900 JOURNAL OF PHYSICAL OCEANOGRAPHY VOLUME 40 A Trigger Mechanism for Loop Current Ring Separations WILTON STURGES Department of Oceanography, The Florida State University, Tallahassee, Florida NICHOLAS G. HOFFMANN AND ROBERT R. LEBEN Colorado Center for Astrodynamics Research, University of Colorado, Boulder, Colorado (Manuscript received 19 February 2009, in final form 14 December 2009) ABSTRACT The Loop Current in the Gulf of Mexico sheds large anticyclonic rings on an irregular basis. The authors attempt to show what actually triggers the ring separations. Pulses of increased transport through the Florida Straits, as observed by the cable data, are observed prior to each ring separation. This finding is consistent over all separation events observed in the satellite altimetry record. The pulses of transport occur approximately two to four weeks before the rings separate. The increase in transport is usually accompanied by a corre- sponding increase in offshore sea level, suggesting forcing from the open ocean. The delay times between the pulses of increased transport and ring separations can be shown to be significantly correlated with the length of the Loop Current. Mean sea levels over the Caribbean and Gulf also peak before the separations, on average. 1. Introduction ring formation from the point of view of numerical modeling. Of course, there is the possibility that the ring As the Loop Current flows into the Gulf of Mexico, it formations are chaotic, but Lugo-Fernandez (2007) con- presses against the western side of the Yucatan Straits. cludes that they are not. There are many similarities be- The outflow from the Gulf constrains the Loop Current tween Loop Current and Gulf Stream ring separation to form a large anticyclonic pattern, as shown in Fig. 1; events, although one major difference is that the Gulf eventually an anticyclonic ring separates from the main Stream sheds both anticyclonic and cyclonic rings. flow. Figure 1a shows a typical ring separation. However, as The ring shedding is quite erratic. A ring will sometimes often happens, the ring may become reattached to the separate quickly after the Loop Current has extended main body of the Loop Current and separate later; such into the Gulf, but at other times several months will an event is shown in Fig. 1b. elapse before the ring detaches completely. In contrast, In this paper, we have used the data and the metrics rings detach from the Gulf Stream almost immediately employed by Leben (2005) to determine when a ring has after they are formed (e.g., Auer 1987; Olson 1991). become detached: the abrupt decrease in area enclosed Leben (2005) has described as well as quantified the er- by the Loop Current, as described by the 17-cm height ratic behavior of ring separations based on sea surface contour on SSH maps. We have compared these ring height (SSH) from altimetry. Schmitz et al. (2005), al- separations with the transport of the Florida Current as though discussing the distinction between ring formation measured by the cable across the Florida Straits (see, e.g., and detachment, show that rings do not separate unless DiNezio et al. 2009; Baringer and Larsen 2001). We have there is a cyclonic ring on both sides of the flow to aid in used other data as well, but the transport measurements the pinching-off process. Oey et al. (2005) have discussed emerge as the significant variable of interest. When comparing these records, we noticed a remark- able and consistent feature: the transport of the Florida Corresponding author address: W. Sturges, Department of Oceanography, The Florida State University, Tallahassee, FL Current, as measured by the cable data, shows pulses of 32306-4320. increased flow shortly before each separation event. E-mail: [email protected] These increases in transport are usually accompanied by DOI: 10.1175/2009JPO4245.1 Ó 2010 American Meteorological Society Unauthenticated | Downloaded 10/05/21 12:12 PM UTC MAY 2010 S T U R G E S E T A L . 901 increases in sea level on the offshore side of the flow; in this study. A more complete description of the data these have ;5–10-cm amplitude and are of 20–25-days processing as it relates to Loop Current monitoring may duration, suggesting that waves propagating in from the be found in Leben (2005). open Atlantic are instrumental in the ring separation The transport of the Florida Current is monitored by trigger process. a cable between the U.S. east coast and the Bahamas, We wish to emphasize that we are describing a set of and data have been made easily available on a Web page observations that are associated with ring separations. maintained by C. Meinen (available online at http://www. Whether these events are indeed the operative mechanism aoml.noaa.gov/phod/floridacurrent/FC_cable_transport_ requires further exploration. It is generally understood 2008.dat). that ring separations result from a flow instability. Our Sea level data at Settlement Point, Bahamas, are study has focused on a decades-old question: Why is there available from the Hawaii Sea Level Center (available often such a long and irregular delay between ring for- online at http://uhslc.soest.hawaii.edu/uhslc), who also mation and the eventual detachment? An important mo- have the responsibility for maintaining that tide gauge. tivating factor for our investigation is the fact that, to the Key West tidal data are available at the primary Na- best of our knowledge, modern numerical models are not tional Oceanic and Atmospheric Administration (NOAA) able to predict when a ring will separate unless sea surface data center (available online at http://tidesandcurrents. height data are assimilated. Thus, we have searched for noaa.gov). mechanisms or processes that might be difficult to capture In many of the plots shown here the data have been in a free-running model simulation. The trigger mecha- normalized by their individual standard deviations (std nism discussed here, in fact, appears to be pulses of dev) for the full record. For Loop Current area, the std transport that could easily be lost in the background of dev is 2.89 3 1010 km2. For transport by cable, the std dev ‘‘ocean noise.’’ Also, the mechanism acts from the is ;3.5 Sv for the ;2000 days before an approximately ‘‘downstream’’ direction in contrast to the potential up- 3-yr cable break near the middle of the record. The stream influences identified in both modeling (Murphy cable data have extremes of 613 Sv, but some of this is et al. 1999; Oey et al. 2003) and observational studies attributed to the side-to-side fluctuations of the flow that (Candela et al. 2002; Abascal et al. 2003). are difficult to correct for, as well as for contributions Therefore, we emphasize that what we describe here from shelf waves propagating down the coast (Johns and as a possible trigger mechanism could also appear to Schott 1987; W. Schmitz 2009, personal communication). a critical observer as merely noise in the data. The es- The std dev of sea level at Settlement Point is ;8.3 cm for sential point is that these seemingly random pulses of the duration we use here. We have used daily means of all transport occur throughout the record, and only when records. they occur when a ring is otherwise poised for de- tachment do they become an operative mechanism. 3. Results Figure 1a shows the position of a LC ring as it is 2. Data separating. It reattaches, as is sometimes the case, and its The primary variable we use to describe the Loop subsequent separation is shown in Fig. 1b. Figure 1c Current is the sea surface height from the combined shows the positions of the various locations described satellite altimetric record available since 1993 from the here. Figure 2a shows a set of data for the first separation Ocean Topography Experiment (TOPEX)/Poseidon, interval, highlighting both the typical signal contained in European Remote Sensing Satellite-1 (ERS-1)andERS-2, the individual time series and the noise levels of the Geosat Follow-On, Jason-1,andEnvisat satellite mis- data. This ring appears to separate in March 2006, when sions. Processing of the SSH data is based on near-real- the Loop Current area drops abruptly, as shown by the time mesoscale analysis techniques designed to exploit vertical red line. The arrow in mid-February indicates an the multisatellite altimetric sampling (Leben et al. 2002). abrupt pulse of increased transport in the Florida Cur- All along-track data were referenced to a mean sea sur- rent cable data. Cyclonic features are found on both face (Wang 2001) and detrended using an along-track sides of the flow as the necking-down process progresses, loess filter that removes a running least squares fit of a tilt as discussed by Schmitz et al. (2005). The amplitude of and a bias from the along-track data within a sliding 200-s the transport pulse is ;5 Sv, or ;1.5 standard deviations window. Daily analysis maps of the detrended SSH of the transport, which is a substantial amount. anomaly were estimated using an objective analysis pro- Figure 2b shows a slightly different set of data for the cedure (Cressman 1959) and added to a model mean to separation in September 2001. The cable was down, but calculate the synthetic sea surface height estimates used sea level data are available on both sides of the flow. The Unauthenticated | Downloaded 10/05/21 12:12 PM UTC 902 JOURNAL OF PHYSICAL OCEANOGRAPHY VOLUME 40 FIG. 1a. Three perspectives of the initial detachment of Eddy Xtreme: (left) CCAR SSH data, (right) the simultaneous GOES sea surface temperature (SST) data, and (middle) both for 1, 8, and 15 Mar 2006.