MARCH 2013 C H A N G A N D O E Y 669
Loop Current Growth and Eddy Shedding Using Models and Observations: Numerical Process Experiments and Satellite Altimetry Data
YU-LIN CHANG National Taiwan Normal University, Taipei, Taiwan, and Princeton University, Princeton, New Jersey
L.-Y. OEY Princeton University, Princeton, New Jersey, and National Central University, Jhongli, Taiwan
(Manuscript received 1 August 2012, in final form 10 October 2012)
ABSTRACT
Recent studies on Loop Current’s variability in the Gulf of Mexico suggest that the system may behave with some regularity forced by the biannually varying trade winds. The process is analyzed here using a reduced-gravity model and satellite data. The model shows that a biannual signal is produced by vorticity and transport fluctuations in the Yucatan Channel because of the piling up and retreat of warm water in the northwestern Caribbean Sea forced by the biannually varying trade wind. The Loop grows and expands with increased northward velocity and cyclonic vorticity of the Yucatan Current, and eddies are shed when these are near minima. Satellite sea surface height (SSH) data from 1993 to 2010 are analyzed. These show, con- sistent with the reduced-gravity experiments and previous studies, a (statistically) significant asymmetric biannual variation of the growth and wane of Loop Current: strong from summer to fall and weaker from winter to spring; the asymmetry being due to the asymmetry that also exists in the long-term observed wind. The biannual signal is contained in the two leading EOF modes, which together explain 47% of the total variance, and which additionally describe the eddy shedding and westward propagation from summer to fall. The EOFs also show connectivity between Loop Current and Caribbean Sea’s variability by mass and vor- ticity fluxes through the Yucatan Channel.
1. Introduction of eddy shedding through model and observational analyses. The intrusion and retraction of the Loop Current The shedding of Loop Current eddies can be in- in the eastern Gulf of Mexico and eddy shedding (i.e., terpreted as being a result of competing imbalance separation of warm rings from the Loop) constitute between the volume influx (Q) through the Yucatan one of the most fascinating geophysical fluid dynam- Channel,whichgrowstheLoop,andwestwardRossby ical phenomena in the ocean (see Oey et al. 2005 for wave (velocity C ’2bR2, R 5 Rossby radius based a review). The resulting circulation is the source of i o o on the matured eddy), which tends to ‘‘peel’’ the eddy much of the variability in the Gulf of Mexico. Sea from the Loop; this will be referred to as the Pichevin– surface height (SSH) constructed from satellite al- Nof mechanism (Pichevin and Nof 1997; Nof 2005).1,2 timetry data shows that the Loop Current behaves in The Loop and eddies are approximated as being ac- a complex and seemingly chaotic fashion. An improved tive in the upper layer only (i.e., reduced-gravity). understandingofwhen an eddy is likely to shed from the Loop Current is of interest both scientifically and for practicalapplications.InthisworkandinXuetal. 1 Hurlburt and Thompson (1980) demonstrated the process nu- (2013), we attempt to contribute to the knowledge merically, while Pichevin and Nof solved it analytically. 2 The growth rate should be said more accurately to be a func- b 2 tion of Q and the eddy’s westward velocity a function of Ro. Corresponding author address: L.-Y. Oey, Princeton University, Formulae for zero-PV eddy (e.g., from Nof 2005) are growth rate 5 0 p 2 3 52 b 2 ; 70 Washington Road, Princeton, NJ 08540. 8g Q/3 f RE and westward velocity 2 Ro/3 , where RE(t) E-mail: [email protected] (Qt)1/4 is the (growing) eddy’s radius, which itself depends on Q.
DOI: 10.1175/JPO-D-12-0139.1
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FIG. 1. A schematic plot of (left) an extended Loop Current when the Yucatan inflow is strong following, say, a maximum westward wind in the Caribbean Sea; and (right) when the inflow weakens and westward Rossby wave dynamics (squiggly arrow represents Rossby wave) overcomes the inflow rate, and an eddy may be shed.