Oceanic mixed layer heat budget

in the Eastern Equatorial Atlantic using ARGO floats

and PIRATA buoysM. Wade (1,2,3), G. Caniaux (1) and Y. du Penhoat

(2)

1. CNRM/GAME (Météo-France / CNRS), Toulouse, France2. IRD LEGOS, Toulouse, France3. LPAOSF, Dakar, Sénégal

Introduction

1. SST anomalies in the Eastern Equatorial Atlantic (EEA) condition precipitation over the West African continent ( Vizy and Cook, 2002)

2. Interaction between the Atlantic Cold Tongue (ACT) and the West African Monsoon (e.g. Okumura and Xie, 2004; Gu and Adler, 2004; Caniaux et al., 2011)

3. Need to understand the physical processes affecting SSTs in the EEA

SAHEL

ACT

Equatorial Front

Reynolds et al. (2007)’s SSTs – 15 June 2005

Introduction

1. Observation (e.g. Merle et al., 1980; Foltz et al., 2003) and modeling studies (Philander and Pacanowski, 1986; Yu et al., 2006; Peter et al., 2006) suggest that subsurface processes are at the origin of upper temperature decrease in the ACT

2. Further south SSTs are largely dominated by surface heat fluxes

3. Observations fail to close heat budgets within 150 Wm-2 in the EEA (Foltz et al., 2003)

We present an oceanic mixed layer heat budget combining

in-situ observations from ARGO floats and PIRATA buoys

and outputs from atmospheric NWP models.

ARGO Floats

October 2007

Method

nsolsoleptp FhIFhwhTTSSTUhcThc )(1)()(.00

Heat storage Hori. advec. Entrainment Solar h. flux Non-solar h. flux

with

SSTUTU ..

hhUhhwhw te ).()()(

Surface heat fluxes: ECMWF

)(2

)(10 21 ).().()( chlhchlh echlVechlVIhI

OSCAR data set (Helber et al., 2007)+ Reynolds et al. (2007)’s SSTs

Ohlmann (2003)

0

0

1

1

dt

dzh hand

Subdivision of the EEA

9 boxes reflecting the regional characteristics of

the dynamics and thermodynamics

Northern boxes (warmer SSTs, Guinea Current, heavy precipitation)

ACT boxes (SEC and EUC)

Southern boxes (SECC, Angola dome)

De Boyer Montégut et al. (2004)

MLD climatology

1

2

3

7

8

4

5

69

Spatial and Temporal Distributions of Profiles

1. 2400 profiles over 3 years (2005-2007)

2. The number of floats increased during the EGEE cruises

3. 20 profiles/mon. in Jan. 2005 and 100 profiles/mon. in Jan. 2007

4. Lower number of floats (4-9) and profiles in boxes 2, 3, 6 than in the other boxes (11-18)

5. 4 PIRATA buoys included

EGEE1EGEE2 EGEE3

EGEE4 EGEE5EGEE6

1

2

3

4

5

6

7

8

9Latitude vs. time

Number vs. time

Comparison ARGO-Reynolds’ SSTs

1. Good agreement between ARGO and Reynolds’ SSTs after quality control

2. Important seasonal cycle in each box, weaker north (3°C-4°C) than south of the equator (4°C-6°C) with north-south SST gradients

3. 2005 was a year with anormally cold SSTs in the ACT (boxes 2, 5, 8)

ARGO SSTsReynolds et al. (2007)’s SSTs

7

3

2

14

5

6

8

9

SSTs vs. time

Mean Annual MLD Heat Budget (2005-2007)

1. Mean budgets are positive except in boxes 6 and 9

2. Surface heat fluxes dominate

3. A significant part of solar heat flux goes out of the ML

4. Mean horizontal advection stronger in boxes 1, 2, 8, 9

5. Weak contribution of entrainment

6. The residuals are strong, meaning that one or several terms are missing

+58

+92

+27 +38

+89

+68

+94

+95

+90

+6

+16

-18 -2

-4

+2 -16

-24

-9

-27

-20

-13

-21

-28

-27

-27

-38

-40

-3 -9

-11

-4-4

-13

-1

-6

-3-47

-15

-32-25

-40

-17

-38

-53

-3

BOX 7

BOX 8

BOX 6

BOX 5

BOX 3

BOX 2

BOX 4 BOX 1

BOX 9

0

-3

+12

+1

-11

+4+4

+3

-8

Horizontal advection

Net heat flux

Fsol*I(-h)Residual Entrainment

Heat storage

Seasonal Cycle

1. Amplitude of cooling / warming: -100 to +50 Wm-2

2. Cooling from April to July, weaker in the ACT

3. Semi-annual signal

4. The Σ ( calculated terms) > storage term (except in box 9): the residual term has a negative contribution

Heat storageSum of the calculated terms

1. The seasonal cycle of heat storage is mainly due to the seasonal cycle of the surface heat fluxes

2. The surface heat fluxes have a >0 contribution, except in austral winter in boxes 6 and 9

3. Entrainment is insignificant throughout the year

Seasonal Cycle

Horizontal advectionNet heat flux Fsol*I(-h)

Entraînment

Seasonal Cycle of the Residuals 1. Negative residuals

(reaching -120 Wm-2)2. Weak residuals in the

southernmost boxes 3. The residuals are due

to errors and missing terms

4. Turbulence data collected during AMMA/EGEE (Dengler et al., 2010) and other studies (Rhein et al., 2010) compare well with the residuals (phase, amplitude, spatial distribution)

5. Vertical mixing is probably the leading missing term

-38 -25 -32

-15-40-53

-3 -17 -47

Heat Storage 5 Wm-2

Horizontal Advection 5 Wm-2

Entrainment 18 Wm-2

Net surface heat flux 17 Wm-2

Fsol I(-h) 7 Wm-2

Residual 27 Wm-2

Statistical errors

Conclusions1.ARGO floats provide realistic ML budgets in

the EEA at seasonal time scales2.Cooling in April-July and warming the

remainder of the year with a short cold season3.The ML cooling rate in the ACT is weaker and

shorter than elsewhere 4.Strong residuals suggest that vertical

diffusion is the most important process during the ACT setup (shear between the SEC and the EUC in the ACT and between the Guinea Current and the Guinea Undercurrent in the northern boxes)