologyMethodology

The goal is to The goal is to understand the major underlying processesunderstand the major underlying processes. The . The

understanding comes around when understanding comes around when observational dataobservational data, , numerical modelsnumerical models

and and theorytheory are combined to give a consistent picture 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 Numerical Modeling AdvantagesAdvantages::

• complete data coveragecomplete data coverage

• ability to run experiments with various conditions and model changes ability to run experiments with various conditions and model changes

in the systemin the system

DisadvantagesDisadvantages: :

• insufficient spatial resolutioninsufficient spatial resolution

• errors in representation of processeserrors in representation of processes

Ocean General Circulation Models (Ocean General Circulation Models (OGCMsOGCMs) used in these studies:) used in these studies:

• Based on Modular Ocean Model (Based on Modular Ocean Model (MOMMOM) of ) of GFDLGFDL

• Global realistic geometry and topographyGlobal realistic geometry and topography

• Coarse spatial resolution: 4 to 2 degrees in latitude and longitude; 25 vertical Coarse spatial resolution: 4 to 2 degrees in latitude and longitude; 25 vertical

levelslevels

• 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 SAMWMixed layers and SAMW

Winds over the Southern Ocean are strong (5-7 msecWinds over the Southern Ocean are strong (5-7 msec-1-1); storms are frequent ); storms are frequent

and powerful with wind speeds exceeding 15msecand powerful with wind speeds exceeding 15msec-1-1

ObservationsObservations: An isolated hurricane in the Northern Hemisphere Pacific : An isolated hurricane in the Northern Hemisphere Pacific

causes episodic cooling of the surface and deepening of the mixed layer 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, (Price 1981; Large et al. 1986; Price et al. 1994; Large and Crawford 1995,

etc.)etc.)

What is the time-mean response of the ocean to these storms?What is the time-mean response of the ocean to these storms?

Subantarctic Mode Water (Subantarctic Mode Water (SAMWSAMW) is formed ) is formed

by convection during local winter at the by convection during local winter at the

northern edge of the Southern Oceannorthern edge of the Southern Ocean

Characterized by uniform density and high Characterized by uniform density and high

concentration of oxygen concentration of oxygen

Affected by the Affected by the windswinds and air-sea fluxes of and air-sea fluxes of

heat/freshwaterheat/freshwater

WOCE section SO3

Response of the mixed layer to storms Response of the mixed layer to storms ((Kamenkovich 2005Kamenkovich 2005))

This study is based on a comparison of This study is based on a comparison of two numerical simulations of the two numerical simulations of the

Southern Ocean: one with and one without wind stormsSouthern Ocean: one with and one without wind storms

Effects of storms on the mixed layer during the local summer – the surface Effects of storms on the mixed layer during the local summer – the surface cools, subsurface ocean warms, the mixed layer deepens: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 Main cause is the vertical mixing enhanced by stormsvertical 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 Response during the local winter – the mixed layer in the most of the Pacific sector is Pacific sector is more shallowmore shallow in the presence of storms: in the presence of storms:

Difference in the mixed-layer depth between a run with and without daily forcing

ExplanationExplanation In the presence of storms: the mixed layer in In the presence of storms: the mixed layer in

summer/autumn issummer/autumn is warmer warmer ⇒⇒ density contrast with the ocean beneath the density contrast with the ocean beneath the

mixed layer is mixed layer is largerlarger ⇒⇒ convection-driven deepening is convection-driven deepening is slowerslower

Antarctic Intermediate Water (AAIW)Antarctic Intermediate Water (AAIW) Cold and fresh AAIW is found in the southeast Pacific and southwest Cold and fresh AAIW is found in the southeast Pacific and southwest

Atlantic (Atlantic (McCartney 1982; Talley 1996McCartney 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 AAIW formation is complicated and still a poorly understood process controlled by convection (McCartney, 1977), subduction (Sørensen et al., controlled by convection (McCartney, 1977), subduction (Sørensen et al., 2001), mixing (Piola and Georgi, 1982)2001), mixing (Piola and Georgi, 1982)

AAIW carries significant amount of heat into the Atlantic (e.g., Sloyan and AAIW carries significant amount of heat into the Atlantic (e.g., Sloyan and Rintoul 2001)Rintoul 2001)

What is its role in global thermohaline circulation ?

Eddies in the Southern Ocean Eddies in the Southern Ocean Kamenkovich and Sarachik (2004)Kamenkovich and Sarachik (2004)

In the Southern Ocean, eddies (spatial scale 30-50 km) act to In the Southern Ocean, eddies (spatial scale 30-50 km) act to flattenflatten isopycnalsisopycnals (surfaces of constant density) (surfaces of constant density)

OGCMOGCM In a numerical model (GCM) In a numerical model (GCM) the eddies are not resolved but the eddies are not resolved but are parameterized – expressed in are parameterized – expressed in terms of resolved, large-scale terms of resolved, large-scale properties quantitiesproperties quantities

Advantage: Advantage: We can vary efficiency We can vary efficiency of eddy effects, and analyze of eddy effects, and analyze changes in the global density and changes in the global density and flow patternsflow patterns

Simulated density distribution in the Southern Ocean: OGCM runs with eddy “flattening effect” (red) and without (blue)

Resulting effects on density in the AtlanticResulting effects on density in the Atlantic Changes in the stratification of the Southern Ocean caused by eddy Changes in the stratification of the Southern Ocean caused by eddy

“flattening effects” spread into the entire Atlantic:“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 circulationResulting 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/coolingChanges in AAIW density due to surface heating/coolingKamenkovich and Sarachik (2004, 2005)Kamenkovich and Sarachik (2004, 2005)

Changes in the surface density of the Southern Ocean affect North Atlantic Changes in the surface density of the Southern Ocean affect North Atlantic

through the intermediate waterthrough the intermediate water

Increase in density of AAIW

Higher density at low- and mid-latitudes

Weaker meridionalflow

Maximum THCintensity

decreases from20x106 m3sec-1

to15x106 m3sec-1

How does the surface warming of the Southern Ocean How does the surface warming of the Southern Ocean affect the global ocean?affect the global ocean?

GCM experimentGCM experiment: We : We

impose anomalous surface impose anomalous surface

warming over the Southern warming over the Southern

OceanOcean

Tropical Pacific warms within Tropical Pacific warms within

20-5020-50 yearsyears; fast boundary-; fast boundary-

trapped Kelvin waves and trapped Kelvin waves and

AAIW play a central roleAAIW play a central role

Warming at the Equator Warming at the Equator

deepens the thermocline, deepens the thermocline,

affects ENSOaffects ENSO

Response of the Atlantic Response of the Atlantic

ocean is much slower due to a ocean is much slower due to a

different geometry of the different geometry of the

basinbasin

AABW: global competition with the North Atlantic AABW: global competition with the North Atlantic Deep Water (NADW)Deep Water (NADW)

Antarctic Bottom Water (Antarctic Bottom Water (AABWAABW) is the deepest and densest water mass) is the deepest and densest water mass It forms at the Antarctic coast due to winter-time freezing and resulting It forms at the Antarctic coast due to winter-time freezing and resulting

brine rejectionbrine rejection AABW sinks to the bottom and spreads northwardAABW sinks to the bottom and spreads northward In the Atlantic, it flows beneath the North Atlantic Deep Water (In the Atlantic, it flows beneath the North Atlantic Deep Water (NADWNADW):):

At the Last Glacial Maximum (21,000 years ago) paleoclimate records At the Last Glacial Maximum (21,000 years ago) paleoclimate records

suggest suggest weaker and shallower NADWweaker and shallower NADW and and enhanced AABWenhanced AABW circulation circulation

Hypothesis (Shin et al. 2003): these changes are caused by enhanced Hypothesis (Shin et al. 2003): these changes are caused by enhanced

AABW formationAABW formation

NADW

AABW

Role ofRole of vertical mixingvertical mixing

Vertical (diapycnal) mixing is primarily driven by breaking of Vertical (diapycnal) mixing is primarily driven by breaking of

internal wavesinternal waves

Direct measurements (Polzin et al., 1997) suggest that mixing is Direct measurements (Polzin et al., 1997) suggest that mixing is

the largest near the rough topography the largest near the rough topography

In OGCMS, stronger vertical mixing has been shown to In OGCMS, stronger vertical mixing has been shown to

correspond to enhanced overturning of the NADW correspond to enhanced overturning of the NADW

How does mixing affect AABW?How does mixing affect AABW?

Dependence of AABW on vertical mixingDependence of AABW on vertical mixingKamenkovich and Goodman (2000)Kamenkovich and Goodman (2000)

OGCM studyOGCM study We vary vertical diffusivity – We vary vertical diffusivity –

intensity of the vertical mixing in the intensity of the vertical mixing in the

model – and analyze changes in the model – and analyze changes in the

Atlantic thermohaline circulationAtlantic thermohaline circulation

Increased vertical mixing leads to:Increased vertical mixing leads to:

• Stronger and thickerStronger and thicker NADW cell NADW cell

• Stronger and thinnerStronger and thinner AABW cell AABW cell

Kv = 0.1 cm2 sec-1

Kv = 1.0 cm2 sec-1

Explanation: A conceptual modelExplanation: A conceptual model

Assume that a meridional flow is Assume that a meridional flow is

determined by the meridional determined by the meridional

pressure gradientpressure gradient

Consider a balance in the Consider a balance in the

equation for density between equation for density between

advectionadvection and and diffusiondiffusion

Notations: Notations: TTaa – volume transport – volume transport

of AABW, of AABW, TTuu – upwelling of AABW, – upwelling of AABW,

kkvv – vertical mixing, – vertical mixing, HHaa – thickness – thickness

of AABW cellof AABW cell

aua

avaauaaa TH

AkTTT ))((a

avuaa HAkTT

)(

2/1

2/1

02/1 ava

aaa Hk

L

AgCT

mixing

Results: AABW transport and thicknessResults: AABW transport and thicknessResults 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 ConclusionsSummary and Conclusions

The results point to an important role of the Southern Ocean in global The results point to an important role of the Southern Ocean in global

ocean circulationocean circulation

Water masses of the Southern Ocean are affected by several dynamical Water masses of the Southern Ocean are affected by several dynamical

processes: surface winds, air-sea exchanges of heat and moisture, mixing processes: surface winds, air-sea exchanges of heat and moisture, mixing

by eddies and internal wavesby 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 Antarctic Intermediate Water (AAIW) is sensitive to air-sea exchanges of heat

and by mixing by ocean eddiesand by mixing by ocean eddies

• The transport of the Antarctic Bottom Water (AABW) is controlled by vertical The transport of the Antarctic Bottom Water (AABW) is controlled by vertical

mixingmixing

We have demonstrated that AAIW and AABW are capable of affecting We have demonstrated that AAIW and AABW are capable of affecting

global thermohaline circulation:global thermohaline circulation:

• AAIW strongly affects meridional overturning in the Atlantic as wells as AAIW strongly affects meridional overturning in the Atlantic as wells as

stratification in the Tropicsstratification in the Tropics

• AABW can change deep density and thermohaline circulation in the Atlantic AABW can change deep density and thermohaline circulation in the Atlantic

Future directionsFuture 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-past abrupt climate changes (etc., transitions from glacial periods, Dansgaard-

Oeschger oscillations)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 Better understanding of the physics of interactions between small and

large scales:large scales:

• Role of eddies: eddy-resolving models can help!Role of eddies: eddy-resolving models can help!

• Topography-intensified mixingTopography-intensified mixing