Subsurface Barrier Layer Thickness (BLT)

Example of a Barrier Layer occurence on an ARGO station in Arabian Sea (31st-Jan-2002). Note the stable temperature inversion of order 0.2 °C in the subsurface barrier layer.

When the top of the halocline is shallower than the top of the thermocline, the surface layer of the ocean, above the thermocline, is separated into two layers: the surface mixed layer, and the subsurface so called Barrier Layer (after Godfrey and Lindstrom, 1989; see also section 3.3 in Lukas and Lindstrom, 1991; Sprintall and Tomczak, 1992), see figure on the right. The temperature in the BL is then equal or even sometimes superior to that in the surface mixed layer, salinity stratification controlling the stability of the water column. This BL have strong consequences on air-sea interactions as it thermodynamically insulates the deep cold ocean from the surface layer interacting with the atmosphere. Their thickness is a proxy of the amplitude of their thermodynamic barrier effect.

As for MLDs, the BLTs are estimated directly on instantaneous profiles with data at observed levels. Following most previous authors who studied the Barrier Layer, we define BLT on an individual station measuring both T and S, through the difference between two depths described above: BLT = TTD_DTm02 - MLD_DReqDTm02 (e.g., Vialard and Delecluse, 1998, Cronin and McPhaden 2002; see Figure on the right).

NB1: Note that Barrier Layer as defined above only occurs when the previous difference is positive (BLT≥0). Negative values may also happen and they correspond to vertically density Compensated Layer, whose thickness is then the absolute value of (TTD_DTm02 - MLD_DReqDTm02). See [1] for further discussions about compensated layers.

NB2: The climatology of the two fields associated to BLT estimation (TTD_DTm02, and MLD_DReqDTm02) are provided as they can also be usefull for BLT estimation and model validations. However, remember that BLT is estimated from individual profiles and mapped to construct a climatology. Due to non linearities of the process (e.g. reduction by the median), it is therefore not equal to the difference between the two climatologies based on TTD_DTm02 and MLD_DReqDTm02 criteria.

 1. BLT 

We estimate the subsurface Barrier Layer Thickness (BLT) from both temperature and density profiles. The BLT is the difference between the Warm Layer Depth and the Mixed Layer Depth with the corresponding variable density criterion. The BLT criterion for each couple of temperature/density profiles is the following :

BLT_DT02 = TTD_DTm02 - MLD_DReqDTm02

NB: due to the non linearities of the gridding process (median, smooth, optimal interpolation), be carefull that the final BLT gridded fields given below are not equal to the difference between TTD final gridded fields and MLD_DReqDTm02 final gridded fields.