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.
Interface Exchange as an Indicator for Eddy Heat Transport
The ocean contains many large‐scale, long‐lived vortices, called mesoscale eddies, that are believed to have a role in the transport and redistribution of salt, heat, and nutrients throughout the ocean. Determining this role, however, has proven to be a challenge, since the mechanics of eddies are only partly understood; a standard definition for these ocean eddies does not exist and, therefore, scientifically meaningful, robust methods for eddy extraction, characterization, tracking and visualization remain a challenge. To shed light on the nature and potential roles of eddies, we extend our previous work on eddy identification and tracking to construct a new metric to characterize the transfer of water into and out of eddies across their boundary, and produce several visualizations of this new metric to provide clues about the role eddies play in the global ocean.
Cleavage of a Gulf of Mexico loop current eddy by a deep water cyclone
Eddy Triton, an anticyclonic eddy shed by the Loop Current in late June 1991, drifted SW across the central Gulf of Mexico in the first 6 months of 1992, along the “southern” of the three characteristic drift paths described by Vukovich and Crissman [1986] from their analyses of 13 years of advanced very high resolution radiometer sea surface temperature data. An expendable bathythermograph (XBT) and conductivity-temperature-depth (CTD) transect of opportunity through Triton at eddy age 7 months in January 1992 found that eddy interior stood 23 dyn. cm higher than periphery; this gradient drove an anticyclonic swirl transport of 9–10 Sv relative to 800 dbar. At eddy age 9–10 months and while this eddy was in deep water near 94°W, it interacted with a mesoscale cyclonic circulation and was cleaved into two parts. The major (greater dynamic centimeters) piece drifted NW to end up in the “eddy graveyard” in the NW corner of the gulf, while the minor piece drifted SW and reached the continental margin of the western gulf off Tuxpan. This southern piece of Eddy Triton then turned north to follow the 2000-m isobath to about 24°N and later coalesced with what remained of the major fragment. Because Eddy Triton's cleavage took place just before the start of marine mammals (GulfCet) and Louisiana-Texas physical oceanography (LATEX) field programs, the closely spaced CTD, XBT, and air dropped XBT (AXBT) data that were gathered on the continental margin north of 26°N in support of these programs allow a detailed look at the northern margin of the larger fragment of this eddy. Supporting data from the space-borne altimeters on ERS 1 and TOPEX/POSEIDON allow us to track both pieces of Eddy Triton in the western Gulf and follow their spin down in dynamic height, coalescence, and ultimate entrainment in January 1993 into another anticyclonic eddy (Eddy U).