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Southern Ocean Surface Measurements and the Upper Ocean Heat Balance

Southern Ocean Surface Measurements and the Upper Ocean Heat Balance. Janet Sprintall Sarah Gille Shenfu Dong Scripps Institution of Oceanography, UCSD. Challenges in the Southern Ocean. Talk Outline: Status of shipboard observations in the Southern Ocean Science Applications:-

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Southern Ocean Surface Measurements and the Upper Ocean Heat Balance

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  1. Southern Ocean Surface Measurements and the Upper Ocean Heat Balance Janet Sprintall Sarah Gille Shenfu Dong Scripps Institution of Oceanography, UCSD

  2. Challenges in the Southern Ocean Talk Outline: Status of shipboard observations in the Southern Ocean Science Applications:- a. Variability in the Antarctic Polar Front b. The upper ocean heat budget in the Southern Ocean 3. Conclusions: Implications for data sampling requirements in the Southern Ocean

  3. IX21 PX81 IX15 AX18 PX50 AX25 PX08 IX28 PX14 AX22 Southern Ocean HR-XBT Measurements USA-SIO; Aust-CSIRO; NZ-NIWA; France www-hrx.ucsd.edu NOAA-AOML www.aoml.noaa.gov/phod/hdenxbt/high_density_home.html U.S-Chinese moon.ldgo.columbia.edu/~xiaojun/xbt/ Italian CLIMA

  4. wind speed (m/s) pCO2 (atm) salinity (psu) XBT temperature Drake Passage Measurements Depth averaged ADCP velocity PIs: Sprintall (XBT); Takahashi, Sweeney (pCO2); Chereskin, Firing (ADCP)

  5. MODIS (Infrared) AMSR-E (Microwave) Lots of cloudy or bad data AMSR-E: cloud penetration Science Application:1. Variability in the Antarctic Polar Front* *Dong, Sprintall & Gille, Location of the Antarctic Polar Front from AMSR-E Satellite Sea Surface Temperature measurements, J. Phys. Oceanogr., in press, 2006.

  6. Comparison of the PF Location from XBT and AMSR-E SST  Subsurface Polar Front from XBT (northern extent of the 2°C isotherm at 100-300m depth)  Surface Polar Front from AMSR-E (southernmost location of an SST gradient above 1.5x10-2°C km-1)

  7. Mean Polar Front Location AMSR-E (2002-05) Dong et al. (2006) AVHRR SST (1987 – 1993) Moore et al. (1999) Deep ocean basin with weak bottom slope: large PF variability

  8. phase What controls the Polar Front Variability? 1. Response of PF location to meridional shifts in the wind field (∂PF/∂t ~ ∂(x)/∂t) coherence 60% > 95%CI wind PF phase wind PF histogram of phase negative phase: shift in latitude of maximum zonal wind stress leads meridional shift in PF

  9. Science Application:2. Southern Ocean Upper Ocean Heat Budget* Domain Averaged Surface Layer Heat Balance(weekly resolution on 1°x1° grid) * Dong, Gille and Sprintall, Heat budget of the Southern Ocean, in prep, 2006

  10. Horizontal Advection geostrophic advection (AVISO SSHa plus GRACE) Ekman advection (COAPS wind stress) Tm mixed layer temperature AMSR-E SST

  11. Imbalance of the Heat Budget Analysis Largest imbalance in winter (~100 W m-2) “Best Case” rms of the imbalance is 146 Wm-2 (0.031°C/day) (NCEP1 air-sea heat fluxes; ARGO density MLD; diffusion =500 m2s-2; spatially-variable ∆T from ARGO)

  12. Sensitivity: 1. Surface heat flux products Spatial rms of Qnet (Wm-2) (1 Jan 2000 - 31 August 2002) NCEP1-NCEP2 RMS of heat balance: NCEP1: 146 Wm-2 NCEP2: 148 Wm-2 (June 02 - Dec 05) NCEP1-ECMWF NCEP1-SOC (monthly clim)

  13. Variable hm Time-Mean hm Sensitivity: 2. Mixed Layer Depth (hm) hm from ARGO float data (∆=0.03kg/m3)

  14. Sensitivity: 2. Mixed Layer Depth (hm) • RMS of Heat Balance for Various MLD: • Argo Floats climatology (Jun 02 - Dec 05): • density diff 0.03kgm-3: 0.031°C/day (146 Wm-2) “best” case • temp diff 0.2°C: 0.033°C/day (157 Wm-2) • de Boyer Montegut et al. (2004) climatology: • density diff 0.03kgm-3: insufficient data • temp diff 0.2°C: 0.037°C/day (151 Wm-2) • WOA 2001 climatology: - density diff 0.125kgm-3: 0.038°C/day (235 Wm-2) NB: Density MLD criteria uses variable ∆T in entrainment term; temperature MLD criteria uses ∆T=0.02°C.

  15. Sensitivity: 3. ∆T across the base of the MLD Annual Average ∆T from Argo floats • many studies use ∆T=0.2°C (eg. Qiu and Kelly, 1993) • for ∆T=0.2, rms of imbalance is 159 Wm-2 cf “best” case of 146 Wm-2. • largest improvement in Indian Ocean where ∆T can be negative: salinity matters! • differences in ∆T are largest in fall and winter when entrainment is strongest

  16. Spatial Variability in the Imbalance of the Heat Budget • Largest imbalance north of ACC: hm is large & temperature gradient is strong • complex upper ocean processes not well resolved by existing measurements

  17. March Average Regional Heat Budget: The Agulhas Retroflection (x10-6 °Cs-1) warming cooling • strong SST gradient and large meanders in Agulhas region • reanalysis Qnet has long length scales, while we & advection have smaller scales: net effect on imbalance is small scale structure. • large imbalances related to small-scale coupling of the wind field and SST? • (e.g O’Neill et al. (2003) find wind stress curl (divergence) related to cross (down) wind components of SST gradient)

  18. Conclusions: Implications for data sampling requirements Shipboard Measurements - met & pCO2 sampling opportunity: need identified PIs. 2. Science Application: Polar Front Variability - weekly and daily SST fields resolve similar PF locations - winds are important! Global forcing fields from satellite. - winds more energetic at higher frequencies 3. Science Application: Upper Ocean Heat Balance - large uncertainties in all terms! - Qnet varies enormously. Need validation with in situ data and winter time measurements - salinity matters! Need Argo floats for MLD and ∆T - need spatial resolution ~0.25° for small-scale coupling - weekly and daily SST fields give similar heat balance

  19. wind wind ACC ACC What controls the Polar Front Variability? 2. Response of PF Transport to changes in zonal wind stress du/dt (~ u==dT/dy) and x coherence 70% > 95%CI phase histogram of phase negative phase: change in zonal wind stress leads to changes in PF transport

  20. Southern Ocean Heat Balance by Basin

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