Mixed layer

The atmospheric mixed layer is a zone having nearly constant potential temperature and specific humidity with height.

The depth of the mixed layer is thus very important for determining the temperature range in oceanic and coastal regions.

The mixed layer is also important as its depth determines the average level of light seen by marine organisms.

In very deep mixed layers, the tiny marine organisms known as phytoplankton are unable to get enough light to maintain their metabolism.

The deepening of the mixed layer in the wintertime in the North Atlantic is therefore associated with a strong decrease in surface chlorophyll a.

[1] Although this process injects a great deal of energy into the upper few meters, most of it dissipates relatively rapidly.

In coastal zones, large velocities due to tides may also play an important role in establishing the mixed layer.

Oceanographers use various definitions of the number to use as the mixed layer depth at any given time, based on making measurements of physical properties of the water.

Additionally, sharp gradients in nutrients (nutricline) and oxygen (oxycline) and a maximum in chlorophyll concentration are often co-located with the base of the seasonal mixed layer.

Two criteria often used to determine the mixed layer depth are temperature and sigma-t (density) change from a reference value (usually the surface measurement).

Large values of the BLT are typically found in the equatorial regions and can be as high as 50 m. Above the barrier layer, the well mixed layer may be due to local precipitation exceeding evaporation (e.g. in the western Pacific), monsoon related river runoff (e.g. in the northern Indian Ocean), or advection of salty water subducted in the subtropics (found in all subtropical ocean gyres).

[9] In particular, the barrier layer is formed in winter season in the equatorward flank of subtropical salinity maxima.

During early winter, the atmosphere cools the surface and strong wind and negative buoyancy forcing mixes temperature to a deep layer.

Using data from the ship of opportunity and Tropical Atmosphere – Ocean (TAO) moorings in the western Pacific, the east and west migration of the warm pool was tracked over 1992-2000 using sea surface salinity (SSS), sea surface temperature (SST), currents, and subsurface data from Conductivity, temperature, depth taken on various research cruises.

[12] This work showed that during westward flow, the BLT in the western Pacific along the equator (138oE-145oE, 2oN-2oS) was between 18 m – 35 m corresponding with warm SST and serving as an efficient storage mechanism for heat.

Barrier layer formation is driven by westward (i.e. converging and subducting) currents along the equator near the eastern edge of the salinity front that defines the warm pool.

[14] Later work, including Argo drifters, confirm the relationship between eastward migration of the warm pool during El Nino and barrier layer heat storage in the western Pacific.