An extreme internal solitary wave event observed in the northern South China Sea
With characteristics of large amplitude and strong current, internal solitary wave (ISW) is a major hazard to marine engineering and submarine navigation; it also has significant impacts on marine ecosystems and fishery activity. Among the world oceans, ISWs are particular active in the northern South China Sea (SCS). In this spirit, the SCS Internal Wave Experiment has been conducted since March 2010 using subsurface mooring array. Here, we report an extreme ISW captured on 4 December 2013 with a maximum amplitude of 240 m and a peak westward current velocity of 2.55 m/s. To the authors’ best knowledge, this is the strongest ISW of the world oceans on record. Full-depth measurements also revealed notable impacts of the extreme ISW on deep-ocean currents and thermal structures. Concurrent mooring measurements near Batan Island showed that the powerful semidiurnal internal tide generation in the Luzon Strait was likely responsible for the occurrence of the extreme ISW event. Based on the HYCOM data-assimilation product, we speculate that the strong stratification around Batan Island related to the strengthening Kuroshio may have contributed to the formation of the extreme ISW.
Simulation of internal solitary waves with negative polarity in slowly varying medium
We consider the propagation of an internal solitary wave over two different types of varying depth regions, i.e. a gentle monotonic bottom slope connecting two regions of constant depth in two-layer fluid flow and a smooth bump. Here, we let the depth of the upper layer is smaller than the lower layer such that an internal solitary wave of negative polarity is generated. The appropriate model for this problem is the variable-coefficient extended Korteweg-de Vries equation, which is then solved numerically using the method of lines. Our numerical results show different types of transformation of the internal solitary wave when it propagates over the varying depth region depending on the depth of the lower layer after the varying depth region including generation of solitary wavetrain, adiabatic and non-adiabatic transformation of the internal solitary wave.We consider the propagation of an internal solitary wave over two different types of varying depth regions, i.e. a gentle monotonic bottom slope connecting two regions of constant depth in two-layer fluid flow and a smooth bump. Here, we let the depth of the upper layer is smaller than the lower layer such that an internal solitary wave of negative polarity is generated. The appropriate model for this problem is the variable-coefficient extended Korteweg-de Vries equation, which is then solved numerically using the method of lines. Our numerical results show different types of transformation of the internal solitary wave when it propagates over the varying depth region depending on the depth of the lower layer after the varying depth region including generation of solitary wavetrain, adiabatic and non-adiabatic transformation of the internal solitary wave.
Lake Muzahi, Rwanda: limnological features and phytoplankton production
Lake Muhazi, a small lake of Rwanda (East Africa) was studied from 1986 to 1990. A dramatic decrease of the catch of Oreochromis niloticus (350 T y−1 in the fifties vs 30 T y−1 in 1982) suggested a loss of productivity or overfishing. In the same period, other ecological changes occurred: the submerged macrophytes regressed and there was a decrease in Secchi depth (0.65 m in 1987 vs 1.5 m in the fifties). Compared to other lakes of the same area, the plankton production seemed low. The results of the present study characterize lake Muhazi as a shallow lake with a rather unstable diurnal stratification and with slight differences in mixing regime between its eastern, deepest part and its western, shallowest part. Secchi disk depth does not vary seasonally to a large extent. The water has a rather high mineral content (conductivity of about 500 µS cm−1 at 25 °C) and low concentrations of dissolved N and P, except in the hypolimnion, where NHinf4sup+-N can be high.Two species, Microcystis aeruginosa and Ceratium hirundinella, account for most of the phytoplankton biomass, which is about 50–80 mg chlorophyll a m−2 in the euphotic zone, usually with little seasonal variation. Daily gross production estimates amount to about 6 to 9.5 g O2 m−2 d−1 with a significant difference between the two parts of the lake. Data on C:N and C:P ratio in the phytoplankton suggest that some N deficiency might occur in the eastern part. Moreover, the Zm:Zc ratio could also lead to rather low net production rates (0.21–0.25 d−1 for a mixed layer of 4 m)In conclusion, the primary production of lake Muhazi is medium for African lakes and the hypothesis that decreased planktonic production could account for a reduced fish production should be discarded. Whereas the present yield of the fishery is only 20 kg ha−1 y−1, the yield estimated from primary production ranges between 46 and 64 kg ha−1 y−1. This could be reached through proper management. Finally, some hypotheses are given to explain the ecological changes which occurred in the lake.