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