Top of Thermocline Depth (TTD)

The Top of Thermocline Depth (TTD) is defined as the depth above which the temperature is superior than the surface temperature (at 10m depth) minus 0.2 °C (cf. [4], [5]). The surface layer of the ocean above the TTD is warm relative to the deeper ocean and the TTD may also be called the surface Warm Layer Depth (WLD). Due to potential salinity effects on density, this layer may contain T°C inversions which are still convectively stable (e.g. in Bay of Bengal in winter). This surface layer is therefore NOT necessarily an isothermal layer. With such a criterion (differing notably from an absolute difference threshold, see MLD_DT02), that layer cannot be called an isothermal nor a temperature-mixed layer.

The definition of the TTD is often linked to the computation of the subsurface Barrier Layer Thickness (BLT) presented here. This is the reason why we used stations where both T and S are available to construct this climatology (same stations as for MLD_DReqDTm02 dataset from the MLD page).

NB:The terms Top of Thermocline Depth (TTD), or surface Warm Layer Depth (WLD) apply generally well in the tropics and mid-latitudes where they make sense. However, in high latitudes, salinity may greatly control the stability of water column, and large temperature inversions may result in finding great TTD, not really representing what we would visually call the top of the thermocline or the surface "warm" layer (see paragraph [22] in [4] for further details about this).

 1. TTD 

As noted above, the TTD is a NOT an isothermal layer depth as T°C inversions may be contained in the surface layer above it! We estimate the Top of Thermocline Depth (TTD) from a fixed threshold on temperature profiles. The criterion is the following :

TTD_DTm02 = depth where (θ = θ_10m - 0.2 °C)