Distribution Of Mixed Layer Depth Research Articles

Seasonal variability of Red Sea mixed layer depth: the influence of atmospheric buoyancy and momentum forcing

The seasonal and spatial evolution of the mixed layer (ML) in the Red Sea (RS) and the influence of atmospheric buoyancy and momentum forcing are analyzed for the 2001–2015 period using a high-resolution (1/100°, 50 vertical layers) ocean circulation model. The simulation reveals a strong spatiotemporal variability reflecting the complex patterns associated with the air–sea buoyancy flux and wind forcing, as well as the significant impact of the basin’s general and mesoscale circulation. During the spring and summer months, buoyancy forcing intensifies stratification, resulting in a generally shallow ML throughout the basin. Nevertheless, the results reveal local maxima associated with the influence of mesoscale circulation and regular wind induced mixing. Under the influence of surface buoyancy loss, the process of deepening of the ML commences in early September, reaching its maximum depth in January and February. The northern Gulf of Aqaba and the western parts of the northern RS, exhibit the deepest ML, with a gradual shoaling toward the south, primarily due to the surface advection of relatively fresh water that enters the basin from the Gulf of Aden. The mixed layer depth (MLD) variability is primarily driven by atmospheric buoyancy forcing, especially its heat flux component. Although evaporative fluxes dominate the annually averaged surface buoyancy forcing, they exhibit weak seasonal and spatial variability. Wind induced mixing exerts a significant impact on the MLD only locally, especially during summer. Of particular importance are strong winds channeled by topography, such as those in the vicinity of the Strait of Bab-Al-Mandeb and the straits connecting the two gulfs in the north, as well as lateral jets venting through mountain gaps, such as the Tokar Jet in the central RS. The analysis highlights the complex patterns of air-sea interactions, thermohaline circulation, and mesoscale activity, all of them strongly imprinted on the MLD distribution.

A metric for surface heat flux effect on horizontal sea surface temperature gradients

Understanding what controls horizontal variations in sea surface temperatures (SSTs) is one of the key science questions in climate research. Although various oceanic effects contribute to reinforcement/relaxation of horizontal variations in SSTs, the role of surface heat fluxes is surprisingly complex and can lead to significant biases in coupled models if improperly represented. In particular, the contribution of surface heat fluxes to surface frontogenesis/frontolysis depends not just on their gradients, but also on the distribution of mixed layer depth, which controls the effective heat capacity of the upper ocean. In this study, a new metric, referred to as the surface flux frontogenesis metric, is proposed that quantifies the relative importance of horizontal variations in surface heat fluxes and mixed layer depth. Global maps of this metric reveal that the role of surface heat fluxes in determining the horizontal SST gradient is highly variable geographically and by season. Furthermore, the metric can help explain characteristics of SST fronts in the northwestern Pacific, the Southern Ocean, the eastern equatorial Pacific, and the west coast of North America. Implications of this metric in coupled models will also be discussed.

Dynamical roles of mixed layer in regulating the meridional mass/heat fluxes

The mixed layer is an important component of the oceanic circulation system. Recent progress in energetics of the oceanic circulation suggests that the amount of external mechanical energy available for mixing is directly linked to the strength of the meridional overturning circulation. Using an analytical two‐dimensional model and a three‐dimensional numerical model, it is shown that the meridional distribution of mixed layer depth plays an important role in regulating the meridional overturning circulation and poleward heat flux. In fact, if the mixed layer at low and middle latitudes is deeper because of increase in mechanical energy input to the turbulence in the upper ocean, the meridional overturning circulation and poleward heat flux are enhanced in a steady circulation system, and at the same time, it may take less mechanical energy to support the subsurface diapycnal mixing.