Vorticity flux through the Yucatan Channel and Loop Current variability in the Gulf of Mexico
[1] Recent observations (the CANEK Program [Candela et al., 2002]) suggest that potential vorticity (PV) flux anomaly (VFA) at Yucatan Channel may serve as a useful indicator of Loop Current variability, including Loop Current extension, retraction, and eddy shedding. Intuitively, anticyclonic VFA extends the Loop Current into the Gulf of Mexico and cyclonic VFA causes retraction or even shedding. However, this intuition is inconsistent with PV conservation. The problem is reexamined here by careful analyses of the relation between VFA and Loop Current variability using (1) the results of a 15-year numerical simulation of shedding specified with simple forcing, and (2) CANEK and satellite observations. Both model and observations indicate that Loop Current eddy shedding or retraction tends to occur shortly (1� 2 months) after the influx of VFA at Yucatan has turned anticyclonic, and that these events are sometimes preceded by a more prolonged period of influx of cyclonic VFA. These findings suggest that contrary to intuition, influx of cyclonic VFA tends to extend the Loop Current into the Gulf, thus making the Loop Current more susceptible to retract or shed an eddy, and influx of anticyclonic VFA may then ‘‘trigger’’ retraction or eddy shedding. However, the Loop Current’s behaviors are much more complex than can be prescribed by these simple rules. A much longer observational data set, coupled with more refined model experiments and sophisticated analyses, is required to further quantify the phenomenon. INDEX TERMS: 4520 Oceanography: Physical: Eddies and mesoscale processes; 4255 Oceanography: General: Numerical modeling; 4576 Oceanography: Physical: Western boundary currents; 4243 Oceanography: General: Marginal and semienclosed seas; KEYWORDS: vorticity flux, Loop Current, Yucatan Channel Citation: Oey, L.-Y. (2004), Vorticity flux through the Yucatan Channel and Loop Current variability in the Gulf of Mexico, J. Geophys. Res., 109, C10004, doi:10.1029/2004JC002400.
Altimetry and drifter data assimilations of loop current and eddies
and direction (absolute) errors are 0.4–5 cm s � 1 (1� 10% errors) and 10� 20, respectively. The correlations of the two dominant empirical orthogonal function (EOF) modes with the corresponding observation modes at a yearlong mooring over the northern slope are: mode-1 � 0.88� 0.93 and mode-2 � 0.5� 0.63. Both show vertically coherent but strongly sheared modes 1 and 2 representing propagation eddies and reversing mode-3 that intensifies for z < � 300 m.
Germination ecology of Central Asian Stipa spp: differences among species, seed provenances, and the importance of field studies
Stipa-species are wide-spread in Central Asia, but sexual reproduction in the dry steppes is rare. To facilitate conservation and restoration of these important rangelands, we studied germination characteristics of three common Mongolian Stipa-species under field- and lab conditions. Seeds of Stipa krylovii, Stipa gobica and Stipa glareosa were sown at the study site in Southern Mongolia over two consecutive years during which period tests were carried out to ascertain whether competition or herbivory are the main constraints of seedling establishment. In addition, we tested germination and seed viability in the laboratory under two different temperature regimes (20/10°C and 8/4°C), as well as the effect of cold-stratification. The lab experiments also included S. krylovii seeds originating from three climatically different provenances. None of the three Stipa-species seedlings emerged during the first 2 years of the field study. However, after an unusually intense rain event in the third year, 3% of S. krylovii, 0.6% of S. glareosa and 0.1% of S. gobica seeds germinated in the study plots. The factors ‘sowing-year’ and ‘vegetation’ significantly affected seedling emergence, whereas grazing had no effect at all. Under laboratory conditions a high percentage of viable seeds of S. gobica and S. glareosa germinated at both incubation temperatures, and cold-stratification had no effect on germination or viability. Germination of S. krylovii seeds required warmer temperatures and cold-stratification had a positive effect. Such evidence for dormancy was more pronounced in seeds from the moister, northern provenances. Germination of Stipa-species in the field is rare and only possible under exceptionally moist conditions. Conservation should thus concentrate on steppe conservation rather than on restoration. Where artificial reseeding is necessary, differences among species and also among different seed provenances should be taken into account.