Since some profiles provide the hydrographic properties on depth levels while others provide them on pressure levels all depths are converted to pressure to make a joint analysis possible.

To eliminate bad profiles the data passed through automatic quality control tests.

The automatic quality control of float profiles follows the standard procedures approved by the international Argo data management team (a document describing the Argo quality control tests is available at www.ifremer.fr/coriolis/cdc/argo_rfc.htm; the main tests are: speed check, gross range tests, spike tests, pressure increasing test, and a vertical gradient test). Some of the profiles that failed the test are inspected visually. Profiles from other instruments go through a similar set of tests.

In addition, the profiles are compared with the Levitus climatology (World Ocean Atlas [WOA] 1998, Conkright et al., 1998, and, more recently, WOA 2001, Conkright et al., 2002) and the NCEP ocean reanalysis (Global Ocean Data Assimilation System, GODAS, Dave Behringer, personal communication, 2005) which helps in the detection of sensor drifts or offsets. A similar quality control procedure is applied to all profiles that were collected with XBTs or other instruments. More details about the quality control procedures as well as results from a comparative analysis of XBT and float profiles can be found in Schmid (2005).

To get gridded fields, the data are grouped into 5˚ longitude by 1.5˚ latitude boxes. Then the mixed layer thickness and temperature are derived for each profile. Herein, the bottom of the mixed layer was defined as the depth where the temperature gradient exceeds 0.05˚C/dbar. An additional quality control is performed on the mixed layer properties themselves. This is a statistical analysis of the mixed layer thickness, for example. If a value is outside of 2 standard deviations around the median of all values in each box for each month it is considered unreliable and excluded from the analysis. If not enough data are available within one box, then the data in immediately adjacent boxes are considered as well.

In the next step, the data and positions within each box are averaged over the time period of interest. Then an objective analysis is performed with a grid resolution of 1˚ by 1˚. The correlation length scales were set to 20˚ in the zonal and 4˚ in the meridional direction. The error of the observations was assumed to be 0.01˚C, 0.01 psu and 10 m for temperature, salinity and mixed layer thickness, respectively. The error of the climatological field was assumed to be 1˚C, 1 psu and 10 m. The rms error from the objective analysis is used to exclude regions with insufficient data coverage from the contour maps and from further analysis. The limits chosen for this purpose are 0.05˚C, 0.03 psu and 6 m for the three parameters.

The heat balance is given by heat storage rate + horizontal advection + entrainment = net surface heat flux. The full equation (Stevenson and Niiler, 1983) for the heat balance is:

where  ρ= density, C _p = specific heat of sea water, h = mixed layer thickness, T = temperature, w_e = entrainment velocity, v = horizontal velocity, q_t = q _lat + q_sens + q_lw + q_sw = net surface heat flux q _sw = heat flux due to shortwave radiation, q _lw = heat flux due to longwave radiation, q_lat = latent heat flux, q _sens = sensible heat flux, q _sw (-h) = heat flux due to shortwave penetration and t = time. Overline indicates (mixed layer) means and primes indicate deviations from the mean.

The heat storage rate is derived as ρC _ph∂T/ ∂t from the gridded mixed layer properties. Before this is done the time series in each box are smoothed with a three-point running mean after a linear interpolation over short gaps (less than 4 months), which is especially important for the time derivative of the temperature.

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