# Presentation: 640931

## Ocean transports of heat and freshwater:How good are our estimates?Has WOCE changed the values and uncertainties?

**Ocean transports of heat and freshwater:****How good are our estimates?****Has WOCE changed the values and uncertainties?**

**ALEXANDRE GANACHAUD**
**IRD, Noum****é****a****, New Caledonia**
**(****MIT/WHOI joint program**** / C. Wunsch)**

## Ocean-atmosphere heat transports

**Keith (1995)**

**1PW=10****15****W**
**=****10****6 ****nuclear plant**

**latitude**

## Outline

**Heat transports**
**3 and more methods**
**Hydrographic method**

**Freshwater transports**
**3 and more methods**
**Hydrographic method**

**Uncertainties**
**What did WOCE bring**** ****?**
**Future ?**

## Heat flux estimationMethod 1 « BULK »

- Heat flux estimation**Method 1 « BULK »**

**Data from voluntary observing ships and buoys**
**Empirical formulae:**
**da Silva (1945-1989)**
**SOC (1980-1993)**
**Uncertainties: **
**Measurement accuracies: ****15 W m****-2**
**Formula is not universal**
**Uneven spatial coverage**

## Heat flux estimationMethod 2: « atmospheric residuals »

**High resolution products**
**Depends on atmospheric (data-assimilating) model and internal parameterizations**
**Uncertainties ****not possible to compute**

**Atmosphere:**
**NCEP**

- Heat flux estimation**Method 2: « atmospheric residuals »**

## WOCE (1990-1995)World Ocean Circulation Experiment

**Goal 1: to develop models useful for predicting climate change ****and to collect the data necessary to test them**
**Objective 1: ****to determine and understand the global large-scale fluxes of heat and freshwater****; their divergence over 5 years and their annual and interannual variability**

**Net air-sea fluxes between sections *****w******ith uncertainties***** **
**T, S, ****o****xygen,**** ****nutrients; ****moored arrays;****XBT, ADCP; floats;****repeat sections**

## Method 3:Hydrographic method

**Ekman Transport**** **

**Method 3:****Hydrographic method**

- Absolute velocity

- Relative thermal wind

- Reference velocity

- NCEP reanalysis

- Correction

*b(x)*

- z

- v

**WOCE subset of Ganachaud & Wunsch**

## Constraints

**Conservation**
**mass**
**salt**
**heat**** (****deep****)**
**silicate (****top-bottom****)**
**170PO****4****+O****2** **(deep)**
**Net fluxes**
**straits**** (****Florida****...)**
**WBC**** (****arrays****)**
**t****opograph****y / ridges**

**Reference surfaces**
**Current meters, water mass and previous works**

## Linear model

**L****in****ear model**

**1200 con****s****traints**
**3000 ****unknowns**

**Ekman**

**Thermal wind**

**unknowns**

** ****b=**

** ****b(x)****	****=0****±****1cm/s**
**w*(z)****	****=0 ****±**** 10****-4**** cm/s**
*******(z)****	****=0 ****±**** 100 cm****2****/s**
******Tekman=  ****±**** 50%**
**Freshwater**** = 0 ****±**** 1Sv**

**Gauss-Markov**
**estimator**

## Absolute velocity field

**Indian Ocean****, 18S**

## Water mass transports

**NADW: ****15±2 to 23±3  Sv**
**AABW: 6±1 Sv **
**CDW ****Indian****:  11±4 **
**CDW ****Pacific****: 7±2**
**CDW ****formation****: ****21 ±6 Sv**
** ****ITF : 16±5 Sv**

**1Sv=10****6****m****3****s****-1**

**Notes:**
The details of water mass conversion will be presented in the next talk by Eli Tziperman and Kevin Speer

## Atlantic Heat transport

**Atlantic ****Heat transport**

**Coherent hydrographic estimates**
**Divergences=air-sea fluxes**
**NCEP-ERBE air-sea flux consistent with GW ****within**** 0.1 PW**
**No uncertainties on climatologies**
**Adjustment at 6°N**

- hydrography

## Indian + Pacific

**Indi****a****n + Pacifi****c**

**Less data**
**Up to 1PW differences in climatologies due to low southern coverage****. **

## Global heat fluxes

**Southern** -0.7PW** /**** Tropical**** **+2.3PW** / ****Northern **-1.7PW
**70% ****of the global cooling takes place in the North**
**Flux products and ocean numerical models are getting closer to hydrographic estimates**

## Uncertainties

**Ergodic hypothesis****:**
**Large scale circulation ****= time mean**
**Measurement noise**
**Calculations**

**To which extent can one section sample time-mean T,S ?**
**Quantification on numerical ocean model**
**Repeat sections**
**Internal waves (****Garrett-Munk****)**
**Experiments (bottom triangles, gaps...)**

## Seasonal bias in hydrographic sections

**WOCE number of months-at-sea**
**At mid and subtropical latitudes**
**(year shifted by 6 months in the **
**Southern Hemisphere)**

## Part II: Freshwater (>80% of greenhouse effect)

***Precipitation***
**Difficult observation: sporadic and orographic effects (islands)**
**Satellite radiometers**** (SSM/I, TOVS)****:**** ****liquid and ice water content. Difficult relation with precipitations, lack of calibration**
**Atmospheric operational models:****-few oceanic observations****-cloud parameterization**
**River runoff: uncertain and unknown groundwater transports**
**Adler et al. (2001): 2-fold changes from one quasi-standard  product to the next**

***Evaporation (all empirical formulae)***
**Bulk****: local (v, q, T). Variable conditions, systematic biases**
**Satellite radiometers**
**Operational atmospheric models**

**Large and Nurser (2001)**

**Notes:**
Greenhouse effect
Evaporation moderates ocean heating
Redistribution of latent heat by the atmosphere
Large and Nurser: compilation of products with adjustments dictated by the purpose-to force ocean models

Lack of reliable benchmarks has retarded climate model development (Wijffels). Gaffen et al (AMIP, 1997): majority of atmospheric models overestimate poleward moisture transport.
Ocean models need FW forcing, and observed products lead to drift -> relaxation which is convinient but not very physical (Wijffels).

Rain gauge measurements are available only in space locations where people live
Since 1987, SSMI= precipitable water
TOVS= TIROS Operational Vertical Sounder
GPCP/CMAP: compilation of fadiosonde, SSM/I andTOVS
Over the ocean, based on OLR and scattering and emission of uwave radiation / based on algorithmes that are not accurate everywhere, for instance in the Pacific ITCZ (Xie and Arkin, 1996) were little in situ calibration exist
Existing products= combination of in situ + satellites, including infrared, MW scattering and MW emission.
OLR: correlated with cloudiness and therefore -P in the tropics but with SST and thus +P in mid-latitudes

## Existing products

**Existing products**

** ****Wijffels, 2001: Comparison of 13 products: deviations largely exceeding the signal**
** ****Need for large scale integrals for reference and calibration**

**Pacific Ocean**

## Mass divergence between two sections
Signal: FW0.3 Sv (0.3 106 m3s-1)
Residual mass noise: nM=±7Sv

Use of mass and salt correlation:

Noise in the difference n ’M  ±0.2 Sv(Wijffels, 2001)

**Signal: FW********0.3 Sv (0.3 10****6**** m****3****s****-1****)**
**Residual mass noise****: ***n**M**=±7Sv*
**Use of mass and salt correlation****:**
**Noise in the difference**** ***n ’**M **** ±0.2 Sv***(Wijffels, 2001)**

**Freshwater transports from hydrographic data**

## Freshwater transports across hydrographic lines

**ATLANTIC**

**PACIFIC**

**INDIAN**

**Zero FW ITF**

**Significant over large areas:**
**Globally (GW): ****30°S - 47°N: E=1.2±0.5 Sv**
**Southern Ocean****: P=0.8±0.9 Sv**
**Too much evaporation in satellites**
**Most other products within large uncertainties**

**From Wijffels (2001)**

## In conclusion

**Much remains to be done**** with WOCE data****: evaluation of uncertainties from OGCM simulation**
**More in-situ ****measurements****: ****VOS radars ?***

**0**

**?**

## Transport processes1. Steady / instantaneous

**	****	****	**
**NET**
**OVERTURNING**
**HORIZONTAL**

**Heat is transported by the overturning term *****in density coordinate***
**Freshwater  is transported by overturning and horizontal; a 2-D model would be wrong**

**Notes:**
1-Quantifying transports is one thing, but understanding how properties are transported is as important because an incorrect process would give an incorrect climate model.
2-To distinguish physical processes, a decomposition of the heat transport into net, overturning and horizontal contributions is usually done and one finds that...
3-However this partition does not really correspond to physical processes and a 2-D density model would lack all the dynamics that generate the air sea fluxes
4-Lynne Talley proposed a more physically based decomposition putting forward the transport of warm water by the Gulf Stream...

## Transport processes2. Rectified time variability

**Roemmich ****&**** Gilson ***** ****(XBT, 2001): ****rectified ****eddy transports (v’T’) may reach 30% of the mean in the North Pacific**

**Notes:**
Will know more with Prof. Marshall’s talk
Jayne and Marotzke: high frequency variability is Ekman-Barotropic balance

## Ocean transport of heat and Freshwater

**Subpolar Cooling (24-47N): 1.1****±0.2PW=32 ±6Wm****-2**
**Bulk: global bias +30Wm****-2**
**Reanalyses: ? (rms 30Wm****-2**** for 500km scales)**
**Freshwater fluxes: bounds on large scale values: ****1.2 ±0.5 Sv net tropical ****e****vaporation**

***How good are our estimates ?***

**New data:**
**Unprecede****n****ted coverage***** with**** ****broad**** accessibility****; m****ore studies from varied methods**
**Higher resolution, supplementary data**
**More bulk**** and satellite**** observations**
**New results:**
**Higher quality estimate, better control of uncertainties (model error)**
**Reduction of uncertainties difficult to quantify from the litterature**
**In repeat sections n****o ****significant**** changes (*)**
**Stimulation of flux products; ocean and climate GCM**

***Has WOCE changed values and uncertainties?***

**Ocean transport of heat and Freshwater**

## Will we be able to detect transport changes?

**Year of publication**

**Notes:**
An interesting question is...
Prelude to Prof. Marotzke’s talk about the Climate stability and unstabilities

## Perspectives 1improvements

**Remaining**** uncertainties**
**Indonesian Throughflow**
**Variability (nutrients) -> repeat observations**
**Error behavior: experiments on GCMs**
**Winds**

**Possible further work**
**Increasing resolution ***
**Global combination of regional studies**

## Perspectives 2future hydrographic lines

**Perspectives ****2****future hydrographic lines**

**Optimizing the signal**

## Perspective 3Constraining flux products

**Grist & Josey (2002)**
**Bulk,**** ****constrained climatology**

**constraint**

## Other applications of WOCE hydrographic program

**Biogeochemical cycles:**
**Oxygen and nutrients transports, **
**diagnostics of sources and sinks, **
**quantification of export production **

## Thanks To

*Macdonald*
*S. Josey*
*J. Marotzke*
*H. Bryden*
*C. Wunsch *
*K. Trenberth*
*... **and many other WOCE actors !*

**Inspired from**
**H. Bryden, S. Imawaki and S. Wijffels**
**Ocean Circulation and Climate**

## Atlantic Ocean

**Atlantic Ocean**

**Coherent hydrographic estimates**
**Divergences=air-sea fluxes**
**NCEP-ERBE air-sea flux consistent with GW ****within**** 0.1 PW**
**No uncertainties on climatologies**
**Adjustment at 6°N**

- bulk

- reanalysis

- hydrography