southern ocean surface measurements and the upper ocean heat balance janet sprintall sarah gille...
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Southern Ocean Surface Measurements Southern Ocean Surface Measurements and the Upper Ocean Heat Balanceand the Upper Ocean Heat Balance
Janet Sprintall Janet Sprintall Sarah GilleSarah GilleShenfu DongShenfu Dong
Scripps Institution of Oceanography, UCSDScripps Institution of Oceanography, UCSD
Challenges in the Southern OceanChallenges in the Southern Ocean
Western Boundary CurrentsWestern Boundary Currents Southern OceanSouthern Ocean
data rich data poor
sampling possible all year round very few winter observations
heat transport from low to high latitudes
circumpolar SST probably means little net heat transport
ocean heat transport primarily geostrophic
strong westerlies drive strong meridional Ekman transport
fairly reliable, validated surface heat flux products
heat flux products with very large uncertainties
Talk Outline:1. Status of shipboard observations in the Southern Ocean2. Science Applications:-
a. Variability in the Antarctic Polar Frontb. The upper ocean heat budget in the Southern Ocean
3. Conclusions: Implications for data sampling requirements in the Southern Ocean
Southern Ocean HR-XBT Measurements
USA-SIO; Aust-CSIRO;NZ-NIWA; Francewww-hrx.ucsd.edu
U.S-Chinesemoon.ldgo.columbia.edu/~xiaojun/xbt/
Italian CLIMA
PX08
IX15
IX21
PX50PX81
AX22
IX28 AX25
AX18
PX14
NOAA-AOMLwww.aoml.noaa.gov/phod/hdenxbt/high_density_home.html
Drake Passage MeasurementsDrake Passage Measurements
wind speed (m/s)
pCO2 (atm)
salinity (psu)
XBT temperature
Depth averaged ADCP velocity
PIs: Sprintall (XBT); Takahashi, Sweeney (pCO2); Chereskin, Firing (ADCP)
Science Application: 1. Variability in the Antarctic Polar Front*
AMSR-E (Microwave)
AMSR-E: cloud penetration
MODIS (Infrared)
Lots of cloudy or bad data
*Dong, Sprintall & Gille, Location of the Antarctic Polar Front from AMSR-E Satellite Sea Surface Temperature measurements, J. Phys. Oceanogr., in press, 2006.
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)
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
What controls the Polar Front Variability?
1. Response of PF location to meridional shifts in the wind field
(∂PF/∂t ~ ∂(x)/∂t)
negative phase: shift in latitude of maximum zonal wind stress leads meridional shift in PF
histogramof phase
coherence60% > 95%CI
phase
wind PF
PFwind
phase
Science Application:
2. Southern Ocean Upper Ocean Heat Budget*
€
∂Tm
∂t=
Qnet − q(−hm)
ρ ocphm
− um ⋅∇Tm + κ∇ 2Tm −weΔT
hm
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
Horizontal Advection
geostrophic advection (AVISO SSHa plus GRACE) Ekman advection (COAPS wind stress)Tm mixed layer temperature AMSR-E SST
€
∂Tm
∂t=
Qnet − q(−hm)
ρ ocphm
− um ⋅∇Tm + κ∇ 2Tm −weΔT
hm
Imbalance of the Heat Budget Analysis
Largest imbalance in winter (~100 W m-2)
€
∂Tm
∂t=
Qnet − q(−hm)
ρ ocphm
− um ⋅∇Tm + κ∇ 2Tm −weΔT
hm
“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)
Spatial rms of Qnet (Wm-2)(1 Jan 2000 - 31 August 2002)
Sensitivity: 1. Surface heat flux products
NCEP1-NCEP2
NCEP1-ECMWF
NCEP1-SOC(monthly clim)
RMS of heat balance:NCEP1: 146 Wm-2
NCEP2: 148 Wm-2
(June 02 - Dec 05)
€
∂Tm
∂t=
Qnet − q(−hm)
ρ ocphm
− um ⋅∇Tm + κ∇ 2Tm −weΔT
hm
Sensitivity: 2. Mixed Layer Depth (hm)
€
∂Tm
∂t=
Qnet − q(−hm)
ρ ocphm
− um ⋅∇Tm + κ∇ 2Tm −weΔT
hm
hm from ARGO float data (∆=0.03kg/m3)
Variable hm Time-Mean hm
Sensitivity: 2. Mixed Layer Depth (hm)
€
∂Tm
∂t=
Qnet − q(−hm)
ρ ocphm
− um ⋅∇Tm + κ∇ 2Tm −weΔT
hm
RMS of Heat Balance for Various MLD:1. 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)
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)
3. 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.
Sensitivity: 3. ∆T across the base of the MLD
• 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
€
∂Tm
∂t=
Qnet − q(−hm)
ρ ocphm
− um ⋅∇Tm + κ∇ 2Tm −weΔT
hm
Annual Average ∆T from Argo floats
Spatial Variability in the Imbalance of the Heat BudgetSpatial 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
Regional Heat Budget: The Agulhas RetroflectionRegional Heat Budget: The Agulhas RetroflectionMarch Average
• 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)
(x10-6 °Cs-1)
cooling warming
Conclusions: Implications for data sampling requirementsConclusions: Implications for data sampling requirements
1. 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
What controls the Polar Front Variability?
2. Response of PF Transport to changes in zonal wind stress du/dt (~ u==dT/dy) and x
negative phase: change in zonal wind stress leads to changes in PF transport
coherence70% > 95%CI
phase
histogramof phase
ACCwind
ACCwind
Southern Ocean Heat Balance by Basin