Climate Impacts on the Southern Ocean

Ecosystem(s)Eileen Hofmann, John Klinck, Mike

DinnimanWalker O. Smith

Eugene Murphy, Nadine Johnston, Rachel Cavanaugh (BAS)

SO GLOBEC Investigators

Presentation Outline

• Background on Southern Ocean GLOBEC program• Southern Ocean food webs• Consider potential climate change

effects on mesopelagic-shelf coupling• Summarize possible effects of climate

change on physical habitat and consequences for biological production and food webs

UK

AustraliaUS, Germany

Germany

Korea

SO GLOBEC Field Study Sites

Target Species

Circumpolar System

Not similar food webthroughout

Considerable heterogeneity in forcingand habitat structure

Regional differences inresponses

Southern Ocean Food Webs

Upper ocean temperatures have increased by 1ºC in the

last 50 years -WAP most rapidly warming region on

planet

Southern Ocean is Undergoing Major Environmental Changes

Parkinson (2002)

19761978198019821984198619881990199219941996199820002002

Year

Density (no. m-2)

1

10

100

1000

30% decline in Antarctic krill in SouthAtlantic in last 30 years

What happened in the past?

Harvesting has generated massive perturbations over more than 2

centuries

Fur-sealsFrom 1778; economic extinction

within 35 years

Whales1906 to 1966, residual thereafter

Fin-fish, krillFrom late 1960s, continuing

Top-down effects => Krill surplus?

Challenges for Southern Ocean

• Climate Impacts• Harvesting effects • Biogeochemistry• Food Webs

Can we develop experimental and modeling programs to address these effects and

interactions at a circumpolar scale?

What is a Southern Ocean Food Web?

Is This the Only Food Web?

Ross Sea

Western AntarcticPeninsula

Classical Food Web

Why the Differences?

HighAntarctic

SubAntarctic

LowProduction High Production

Seasonal length

Differences due toCirculation

Sea-iceBiogeochemistry

ProductionSeasonality

Mesopelagic Environment

• Region between about 200 m and 700 m

• For much of the Antarctic this is the depth of the continental shelf

• Shelf region is flooded with oceanic water, Circumpolar Deep Water (CDW), between 200-700 m - various forms of CDW

• Provides a direct connection between epipelagic and mesopelagic regions

• Focus on western Antarctic Peninsula

Shelf depth ~400 m

ACC flows along shelf edge

Deep trenches that provide connections between shelf and oceanic environments

WAP Circulation

(Klinck et al., 2004)

Southern Ocean Sentinel Workshop Hobart, Tasmania, 20-24 April

2009

Fall 2001 Warm and salty water mass

Floods shelf below 200 m

Extends across shelf atspecific sites

CDW Effects

Inputs of heat and salt

Surface water abovefreezing in winter

Salt excess Klinck et al. (2004)

Phytoplankton assemblagedominated by diatoms

CDW - regions of highprimary production

Prezelin et al. (2002)

Not all parts of the shelf are biologically similar

Biological Hot SpotsBiological Hot Spots

(Costa et al., 2007)

Climate Change Effects on CDW

• Effects of increased and decreased wind strength and increased transport of Antarctic Circumpolar Current on CDW intrusions onto the WAP shelf • Modified wind scenarios represent

regional effects - positive Southern Annular Mode gives stronger westerlies• Change in ACC transport represents

large-scale circulation effects - global thermohaline circulation

Circulation Model Characteristics

• ROMS: 4 km horizontal resolution, 24 levels• Ice shelves (mechanical and thermodynamic)• Dynamic sea ice• Bathymetry: ETOPO2v2 + WHOI SOGLOBEC region +

Padman grid + BEDMAP + Maslanyj• Open boundaries: T + S set to SODA, barotropic V

relaxed to SODA, baroclinic V pure radiation• Daily wind forcing from a blend of QSCAT data and

NCEP reanalyses • Other atmospheric parameters from several sources,

including Antarctic Mesoscale Prediction system (AMPS)

Simulation Configuration

• Track dye concentration as proxy for CDW• Dye concentration off the shelf set to 100 below

200 m and at temperatures > 0ºC• Allow 4-year spin up of circulation model • Simulations begin in January and run for 2 years

that correspond to 2000-2002 • Set up a reference case using current conditions

to provide comparisons

Model Domain

Includes iceshelves

Focus onMarguerite Bayand Crystal Sound regionsof WAP

Dye distribution for current conditions - FebruaryLevel of CDW (210-420 m)

50% increase in wind speed

20% decrease inwind speed

20% increase in wind speed

20% increase inwind speed andincrease in ACCtransport

Vertical dye distribution

Current conditions

50% increase in windspeed

Vertical temperaturedistribution

Current conditions

50% increase in wind speed

Summer sea ice distribution

50% increase in windspeed

Current conditions

Winter sea ice distribution

50% increase in wind speed

Current conditionsSummer sea ice

Dye concentrationfor Crystal Sound

Inner portion ofWAP shelf

Stronger winds andACC provide more CDW to region

Is this beneficial?

Will region persistas a biological hotspot?

Summary

• Strong coupling between mesopelaic and epipelagic environments

• Intrusions of CDW are controlling habitat structure and biological production

• Modified by winds and circulation changes• Biological hot spots are coincident with

intrusions of CDW • What are the consequences of changes in

CDW intrusions?• Is this specific to WAP region?

Biological continuum that is driven by subsurface intrusions of CDW

Prezelin et al. (2004)

Shift to a diatom-dominated system?

Alternative pathways buffer change - reflect/support long-term change?Need better quantification of alternative pathways

Alternative Food Web Pathways

High krill Low krill

Salps Zooplankton Krill

P

60%20%

20%

Salps Zooplankton Krill

P

20%

60%20%

P

20%60%20%

Krill

Salps

Benthos

Penguins

Penguins

Krill

Zooplankton Salps

Benthos

Zooplankton

Detritus

Killer Whales

Salps

Zooplankton

Killer Whales

Change in production

Salps Zooplankton Krill

Ballerini et al. (in prep)

Change in production

Z K

P

Fish Cephalopods

Z K

P

Fish Cephalopods

14% 3%

83%

80% 20%

0%Ballerini et al. (in prep)

Large-scale distribution of ACC fronts

Potential Consequences

• Reduction in winter sea ice- current food web components disappear? • Time history of seasonal

heating/cooling of surface layers changed - implications for air-sea exchanges and sea ice formation? • Timing of productivity changed -

same annual production but different time distribution?

Potential Consequences

• Larger areas of shelf influenced by warm CDW - change in habitat structure and food web linkages?

• Open/close more habitat - more regions where Antarctic krill can reproduce, reduced regions for Adélie penguins?

• More emphasis on benthic system - warmer bottom temperatures

• Mixing processes of CDW still a matter of research and debate - basic physical understanding still needs to be developed

Relevance to Global Ecosystems

Global carbon budget models lack biological detail

Current models do not capture what is known about SO ecosystems

• Circumpolar, interdisciplinary program focused on climate interactions and feedbacks to ecosystem function and biogeochemical cycles

• Extend and further develop circulation, ecosystem, and biogeochemical models

• Focus on end-to-end food web models

• Combine food web and biogeochemical communities

Joint program under IMBERand GLOBEC - 10 year effort

Thank you!

Photos byD. Costa