scholarly journals Double diffusion in meromictic lakes of the temperate climate zone

2009 ◽  
Vol 6 (6) ◽  
pp. 7483-7501 ◽  
Author(s):  
C. von Rohden ◽  
B. Boehrer ◽  
J. Ilmberger
Keyword(s):  
Double Diffusion ◽  
Heat Fluxes ◽  
Temperate Climate ◽  
Seasonal Temperature ◽  
Meromictic Lakes ◽  
Temperature Changes ◽  
Convective Mixing ◽  
Lake Bottom ◽  
Diffusive Processes ◽  
Double Diffusive

Abstract. Meromictic lakes are characterized by strong stable density stratification in and below the chemocline, which separates the oxic mixolimnion from the mostly anoxic monimolimnion. Stable density gradients involve slow vertical exchange, especially in the chemocline, where vertical transport can be as low as molecular. Typically, destabilizing temperature gradients establish in the monimolimnion as a consequence of seasonal changing heat fluxes. At the same time, gradients of solutes extending to the lake bottom stabilize the overall stratification. Double diffusive processes may create local instabilities and subsequently cause convective mixing when the destabilization due to heat gradients exceeds the stabilization by solutes (diffusive regime). This configuration can annually occur in the upper part of the monimolimnion, if seasonal temperature changes in the mixolimnion reach the top of the monimolimnion. We present CTD-measurements from two meromictic mining lakes in Germany, which document the seasonal occurrence of convective mixing in discrete horizontal homogeneous layers within the monimolimnion which can be identified by the characteristic step-like structure. In the deeper layers, the steps emerge with a time delay which is determined by the progression of the mixolimnetic temperature changes into the monimolimnion. Interestingly, the chemocline interface is not degraded by these processes. However, double diffusive convection is essential for the redistribution in the inner parts of the monimolimnion at longer time scales, which is crucial for the assessment of the ecologic development of such lakes.

scholarly journals Evidence for double diffusion in temperate meromictic lakes

2010 ◽  
Vol 14 (4) ◽  
pp. 667-674 ◽  
Author(s):  
C. von Rohden ◽  
B. Boehrer ◽  
J. Ilmberger
Keyword(s):  
Double Diffusion ◽  
Stability Ratio ◽  
Temperature Changes ◽  
Convective Mixing ◽  
Diffusive Convection ◽  
Seasonal Basis ◽  
Mixing Patterns ◽  
Diffusive Effects ◽  
The Stability ◽  
Double Diffusive

Abstract. We present CTD-measurements from two shallow meromictic mining lakes. The lakes, which differ in size and depth, show completely different seasonal mixing patterns in their mixolimnia. However, the measurements document the occurrence of similar seasonal convective mixing in discrete layers within their monimolimnia. This mixing is induced by double diffusion and can be identified by the characteristic step-like structure of the temperature and electrical conductivity profiles. The steps develop in the upper part of the monimolimnion, when in autumn cooling mixolimnion temperatures have dropped below temperatures of the underlying monimolimnion. The density gradient across the chemocline due to solutes overcompensates the destabilizing temperature gradient, and moreover, keeps the vertical transport close to molecular level. In conclusion, preconditions for double diffusive effects are given on a seasonal basis. At in general high local stabilities N2 in the monimolimnia of 10−4–10−2s−2, the stability ratio Rρ was in the range of 1–20. This quantitatively indicates that double diffusion can become visible. Between 1 and 6 sequent steps, with sizes between 1 dm and 1 m, were visually identified in the CTD-profiles. In the lower monimolimnion of the deeper lake, the steps systematically emerge at a time delay of more than half a year, which matches with the progression of the mixolimnetic temperature changes into the monimolimnion. In none of the lakes, the chemocline interface is degraded by these processes. However, double diffusive convection is essential for the redistribution of solutes in the inner parts of the monimolimnion at longer time scales, which is crucial for the assessment of the ecologic development of such lakes.

Finescale Instabilities of the Double-Diffusive Shear Flow*

2011 ◽  
Vol 41 (3) ◽  
pp. 571-585 ◽  
Author(s):  
Timour Radko ◽  
Melvin E. Stern
Keyword(s):  
Stability Analysis ◽  
Shear Flow ◽  
Velocity Profile ◽  
Turbulent Mixing ◽  
Richardson Number ◽  
Floquet Theory ◽  
Double Diffusion ◽  
Diffusive Processes ◽  
Main Thermocline ◽  
Double Diffusive

Abstract This study examines dynamics of finescale instabilities in thermohaline–shear flows. It is shown that the presence of the background diapycnal temperature and salinity fluxes due to double diffusion has a destabilizing effect on the basic current. Using linear stability analysis based on the Floquet theory for the sinusoidal basic velocity profile, the authors demonstrate that the well-known Richardson number criterion (Ri < ¼) cannot be directly applied to doubly diffusive fluids. Rigorous instabilities are predicted to occur for Richardson numbers as high as—or even exceeding—unity. The inferences from the linear theory are supported by the fully nonlinear numerical simulations. Since the Richardson number in the main thermocline rarely drops below ¼, whereas the observations of turbulent patches are common, the authors hypothesize that some turbulent mixing events can be attributed to the finescale instabilities associated with double-diffusive processes.

The impact of thermohaline staircases: estimates from a global analysis of Argo floats

2020 ◽  
Author(s):  
Carine van der Boog ◽  
Julie D Pietrzak ◽  
Henk A Dijkstra ◽  
Caroline A Katsman
Keyword(s):  
Heat Transfer ◽  
Indian Ocean ◽  
Atlantic Ocean ◽  
Water Mass ◽  
Double Diffusion ◽  
The Indian Ocean ◽  
Diffusive Processes ◽  
The Impact ◽  
Double Diffusive ◽  
Water Mass Transformation

<p>Thermohaline staircases are stepped structures in the temperature and salinity stratification that result from double diffusive processes. In the open ocean, double diffusive processes enhance the downgradient diapycnal heat transfer compared to turbulent mixing. However, in combination with salinity effects, the resulting buoyancy flux within the thermohaline staircases is counter gradient. This vertical density transport strengthens the stratification and, consequently, affects the density of the water masses above and below the staircase layer. Although 44 percent of the world’s oceans is susceptible to double diffusion and thermohaline staircases are ubiquitous in these regions, the impact of double diffusion on diapycnal heat transfer and on water mass transformation has not been quantified yet. Here, we analyse a dataset of Argo float profiles to obtain a global overview of the occurrence of thermohaline staircases and to estimate their impact on diapycnal heat transfer and water mass transformation. Several regions with a high staircase occurrence are identified. Besides the well-known regions in the Caribbean Sea, the Mediterranean Sea and the subtropical Atlantic Ocean, our analysis reveals a new staircase region in the Indian Ocean. Using this global overview, we estimate, for the first time, the contribution of downgradient diapycnal heat transfer by the staircases. It appears that this contribution is very low compared to the dissipation required to maintain the observed temperature stratification. However, each staircase region can potentially impact the global circulation by affecting the density of the water masses above and below. In particular, the staircase region in the Indian Ocean overlies the waters of the Tasman Leakage. These waters flow westward from Australia towards the Agulhas region and affect the properties of waters entering the Atlantic Ocean. This implies that the vertical flux of salt into the Tasman Leakage waters induced by the presence of thermohaline staircases above can impact the salt transport into the Atlantic Ocean, which in turn is expected to impact the Atlantic Meridional Overturning Circulation. </p>

scholarly journals Double-diffusive mixing makes a small contribution to the global ocean circulation

2021 ◽  
Vol 2 (1) ◽  
Author(s):  
Carine G. van der Boog ◽  
Henk A. Dijkstra ◽  
Julie D. Pietrzak ◽  
Caroline A. Katsman
Keyword(s):  
Ocean Circulation ◽  
Mechanical Energy ◽  
Global Ocean ◽  
Small Contribution ◽  
Global Ocean Circulation ◽  
Diffusive Fluxes ◽  
Diffusive Mixing ◽  
Diffusive Processes ◽  
Flowing Waters ◽  
Double Diffusive

AbstractDouble-diffusive processes enhance diapycnal mixing of heat and salt in the open ocean. However, observationally based evidence of the effects of double-diffusive mixing on the global ocean circulation is lacking. Here we analyze the occurrence of double-diffusive thermohaline staircases in a dataset containing over 480,000 temperature and salinity profiles from Argo floats and Ice-Tethered Profilers. We show that about 14% of all profiles contains thermohaline staircases that appear clustered in specific regions, with one hitherto unknown cluster overlying the westward flowing waters of the Tasman Leakage. We estimate the combined contribution of double-diffusive fluxes in all thermohaline staircases to the global ocean’s mechanical energy budget as 7.5 GW [0.1 GW; 32.8 GW]. This is small compared to the estimated energy required to maintain the observed ocean stratification of roughly 2 TW. Nevertheless, we suggest that the regional effects, for example near Australia, could be pronounced.

A parametrized model of seasonal temperature changes in lakes

1990 ◽  
Vol 5 (1) ◽  
pp. 12-25 ◽  
Author(s):  
S.S. Zilitinkevich ◽  
V.A. Rumyantzev
Keyword(s):  
Seasonal Temperature ◽  
Temperature Changes

scholarly journals The Seasonal Cycle of Blocking and Associated Physical Mechanisms in the Australian Region and Relationship with Rainfall

2013 ◽  
Vol 141 (12) ◽  
pp. 4534-4553 ◽  
Author(s):  
M. J. Pook ◽  
J. S. Risbey ◽  
P. C. McIntosh ◽  
C. C. Ummenhofer ◽  
A. G. Marshall ◽  
...  
Keyword(s):  
Southern Ocean ◽  
Seasonal Cycle ◽  
Seasonal Temperature ◽  
Ocean Fronts ◽  
Temperature Changes ◽  
Physical Mechanisms ◽  
Australian Region ◽  
Continental Antarctica ◽  
Positive Correlations ◽  
Blocking Index

Abstract The seasonal cycle of blocking in the Australian region is shown to be associated with major seasonal temperature changes over continental Antarctica (approximately 15°–35°C) and Australia (about 8°–17°C) and with minor changes over the surrounding oceans (below 5°C). These changes are superimposed on a favorable background state for blocking in the region resulting from a conjunction of physical influences. These include the geographical configuration and topography of the Australian and Antarctic continents and the positive west to east gradient of sea surface temperature in the Indo-Australian sector of the Southern Ocean. Blocking is represented by a blocking index (BI) developed by the Australian Bureau of Meteorology. The BI has a marked seasonal cycle that reflects seasonal changes in the strength of the westerly winds in the midtroposphere at selected latitudes. Significant correlations between the BI at Australian longitudes and rainfall have been demonstrated in southern and central Australia for the austral autumn, winter, and spring. Patchy positive correlations are evident in the south during summer but significant negative correlations are apparent in the central tropical north. By decomposing the rainfall into its contributions from identifiable synoptic types during the April–October growing season, it is shown that the high correlation between blocking and rainfall in southern Australia is explained by the component of rainfall associated with cutoff lows. These systems form the cyclonic components of blocking dipoles. In contrast, there is no significant correlation between the BI and rainfall from Southern Ocean fronts.

scholarly journals Eastern Arctic Ocean Diapycnal Heat Fluxes through Large Double-Diffusive Steps

2019 ◽  
Vol 49 (1) ◽  
pp. 227-246 ◽  
Author(s):  
Igor V. Polyakov ◽  
Laurie Padman ◽  
Y.-D. Lenn ◽  
Andrey Pnyushkov ◽  
Robert Rember ◽  
...  
Keyword(s):  
Arctic Ocean ◽  
Dissipation Rate ◽  
Critical Role ◽  
Density Ratio ◽  
Atlantic Water ◽  
Heat Fluxes ◽  
The Arctic ◽  
Eurasian Basin ◽  
Average Flux ◽  
Double Diffusive

AbstractThe diffusive layering (DL) form of double-diffusive convection cools the Atlantic Water (AW) as it circulates around the Arctic Ocean. Large DL steps, with heights of homogeneous layers often greater than 10 m, have been found above the AW core in the Eurasian Basin (EB) of the eastern Arctic. Within these DL staircases, heat and salt fluxes are determined by the mechanisms for vertical transport through the high-gradient regions (HGRs) between the homogeneous layers. These HGRs can be thick (up to 5 m and more) and are frequently complex, being composed of multiple small steps or continuous stratification. Microstructure data collected in the EB in 2007 and 2008 are used to estimate heat fluxes through large steps in three ways: using the measured dissipation rate in the large homogeneous layers; utilizing empirical flux laws based on the density ratio and temperature step across HGRs after scaling to account for the presence of multiple small DL interfaces within each HGR; and averaging estimates of heat fluxes computed separately for individual small interfaces (as laminar conductive fluxes), small convective layers (via dissipation rates within small DL layers), and turbulent patches (using dissipation rate and buoyancy) within each HGR. Diapycnal heat fluxes through HGRs evaluated by each method agree with each other and range from ~2 to ~8 W m−2, with an average flux of ~3–4 W m−2. These large fluxes confirm a critical role for the DL instability in cooling and thickening the AW layer as it circulates around the eastern Arctic Ocean.

Seasonal Changes in the Germination Responses of Buried Witchgrass (Panicum capillare) Seeds

Weed Science ◽  
1986 ◽  
Vol 34 (1) ◽  
pp. 22-24 ◽  
Author(s):  
Jerry M. Baskin ◽  
Carol C. Baskin
Keyword(s):  
Seasonal Changes ◽  
Late Summer ◽  
Late Spring ◽  
Seasonal Temperature ◽  
Temperature Changes ◽  
Late Autumn ◽  
Buried Seeds ◽  
Autumn And Winter

Buried seeds of witchgrass (Panicum capillare L., # PANCA) exposed to natural seasonal temperature changes in Lexington, KY, for 0 to 35 months exhibited annual dormancy/nondormancy cycles. Seeds were dormant at maturity in early October. During burial in late autumn and winter, fresh seeds and those that had been buried for 1 and 2 years became nondormant. Nondormant seeds germinated from 76 to 100% in light at daily thermoperiods of 15/6, 20/10, 25/15, 30/15, and 35/20 C, while in darkness they germinated from 1 to 24%. In late spring, seeds lost the ability to germinate in darkness, and by late summer 63 to 100% of them had lost the ability to germinate in light. As seeds became nondormant, they germinated (in light) at high (35/20, 30/15 C) and then at lower (25/15, 20/10, and 15/6 C) temperatures. As seeds reentered dormancy, they lost the ability to germinate (in light) at 15/6 C and at higher thermoperiods 2 to 3 months later.

scholarly journals Building and Urban Cooling Performance Indexes of Wetted and Green Roofs—A Case Study under Current and Future Climates

Energies ◽  
2020 ◽  
Vol 13 (23) ◽  
pp. 6192
Author(s):  
Madi Kaboré ◽  
Emmanuel Bozonnet ◽  
Patrick Salagnac
Keyword(s):  
Thermal Comfort ◽  
Emission Scenario ◽  
Green Roof ◽  
Heat Fluxes ◽  
Temperate Climate ◽  
Passive Cooling ◽  
Thermal Discomfort ◽  
Indoor Thermal Comfort ◽  
Performance Indexes ◽  
The Impact

We developed and studied key performance indexes and representations of energy simulation heat fluxes to evaluate the performance of the evaporative cooling process as a passive cooling technique for a commercial building typology. These performance indexes, related to indoor thermal comfort, energy consumption and their interactions with their surrounding environments, contribute to understanding the interactions between the urban climate and building for passive cooling integration. We compare the performance indexes for current and future climates (2080), according to the highest emission scenario A2 of the Special Report on Emission Scenario (SRES). Specific building models were adapted with both green roof and wetted roof techniques. The results show that summer thermal discomfort will increase due to climate change and could become as problematic as winter thermal discomfort in a temperate climate. Thanks to evapotranspiration phenomena, the sensible heat contribution of the building to the urban heat island (UHI) is reduced for both current and future climates with a green roof. The performance of the vegetative roof is related to the water content of the substrate. For wetted roofs, the impacts on heat transferred to the surrounding environment are higher for a Mediterranean climate (Marseille), which is warmer and drier than the Paris climate studied (current and future climates). The impact on indoor thermal comfort depends on building insulation, as demonstrated by parametric studies, with higher effects for wetted roofs.

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