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.
Frequency of Ring Separations from the Loop Current in the Gulf of Mexico: A Revised Estimate
The most energetic events in the circulation of the Gulf of Mexico are the separation of large anticyclonic rings from the Loop Current. Building on previous work, the authors examine all the apparent rings since July 1973. This new dataset includes the satellite altimetry since 1992, providing a set of 34 consecutive ring formations. The primary advantage of altimetry is that the data remain available in the summer. One finding is that the ambiguity of whether or not a ring has separated is reduced, but not eliminated; the uncertainty with which separation ‘‘events’’ can be specified remains approximately 4 weeks, even with nearly continuous data. Primary peaks in the distribution of separation intervals are found at 6 and 11 months with a smaller peak at 9 months. If the spectrum is smoothed heavily enough, a peak in the distribution can be formed nearer 12 months, but this near-annual peak is a result more of the smoothing than of the data.
Seven-year current meter record in the eastern North Atlantic
Continuous current measurements at the 1000 m level were obtained in the central Canary Basin of the northeast Atlantic near 33°N, 22°W for 2398 days. Even with this very long time series no statistically significant mean current could be estimated at that level, because the energetic fluctuations are large compared to the weak mean. In the eddy scale range, i.e. at current fluctuations are scales between 47 and 455 days, a pronounced anisotropy between zonal and meridional components is apparent. For the first time in the subtropical North Atlantic gyre our data allow confirmation of the expected spectral decrease beyond the eddy scale peak in an eastern basin. With respect to future global experiments we wonder if our results from an eastern basin location are representative for the general circulation at mid-ocean sites?