Cyclonic Eddies Northeast of the Campeche Bank from Altimetry Data

Transcription

1 623 Cyclonic Eddies Northeast of the Campeche Bank from Altimetry Data JORGE ZAVALA-HIDALGO, STEVEN L. MOREY, AND JAMES J. O BRIEN Center for Ocean Atmospheric Prediction Studies, The Florida State University, Tallahassee, Florida (Manuscript received 6 March 2001, in final form 25 September 2002) ABSTRACT Eight cyclonic eddies were identified near the western edge of the Loop Current, at the northeast shelf break of the Campeche Bank, through TOPEX/Poseidon sea surface height anomaly data from January 1993 through March The eddies migration and their lifecycle are described. The formation of the eddies appears to be related to the dynamics of the Loop Current because the timing of their generation corresponds to the last stage of the anticyclone shedding from the Loop Current. The cyclones intensify while moving slightly to the northwest along the shelf break of the Campeche Bank; later, some cyclones are observed to move northward toward the Mississippi shelf break. The cyclones remain next to the Campeche Bank, south of 26 N, from 1.3 to 9.6 months; later, some of them move northward and strengthen by merging with other eddies, extending their life span. The eddies vertical structure is analyzed with hydrographic data, and the connection between the formation of the cyclones and the evolution of the Loop Current is further described using sea surface temperature images. 1. Introduction The Loop Current (LC) and mesoscale features associated with it dominate the circulation of the eastern Gulf of Mexico (GM). The LC enters the Gulf through the Yucatan Strait, turns anticyclonically to the east, and leaves the GM through the Southern Florida Straits. This path is not stationary; the LC evolves as it penetrates toward the northwest in the Gulf, with a mean incursion up to 27.5 N and a variation of more than 100 km to the north and south from its mean location (Vukovich 1988a). After some time, when the LC has penetrated northward, an anticyclonic eddy separates from it (Fig. 1). The distribution of intervals between the separation of eddies has primary peaks at 6 and 11 months with a mean of 9.5 months (Sturges and Leben 2000). The anticyclonic eddies pinched off by the LC have a deep signature of around 1000 m and diameters between 200 and 400 km (Mooers and Maul 1998). Cyclonic features associated with the LC have been identified (Cochrane 1972; Vukovich and Maul 1985; Vukovich 1988b; Lee et al. 1995; Fratantoni et al. 1998). In a study of the LC conditions between May and September 1969, Cochrane (1972) described the presence of two cold meanders that developed before the separation of the anticyclone: one on the western side, be- Corresponding author address: Dr. Jorge Zavala-Hidalgo, COAPS/ The Florida State University, Tallahassee, FL zavala@coaps.fsu.edu tween the LC and the Campeche Bank (CB), and the other on the eastern side, close to the Dry Tortugas near the West Florida Shelf. He observed that these meanders shift eastward and westward, constricting the LC, and join separating the anticyclone. It has been observed that the cyclonic features near the Dry Tortugas come from the northern edge of the LC, moving along the LC boundary off the West Florida Shelf, until they reach the Dry Tortugas where they remain and develop for a period up to four months (Vukovich and Maul 1985; Vukovich 1988b; Fratantoni 1998; Fratantoni et al. 1998). The eddies are termed Loop Current frontal eddies (LCFEs) while they are on the northern edge of the LC, and Tortugas eddies when they have reached the Dry Tortugas (Lee et al. 1995). Using a two-layer numerical model with topography, Hurlburt (1986) obtained cyclones south-southwest of the center of the LC and observed that they prevail only prior to an eddy shedding event. Some cyclones move around the LC and others toward the north, where they stagnate and dissipate. Hurlburt identified another region of cyclonic eddy generation near the eastern side of the LC. Here, eddies are generated at any time of the LC cycle with an irregular period of around 75 days. He also showed that both types of cyclones are generated by local baroclinic instabilities. Cyclonic eddies have been observed in the central and western Gulf as well (Padilla et al. 1990; Hamilton 1992; Vidal et al. 1994). Hamilton (1992) observed cold cyclones with diameters of km and surface swirl velocities of cm s 1 in the vicinity of the 2003 American Meteorological Society

2 624 JOURNAL OF PHYSICAL OCEANOGRAPHY VOLUME 33 FIG. 1. Representative positions of the Loop Current edge: in an early stage (thin line), before an eddy shedding (dashed line), and after an eddy shedding event (thick line). Also shown are the TOPEX/Poseidon tracks over the Gulf of Mexico. The location of track-a from 21 to 29 N is highlighted with a thick gray line. Louisiana continental slope. He found that the cyclones are long-lived, 6 months or more, have little or no effect on the surface temperature, have a strong signature at depths of 200 to 800 m, and move northwestward. The cyclones were observed in both the central deep basin and over the deepest part of the northern slope, but the location of generation has not been established in many cases. Previous studies have identified the generation of cyclonic eddies in the western and northwestern Gulf as a result of the interaction of the anticyclones with the continental platform (Lewis et al. 1989; Vidal et al. 1994), but there is not a clear link with those observed by Hamilton (1992) due to their location and movement. In this paper we suggest that some of the eddies observed by Hamilton could have originated between the western edge of the LC and the CB. Eddies are often identified by means of sea surface temperature (SST) images because an eddy often has a different temperature than its surroundings. However, in the Gulf of Mexico the SST from May to September is very similar inside and outside the eddies, making difficult to distinguish them using this tool (Vukovich and Maul 1985; Vukovich 1988b; Lee et al. 1995; Fratantoni et al. 1998; Sturges and Leben 2000). In addition, it has been reported that the cyclonic eddies observed in the central and western Gulf do not have a clear surface temperature signature (Hamilton 1992). Also, Vukovich and Maul (1985) reported that the western cold feature near the CB observed by Cochrane (1972) was not detected in Advanced Very High Resolution Radiometer (AVHRR) images. Satellite altimetry is an alternative tool to identify eddies because they are associated with sea surface height (SSH) anomalies that can be observed in all seasons. The Colorado Center for Astrodynamics Research continuously monitors SSH in the Gulf of Mexico. These data, combined with hydrography and model data, show the intense mesoscale variability (available online at www-ccar.colorado.edu/ realtime/gom/gom.html). In this paper we study the cyclonic eddies on the southwestern side of the LC, northeast of the CB, using sea surface height anomaly data. It should be considered as a preliminary work intended to describe the formation, development, and drift of the CB eddies. In this study we do not deal deeply with the process of eddy generation nor with their dynamic properties.

3 625 FIG. 2. Sea surface height anomaly in meters along TOPEX/Poseidon track-a for the period Jan 1993 Mar The track position is indicated in Fig. 1. The eight cyclonic eddies identified are indicated by the arrows. 2. The data TOPEX/Poseidon (TP) sea surface height data were extracted from the geophysical data records (AVISO 1992) and analyzed for the period January 1993 through March Corrections described in AVISO (1998) were applied to the data. Also, the TP along track data, x i, were passed through a filter in order to remove data spikes of x i 150 cm and values x i such that both x i 1 x i 10 cm and x i x i 1 10 cm. Last, a multiyear mean was removed and the remaining anomaly was analyzed. In addition, for the purpose of visualization of the LC evolution and its relationship to the formation of cyclonic eddies, the corresponding SSH anomaly of a gridded mean surface dynamic height relative to 1000 m was added to a similarly gridded TP sea surface height anomaly data. The mean dynamic height from historical hydrographic data was obtained from the Naval Research Laboratory (Fox et al. 2002). Other sources of data have been used to complement the SSH data for this study. A series of AVHRR satellite images for the period November 1997 March 1998 were processed to enhance the SST features during the formation of a CB eddy. Additionally, the vertical structure of the cyclones of the CB was analyzed through hydrographic data from Seaward Explorer cruise SE8729 for the period 5 8 November Observations of Campeche Bank cyclonic eddies a. Campeche Bank cyclonic eddies detected by SSH TOPEX/Poseidon data for the period January 1993 March 2000 were reviewed and eight negative SSH anomalies were identified northeast of the CB. The SSH anomaly along track-a (Fig. 1) shows the formation and evolution of the CB cyclones and LC anticyclones (Fig. 2). The eddies are chronologically identified by CE and the corresponding number (Table 1). A striking example of a CB cyclonic eddy is CE7, first identified in SSH data on 7 February It had the largest SSH anomaly and longest life of all the CEs in the record. All the eddies were generated at the northeast shelf break of the CB, near 23.5 N, 86.5 W, within one degree in the north south direction along the slope. The cyclonic eddies were generated at the time the LC anticyclones were separating, as can be observed in the along-track data and contour maps of TP SSH anomaly added to the associated altimetry of the mean dynamic height relative to 1000 m (not shown). Alternatively, generating the contour maps adding the TP SSH anomaly to a 7-yr mean sea level from a numerical simulation using the Navy Coastal Ocean Model (Martin 2000; Morey et al. 2002) does not make any significant difference. The lifecycle and movement of the CB eddies were TABLE 1. Periods in which the cyclonic eddies were identified through the TOPEX/Poseidon SSH data. Reference name Observed period Life span (months) CE1 CE2 CE3 CE4 CE5 CE6 CE7 CE8 * Day not available. 16 Aug Jan Jul Jan Mar 1 Jun Jan 5 Mar Sep May Jul 19 Dec Feb Apr Sep Jan Time spent south of 26 N (months) Shedding date (Sturges and Leben 2000) 19 Sep Sep Apr Apr 1996 Sep 1996* 11 Oct Mar Aug 1999

4 626 JOURNAL OF PHYSICAL OCEANOGRAPHY VOLUME 33 FIG. 3. Trajectories of eight Campeche Bank eddies observed through altimetry data. Dots indicate the eddies positions at 10-day intervals. The positions were determined by the subjective analysis of contour maps. estimated by a subjective analysis of 268 contour maps, as described above, generated every 10 days (Fig. 3 and Table 1). Some weeks after the pinch off of the anticyclones, the cyclones intensified moving slightly along the CB shelf break, between the LC and the recently shed anticyclone (Fig. 3). When the anticyclone moved to the west, frequently the cyclonic eddies moved to the north between the recently pinched off anticyclone and the LC. It was identified that some of the eddies merged with independent cyclonic features, located north of the CB eddies; remarkably, CE7 merged with other cyclones at least three times, one soon after the separation of the LC eddy with a cyclone from the eastern side of the LC, and two others in June 1998 and in October The eddies that moved to the north turned either to the northwest or northeast, those that did not move to the north became weak near the CB shelf break and cannot be clearly traced with TP SSH anomaly data. There is a considerable uncertainty in the estimated trajectories due to the TP track distance (see Fig. 1). The life span of the CB cyclones estimated through TP data was between to 14.9 months, although the eddies that lived longest were enhanced by merging with other cyclones. The period that the cyclones remain next to the Campeche Bank, south of 26 N, was also variable, from to 9.6 months (Table 1). b. The formation of a Campeche Bank eddy through AVHRR images Although the SST signature of the CB eddies is weak, during winter conditions their formation can sometimes be tracked through a careful review of AVHRR images. As a case study, the evolution of the LC during the formation of the CE7 is analyzed through SST satellite images (Fig. 4). In mid-january 1998, before the formation of CE7, the western edge of the LC was close to the 500-m isobath, between 22 and 24 N (Fig. 4a); then it began to separate from the shelf break, moving into deeper waters. The initial displacement was between 22 and 23 N (Figs. 4b d). During the next two weeks the displacement became larger, but more pronounced between 23 and 24 N, with a movement of around 50 km to the east. At that time, the cyclonic circulation had already developed and could be identified by a filament of warm water located close to 87.5 W (Fig. 4e). During the next two weeks the LC edge moved eastward and northward, near 24 N, 86 W, where it sharply bent to the west (Figs. 4f,g). In this period the altimetry data showed an intensification of the cyclone (see Fig. 2). Over the following days the bending process continued, penetrating toward the north and developing a filament of warm

5 627 FIG. 4. Loop Current frontal boundaries determined subjectively from AVHRR infrared images for the period 8 Jan 31 Mar Images are from 7 10-day composites. Temperatures range from 13 (dark blue) to 28 C (dark red). water that barely allowed the detection in the SST satellite images of the CB eddy (Fig. 4i). At that time, by the end of March, the LC eddy seems to have already separated from the LC by the penetration of cold water from the northeast; also, the CB cyclone had formed between the CB slope to the west, the LC to the east, and the anticyclone to the north (Fig. 4i). c. Hydrographic evidence of the Campeche Bank eddies Other sources of data provide supporting evidence to the formation of the CB eddies. Hydrographic data collected in November 1987 show the presence of a cyclonic feature in the same region as those detected in

6 628 JOURNAL OF PHYSICAL OCEANOGRAPHY VOLUME 33 similar to the strongest eddies studied by Hamilton (1992). Also, the surface temperature signature is weak or nonexistent, illustrating the difficulty of detecting these eddies in SST images as was reported by Vukovich and Maul (1985). It is noticeable that the anticyclone has a strong signature at the depth of the 20 to 25 C isotherms while the cyclone does not, but both have a strong middepth signature. Considering the upper 450 m and using the displacement of the 8 C surface as an indicator of the size of the eddy, an estimation of its diameter is around 80 km (note that the cruise track may not have passed over the center of the eddy, so it may actually be larger in diameter). FIG. 5. (a) Location of hydrographic stations from 5 to 8 Nov (b) Vertical distribution of temperature ( C) along the section represented in the upper panel. Isotherms are shifted by the influence of a Loop Current anticyclone and by a Campeche Bank eddy. TP data, and a large recently pinched off anticyclone northwest of the CB cyclone (Fig. 5). The vertical temperature profile across the eddy shows a doming of isotherms in the last five casts indicating the presence of the cyclonic eddy (Fig. 5b). The doming is more intense between 300 and 800 m, which is similar to the vertical structure of the eddies studied by Hamilton (1992). Using the 8 C surface depth as an indication of the eddies, it is found that the surface of the 1987 CB eddy rises from a mean depth of around 550 to 320 m, which is 4. Summary The analysis of the SSH anomaly from TP data shows the formation of eight cyclonic eddies generated northeast of the CB from January 1993 to March The CB cyclonic eddies are always generated in the same region. They are not periodic but have a timed relationship with the formation of major anticyclonic eddies from the LC. The dates on which these cyclones and the associated anticyclones were detected agree with the dates of anticyclone shedding reported by Sturges and Leben (2000) (Fig. 2 and Table 1). These peculiarities show that their generation is related to the geographic characteristics of the Yucatan Strait and the LC dynamics. Previous numerical experiments have shown that the eddies are generated by a local baroclinic instability (Hurlburt 1986). After remaining for several weeks near the generation region, most of the observed CEs moved northward following the westward shift of the LC anticyclones. North of the generation region they frequently merge with other cyclones, enhancing their circulation. In their last stage the cyclonic eddies migrate either to the west or the east, and dissipate. Those that did not move to the north decay near the generation region. The life cycle of the observed eddies varies from 3 to 15 months, although the longest-lived are generally reinforced by merging with other cyclones. The CB eddies are generated in a different region than that reported for the LCFEs. They form southsouthwest of the center of the LC, near 23.5 N, 86.5 W, where the LC bends strongly to the west prior to the anticyclone detachment (Figs. 2 and 3). The LCFE growth regions are north-northwest (north of 26 N) and east of the LC center (Vukovich 1988a; Fratantoni et al. 1998); nevertheless, some could be small perturbations while moving along the CB slope, growing north of 26 N. Another difference is that the frequency of generation of the LCFEs is about few weeks (Fratantoni et al. 1998), while we only observe eight CB eddies in a period of 88 months. A careful analysis of SST satellite images shows an eastward displacement of the LC western edge between 23 and 24 N during the formation of a CB eddy. The

7 629 SST signature of these eddies is small (at most 2 C) at the CB latitude in contrast to the greater thermal gradients farther north in the Gulf during winter (Figs. 2 and 7a in Vukovich and Maul 1985). Hydrographic data from November 1987 supports the existence of the CB eddies in agreement with the timing and location identified through the analysis of altimetry data and with observations reported by Cochrane (1972). Hydrography shows strong vertical displacements of the 5 18 C isotherms. The diameter and vertical structure of the CB eddies, as well as their movement, suggest that some of the cyclones observed in the central and northwestern Gulf by Hamilton (1992) could be generated near the CB, but this requires further investigation. Acknowledgments. Author J. Zavala-Hidalgo is funded by DMEFS, Mississippi State University, and the Secretary of Navy Grant from ONR. The Center for Ocean-Atmospheric Prediction Studies receives its base funding from the Applied Research Center, funded by NOAA Office of Global Programs, the NASA Physical Oceanography Program, and through ONR s Secretary of the Navy Grant to Dr. James J. O Brien. The authors thank Subrahmanyam Bulusu for his help with the preprocessing of the TOPEX/Poseidon data. Special thanks are given to Peter Hamilton who provided the hydrographic data. Mark A. Bourassa, Nobuo Suginohara, Rhonda Cooper, Luis Zamudio, and three anonymous reviewers made important suggestions for the improvement of this manuscript. Erik Marquez and Ranulfo Sobreya from the ICML-UNAM processed the AVHRR images. REFERENCES AVISO, 1992: Merged TOPEX/POSEIDON products. CD-ROM User Manual, ed. 2.1, AVISO, AVI-NT CN., 1998: AVISO user handbook. Corrected Sea Surface Heights, ed. 3.1, AVISO, AVI-NT CN. Cochrane, J. D., 1972: Separation of an anticyclone and subsequent developments in the Loop Current (1969). Contributions on the Physical Oceanography of the Gulf of Mexico, L. R. A. Capurro and J. L. Reid, Eds., Vol. II, Gulf Publishing Co., Fox, D. N., W. J. Teague, C. N. Barron, M. R. Carnes, and C. M. Lee, 2002: The Modular Ocean Data Assimilation System (MO- DAS). J. Atmos. Oceanic Technol., 19, Fratantoni, P. S., 1998: The formation and evolution of Tortugas eddies in the southern Straits of Florida and the Gulf of Mexico. Ph. D. thesis, University of Miami, 181 pp., T. N. Lee, G. Podesta, and F. Muller-Krager, 1998: The influence of the Loop Current perturbations on the formation and evolution of Tortugas eddies in the southern Straits of Florida. J. Geophys. Res., 103, Hamilton, P., 1992: Lower continental slope cyclonic eddies in the central Gulf of Mexico. J. Geophys. Res., 97, Hurlburt, H. E., 1986: Dynamic transfer of simulated altimeter data into subsurface information by a numerical ocean model. J. Geophys. Res., 91, Lee, T. N., K. Leaman, E. Williams, T. Berger, and L. Atkinson, 1995: Florida Current meanders and gyre formation in the southern Straits of Florida. J. Geophys. Res., 100, Lewis, J. K., A. D. Kirwan Jr., and G. Z. Forristal, 1989: Evolution of a warm-core ring in the Gulf of Mexico, Lagrangian observations. J. Geophys. Res., 94, Martin, P., 2000: A description of the Navy Coastal Ocean Model version 1.0. NRL Rep. NRL/FR/ , Naval Research Laboratory, Stennis Space Center, 39 pp. Mooers, C. N. K., and G. Maul, 1998: Intra-Americas sea circulation. The Sea, A. Robinson and K. H. Brink, Eds., The Global Coastal Ocean, Regional Studies and Syntheses, Vol. 11, Wiley and Sons, Morey, S. L., J. J. O Brien, W. W. Schroeder, and J. Zavala-Hidalgo, 2002: Seasonal variability of the export of river discharged freshwater in the northern Gulf of Mexico. Proc. Oceans 2002 MTS/ IEEE Conf., Biloxi, MS, MTS/IEEE, Padilla, A. R., D. A. Salas, and M. A. Monreal, 1990: Evidence of a cyclonic eddy in the Bay of Campeche. Ciencias Marinas, 16 (N3), Sturges, W., and R. Leben, 2000: Frequency of ring separations from the Loop Current in the Gulf of Mexico: A revised estimate. J. Phys. Oceanogr., 30, Vidal, V. M. V., F. V. Vidal, A. F. Hernández, A. Meza, and J. M. Pérez-Molero, 1994: Baroclinic flows, transports, and kinematics properties in a cyclonic-anticyclonic-cyclonic ring triad in the Gulf of Mexico. J. Geophys. Res., 99, Vukovich, F. M., 1988a: Loop Current boundary variations. J. Geophys. Res., 93, , 1988b: On the formation of elongated cold perturbation off the Dry Tortugas. J. Phys. Oceanogr., 18, , and G. A. Maul, 1985: Cyclonic eddies in the eastern Gulf of Mexico. J. Phys. Oceanogr., 15,