There is, I think, something ethereal about i —the square root of minus one. I remember first hearing about it at school. It seemed an odd beast at that time—an intruder hovering on the edge of reality.

Usually familiarity dulls this sense of the bizarre, but in the case of i it was the reverse: over the years the sense of its surreal nature intensified. It seemed that it was impossible to write mathematics that described the real world in …

I and i

A crop phenology detection method using time-series MODIS data

The Aral Sea is a huge terminal lake located among the deserts of Central Asia. Over the past 10 millennia, it has repeatedly filled and dried, owing both to natural and human forces. The most recent desiccation started in the early 1960s and owes overwhelmingly to the expansion of irrigation that has drained its two tributary rivers. Lake level has fallen 23 m, area shrunk 74%, volume decreased 90%, and salinity grew from 10 to more than 100g/l, causing negative ecological changes, including decimation of native fish species, initiation of dust/salt storms, degradation of deltaic biotic communities, and climate change around the former shoreline. The population residing around the lake has also been negatively impacted. There is little hope in the foreseeable future to fully restore the Aral Sea, but measures to preserve/rehabilitate parts of the water body and the deltas are feasible.

The Aral Sea Disaster

If model parameterizations of unresolved physics, such as the variety of upper ocean mixing processes, are to hold over the large range of time and space scales of importance to climate, they must be strongly physically based. Observations, theories, and models of oceanic vertical mixing are surveyed. Two distinct regimes are identified: ocean mixing in the boundary layer near the surface under a variety of surface forcing conditions (stabilizing, destabilizing, and wind driven), and mixing in the ocean interior due to internal waves, shear instability, and double diffusion (arising from the different molecular diffusion rates of heat and salt). Mixing schemes commonly applied to the upper ocean are shown not to contain some potentially important boundary layer physics. Therefore a new parameterization of oceanic boundary layer mixing is developed to accommodate some of this physics. It includes a scheme for determining the boundary layer depth h, where the turbulent contribution to the vertical shear of a bulk Richardson number is parameterized. Expressions for diffusivity and nonlocal transport throughout the boundary layer are given. The diffusivity is formulated to agree with similarity theory of turbulence in the surface layer and is subject to the conditions that both it and its vertical gradient match the interior values at h. This nonlocal “K profile parameterization” (KPP) is then verified and compared to alternatives, including its atmospheric counterparts. Its most important feature is shown to be the capability of the boundary layer to penetrate well into a stable thermocline in both convective and wind-driven situations. The diffusivities of the aforementioned three interior mixing processes are modeled as constants, functions of a gradient Richardson number (a measure of the relative importance of stratification to destabilizing shear), and functions of the double-diffusion density ratio, Rρ. Oceanic simulations of convective penetration, wind deepening, and diurnal cycling are used to determine appropriate values for various model parameters as weak functions of vertical resolution. Annual cycle simulations at ocean weather station Papa for 1961 and 1969–1974 are used to test the complete suite of parameterizations. Model and observed temperatures at all depths are shown to agree very well into September, after which systematic advective cooling in the ocean produces expected differences. It is argued that this cooling and a steady salt advection into the model are needed to balance the net annual surface heating and freshwater input. With these advections, good multiyear simulations of temperature and salinity can be achieved. These results and KPP simulations of the diurnal cycle at the Long-Term Upper Ocean Study (LOTUS) site are compared with the results of other models. It is demonstrated that the KPP model exchanges properties between the mixed layer and thermocline in a manner consistent with observations, and at least as well or better than alternatives.

Oceanic vertical mixing: a review and a model with a nonlocal boundary layer parameterization

Historical hydrographic data from the continental shelf off eastern South America are used to examine the thermohaline properties of the water masses in the region between 20°S and 40°S. The continental shelf water masses are originated by dilution of open ocean waters of the western boundary currents of the South Atlantic Ocean. On the basis of temperature-salinity relation, two distinct water masses are identified, namely, the Subantarctic Shelf Water and the Subtropical Shelf Water. Subantarctic Shelf Water originates by dilution of Subantarctic Water, primarily in the southeast Pacific, due to excess precipitation and continental runoff and enters the continental shelf near 55°S. The Subtropical Shelf Water is modified South Atlantic Central Water diluted by continental runoff from the coast of Brazil. In addition, substantial dilution of the upper shelf waters takes place at the mouth of Rio de la Plata (approximately located at 36°S) and, in a lesser extent, at the Patos-Mirim Lagoon (at 32°S). The Rio de la Plata and the Patos outflows form a low-salinity tongue that caps the shelf water leading to a salinity decrease to values <30. The low-salinity tongue extends northward over the shelf penetrating farther north in winter than in summer. The extent of the low-salinity water has a strong impact on the vertical stratification and acts to limit winter convection to the layer above the halocline. There is little or no indication of mixing between Subantarctic Shelf Water and Subtropical Shelf Water. An intense temperature, salinity, and nutrient front separates these water masses. The front is oriented along the north-south direction, located on average near the 50 m isobath at 32°S and extends southward toward the shelf break near 36°S. Between 32° and 34°S the Subtropical Shelf Front follows the 100 to 200 m isobaths and separates Subantarctic Shelf Water from the oceanic South Atlantic Central Water. On the basis of the temperature and salinity distributions, beneath the low-salinity surface layer, the Subtropical Shelf Front appears as an extension of the Brazil-Malvinas Confluence over the continental shelf of South America. Thus the location of the Subtropical Shelf Front may be linked to the migrations of the separation point of the Brazil-Malvinas Confluence from the continental slope.

/pdf/subtropical-shelf-front-off-eastern-south-america-4197wsghub.pdf

Subtropical shelf front off eastern south america

1. What do we know about dead, dying and endangered lakes and seas A.G. Kostianoy, P.O. Zavialov. 2. Water management aspects of Amu Darya J. Froebrich, O. Kayumov. 3. Morphological changes in the Aral Sea. Satellite imagery and water balance model R. Ressl, Ph. Micklin. 4. Aral Sea basin evolution: geodynamic aspect B. Nurtaev. 5. The Aral Sea crisis Ph. Micklin. 6. Hydrobiology of the Aral Sea N.V. Aladin, I.S. Plonitkov, R. Letolle. 7. Main features of the Caspian Sea hydrology A.N. Kosarev, V.S. Tuzhikin, A.G. Kostianoy. 8. Hydrobiology of the Caspian Sea N.V. Aladin, I.S. Plotnikov. 9. Environmental contamination of the Caspian Sea. An overview of recent findings M.R. Sheikholesmani, S.de Mora, V. Vladymyrov. 10. 1000 years of environmental history of Lake Issyk-Kul S. Giralt, S. Riera, S. Leroy, T. Buchaca, J. Klerkx, M.de Batist, C. Beck, V. Bobrov, J. Catalan, V. Gavshin, R. Julia, I. Kalugin, R. Kipfer, V. Lignier, S. Lombardi, V. Matychenkov, F. Peters, V. Podsetchine, V. Romanovsky, F. Shukonikov, N. Voltattorni. 11. The Dead Sea as a dying lake I. Gavrieli, A. Oren. 12. The continual degradation of Lake Corangamite, Australia B.V. Timms. 13. Lake Chad: a challenging environment J. Lemoalle. 14. Genetic traces of environmental variations in ancient lakes S.V. Semovski, E. Verheyen, D.Y. Sherbakov. 15. Hypoxia and the physics of the Louisiana Coastal Current W.J. Wiseman, N.N. Rabalais, R.E. Turner, D. Justie. List of Participants.

Dying and dead seas : climatic versus anthropic causes

Measurements of velocity, hydrography, surface meteorology, and microstructure were made through several squall events during a westerly wind burst that occurred in the Western Pacific warm pool in December 1992. Sustained wind forcing generated a weakly stratified turbulent surface layer that extended to the top of the main thermocline. Following each rain event, freshwater formed a statically stable layer in the upper 4‐12 m. The subsequent evolution of the mixing profile was strongly depth-dependent. Turbulence increased dramatically in the fresh layer adjacent to the surface but decreased in the underlying layer. The factor by which turbulence decreased following a given squall was strongly correlated with the net rainfall. The observed timescale for the decay of the turbulence was about 0.7 buoyancy periods, similar to decay times observed near the surface after sunrise. However, these decay times are significantly larger than those estimated indirectly (as the ratio of dissipation rate to turbulent kinetic energy) from turbulent patches in the thermocline. To account for the discrepancy, the authors hypothesize that turbulence production continues to act during the observed decay process, partially counteracting the effect of dissipation.

/pdf/decay-of-turbulence-in-the-upper-ocean-following-sudden-10jqyu90ir.pdf

Decay of Turbulence in the Upper Ocean following Sudden Isolation from Surface Forcing

An area of about 40000 km2 of desalinated upper layer waters with a salinity of less than 25 psu was found during cruise 54 of the R/V Akademik Mstislav Keldysh in the southwestern part of the Kara Sea (September 2007). Close to the boundary of this region located near the eastern coast of Novaya Zemlya, the salinity was less than 16 psu. The thickness of the desalinated layer was about 10 m. The results of the chemical analysis revealed that the observed desalination of the sea water was produced, first of all, by the Yenisei River, while the contribution of the Ob River’s waters was secondary. However, the most desalinated region near the eastern coast of Novaya Zemlya was separated from the Ob-Yenisei estuary and corresponded to a quasi-isolated lens. It is likely that the formation of this lens, as well as the major part of the desalinated upper layer waters, occurred in June when the flood of the Yenisei was maximal, while the further drift of the desalinated waters to the west of the Ob-Yenisei estuary was forced by the prevailing northern wind. The additional desalination (by 2–3 psu and even more) of the upper layer waters near the eastern coast of Novaya Zemlya might be related to the melting of the Novozemelskiy ice massif. The regularities of the temporal evolution of the upper desalinated layer, as well as the influence of this layer on the hydrological structure and dynamics of the southwestern Kara Sea, are discussed.

https://link.springer.com/content/pdf/10.1134/S0001437010050036.pdf

The upper desalinated layer in the Kara Sea

[1] We report the results of a hydrographic survey conducted in November, 2002, in the Uzbekistan part of the western basin of the dying Aral Sea. There were very few hydrographic measurements in this region since at least early 1990s. The salinity in the western deep basin of the Aral Sea varied from about 82 psu at the surface to over 94 psu at the bottom. The absolute lake surface level was about 30.5 m. Hence, the observed salinity values were much higher, and the level much lower, than expected according to earlier predictions. The density in the western basin exhibited an extremely strong stratification of ∼11 kg/m3 per ∼20 m in the bottom layer. The picnocline was accompanied by a temperature inversion whose magnitude was ∼4°C. The observed density stratification effectively isolating the lower part of the water column from surface exchanges may be responsible for the increase of summer SSTs and evaporation rates reported in previous studies. We discovered the hydrogen sulphide contamination in the bottom layer whose upper limit was at the depth of approximately 22 m. Estimates suggest that the western basin salinization occurs not only because of the local evaporation, but also because of the assimilation of the saltier eastern basin water in the course of the interbasin exchange through the connecting channel. The satellite imagery analysis, in particular the Maximum Cross-Correlation method, suggests that the circulation pattern in the Aral Sea in its present limits is cyclonic under the eastern winds that are predominant in the region throughout the year.

https://agupubs.onlinelibrary.wiley.com/doi/pdf/10.1029/2003GL017427

Hydrographic survey in the dying Aral Sea

A brief historical overview.- The immediate past: A summary of the pre-desiccation state.- Present-day desiccation.- The future: What happens next?.- The Aral crisis in global perspective.- Concluding remarks.

Peter Zavialov

Papers

Dying and dead seas : climatic versus anthropic causes

Decay of Turbulence in the Upper Ocean following Sudden Isolation from Surface Forcing

The upper desalinated layer in the Kara Sea

Hydrographic survey in the dying Aral Sea

Physical Oceanography Of The Dying Aral Sea