Diurnal variations of air-sea flux of CO2, ocean CO2 partial pressure, SST and mixed layer
depth along drifter trajectories in the North-Eastern Atlantic
Merlivat.L1 ., Caniaux.G 2 ., and Barbero-Munoz. Leticia 1,3
1,,Laboratoire d’Océanographie et du Climat par Expérimentation et Analyse Numérique,
LOCEAN,InstitutPierreSimonLaplace,PARIS ,FRANCE
2,CNRM ,Toulouse, FRANCE
3, UNIVERSITY, Las Palmas, SPAIN
During the POMME experiment (North East Atlantic, February to September 2001), four
CARIOCA drifters were launched in the area 20W-15W, 39N-45N in order to collect data at air-sea
surface and compute CO2 flux .In this presentation, we will focus on the analysis of the mechanisms
responsible for the short term variability of pCO2 at the sea surface and the CO2 flux at the air water
interface during winter months, February and March.
On the water side, hourly measurements of pCO2, sea surface temperature (SST ) salinity
and fluorescence are made by CARIOCA buoys on samples pumped at a depth of 2 meters. Wind
speed is measured at a height of 2 meters in the atmosphere.The value of the dissolved inorganic
carbon, DIC,is computed from the distribution of pCO2 and salinity, knowing the relationship which
links alkalinity and salinity in the studied area. In some situations ,a regular diurnal cycle of pCO2,
SST and DIC is observed.The minimum of SST around sunrise coincides with the maximum of
pCO2 and DIC while the maximum of SSTwhich occurs in the middle of the afternoon, around
sunrise plus 9 hours is accompanied by a minimum of pCO2 and DIC. The thermodynamical effect
due to the influence of SST would have induced a pCO2 change in an opposite direction. Mixing due
to either nocturnal convection or wind action and biological processes are the likely candidates to
interpret the observations
An estimate of the diurnal cycle of mixed layer depth and SST along the CARIOCA
drifter trajectories has been computed by using a one dimensional model forced with hourly heat,
salt and wind fluxes. Radiative fluxes (solar and infrared) are derived from satellite. Turbulent fluxes
are calculated with bulk formulae from ECMWF air temperature, humidity, wind, precipitation and
pressure fields and interpolated along the buoy trajectories (Caniaux et al., 2004)., Initialization of
the model is provided by selecting the appropriate nearest CTD cast launched during one of the four
mesoscale hydrological surveys performed during the experiment. Mixed layer depths were
computed with Thomson and Fine’s algorithm
Circumstances are identified when a good retrieval of both daily cycles of SST and SSS
computed by the model compared to buoys’ data is observed.This indicates that the 1D modeling
hypothesis is valid. In winter large diurnal MLD cycles (30m to 120m) occur during weak wind/large
solar radiation periods, even by strong nocturnal convection .Stratification occurs at the beginning of
March, coincident with SST increase.The daily cycle of mixed layer depth and DIC are tightly
linked, the maximum depletion of DIC occurring when the mixed layer depth is the shallower. An
estimate of the net community metabolism, NCM is computed under the studied winter conditions.
The value of pCO2 at the sea surface is always smaller than its value in the atmosphere. Processes
which control the daily variability of pCO2 at the sea surface induce a variability of at most 20% of
the gradient of pCO2 between atmosphere and water and consequently on the flux exchanged at the
interface. However,the variability of the wind speed remains the main controlling factor , even at
short time scale
References
Caniaux.G et al, submitted to JGR, 2004