Mean Mixed Layer Depth Research Articles

Upper ocean modeling in a coastal bay

A numerical model of the upper ocean is developed to study the dynamics and thermodynamics of the Baie des Chaleurs (Gulf of St. Lawrence, Canada). The model has primitive equation dynamics with two active layers embedded with a Niiler‐Kraus (Niller and Kraus, 1977) type mixed layer model at the top. Proper open boundary conditions and forcing functions are constructed. The model is eddy‐permitting, with a horizontal grid spacing of 2×4 km. An Arakawa C grid scheme is used. Forced by observed wind, atmospheric heat fluxes, river runoff, and appropriate remote forcing (in particular, the Gaspé Current, (GC)), the model demonstrates that the mean cyclonic general circulation pattern in the bay is a consequence of the intrusion of the GC. The strength of the circulation depends on the resultant stress of prevailing westerly winds and the opposing GC intrusion. In the mixed layer, atmospheric heat fluxes and horizontal thermal advection play a key role in the thermal balance at the eastern part of the bay. The local mixed layer fluctuations are controlled by wind and GC induced divergence. The entrainment (and its corresponding heat flux) is important at the western part of the bay and changes the mean mixed layer depth on a timescale of more than a week. Varying GC intensifies the flow variations induced by the wind in the bay and improves simulation results as compared with observations.

Variations in Mixed-Layer Depths Arising from Inhomogeneous Surface Conditions

Abstract Current approaches to parameterizations of sub-grid-scale variability in surface sensible heat fluxes in general circulation models normally neglect the associated variability in mixed-layer depths. Observations and a numerical mesoscale model are used to show that the magnitude of such variability can be significant. Over a domain of (41 km)2, the standard deviation of simulated mixed-layer depths was found to be 21%–24% of the mean noontime values on three days, and the mean depths were not simply related to the mean sensible heat fluxes. Results obtained with two-dimensional simulations over idealized distributions of warm, dry and cool, or moist surfaces show that as the characteristic sizes of individual patches increase, the distributions of mixed-layer depths tend to assume a bimodal nature. under these conditions, the mean mixed-layer depth may have little physical relevance. Finally, the use of domain-averaged values of wind and temperature to compute surface fluxes is shown to be anothe...

Solar radiation, phytoplankton pigments and the radiant heating of the equatorial Pacific warm pool

Recent optical, physical, and biological oceanographic observations are used to assess the magnitude and variability of the penetrating flux of solar radiation through the mixed layer of the warm water pool (WWP) of the western equatorial Pacific Ocean. Typical values for the penetrative solar flux at the climatological mean mixed layer depth for the WWP (30 m) are ∼23 W m−2 and are a large fraction of the climatological mean net air‐sea heat flux (∼40 W m−2). The penetrating solar flux can vary significantly on synoptic timescales. Following a sustained westerly wind burst, in situ solar fluxes were reduced in response to a near tripling of mixed layer phytoplankton pigment concentrations. This results in a reduction in the penetrative flux at depth (5.6 W m−2 at 30 m) and corresponds to a biogeochemically mediated increase in the mixed layer radiant heating rate of 0.13°C per month. These observations demonstrate a significant role of biogeochemical processes on WWP thermal climate. We speculate that this biogeochemically mediated feedback process may play an important role in enhancing the rate at which the WWP climate system returns to normal conditions following a westerly wind burst event.

On the variations in the mean convective mixed-layer depth over West Africa

Using the well-known method of Holzworth (1964) which is based on finding the intersections of upper-air soundings with the dry-adiabat of maximum surface temperatures to estimate the convective mixed-layer depth (MLD), we have obtained in this study representative values of this parameter from an 11-year monthly mean tropospheric data compiled for the West African sub-region. The mean MLD shows recognizable seasonal and spatial variations which is in conformity with the synoptic-scale circulation conditions over the study area. The annual variation indicates a bimodal distribution with the maximum depth of about 1500 m in February (or April for the areas north of 12°N) and in November. The lowest depth of about 650 m is found during the month of August which coincides with the peak of the monsoon season in West Africa. A quantitative relationship for the annual variation of the MLD is also obtained. The MLD values realised are then discussed in relation to the air pollution potential of the West African area

Construction and accuracy analysis of images of the daily-mean mixed-layer depth

Abstract Using satellite data only, a unique model of the upper-ocean mixed-layer has been developed recently by Yan for calculating the thermal inertia (or the heat flux change per unit temperature change per day) of the mixed-layer from which daily mean mixed-layer depth is estimated. Thus digital maps of the oceanic daily mixed-layer depth in the vicinity of the Sargasso Sea for 14 May-30 August, 1982 have been generated from visible, near-infrared, thermal-infrared and microwave observations made by radiometers on board the NOAA-7, -8, GOES and Nimbus-7 satellites. The errors incurred in producing such maps are of two general type: measurement from satellites and model simplification. The sensitivity of the modelled mixed-layer depth to simplifications in the model (modelling errors) and to errors in data used for forcing the model (data errors) is considered here. To emph isize the oceanographic relevance of these errors, we express the errors in mixed-layer depth as a function of model errors and da...

On water mass exchange between the Southern Ocean and the World Ocean; emphasis on the Atlantic sector

We characterise the representation of the Southern Ocean water mass structure and sea ice within a suite of 15 global ocean-ice models run with the Coordinated Ocean-ice Reference Experiment Phase II (CORE-II) protocol. The main focus is the representation of the present (1988–2007) mode and intermediate waters, thus framing an analysis of winter and summer mixed layer depths; temperature, salinity, and potential vorticity structure; and temporal variability of sea ice distributions. We also consider the interannual variability over the same 20 year period. Comparisons are made between models as well as to observation-based analyses where available.The CORE-II models exhibit several biases relative to Southern Ocean observations, including an underestimation of the model mean mixed layer depths of mode and intermediate water masses in March (associated with greater ocean surface heat gain), and an overestimation in September (associated with greater high latitude ocean heat loss and a more northward winter sea-ice extent). In addition, the models have cold and fresh/warm and salty water column biases centred near 50°S. Over the 1988–2007 period, the CORE-II models consistently simulate spatially variable trends in sea-ice concentration, surface freshwater fluxes, mixed layer depths, and 200–700 m ocean heat content. In particular, sea-ice coverage around most of the Antarctic continental shelf is reduced, leading to a cooling and freshening of the near surface waters. The shoaling of the mixed layer is associated with increased surface buoyancy gain, except in the Pacific where sea ice is also influential. The models are in disagreement, despite the common CORE-II atmospheric state, in their spatial pattern of the 20-year trends in the mixed layer depth and sea-ice.

The Meridional and Seasonal Structures of the Mixed-Layer Depth and its Diurnal Amplitude Observed during the Hawaii-to-Tahiti Shuttle Experiment

We describe the meridional and seasonal structures of daily mean mixed-layer depth and its diurnal amplitude and their relation to atmospheric fluxes by compositing mixed-layer depth estimates derived from density observations. The diurnal mean mixed-layer depth shows a ridge at the equator, troughs, which vary seasonally in intensity, at 10° to 15°N and 5° to 10°S, and a trough appearing just north of the equator in the second half of the year. This is in contrast to the ridge-trough structure of the top of the main thermocline, which reflects the dynamic topography associated with the equatorial current system. The diurnal amplitude is significantly different from zero for most latitudes year-round, indicating that the diurnal cycle of mixed-layer depth is a widespread phenomenon. For sufficiently strong heating, both the mixed-layer depth and its diurnal amplitude are significantly correlated with Monin-Obukhov length scales based on the mean net heat flux, mean wind stress, and mean shortwave radiation. This suggests a possible parameterization of the mixed-layer depth and diurnal amplitude in terms of the mean atmospheric fluxes for meridional scales of a few degrees and seasonal time scales.

Oceanic upper mixed layer depth determination by the use of satellite data

We have developed a method to determine the oceanic daily mean mixed layer depth from satellite observations and a mixed layer thermal inertia (MLTI) model. Temporal variations in the spatial distribution of the thermal inertia of the mixed layer provide information applicable to studies of variations in the upper ocean daily mean mixed layer depth in regions of the ocean where diurnal variations in the thermal structure of the upper layer due to horizontal advection and horizontal diffusion are relatively small. The algorithms were developed to use remotely-sensed values of sea surface temperature, albedo, and surface wind speeds to calculate the thermal inertia and to predict changes in subsurface diurnal mixed layer depth. The MLTI model, based on a mixed layer model of the upper ocean, has been used to simulate the diurnal mixing process and thermal inertia distribution in the Sargasso Sea around 34°N, 70°W. Sea surface temperature and albedo have been obtained from the NOAA7-AVHRR images. Surface wind speeds have been derived from the Scanning Multichannel Microwave Radiometer (SMMR) aboard Nimbus 7. Image processing was performed for images gathered between June and July 1982. The daily mean mixed layer depths predicted by the MLTI model agree well with data gathered at the LOTUS mooring located in the Sargasso Sea. This suggests that vertical mixing is the dominant physical process that controls the thermal inertia distribution in the midocean, far from major current systems, and that remote sensing is a promising tool to study such upper ocean processes. The MLTI model, which also provides information on the total heat flux into the upper ocean within the diurnal cycle, could be improved by using other data such as Altimeter data from improved satellite sensors, thus opening up the possibility of applications to operational upper ocean forecasting.

The structure of radiatively driven convection in stratocumulus

AbstractData from a series of five research flights in marine stratocumulus are used to investigate the structure of radiatively driven, free convective layers. Conditional sampling methods are used to determine the properties of the primary convective elements: negatively buoyant downdraughts. The turbulent velocity data are also subjected to spectral analysis and the turbulent kinetic energy (TKE) balance is evaluated for each case. Results from each case are found to be very similar if scaled using appropriate mixed layer quantities.Downdraughts are selected using a criterion based on vertical velocity. The distributions of their intersected widths at cloud top, and the velocity spectra, are consistent with downdraughts occupying narrow regions (∼0.1‐0.15 h wide, h being the mean mixed layer depth) around the periphery of larger, regular (∼0.5 h‐0.75 h diameter) updraughts as suggested by cellular patterns observed in the cloud tops. Downdraughts are found to carry over half of the total turbulent fluxes of heat and water substance within cloud, and also transport variance down the mean gradients. Their thermodynamic properties are consistent with their containing a small fraction of air from above cloud top although their negative buoyancy is almost entirely due to the incorporation of radiatively cooled air. The properties of the downdraughts near cloud top suggest they are formed primarily as a result of the local horizontal convergence of upwelling motions constrained by the inversion. While being cooled radiatively, these quasi‐horizontal motions scour the base of the inversion, incorporating some drier air before being forced downwards in relatively narrow zones. Comparisons with other convective layers over land or sea suggest that convection in stratocumulus proceeds with a greater mass flux and correspondingly reduced differences between convective elements and the environment, possibly reflecting the improved ventilation possible at an inversion interface compared with much rougher surfaces.The TKE balance divides into three main regions and is consistent with the interpretations given above. The pressure transport is implied to be the largest source term close to cloud top. Comparisons with model results reveal some important differences.

A Calculation of Ocean Heat Storage and Effective Ocean Surface Layer Depths for the Northern Hemisphere

Abstract In the hierarchy of simple ocean formulations available for coupling to atmospheric GCMs, a scheme whereby ocean surface-layer depths vary geographically and seasonally is deemed better than a fixed depth layer at all locations and seasons, but still is less sophisticated than dynamic ocean models. Yet such simple ocean formulations are useful for basic sensitivity studies. Here, a calculation of varying surface layer depths is done by first performing an ocean heat storage calculation using gridded, long-term mean mixed-layer depths and sea surface temperatures with a parameterized temperature structure beneath the mixed layer derived from weather ship data. Heat storage values in the midlatitudes are larger in the Atlantic than in the Pacific, which is in qualitative agreement with the weather ship data. Variants of the basic calculation show that neither mixed layer data nor SST data alone are sufficient to compute heat storage adequately. Using the results from the parameterized heat storage ...