900 JOURNAL OF PHYSICAL 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 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 level, suggesting forcing from the open . 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 , 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 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 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 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 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

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FIG. 1a. Three perspectives of the initial detachment of Xtreme: (left) CCAR SSH data, (right) the simultaneous GOES (SST) data, and (middle) both for 1, 8, and 15 Mar 2006. GOES SST data with the CCAR SSH contours are overlaid. The SSH data are in centimeters, and the SST data are in degrees Celsius. The detachment took place on 8 Mar and is marked by the breaking of the 17-cm height contour. red curve shows sea level on the offshore side, at Settle- separation, there are two jumps in the transport of the ment Point, and the green curve shows the difference Florida Current, which is indicated by the two arrows. between Settlement Point and Key West. Because the The figure also shows the sea level signal at Settlement two signals are so similar, it is reasonable to conclude Point and the sea level difference across the flow, as in that they mimic the transport between them, as we might Fig. 2b, as well as the noise level in the data. expect, although that is sometimes not the case with these Although there is a troublesome level of noise in these data. data (by which we refer mainly to real oceanic variability, Figure 2c shows a larger dataset for a different sepa- not to errors in measurements), in most cases the pulses ration event when more of the relevant variables are stand out clearly from the noise. For those cases where available. A ring separates in July 1993; prior to the the pulses of transport are ;2 std dev, the sea level

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FIGS. 1b and c. (b) As in (a), but for reattachment and final separation of Eddy Extreme for 10, 17, and 24 Apr 2006. Eddy Xtreme briefly reattached to the Loop Current on 17 April, as shown by the reconnection of the 17-cm height contours; warm Loop Current water is seen being entrained by the eddy. It separated completely from the Loop Current, based on the 17-cm contour, on 18 Apr. (c) Locations of the datasets used here: the dashed line from Florida across the Florida Straits indicates the approximate location of the undersea cable, Settlement Point is at the northwest tip of the Bahamas island at the eastern end of the cable, and Key West is at the southernmost tip of Florida. Contours show SSHA at an essentially arbitrary time to show variability in a single daily map, in 5-cm increments; dashed contours are negative. SSHA contours are not shown where depth is less than 50 m.

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FIG. 2. (a) Loop Current area and Florida Current transport at the time of a Loop Current ring separation. The Florida Current transport is from cable data. Daily values are used for all variables. The vertical dashed red line indicates the time of ring separation. The vertical black arrow points to a pulse of increased transport, as shown in Table 1. (b) As in (a), also showing sea level at Settlement Point, Bahamas, but for a time when the cable transport data are not known. The similarity between the Settlement Point sea level and the sea level difference across the flow suggests that the transport signal would be similar. (c) As in (b), but at the time of the Loop Current ring separation of July 1993, also showing the sea level difference between Settlement Point and Key West. The vertical black arrows point to pulses of increased transport, as shown in Table 1.

Unauthenticated | Downloaded 10/05/21 12:12 PM UTC MAY 2010 S T U R G E S E T A L . 905 difference is also essentially ;2 std dev. Two std dev of TABLE 1. Delays between the increased pulses of Florida Current transport represents ;7Sv(atypicalvalueinFig.4), transport and the subsequent separation of a ring, as shown in Fig. 2. whereas the sea level difference represents an increased The ‘‘previous’’ column shows the delay for the previous pulse of increased transport for those occasions when one was apparent. difference of ;17 cm. The means of these primary vari- When the transport values were not available because the cable ables are ;30 Sv and ;75 cm, so these pulses represent was down, Settlement Point sea level was used as a surrogate for (as would be expected) fluctuations of ;23% in both separations 10–12. values. This close agreement is, of course, reassuring. Event Separation date Delay days Previous Julian day Another obvious comparison would be to examine sea level on the inshore side of the stream. Because the 1 July 1993 10 27 728121 2 September 1993 17 — 728182 Miami has many breaks during this time pe- 3 August 1994 8 28 728533 riod, we chose the record from Key West. It turns out 4 April 1995 15 26 728768 that Key West sea level does not look at all like the re- 5 September 1995 13 — 728910 cord of transport or of ring separations. This holds true 6 March 1996 6 21 729098 both for individual events and for the mean. 7 October 1996 13 — 729310 8 September 1997 13 29 729662 From a more basic point of view, the comparison be- 9 March 1998 13 27 729836 tween the transport as observed by cable and the sea 10 October 1999 9 17 730394 level difference between the two tide gauges is worth 11 April 2001 11 — 730941 examining and is shown in Figs. 2b,c. There are many 12 September 2001 6 — 731115 cases during which the cable transport and sea level 13 February 2002 21 — 731275 14 March 2002 9 32 731286 difference are quite similar, but there are a few times 15 August 2003 27 — 731798 when there is no resemblance whatsoever. Although this 16 December 2003 15 42 731945 is an interesting issue, it is not the point of the present 17 August 2004 9 20 732181 work and is not pursued further here. 18 September 2005 20 — 732576 For completeness, plots of approximately half of the 19 February 2006 12 24 732714 20 March 2006 20 — 732743 remaining separations covered by this analysis period Mean 13 27 are shown in the appendix. We examined each separa- Std dev 5 7 tion event to determine the delays between the pulses of transport and the following ring separation, knowing a priori that the ring had separated. Table 1 shows these Loop Current before each ring separation. Because this values; the first entry in the table is for the first transport result was encouraging, we examined the average of jump prior to the ring separation; the second transport these events further in Fig. 4. jump, if it occurs within a reasonable time, is also shown. Figure 4a shows the transport anomaly during each of In three cases, the cable was out of commission, so sea the shedding events relative to the time of separation as level height at Settlement Point was used as a surrogate well as their mean; that is, the vertical line in the middle variable. Using the tide gauge data in those cases re- of the figure is the time of ring separation, the transport mains a bit speculative, but it allows us to examine all the anomaly of each separation event is synchronized rela- separation events. The mean of all the first delay times is tive to that time, and all events are shown. A transport 13 6 5 days. (If the three events using Settlement Point pulse in the mean (the blue curve) is seen ;14 days data were removed, the mean would change only in the preceding the ring separations and again at about twice first place beyond the decimal.) that, which is consistent with the results in Table 1. Note There may appear to be a certain amount of subjectivity also that a third pulse is seen shortly after the ring has here in deciding exactly where the transport jumps occur. separated, which is suggestive of a wave train. However, after examining all these separation events, the Figure 4b similarly shows a set of all the data, again jumps in transport emerge as a consistent feature even if relative to the time of separation; the individual plots of occasionally obscured by the noise in the different variables. sea level anomaly at Settlement Point, relative to the The distribution of the delay times is shown in Fig. 3. separations, make up the noisy dataset. The mean curve, Although the individual pulses prior to the separations however, is consistent with Fig. 4a. Sea level is high, in are a noisy dataset, in the mean they clearly are grouped the mean, roughly two weeks before the ring separation into the bimodal pairing shown here; a pulse is seen events and again almost two weeks earlier than the first roughly two weeks prior to each ring separation and rise. High sea level on the offshore side of the flow of a second one is seen roughly two weeks before that. We course is consistent with increased transport. Note that, show, in a later section, that the variability in these delay in these figures, the nonnormalized values are shown so times is significantly correlated with the length of the that the absolute amplitudes can be seen.

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FIG. 3. A histogram of the delay times given in Table 1, as derived from the individual separation events.

It is not surprising that the transport increases that data with only daily resolution it is difficult to assign too appear here have a much reduced amplitude in the much meaning to the one-day delay. mean. We also find that third pulse of increased transport 1) THE INCOMING WAVES shortly after the ring separation. The three pulses are not always present in the individual separation events, in Waves with a period of order one month are such part because of noise in the data or because one or more ubiquitous features in the open ocean that one does not of the individual data sources is not available. know whether to call them eddies, waves, or simply noise. In an early but profound investigation of such variability Which comes first? in ocean signals, Schroeder and Stommel (1969) referred The increased sea levels on the offshore side can be to ‘‘month persistent’’ features, using the hydrographic found as a robust feature, as described in a later section, and tide gauge data at Bermuda to show that they were but it is a fair question to ask the following: Might the indeed genuine features of oceanic processes and not increased sea level on the right-hand side of the flow merely errors or unexplained noise in the data. simply be a result of the increase in transport rather than The pulses of increased transport are similar to simple a cause? To explore this question, we computed the background noise, so we investigated the spectrum of cross-spectrum between sea level at Settlement Point sea level at Settlement Point. Figure 5 shows the spec- and the cable transport (not shown). There is a small but trum computed from two data segments, each two years consistent phase shift between the two, with sea level long (to avoid gaps in the data). The individual raw leading transport by roughly 108 at periods of ;30 days. spectra were combined and smoothed slightly. This phase shift is equivalent to roughly one day of lead One feature of the spectrum should be emphasized. and is in the correct sense to assure us that the increase The power in the pulses we have been describing, having in offshore sea level is a causative mechanism and is not periods of order 20–30 days, is ubiquitous. A more de- a result of the increased transport. Although we can tailed examination shows that there is a great deal of assume that this delay represents the response time of variability in the amplitude of these but no obvious sea- the flow within the straits, because this study is based on sonal dependence. To explore these incoming waves, we

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FIG. 4. (a) The transport anomaly of the Florida Current from cable data (black lines) for each of the Loop Current shedding events, relative to the time of shedding. The blue curve shows the mean over all separations. Note that the cable transport values are not normalized here. (b) Sea level height at Settlement Point (black lines) for each of the Loop Current shedding events, relative to the time of shedding. The blue curve shows the mean over all separations. Note that the sea level values are not normalized here. The mean value for this record is 3064 mm. made animations of the (high passed) SSHA data from cross the ocean; by contrast, these 20–30-day waves the AVISO open ocean product. These waves come in arise abruptly in the last 400 km or so from the coast and directly from the east and do not propagate across the have wavelengths of the order of 200–250 km. We as- ocean as typical low-frequency Rossby waves. The typi- sume that they are locally wind driven, but we have not cal Rossby waves usually increase in amplitude as they explored this further.

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FIG. 5. A variance-preserving spectrum of sea level at Settlement Point using two segments of 2 yr each. The 12.0- and 6.0-month terms were removed before computing the spectrum.

2) ONE SPECULATIVE MECHANISM sea levels, which is (strictly) a measure of the surface velocity, follows the trend of the two on either side, with There is an old idea that sea level in the Gulf must the difference rising ;4cm. be slightly higher than in the Caribbean because (i) In terms of the sea surface velocity, the ;4-cm in- the northward inflow at Yucatan has to be reversed and crease is only a ;5% increase. A possibly more signifi- (ii) the increased static head in the Gulf provides the cant factor, however, is the pressure head between the necessary momentum to overcome the turbulent fric- Gulf and open Atlantic. The downstream slope is on the tional losses on the inshore side of the Florida Cur- order of 5 cm between the Keys and Cape Hatteras. As rent. We explored this idea by examining the mean sea this pressure head in the Gulf increases, it has three level over the entire Gulf and over the entire Caribbean; possible consequences: first, it could increase the trans- Fig. 6 shows the mean values, plotted relative to the port out of the Gulf in the Florida Current; second, it time of each separation. Although the resolution here could decrease the transport into the Gulf from the is in 10-day increments, we see that sea level peaks Caribbean; and third, it could increase the deep trans- approximately 20 days before the ring separations, in port out of the Gulf back to the Caribbean. Because we the mean. do not have adequate measurements of either the upper A remarkable process shows up in Figs. 2 and 6; sea inflow or the deep outflow, much of our conclusions level rises on both sides of the outflowing current—but about the process here have to be speculative. not by the same amount—before a ring separates. Sea In a ground-breaking result from their ex- level rises roughly 4 cm at the coast (Key West) and periment in Yucatan, Bunge et al. (2002) showed that, as ;8 cm on the offshore side (Settlement Point) during the Loop Current extended into the Gulf displacing large the 2-month interval shown here. The peaks in sea level volumes of water in the upper layers, a compensating at Settlement Point occur ;10 days before the sep- outflow was observed in the deep layers in Yucatan. What aration; the difference signal looks remarkably similar we see here is that the compensation is not always perfect to sea level at Settlement Point near the two peaks and that before a ring separation there also could be some prior to the separation. The difference between the two increase in the Florida Straits outflow.

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FIG. 6. Mean sea level (from SSH) over the Gulf of Mexico, the Caribbean Sea, and their difference, relative to the time of separation of all rings in this discussion. This dataset has only 10-day resolution. The mean values (mm) for the Gulf and Caribbean are shown.

One final part of the scenario comes into play. As the Nevertheless, the result of these observations is robust Loop Current extends farther and farther into the Gulf and thus seems worth presenting. The maximum in cor- before a ring has detached, the result is that a substantial relation is found in a broad region between 10 and 15 days. fraction of the transport that will go out through the The delay time, as shown in Fig. 7, is significantly Florida Straits has a longer pathway within the Gulf. The correlated (0.7 6 0.002) with the length of the Loop detachment process can be thought of merely as the point Current at the time of the separation. The slope of the at which none of the Florida Straits outflow goes around line in Fig. 7 is 0.0084 days km21, or 1.38 6 0.34 m s21. the nascent Loop Current ring. The length of the Loop Current here, from the statistics Although there is a great deal of scatter in the in- we have used previously, is actually the total length dividual cases that form Fig. 6, the results from each around the entire Loop Current; the relevant length for separation are similar to the mean in 13 of the 20 events propagation of the signal is roughly half the value used in our data. Thus, the idea of large-scale sea level pres- for these points, because the trigger signal does not sure forcing remains a plausible result. propagate all the way around the circumference of the Loop Current but propagates only to the point of sep- 3) A TESTABLE, IF SPECULATIVE, HYPOTHESIS aration of the ring. These values lead to an effective To provide a specific mechanism for the ring separa- speed of approximately 2.8 6 0.7 m s21, a reasonable tion triggers, we first review the details of a possible value for the speed of the Loop Current. This value can mechanism. A pulse of increased transport is observed. be most easily understood as a time that the trigger pulse This change in transport must be propagated upstream. is carried along with the flow of the Loop Current from The path this pulse follows and the precise dynamics of Yucatan to the position of nascent ring separation. A the transmission are not understood and remain to be travel time of only ;14 days for the initial pulse to go explored. The pulse follows an unspecified path until it from the Florida Straights to Yucatan, if it were to reaches the region where the ring is about to separate. To propagate as a baroclinic around the pe- test this possibility, we have compared the observed de- rimeter, seems too short by at least a factor of 2. Note, lays with the length of the Loop Current, as shown in Fig. 7. however, that in the insightful work of Bunge et al. The scatter in this comparison is reduced dramatically (2002) they found an unexplained delay of ‘‘approxi- if we examine only delays longer than 13 days. That is, mately a week’’ between the intrusion cycle of the Loop the first 12–14 days of the delay appear to be an initi- Current and the deep outflow associated with it. ation phase, which we do not attempt to explain here; The intercept of the line in Fig. 7 is, to within one this feature requires further understanding and analysis. standard error, the 13-day cutoff we have chosen. Thus,

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FIG. 7. Correlation plot of delay time (y axis) as a function of the length of the Loop Current (x axis). The line shows the least squares fit, with a correlation coefficient of 20.7. the results in Fig. 7 give increased confidence that the accompanied by increased sea level on the offshore side mechanism is consistent with the observed data. of the stream, appear to be an operative mechanism. Al- though what we are describing here as signal is frequently obscured by noise, this may well be its most important 4. Discussion feature. For this analysis, the time when the rings separate is It may be worth mentioning that there is a contribu- known a priori. The analysis cannot proceed without this tion of sea level variability near 14 days associated with knowledge. the fortnightly tide. However, repeating the calculation The idea that pulses of transport could be the trigger for Fig. 5 with the fortnightly tide removed does not for ring separations has been discussed for many years. change the power in the 20–30-day band. The fact that, Pulses of increased transport in the , for on average, the trigger pulses occur about 14 days before example, have been shown to be associated with the a separation suggests that the additional energy con- separations of rings from the Agulhas Current (e.g., van tributed by the fortnightly tide may possibly be enough Leeuwen et al. 2000, Lutjeharms et al. 2001), although to change what would otherwise be merely noise into an many of the details there are different. This effect has effective trigger mechanism; that is, although this is in- been observed for nearly two decades. deed speculative, when the essentially random ;25-day It is noteworthy that Pichevin and Nof’s (1997, their signals coincide with the fortnightly tide, the pulses be- Fig. 8) results show a similar effect. In their analytical come large enough to be a trigger. Whether this is an model, there is a clear correlation between the pinching important aspect of the process we cannot say. off of rings and the transport of the flow. It often (but not always) appears that there is a drop Similarly, in Leben’s (2005) analysis, his Fig. 6 shows in transport about the time the ring separates. Although a striking correlation between the ring separation times there is no doubt that this is an accurate observation, the and the length of the Loop Current. Perhaps this cor- point of our study is to find a mechanism or event that relation can be understood as a result of the greater time precedes the separation event and that could possibly be that has elapsed before the separation event occurs, a causative mechanism. The fact that the amplitudes of giving the Loop Current more time to grow. the transport pulses agree, via geostrophy, with the Our basic motivation here is to find a mechanism that amplitudes of the sea level difference signal is a mean- could serve as a trigger for ring shedding but may not ingful finding, and it suggests that these pulses are real be apparent or computed well enough in an otherwise oceanic signals. So, these pulses of transport might in- quite capable global circulation model. What emerges deed appear merely to be noise in the data. And yet if here is that pulses of transport in the Florida Current, the Loop Current is in a position where a ring-shedding

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FIG. A1. (a) Data for the September 1993 Loop Current ring separation. The ‘‘Sett Pt’’ curve (red) is sea level anomaly at Settlement Point, Bahamas. All data are normalized by their own std dev. The vertical arrow shows the position of the pulse of transport prior to ring separation as entered into Table 1 of the text. For this separation, there is little or no agreement between the transport pulse and the Settlement Point sea level. (b) The August 1994 separation. (c) Data for the March 1996 ring separation. The sea level difference curve (green) is for Settlement Point minus Key West. (d) Data for the March 1998 ring separation. event is poised to happen—but the separation has not all the time. The eastern Gulf of Mexico is relatively yet occurred—we suggest that these pulses provide the shielded from such incoming wave radiation; therefore, mechanism that causes the instability to go forward and the Loop Current rings appear to require the sort of the rings to separate. extra stimulus we have described. One of the puzzling differences between the events shown here and the shedding of Gulf Stream rings is that Acknowledgments. During the course of this work, Gulf Stream rings detach almost immediately after they we have benefited from discussions with our colleagues have formed. Because the Gulf Stream, on the western A. Clarke, W. Dewar, D. Nof, and W. Schmitz. C. Meinen side of the ocean, is exposed to a nearly constant barrage was most helpful in arranging for our use of the Florida of eddy noise, this trigger mechanism may be present Current cable data; these data are available on their

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FIG. A2. (a) Data for the October 1999 ring separation. The cable was down for this separation, so the delay times shown in Table 1 were determined from the two pulses indicated at the Settlement Point tide gauge. (b) Data for the March 2002 Ring separation. No data are available at Settlement Point. The flat spot at the beginning of the plot is when the cable was out. (c) Data for the August 2004 Ring separation. No data are available at Settlement Point. The flat spot at the end of this plot is for a time when the cable was out. (d) Data for the February 2006 Ring separation. No data are available at Settlement Point.

Web site (available online at http://www.aoml.noaa.gov/ Topography Project Scientist is also greatly appreciated. phod/floridacurrent/) and funded by the NOAA Office W. Sturges is grateful for support by the MMS as well as of Climate Observations. R. Leben acknowledges the by NSF OCE-0326233 and OCE-0925404. support of Minerals Management Service, Gulf of Mexico OCS Region Contract M08PC20043 to Science Applica- tions International Corp. and NASA Ocean Surface To- APPENDIX pography Mission Science Team Grant NNX08AR60G. Altimetry was obtained from ESA, NOAA, and the Additional Plots Showing All Available Data for Physical Oceanography Distributed Active Archive Cen- More Separation Events ter (PO.DAAC) at the NASA Jet Propulsion Laboratory, Pasadena, California (available online at http://podaac. Figures A1 and A2 show the observations for eight jpl.nasa.gov), and support as PO.DAAC Ocean Surface additional separation events.

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