Mixed Layer Density Research Articles

Verification of the Ekman Upwelling Criterion with In Situ Temperature Measurements in the Southeastern Baltic Sea

Upwelling leads to a sharp and strong decrease in water temperature in the coastal zone of the southeastern Baltic Sea. The quality of existing hydrodynamic models cannot fully meet the requirements of accurate upwelling forecasts. This study provides insight into the applicability of the simplified Ekman upwelling criterion method for the southeastern Baltic Sea. The upwelling criterion is the ratio of the vertical velocity and the duration of the upwelling wind to the mixed layer density. The vertical velocity was determined by the divergence of the integral Ekman transport in the transverse direction. Calculation of the criterion was based on wind data from NCEP/CFSR reanalysis. The upwelling criterion was compared with in situ temperatures from direct measurements near the D-6 oil platform taken in 2015–2017. Only 46% of calculated upwelling cases were confirmed by temperature decreases in the sub-surface. It was found that more than half of the cases of strong temperature decreases were caused by a northern wind (Ekman upwelling), when the criterion exceeded the threshold value. Comparison of the hydrodynamic model results and direct measurements shows that the model’s quality is far from perfect, and the simplified methods can be used as alternatives to models. Some recommendations were made for future upwelling research.

The impact of a parameterisation of submesoscale mixed layer eddies on mixed layer depths in the NEMO ocean model

A parameterisation scheme for restratification of the mixed layer by submesoscale mixed layer eddies is implemented in the NEMO ocean model. Its impact on the mixed layer depth (MLD) is examined in 30-year integrations of “uncoupled” ocean–ice (GO5) and “coupled” atmosphere–ocean–ice–land (GC2) 1/4° global climate configurations used by the Met Office Hadley Centre. The impact of the scheme on the MLD in GO5 is up to twice as large in subtropical and mid-latitudes when the mixed layer Rossby radius is not limited to guard against CFL-type instabilities and excessively strong volume overturning. Such a limit is not found to be necessary for stable integration of the scheme in NEMO. An alternative form of the scheme is described that approximates the mixed layer Rossby radius as a function only of latitude. This formulation is more generally robust to instability and has a comparatively larger impact on the MLD than the original formulation, but yields qualitatively similar results. The global mean impact of the scheme on the MLD is found to be almost twice as large in 1° and 2° configurations of GO5 as it is in the 1/4° configuration. This is shown to be the result of the scheme overcompensating for the decay in strength of resolved mixed layer density fronts in this model with decreasing horizontal grid resolution. The MLD criterion defining the depth scale of the scheme is shown to affect its global mean impact on the MLD by nearly a factor of 3 in GO5 and GC2, depending on whether the criterion is chosen to capture the actively mixing layer or the well-mixed layer. The parameterisation reduces the magnitude of deep MLD biases while increasing the magnitude of shallow biases. The globally averaged winter MLD bias is reduced from 17% to 9% of climatological values in GO5 but changes from +3% to −4% in GC2. Summer mixed layers are too shallow on average in both configurations and their average magnitude is increased by the parameterisation.

Higher Sea Levels at Hawaii Caused by Strong El Niño and Weak Trade Winds

AbstractHawaii experienced record-high sea levels during 2017, which followed the 2015 strong El Niño and coincided with weak trade winds in the tropical northeastern Pacific. The record sea levels were associated with a combination of processes, an important contributing factor of which was the persistent high sea level (~10 cm above normal) over a large region stretching between Hawaii and Mexico. High sea levels at Mexico are known to occur during strong El Niño as the coastal thermocline deepens. Planetary wave theory predicts that these coastal anomalies propagate westward into the basin interior; however, high sea levels at Hawaii do not occur consistently following strong El Niño events. In particular, Hawaii sea levels remained near normal following the previous strong El Niño of 1997. The processes controlling whether Hawaii sea levels rise after El Niño have so far remained unknown. Atmosphere-forced ocean model experiments show that anomalous surface cooling, controlled by variable trade winds, impacts sea level via mixed layer density, explaining much of the difference in Hawaiian sea level response after the two recent strong El Niño events. In climate model projections with greenhouse warming, more frequent weak trade winds following El Niño events are expected, suggesting that the occurrence of high sea levels at Hawaii will increase as oceanic anomalies more often traverse the basin.

Automatic Detection of Internal Waves on Satellite Images and Estimates of the Mixed Layer Density

This work presents an automatic algorithm for detecting internal waves (IWs) using the visible channel images of the MODIS radiometer. The algorithm is based on the local Hough transform. The sources of detection errors and the possibility of excluding false detections are considered. If a train of IWs is detected, it is possible to estimate the wavenumber. The wavelength in the images used in our experiments is within the range 1300–2200 m. The speed of IWs is estimated using two adjacent passes of the Aqua and Terra satellites. The speed and wavenumber are used for estimating the density characteristics of the mixed layer in a two-layer model with constant density. The estimated density difference between the layers based on satellite data is in a good agreement with the in situ data.

Izmenchivost' tolshchiny peremeshannogo sloya v Chernom more i ee svyaz' s dinamikoy vod i atmosfernym vozdeystviem

Purpose . Spatio-temporal variability of the mixed layer depth (MLD) in different regions of the Black Sea in 1985–2017, its relationship with large- and meso-scale dynamics and wind velocity are investigated. Metods and Results . The study is based on the hydrological data archive for 1985–2017 including the measurements of the ship expeditions, the Argo buoys and the moored buoy “Aqalog”. Seasonal and interannual variability of the mixed layer depth was determined using the density criterion ( dr = = 0,07kg/m 3 ) between the surface layers and the base of the upper mixed layer. Conclusions . In January – March, the large-scale and mesoscale dynamics significantly affects the mixed layer depth variability. Minimum monthly average values of the mixed layer depth in winter are observed in the mesoscale cyclonic eddies and in the center of the sea (20–30 m), the moderate values – on the periphery of the basin (40–45 m) and the maximum ones – in the mesoscale anticyclones (60–70 m). Several times the mixed layer depth values exceeding 150 m were detected in the downwelling areas of the basin. Analysis of the whole period (1985–2017) shows that the mixed layer density was never more than 1015 kg/m 3 . This isopycnal limits the maximum possible depth of the upper mixed layer. The impact of wind velocity on the spatial and temporal variability of the mixed layer thickness is the largest in spring and autumn when the seasonal thermocline is weak. It is less important in summer when solar heating stabilizes the upper layer, and in winter when the mixed layer depth is large. Rise of the mixed layer depth in summer is observed in recent years that is associated with rise of the wind speed in a warm period of a year.

Observations and numerical simulations of large‐eddy circulation in the ocean surface mixed layer

AbstractTwo near‐surface dye releases were mapped on scales of minutes to hours temporally, meters to order 1 km horizontally, and 1–20 m vertically using a scanning, depth‐resolving airborne lidar. In both cases, dye evolved into a series of rolls with their major axes approximately aligned with the wind and/or near‐surface current. In both cases, roll spacing was also of order 5–10 times the mixed layer depth, considerably larger than the 1–2 aspect ratio expected for Langmuir cells. Numerical large‐eddy simulations under similar forcing showed similar features, even without Stokes drift forcing. In one case, inertial shear driven by light winds induced large aspect ratio large‐eddy circulation. In the second, a preexisting lateral mixed layer density gradient provided the dominant forcing. In both cases, the growth of the large‐eddy structures and the strength of the resulting dispersion were highly dependent on the type of forcing.

Relative contributions of temperature and salinity to seasonal mixed layer density changes and horizontal density gradients

Temperature and salinity both contribute to ocean density, including its seasonal cycle and spatial patterns in the mixed layer. Temperature and salinity profiles from the Argo Program allow construction and analysis of a global, monthly, mixed layer climatology. Temperature changes dominate the seasonal cycle of mixed layer density in most regions, but salinity changes are dominant in the tropical warm pools, Arctic, and Antarctic. Under the Intertropical Convergence Zone, temperature and salinity work in concert to increase seasonal stratification, but the seasonal density changes there are weak because the temperature and salinity changes are small. In the eastern subtropics, seasonal salinity changes partly compensate those in temperature and reduce seasonal mixed layer density changes. Besides a hemispheric seasonal reversal, the times of maximum and minimum mixed layer density exhibit regional variations. For instance, the equatorial region is more closely aligned with Southern Hemisphere timing, and much of the North Indian Ocean has a minimum density in May and June. Outside of the tropics, the maximum mixed layer density occurs later in the winter toward the poles, and the minimum earlier in the summer. Finally, at the times of maximum mixed layer density, some of the ocean has horizontal temperature and salinity gradients that work against each other to reduce the horizontal density gradient. However, on the equatorial sides of the subtropical salinity maxima, temperature and salinity gradients reinforce each other, increasing the density gradients there. Density gradients are generally stronger where either salinity or temperature gradients are dominant influences.

Formation and Subduction of Central Mode Water Based on Profiling Float Data, 2003–08

Abstract Temperature and salinity data from Argo profiling floats in the North Pacific during 2003–08 have been analyzed to study the structure of winter mixed layer north of the Kuroshio Extension and the subsurface potential vorticity distribution in the subtropical gyre in relation to the formation and subduction of the central mode water (CMW). In late winter, two zonally elongated bands of deep mixed layer extend at 33°–39° and 39°–43°N, from the east coast of Japan to 160°W. These correspond to the formation region of the lighter variety of CMW (L-CMW) and that of the denser variety of CMW (D-CMW) and the recently identified transition region mode water (TRMW), respectively. In the western part of the L-CMW and D-CMW–TRMW formation regions west of 170°E, the winter mixed layer becomes deeper and lighter to the east (i.e., to the downstream). As a result, the formed mode water is reentrained into the mixed layer in the farther east in the following winter and modified to the lighter water and is thus unable to be subducted to the permanent pycnocline. In the eastern part of the formation regions between 170°E and 160°W, on the other hand, the winter mixed layer becomes shallower and lighter to the east. From these areas, the L-CMW with potential density of 25.7–26.2 kg m−3 and the D-CMW–TRMW (mostly the former) of 26.1–26.4 kg m−3 are subducted to the permanent pycnocline, and they are then advected anticyclonically in the subtropical gyre. These results imply that during the analysis period large-scale subduction to the permanent pycnocline occurs in the density range up to 26.4 kg m−3 in the open North Pacific, whereas the winter mixed layer density reaches the maximum of 26.6 kg m−3. This is supported by the vertical distribution of apparent oxygen utilization in a hydrographic section in the subtropical gyre.

Buoyancy and Wind-Driven Convection at Mixed Layer Density Fronts

Abstract In this study, the influence of a geostrophically balanced horizontal density gradient on turbulent convection in the ocean is examined using numerical simulations and a theoretical scaling analysis. Starting with uniform horizontal and vertical buoyancy gradients, convection is driven by imposing a heat loss or a destabilizing wind stress at the upper boundary, and a turbulent layer soon develops. For weak lateral fronts, turbulent convection results in a nearly homogeneous mixed layer (ML) whose depth grows in time. For strong fronts, a turbulent layer develops, but this layer is not an ML in the traditional sense because it is characterized by persistent horizontal and vertical gradients in density. The turbulent layer is, however, nearly homogeneous in potential vorticity (PV), with a value near zero. Using the PV budget, a scaling for the depth of the turbulent low PV layer and its time dependence is derived that compares well with numerical simulations. Two dynamical regimes are identified. In a convective layer near the surface, turbulence is generated by the buoyancy loss at the surface; below this layer, turbulence is generated by a symmetric instability of the lateral density gradient. This work extends classical scalings for the depth of turbulent boundary layers to account for the ubiquitous presence of lateral density gradients in the ocean. The new results indicate that a lateral density gradient, in addition to the surface forcing, can affect the stratification and the rate of growth of the surface boundary layer.

Wintertime mixed layer measurements at Maud Rise, Weddell Sea

Sea ice plays a crucial role in the exchange of heat between the ocean and the atmosphere, and areas of intense air‐sea‐ice interaction are important sites for water mass modification. The Weddell Sea is one of these sites where a relatively thin first‐year ice cover is constantly being changed by mixing of heat from below and stress exerted from the rapidly changing and intense winds. This study presents mixed layer turbulence measurements obtained during two wintertime drift stations in August 2005 in the eastern Weddell Sea, close to the Maud Rise seamount. Turbulence in the boundary layer is found to be controlled by the drifting ice. Directly measured heat fluxes compare well with previous studies and are well estimated from the mixed layer temperatures and mixing. Heat fluxes are also found to roughly balance the conductive heat flux in the ice; hence, little freezing/melting was observed. The under‐ice topography is estimated to be hydraulically very smooth; comparison with a steady 1‐D model shows that these estimates are made too close to the ice‐ocean interface to be representative for the entire ice floe. The main source and sink of turbulent kinetic energy are shear production and dissipation. Observations indicate that the dynamics of the under‐ice boundary layer are influenced by a horizontal variability in mixed layer density and an increasing amount of open leads in the area.

Mixed layer depth climatology of the North Pacific based on Argo observations

A monthly mean climatology of the mixed layer depth (MLD) in the North Pacific has been produced by using Argo observations. The optimum method and parameter for evaluating the MLD from the Argo data are statistically determined. The MLD and its properties from each density profile were calculated with the method and parameter. The monthly mean climatology of the MLD is computed on a 2° × 2° grid with more than 30 profiles for each grid. Two bands of deep mixed layer with more than 200 m depth are found to the north and south of the Kuroshio Extension in the winter climatology, which cannot be reproduced in some previous climatologies. Early shoaling of the winter mixed layer between 20–30°N, which has been pointed out by previous studies, is also well recognized. A notable feature suggested by our climatology is that the deepest mixed layer tends to occur about one month before the mixed layer density peaks in the middle latitudes, especially in the western region, while they tend to coincide with each other in higher latitudes.

Buoyancy-Forced Downwelling in Boundary Currents

Abstract The issue of downwelling resulting from surface buoyancy loss in boundary currents is addressed using a high-resolution, nonhydrostatic numerical model. It is shown that the net downwelling is determined by the change in the mixed layer density along the boundary. For configurations in which the density on the boundary increases in the direction of Kelvin wave propagation, there is a net downwelling within the domain. For cases in which the density decreases in the direction of Kelvin wave propagation, cooling results in a net upwelling within the domain. Symmetric instability within the mixed layer drives an overturning cell in the interior, but it does not contribute to the net vertical motion. The net downwelling is determined by the geostrophic flow toward the boundary and is carried downward in a very narrow boundary layer of width E1/3, where E is the Ekman number. For the calculations here, this boundary layer is O(100 m) wide. A simple model of the mixed layer temperature that balances horizontal advection with surface cooling is used to predict the net downwelling and its dependence on external parameters. This model shows that the net sinking rate within the domain depends not only on the amount of heat loss at the surface but also on the Coriolis parameter, the mixed layer depth (or underlying stratification), and the horizontal velocity. These results indicate that if one is to correctly represent the buoyancy-forced downwelling in general circulation models, then it is crucial to accurately represent the velocity and mixed layer depth very close to the boundary. These results also imply that processes that lead to weak mixing within a few kilometers of the boundary, such as ice formation or freshwater runoff, can severely limit the downwelling forced by surface cooling, even if there is strong heat loss and convection farther offshore.

Subduction of South Pacific waters

Using existing high‐resolution CTD observations, complemented by a large amount of recently available Argo floating data, this study provides a detailed description of the subduction of South Pacific waters. With a significantly improved climatological dataset on the mixed layer properties, we obtain an annual subduction rate of 48.8 Sv (1 Sv = 106 m3 s−1) from 10°S to 60°S in the South Pacific. Two peaks stand out in this subduction rate sorted by winter mixed layer density: one corresponds to the formation of eastern Subtropical Mode Water (STMW) and part of the Subtropical Underwater (SUW) between 25.0 and 25.5 σθ, and the other has a density range between 26.6 and 27.1 σθ, representing the formation of Sub‐Antarctic Mode Water and Antarctic Intermediate Water (SAMW/AAIW). The subduction in eastern STMW/SUW range is 12.7 Sv, only slightly smaller than that (14.6 Sv) associated with the SAMW/AAIW. Sandwiched between these two peaks, roughly in the density range between 26.0 and 26.5 σθ, is the southwestern STMW, with a relatively small annual subduction of 5.6 Sv. Uncertainties of these estimates are discussed.

Modeling Decadal Changes on the Indian Ocean Section I5 at 32°S

Abstract A near-global ocean model with resolution enhanced in the southern Indian Ocean has been spun up to seasonal equilibrium and then driven by NCEP–NCAR reanalysis 1 monthly mean forcings and Hadley SSTs over the period 1948–2002. The aim was to simulate changes in the subsurface properties observed in hydrographic surveys at 32°S in the Indian Ocean in 1965, 1987, and 2002. These surveys showed a zonally averaged cooling on isopycnals of 0.5° and 0.3°C in mode and intermediate waters between 1965 and 1987 and a warming of the mode water coupled with a continued cooling of the intermediate water between 1987 and 2002. The major changes in isopycnal depth and temperature modeled in this study were confined to the mode water and were qualitatively similar to those observed but concentrated in a lower density class and in the eastern half of the section. The dominant changes here were multidecadal, with maximum temperatures on the σθ = 26.7 kg m−3 isopycnal being reached in 1968 and minimum temperatures in 1990. The simulations showed a propagation of interannual anomalies toward the section from a region of deep late winter mixed layers in the southeast Indian Ocean within a period of several years. Surface temperatures in this region were lowest in the 1960s and highest in the late 1980s. Temperatures on isopycnals showed the opposite variation, consistent with SST having the controlling effect on mixed layer density and depth. Isopycnal depths within the mode water were strongly correlated with temperature, implying a redistribution of mode water density classes, the greatest volume of mode water being produced in a higher density class (σθ = 26.8–27.0 kg m–3) during the period of cooler surface forcing in the 1960s and 1970s than during the warmer period following (σθ = 26.6–26.8 kg m–3).

Temporal variability of winter mixed layer in the mid-to high-latitude North Pacific

Temperature and salinity data from 2001 through 2005 from Argo profiling floats have been analyzed to examine the time evolution of the mixed layer depth (MLD) and density in the late fall to early spring in mid to high latitudes of the North Pacific. To examine MLD variations on various time scales from several days to seasonal, relatively small criteria (0.03 kg m−3 in density and 0.2°C in temperature) are used to determine MLD. Our analysis emphasizes that maximum MLD in some regions occurs much earlier than expected. We also observe systematic differences in timing between maximum mixed layer depth and density. Specifically, in the formation regions of the Subtropical and Central Mode Waters and in the Bering Sea, where the winter mixed layer is deep, MLD reaches its maximum in late winter (February and March), as expected. In the eastern subarctic North Pacific, however, the shallow, strong, permanent halocline prevents the mixed layer from deepening after early January, resulting in a range of timings of maximum MLD between January and April. In the southern subtropics from 20° to 30°N, where the winter mixed layer is relatively shallow, MLD reaches a maximum even earlier in December–January. In each region, MLD fluctuates on short time scales as it increases from late fall through early winter. Corresponding to this short-term variation, maximum MLD almost always occurs 0 to 100 days earlier than maximum mixed layer density in all regions.

Parameterization of Eddy Effects on Mixed Layers and Tracer Transport: A Linearized Eddy Perspective

Abstract Two aspects of the effects of eddies on ocean circulation have proven difficult to parameterize: eddy effects in regions of neutrally stable (or convecting) fluid and the mixing of passive tracers. The effects of linearized eddies, although a restrictive parameter regime, can be straightforwardly computed in these cases. The eddy effects in areas of neutral stability—for example, mixed layers—blend naturally into those in the stably stratified water below, although losing the concept of bolus velocity. Instead, the mixed layer density is advected by an extra overturning velocity and is diffused laterally by a diffusion that is the same as the eddy diffusion at the top of the stably stratified fluid. Passive tracers are advected by the bolus velocity and mixed by the same diffusivity as is used to compute the bolus velocity at that location, so that two different diffusivities are not needed.

The Structure of the Wind-Driven Circulation in the Subtropical South Pacific Ocean*

The structure of the wind-driven circulation in the subtropical South Pacific is studied using simple diagnostic and analytical models. The diagnostic calculation is based on the Levitus climatology. The analytical model is forced by observed winter mixed layer density and depth calculated from the Levitus climatology and by the surface wind stress data from the Hellerman and Rosenstein climatology. The wind-driven gyre in the South Pacific is relatively deep, reaching 2.4 km along the southern edge of the gyre. The gross feature of subduction obtained from both the data analysis and the analytical model is similar, with an annual ventilation rate of 21.6 Sv (Sv ≡ 106 m3 s−1), including 18.1 Sv from vertical pumping and 3.5 Sv from lateral induction. Although the annual subduction rate in the South Pacific is comparable to that in the North Atlantic, lack of localized subduction leads to relatively weak mode water formation in the region where the East Australian Current separates from its western boundary. In addition, results from the analytical model indicate the existence of an isopycnal slope reversal in the southeastern Pacific.

LEED crystallographic analysis for the Rh(111)-(√7 × √7)R19.1°P surface structure

A recent communication [11] reported a novel structure for the (√7 × √7)R19.1° reconstruction formed by P on the Rh(111) surface, and the present paper details the associated tensor LEED analysis. This reconstruction involves a packed arrangement of Rh pentagons and triangles for the topmost layer, with P atoms at 3 7 monolayer coverage occupying eight-coordinate sites created by the Rh pentagons. The nearly flat, densely packed RhP mixed layer (density of five Rh and three P atoms per seven Rh atoms in the substrate unit mesh) appears to provide a major stabilizing effect on this surface. The average PRh bond distance is indicated to be 2.52 Å, slightly longer than that (2.38 Å) in bulk Rh2P where the P atoms are also eight-coordinate. Some similarities are noted with the corresponding (rv 7 ̄ × √7) R19.1° structures formed by S on the Pd(111), Cu(111) and Ag(111) surfaces.

Pelagic primary production in the Coral and southern Solomon Seas

Phytoplankton primary production was measured around the periphery of the Coral Sea during October 1985 and in the boundary current systems bordering the northern Australian Great Barrier Reef (GBR) and Papuan Barrier Reef (PBR) during October 1985 and June-July 1988. Under strong wind conditions (mean winds 8-12 m s-1), the north-western Papuan Barrier Reef region was characterized by a shallow surface mixed layer, shallow nutriclines (25-75 m) and shallow subsurface chlorophyll maxima. Under low wind stress conditions (mean winds <5 m s-1), the southern and western Coral Sea were also characterized by a shallow surface mixed layer and stable underlying density profiles but deep (>I00 m) nutriclines and deep (60-125 m) subsurface chlorophyll and primary production maxima. Regardless of location, most primary production occurred above the 20% mid-day isolume surface. Phytoplankton standing crop and primary production in all regions were dominated by picoplankton (<2 μm size fraction). Very high primary production rates (1-3 g C m-2 day-1) were measured at a number of stations adjacent to the western margin of the PBR and within the central basin of the Louisiade Archipelago. Evidence for upwelling along the western margin of the PBR was observed under both north-easterly (normal to the reef axis) and south-easterly (parallel to the reef axis) wind regimes; however, surface outcropping of upwelled water did not occur. Oceanic primary production in the Coral Sea is estimated to be between 100 and 200 g C m-2 year-1. Primary production in and around the Louisiade Archipelago appears to be on the order of 200-300 g C m-2 year-1. Near-surface chlorophyll standing crop was generally better correlated with near-surface primary production than was total chlorophyll with total areal primary production.

North Pacific Intermediate Water in the Kuroshio/Oyashio Mixed Water Region

Abstract The North Pacific Intermediate Water (NPIW) originates as a vertical salinity minimum in the mixed water region (MWR) between the Kuroshio and Oyashio, just east of Japan. Salinity minima in this region are examined and related to the water man structures, dynamical features, and winter mixed layer density of waters of Oyashio origin. Stations in the MWR are divided into five regimes, of which three represent source waters (from the Kuroshio, Oyashio, and Tsugaru Current) and two are mixed waters formed from these three inputs. Examination of NPIW at stations just east of the MWR indicates that the mixed waters in the MWR are the origin of the newest NPIW. Multiple salinity minima with much finestructure are seen throughout the MWR in spring 1989, with the most fragmented occurring around the large warm core ring centered at 37°N, 144°E, suggesting that this is a dominant site for salinity minimum formation. The density of the NPIW in the MWR is slightly higher than the apparent late winter surfa...