― Introduction of Potential Conductivity ―

Key words: Argo project, Quality Control, HydroBase, Conductivity sensor correction

1: Frontier Observational Research System for Global Change, Climate Variations
Observational Research Program

2: Japan Marine Science and Technology Center, Ocean Research Development

In this study, we introduce a correcting-method of Argo (salinity) data, which is planed to be used in Japan Argo delayed-mode database in JAMSTEC/FORSGC.

Firstly, we show the horizontal movements of two Argo floats, WMO 29032 and 29033, used in this study. They were deployed in March 2000 and drifted in the sea for more than one year.

Fig. 1: Horizontal distribution of locations observed by the Argo floats (WMO 29032 and 29033) with climatological bottle sampling data. Red circles represent stations of WOCE-CTD data. Note that the location of the profile #11 of WMO29033 was not fixed.

Example 1: Sudden drift (WMO 29033)

Salinity drift occurred between profiles #10 and #12
when it was drifting near sea surface for about 3 months in summer
due to the insufficiency of its buoyancy (here after "summer vacation").

Fig. 2: T-S diagram of WMO 29033 observations. The red and blue circles represent the profiles #1-10 and #12-19, respectively.

Salinity drift became large between profiles #8 and #11. This float also took 3 moths "summer vacation", and its influence to the sensor should appear the data between profiles #6 and #7. Its sensor, however, was hardly damaged at that time, but about 2 months later. The sensor drifting was very slowly, and continued for about 4 months.

Fig. 3: T-S diagram of WMO 29032 observations. The red, blue, and green circles represent the profiles #1-8, #9-10, and #11-13, respectively

Conductivity observed by Argo floats is
calculated with the following equation:

C = C( S, T, P)

S: salinity

T: in situ temperature

P: in situ pressure

In this study, we use conductivity ratio R,
which is written with C as follows:

R = C( S, T, P) / C( 35, 15, 0 ).

Here after, it is written as

R = sal78( S, T, P). .......(1)

Firstly, we examine that sensor drift can be detected in the conductivity data.

However, clear differences between the data before and after of the "summer vacation" cannot be detected in theta-R diagram of WMO 29033.

Fig. 4: Relation of potential temperature and conductivity ratio for the data observed by WMO29033. Red and blue circles represent the data before and after "summer vacation", respectively.

Following to the relation of in situ temperature and
potential temperature,
we introduced a new value "potential conductivity ratio",
R_theta:

theta = theta( S, T, P, Pref), ........(2)

R_theta = sal78( S, theta, Pref). .....(3)

theta: potential temperature

Pref: Reference pressure

In this study, we use simply Pref = 0 dbar.

After that, the conductivity sensor drift can be clearly discriminated in the whole view (see Figs. 5c and 6).

Fig. 6: Same as Fig. 4 but for potential conductivity ratio.

- A linear local climatological relation of theta and R_theta in the middle/deep layers is estimated by the least squares method from the climatological profile data set, HydroBase (Fig. 7a).
- "True" values of R_theta (R_theta_"true") are calculated from its observed theta based on the local climatological relation (Fig. 7a).
- A linear relation of the observed and "true"
values of R_theta, namely a correcting-equation,
is estimated by the least squares method (Fig. 7c).

R_theta_"true" = a * R_theta_obs + b........(4)

Note that value of a in the above equation is restricted within 1 plus/minus 0.001 to obtain a reasonable and stable correction. - Finally, corrected R_theta is calculated from the observed values by the equation (4). Then, corrected salinity data are obtained.

Fig. 7: An example to correct Argo data (yellow) based on the climatology (black). (a) Estimation of a local climatological relation of potential temperature and potential conductivity ratio (the light-blue line), (b) drift of potential conductivity ratio from the local climatological relation, and (c) estimation of a correcting-equation of potential conductivity ratio of Argo data (the light-blue line).

In order to confirm the sensor drift of WMO 29033,
a CTD cast was conducted at 27.502°N 148.086°E
by R/V Mirai on February 17, 2001.

This CTD station is apart six days in time and
75 km in space from 18th station of
WMO 29033 at 27.498°N 148.859°E on
February 11, 2001 (see Fig. 1).

By using this CTD data, we examine
a performance of the above correcting method,
in which the local climatology was historical
profile data in area 1214_ABh.

Fig. 8: Corrected Argo data (red) based on the climatology with WOCE-CTD data (black). Orange and blue circles represent the raw Argo data and CTD data observed by R/V Mirai, respectively.

Fig. 9: Same as Fig. 8 but for climatology without WOCE-CTD data.

Fig. 10: Salinity difference from the CTD data observed by R/V Mirai. Red and blue circles represent the corrected Argo data based on climatology with and without WOCE-CTD data, respectively. Light-blue circles represent the raw Argo data.

Also it is suggested that this method is almost independent on the quality and quantity of the climatological data set.

The method described in this study is a prototype; therefore, we will continue to improve the method to achieve the goal of the Argo program for data accuracy in salinity, within 0.01 psu.