Wind effects on Circumpolar Deep Wind effects on Circumpolar Deep Water intrusions on the West Antarctic Water intrusions on the West Antarctic

Peninsula continental shelfPeninsula continental shelf

Mike DinnimanMike DinnimanJohn KlinckJohn Klinck

Center for Coastal Physical Center for Coastal Physical OceanographyOceanography

Old Dominion UniversityOld Dominion UniversityFebruary 25, 2010February 25, 2010

Outline of PresentationOutline of Presentation

• Introduction

• Description of circulation model

• Timing of Circumpolar Deep Water (CDW) intrusions on the WAP

• “Downscaling” simulations: CDW transport and ice shelf basal melt

• Conclusions

IntroductionIntroduction• Relatively warm, nutrient-rich CDW found at shelf break around most of Antarctica

• CDW exchange provides heat and nutrients that can stimulate primary production

• Changes in either the temperature or quantity of warm CDW entering the ice shelf cavities and increasing the drainage rate of outlet glaciers into the Amundsen Sea have been proposed as a reason for the increased volume loss of the West Antarctic Ice Sheet

• Modifications in the wind forcing have been proposed as a mechanism for changes in the flux of CDW onto the continental shelf and to the cavities beneath the ice shelves

Antarctic Peninsula ModelAntarctic Peninsula Model• ROMS: 4 km horizontal resolution, 24 levelsROMS: 4 km horizontal resolution, 24 levels• Ice shelves (mechanical and thermodynamic)Ice shelves (mechanical and thermodynamic)• Dynamic sea iceDynamic sea ice• Bathymetry: ETOPO2v2 + WHOI SOGLOBEC Bathymetry: ETOPO2v2 + WHOI SOGLOBEC

region + region + PadmanPadman + BEDMAP + Maslanyj+ BEDMAP + Maslanyj• Daily (or better) wind forcing from a blend of Daily (or better) wind forcing from a blend of

QSCAT data and NCEP reanalyses or Antarctic QSCAT data and NCEP reanalyses or Antarctic Mesoscale Prediction System (AMPS) windsMesoscale Prediction System (AMPS) winds

• Lateral boundary conditions from SODA Lateral boundary conditions from SODA ocean reanalysis and SSM/I ice ocean reanalysis and SSM/I ice concentrationsconcentrations

• Experiments w/ dye representing CDWExperiments w/ dye representing CDW

Previous view of WAP intrusion frequency: 4-6 events/year

However, this was based on broad scale hydrographic surveys

Detided, low-pass filtered time series of potential temperature at two moorings within Marguerite Trough => shows much higher intrusion frequency Moffat et al, 2009

Solid line: A2 – north side of MTDashed line: A3 – south side of MT

Model histogram of the intrusion duration (55 dye unit-Sv. threshold) for the entire model run

Model: 2.1 events/month(3.0 events/month w/ 45 dye unit-Sv. threshold)Typical duration: 1-4 d

Observed histogram of the intrusion duration

A2: 3.8 events/monthTypical duration: 1-3 d

Definition of intrusion is arbitrary, but model is much closer to mooring data than previous survey based picture of 4-6 intrusions per year

Duration of intrusions suggests wind forcing (weather band frequency: 2-8 days)

No significant correlation between wind perpendicular to flux “gate”, but significant lagged correlation for wind parallel to cross-section:r = 0.58 (wind leads dye flux by 2-3 days)

Red: Pseudo-stress (x 1/2) parallel to CDW-dye flux cross sectionBlue: CDW-dye flux anomaly (reversed sign)

Wind Change SimulationsWind Change Simulations

• Positive Southern Annular Mode leads to stronger westerlies (and more mesoscale cyclones?) in this area

• Simulate regional effects of this with modified winds

• ACC transport in a regional model must be imposed: Simulate global effects by increasing the ACC transport

• Simulations run for 3 years that correspond to 2000-2002

• Reference case using current conditions (QSCAT/NCEP)

• Winds are simply scaled with both components multiplied by a constant factor

• ACC transport increased by sharpening the T and S gradients in the ACC fronts on the lateral open boundaries

Stronger winds and ACC provide more CDW to open shelf, but not necessarily to the cavity beneath GVI Ice Shelf

Dye concentration for open shelf near GVI Ice shelf

Dye concentrationbeneath GVI Ice Shelf

Melt rate beneathGeorge VI Ice Shelf

Stronger winds reduce melt ratebeneath GVI(stronger ACC makeslittle difference)

Why?

Dye concentrati

on

Temperature

Base winds Wind X 1.2

South GVI entrance: Feb. 2001

ConclusionsConclusions

• Intrusions of CDW on to the shelf have a Intrusions of CDW on to the shelf have a shorter timescale then previously thought shorter timescale then previously thought and the intrusion timescales seem to be and the intrusion timescales seem to be related to wind eventsrelated to wind events

• Increases in winds and ACC transport lead Increases in winds and ACC transport lead to increases in the amount of CDW to increases in the amount of CDW advected onto the continental shelfadvected onto the continental shelf

• However, increases in winds and ACC However, increases in winds and ACC transport do not necessarily lead to transport do not necessarily lead to increased CDW flux underneath the ice increased CDW flux underneath the ice shelves or increased basal meltshelves or increased basal melt

AcknowledgementsAcknowledgements

• BEDMAP data courtesy of the BEDMAP consortium

• Other bathymetry courtesy of Laurie Padman and Tom Bolmer

• AMPS winds courtesy of John Cassano

• Computer facilities and support provided by the Center for Coastal Physical Oceanography

• Financial support from the U.S. National Science Foundation (ANT-0523172).

Tmax below mixed layer for the West Antarctic Peninsula (WAP) shelf

Moffat et al., 2009

Potential temperature and salinity from the SO4P WOCE section (includes the WAP shelf).

Moffat et al, 2009

Ice Shelf ModelingIce Shelf Modeling

• Ice Shelf does not change in time in modelIce Shelf does not change in time in model

• Three equation viscous sub-layer model Three equation viscous sub-layer model for heat and salt fluxes (Holland and for heat and salt fluxes (Holland and Jenkins, 1999)Jenkins, 1999)– Conservation of HeatConservation of Heat– Conservation of SaltConservation of Salt– Freezing point of sea water (function of Freezing point of sea water (function of

pressure and salinity)pressure and salinity)

• PGF calculation assumes the ice shelf has PGF calculation assumes the ice shelf has no flexural rigidity and pressure at the no flexural rigidity and pressure at the base comes from the floating icebase comes from the floating ice

Dye concentration at depth (model level 4): 9/15/2003 – 9/15/2005

Model average velocity(net through flow: 0.14 Sv.)

Estimated velocity from observations (Jenkins and

Jacobs, 2008; net through flow: 0.17-0.27 Sv.)

Model mean basal melt rate (GVI): 6.0 m/yr

Estimated observed basal melt rate:2.1 m/yr (Potter and Paren, 1985)2.8 m/yr (Corr et al., 2002)3.1-4.8 m/yr (Jenkins and Jacobs,2008)

Left: Gate for measuring CDW intrusions (blue) and upstream cross-section (green-red)Right: Model dye flux across Marguerite Trough “gate” and intrusion thresholds

“Intrusion” flow Non-“Intrusion” flow

No significant correlation between upstream volume flux along continental slope and dye flux, but significant lagged correlation for inner along slope volume flux and dye flux (r = 0.74, volume flux leads dye flux by 2 days)

Physical Mechanism?

Dye concentration (Layer 4 - Feb. 2002)

20% decrease in wind speed

20% increase in wind speed

Summer sea ice distribution (Feb. 2002)

20% decrease in wind speed

20% increase in wind speed

Melt rate beneathWilkins Ice Shelf

Highly seasonalmelt rate, butstronger winds increase melt ratebeneath Wilkins(ACC makes littledifference)

Why?

Base winds

Wind X 1.2

South GVI entrance temperature: Feb.

2001

CDW as a source of Fe?

Assume:

Offshelf CDW dissolved Fe concentration = 0.5 nM

C:Fe algal assimilation ratio = 450,000 mol/molScaling from the dye flux into the top 100m of the open shelf areas, the Fe flux can support primary production on the WAP shelf of:

Growing season (11/1-2/28) Entire yearBase: 0.5 gC/m2/yr 7.6 gC/m2/yrWind*0.8: 0.0 gC/m2/yr 1.2 gC/m2/yrWind*1.2: 4.9 gC/m2/yr 19.7 gC/m2/yrWind*1.2+: 7.6 gC/m2/yr 23.7 gC/m2/yrWind*1.5: 20.2 gC/m2/yr 48.7 gC/m2/yr