oceanic mixed layer heat budget in the eastern equatorial atlantic using argo floats and pirata...
TRANSCRIPT
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 advection
Net 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
Conclusions
1.ARGO floats provide realistic ML budgets in the EEA at seasonal time scales
2.Cooling in April-July and warming the remainder of the year with a short cold season
3.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)