Subtropical Mode Waters

A mode water is defined as an anomalous volume of water with homogeneous characteristics, usually temperature and salinity. The global ocean is full of these entities, but subtropical mode waters, because they are located in the thermocline, are tightly linked to surface atmospheric processes and play a key role in biology. The mode water formation process usually occurs at the mid-end of the winter through buoyancy loss at the sea surface (temperature loss and/or salinity gain) which triggers mixing and homogenize the water deep in the water column (then a mode water).This figure shows the thickness of the most voluminous subtropical mode waters:

Among them is the North Atlantic Subtropical Mode Water, characterized by a temperature of 17.8oC, a salinity of 36.5PSU and a potential density of 26.45kg/m3.

Application of the formalism due to Walin (1982): lateral diapycnal volume flux, A, whose divergence drives subduction, is related to ‘diffusive’ fluxes, D, acting across the boundary of the shaded control volume (which includes small-scale and diapycnal eddy fluxes) and air-sea buoyancy fluxes acting across the upper surface, F = ∂B/∂σ.

I used this framework to study the seasonal cycle of the North Atlantic and North Pacific STMW, extending the integral point of view of the initial theory to a complete analysis toolbox with a mapping technic for each terms. 

The most problematic drawback of the Walin's formalism is that it focuses only on buoyancy and doesn't distinguish highly and poorly stratified fluid. However, as the video below highlights, the mode water formation is fundamentally a process of ventilation where the vertical stratification is reset to 0 every winter, and hence potential vorticity reduced.

In order to understand the fundamental processes behind the life cycle of mode water, I use the formalism of PV fluxes. This framework is very powerful because it puts on the same ground advective, buoyancy and frictional processes. I used analyzed fields of ocean circulation and the flux form of the potential vorticity equation to map the creation and subsequent circulation of low potential vorticity EDW in the North Atlantic. I applied novel mapping techniques (i) to render the seasonal cycle and annual-mean mixed layer vertical flux of potential vorticity (PV) through outcrops and (ii) to visualize the extraction of PV from the mode water layer in winter, over and to the south of the Gulf Stream. Both buoyancy loss and wind forcing contribute to the extraction of PV, but I find that the former greatly exceeds the latter. The subsequent path of STMW cam also be mapped using Bernoulli contours on isopycnal surfaces.

References:

Diagnosing the observed seasonal cycle of Atlantic subtropical mode water using potential vorticity and its attendant theorems,  

G. Maze and J. Marshall: JOURNAL OF PHYSICAL OCEANOGRAPHY, doi: 10.1175/2011JPO4576.1, 2011.

On the seasonal cycle of Eighteen Degree Water Volume 

G. Forget, G. Maze, J. Marshal and M. Buckley. JOURNAL OF PHYSICAL OCEANOGRAPHY, doi: 10.1175/2010JPO4257.1, 2011

Observing the cycle of convection and restratification over the Gulf Stream system and the subtropical gyre of the North Atlantic Ocean: preliminary results from the CLIMODE field campaign

J. Marshall, A. Anderson, W. Dewar, S. Doney, J. Edson, R. Ferrari, G. Forget, D. Fratantoni, M. Gregg, T. Joyce, K. Kelly, S. Lozier, R. Lumpkin, G. Maze, J. Paster, R. Samelson, K. Silverthorne, E. Skyllingstad, F. Straneo, L. Talley, L. Thomas, J. Toole and R. Weller. BULLETIN OF THE AMERICAN METEOROLOGICAL SOCIETY, doi: 10.1175/2009BAMS2706.1, 2009.

Using transformation and formation maps to study the role of air-sea heat fluxes in North Atlantic Eighteen Degree Water formation 

G. Maze, G. Forget, M. Buckley, J. Marshall  and I. Cerovecki. JOURNAL OF PHYSICAL OCEANOGRAPHY, doi:10.1175/2009JPO3985.1, 2009.