Jaya Naithani

Origin of intraseasonal variability in Lake Tanganyika

[1] Intraseasonal thermocline oscillations in Lake Tanganyika are analysed using observations near Mpulungu and simple analytical/numerical models, in order to understand their origin. The region around the lake is characterised by strong and persistent southeast winds during the four months dry season, lasting from May to August. The associated wind-stress causes the tilting of the thermocline which oscillates for the whole year. The wavelet transform spectra of temperature at 30 m depth of the lake near Mpulungu indicates the presence of various scales of motion, localised in frequency and time. The dominant modes of thermocline oscillations are intraseasonal variability with 3-4 weeks periods. Similar results are obtained from a reduced-gravity model with various wind forcing, including the observed forcing, and a simple analytical solution. In addition, the model results indicates that the dominant mode of oscillation exhibits one node only. From the study, it is inferred that the free modes of oscillations of the lake are in resonance with wind pulses.

Are there internal Kelvin waves in Lake Tanganyika

It is generally believed that the Earths rotation has negligible impact on the water circulation in basins which are very narrow or located near the Equator. However, herein evidence is presented of the influence of the Earths rotation on the hydrodynamics of Lake Tanganyika, which is both very narrow (width/length approximate to 0.08) and located near the Equator. Numerical simulations exhibit small upwellings at the western shores as a result of the thermocline oscillations induced by the southeasterly winds of the dry season. These structures tend to propagate cyclonically around the lake similar to internal Kelvin waves. Numerical experiments in which f is varied concludes that internal Kelvin waves are present in Lake Tanganyika. It is also evidenced from this study that the internal Kelvin waves cannot be anticipated based on classic scaling arguments.

Marine air intrusion into the Adelie Land sector of East Antarctica: A study using the regional climate model (MAR)

[1] Marine air intrusion and subsequent cloud formation plays a dominant role in the energy budget and mass balance of the Antarctic. However, the intrusion is very difficult to understand using the ground-based measurements alone. In this paper we present simulations of marine air intrusion into the Adelie Land, East Antarctica, using the Modele Atmospherique Regional (MAR), for July 1994 and January 1995. The model is nested into the European Centre for Medium-Range Weather Forecasts (ECMWF) analyses. The simulations show a strong influence of large-scale disturbances, over the ocean, which helped in the penetration of marine air into the interior and the formation of clouds. Each marine air intrusion episode resulted in cloud formation in July 1994. Blocking anticyclones have also been found to be responsible for much of the moisture transport far into the interior elevated locations. MAR simulations, as well as ECMWF analyses, show influence of cyclones in strengthening and prolonging the surface layer flow. The study also indicated that the influence of depressions on surface winds is pronounced during the period when the depression is approaching the Adelie Land coast.

Possible effects of global climate change on the ecosystem of Lake Tanganyika

Any change in the air temperature, wind speed, precipitation, and incoming solar radiation induced by increasing greenhouse gasses and climate change will directly influence lakes and other water bodies. The influence can cause changes in the physical (water temperature, stratification, transparency), chemical (nutrient loading, oxygen) and biological (structure and functioning of the ecosystem) components of the Lake. In this work an influence of the likely effects of the climate change on the above three components of Lake Tanganyika are studied by means of a simple ecological model. Simulations for the years 2002–2009 have been performed using the wind and solar radiation data from the National Centres for Environmental Protection (NCEP) reanalysis. Various possible climatic scenarios are studied by changing the surface layer depth, its temperature and the wind stress. Any change in any of the above physical forcing parameters modifies the timing and intensity of the dry season peaks of the biogeochemical parameters. It is seen that the gross production increases as temperature of the surface layer increases and its depth decreases. High temperature and low wind stress, reduces the biomass. The effects of a slight increase in lake water temperature on the Lake Tanganyika ecosystem might be mitigated by increased windiness, if the latter was sufficient to induce greater mixing.

An ecological model for the Scheldt estuary and tidal rivers ecosystem: spatial and temporal variability of plankton

This paper presents the formulation, structure, and governing equations of an ecosystem model developed for the Scheldt estuary and the tidal river network. The model has twelve state variables: nitrate, ammonium, phosphate, dissolved silica, freshwater and marine phytoplankton (chlorophytes and diatoms), freshwater zooplankton (ciliates, rotifers, and copepods), and benthic detritus. The ecological model is coupled to the 1-D tidal resolving version of the Second-generation Louvain-la-neuve ice-ocean Model (SLIM) (http://www.climate.be/SLIM). The model successfully simulates the observed longitudinal and seasonal variation of plankton in the Scheldt estuary. The phytoplankton production in the estuary is governed by temperature, underwater available light, turbidity, nutrients, and discharge. Of all these factors, discharge seems to be dominant. High discharge increases the turbidity in the water column and thus reduces the underwater light, while low discharge means decreased nutrients. The marine phytoplankton species were present as far to the upstream limits of the brackish waters, with diatoms dominating in the spring and chlorophytes in early summer. The freshwater phytoplankton are seen from late spring to summer. Freshwater zooplankton followed the evolution of freshwater phytoplankton.