A Model Study of the Loop Current Eddy Shedding Event in July, 2011 in the Gulf of Mexico

A model study of the loop current eddy shedding event in July, 2011 in the Gulf of Mexico

Abstract

This paper studies an eddy shedding event of the Loop Current in Jul, 2011 using ampi-version of the Princeton Ocean Model. A real-time forecast was conducted starting from Jul/01, 2011, and predicted the eddy shedding on Jul/22/2011, consistent with the observed shedding time on Jul 22-29 from satellite. We studied the underlying dynamics of the shedding event by analyzing a free-model model run initialized on May/15. The LC was growing due to the Yucatan influx, and ultimately reached a large enough size (200 – 300 km) where β effects became important. In late June, the easterly wind stress in the Caribbean Sea decreased and consequently the Yucatan inflow dropped. The Rossby wave speed (- βRo2) exceeds the LC growth fed by Yucatan inflow, providing a favorable condition for the eddy shedding. The baroclinic instability north of the Campeche Bank accelerated the eddy shedding.

Introduction

The Loop Current (LC) is the dominant feature of the circulation in the eastern Gulf of Mexico (GOM) and the formation region of the Florida Current-Gulf Stream system. It is a component of the meridional overturning circulation, playing a key role in the global climate. The LC episodically sheds large warm-core eddies or rings (200-300km wide, 500-1000m deep) that generally propagate westward at 2~5 km day-1, with maximum swirl speed≈1.8-2 ms-1 (see Oey et al. 2005 for a review). The LC and eddies affect every aspect of oceanography in the GOM directly or indirectly through their smaller-scale subsidiaries.

The LC eddy shedding was observed (e.g. Vukovich 1988; Sturges 1994; Sturges and Leben 2000) and reproduced in several numerical models (e.g. Chang and Oey 2011; Chang and Oey 2012; Le Hénaffet al. 2012). The LC usually experiences the expansion, shedding, retraction cycle. Sometimes, after the LC eddy shedding, it may re-attach to the LC and detach from it several times until final separation. (Henaff et al. 2012).

The LC eddy shedding and the underlying dynamics have beenwell studied in the past decades. A primary eddy shedding dynamics was clearly addressed by Pichevin and Nof (1997) and Nof (2005). The loop current grows larger and deeper withmass influx from the Yucatan Channel. Up to a certain large size, the variations of the Coriolis parameter (f) are significant (β effects), and the westward Rossby wave velocity (- βRo2) exceeds the LC growth rate. At this point, the Loop eddy begins to detach.

The upper and lower layer(shallower and deeper than 1000m) mass coupling in the eastern Gulf may also contributeto eddy shedding (see Fig.5 of Chang and Oey, 2011, hereafter CO2011). They examined the mass conservation in the two layers bounded by 900W (west), Yucatan Channel (South), and the Straits of Florida (east) in the semi-enclosed eastern Gulf. They found that the upper layer becomes divergent while the lower layer becomes convergent during the incipient shedding phase. A strong upward motion induced between the two layers can contribute to the shedding.

Baroclinic instability is another factor which may contribute to the LC eddy shedding. Oey (2008) showed that north of Campeche Bank is a fertile place for baroclinic instability of the LC due to the flow interaction with the deep slope. The instability tends to generate a deep cyclone below 1000m, and the cyclone can induce an upwelling which may accelerate the upper layer LC shedding.

Recently, Chang and Oey (2012) indentify the seasonal preferences in summer and winter of the LC eddy shedding by analyzing long-term satellite observation data and numerical model results. The combined effect induced by the seasonal winds in the Caribbean Sea and the GOM, which are 180 out of phase, is considered as the fundamental mechanism for the seasonal shedding preference. In July, 2011, an eddy shedding was observed in late July. Our operational forecasting model initialized on Jul/01 surprisingly predicted the shedding nearly in the same time. We are interested in the case, and trying to obtain a complete picture of the shedding dynamics using the model results.

Section 2 describes the model and experiment setup, section 3 presents the main results, and section 4 summarizes the paper.

The model description and setup

Princeton Regional Ocean Forecast System (PROFS) is based on the paralledlized Princeton Ocean Model (mpiPOM/sbPOM). The terrain-following three-dimensional (3D) ocean model uses Mellor and Yamada’s (1982) turbulence closure scheme modified by Craig and Banner (1994) to include wave-enhanced turbulence near the surface. An enhanced mixing in bottom boundary layer due to wave-current interaction is considered. A fourth-order pressure gradient scheme (Berntsen and Oey, 2010) is used to guarantee small pressure-gradient errors. Smagorinsky’s (1963) shear and grid-dependent horizontal viscosity is used with eddy viscosity = 0.1 and diffusivity = 0.02. Sea surface temperature (SST) is relaxed daily to AVHRR MCSST (Advanced Very High Resolution Radiometer Multi-Channel Sea Surface Temperature, ).

The orthogonal curvilinear model domain includes the north-west Atlantic Ocean west of 550W and from 50N ~550N, with 3~5 km horizontal resolution and 25 sigma levels in the vertical. At 550W, the World Ocean Atlas data (“climatology”) from NODC ( is used for boundary condition along the eastern open boundary as well as initial condition. The topography was set up according to Etopo2 and NOS digitized map on shelves. The model was first integrated for 17 years from 1993-2009 as spinup, forced by six-houly cross-calibrated Multi-platform (CCMP) wind, monthly climatological NCEP surface fluxes, M2, S2,K1 and O1 tides, and 51 daily river discharge from the U.S. Geological Survey (34 rivers in the Gulf and 17 rivers in the eastern coasts). A hindcasting scheme is used, which projects satellite sea-surface height anomaly (SSHA) data from Archiving, Validation and Interpretation of Satellites Oceanographic data (AVISO) ( on a 1/30×1/30 Mercator grid to the model density field (Yin and Oey, 2007). This run is then continued for 2010 and 2011 by applying NCEP high resolution Global Forecast System winds (0.50 GFS) from March, 12th, 2010. Surface heat and evaporative fluxes are relaxed to monthly climatological values with a time scale of 100 days.

Two simulations were carried out, a14-week forecast run starting from July 1, 2011 anda free-model run starting from May 15, 2011. The reasons for doing these two simulations are discussed in the next section.

Results

The Jul/01 forecast predicted a LC eddy shedding occurred on Jul, 22nd, consistent with the observed eddy shedding period from Jul, 25th to Jul, 30thfrom the AVISO satellite sea surface height (SSH) data. Figure 1 shows the development of daily-averaged SSH (colors) and sea surface currents (black trajectories) from the forecast. The satellite SSH=0 (magenta) contour is also plotted to indicate the position of the LC. On Jul/01, the model position of the LC is in good agreement with theobservation. The LC was extended northwestward in the GOM, but the satellite LC was located slightly more northwestward. Ten days later (Jul/11), the forecast showed the LC extended further northwest, compared well with the observation. On Jul/21, the model developed a thin neck and the LC eddy was about to shed, though the observation didn’t show such a thin neck. At the end of July, the eddy had been separated from both model and observation.

To evaluate the forecast,the time series of the spatial correlation coefficient and root mean square errors between the model and AVISO SSH anomalies were calculated in the deep region of the GOM, north of 23oN and west of 84oW with depth deeper than 500m. The forecast correlation coeffcientsdecreased from 0.8 to 0.4, and RMS errors increased from 0.15 to 0.25over thefirst 8 weeks (black dotted line of the Fig. 1). Persistence was calculated too (grey line of the Fig.1). The model beat the persistence for both correlation coeffcient and RMS errors for the entire 8 weeks with the model mean values 0.63 and 0.21 whease the persistence only 0.28 and 0.26, respectively.

This forecastis a good case to study the underlying dynamics of the LC eddy shedding event. However this forecast initialized on July 1st, which is only about 3-4 weeks ahead of the shedding. The dynamics of eddy shedding may be hampered due to the data assimilation of SSH anomaly used to generate the initial conditions. So we conducted a free-model run starting from the May, 15th. A LC eddy shedding occurred on Jul, 8th, about 2 weeks earlier than the observed eddy shedding. We use the free-model run to study the LC eddy shedding dynamics.

Fig. 2 shows the daily-average SSH on June 10th from the free-model run initialized from May 15th, 2011. It is shown that the LC has been well extended into northwest of the GOM at this time. The shedding occurred a month later. Firstly, weexamine the primary dynamics (Nof, 2005) for the eddy shedding. The LC is fed by the inflow through the Yucatan Channel. When the loop grows to a big enough size200~300km, it is big and deep enough for β to have effects. So the westward Rossby wave velocity = Ci = - βRo2, where Ro = (g’h(t))1/2/f, can then “peel” a portion of the Loop’s mass – i.e. a warm eddy – westward. The velocity, Ci, was calculated at the (-89.40, 260), the star location of Fig. 2. From Fig. 3 (bottom), the Ci kept increasing from March to June, reached its maximum at the end of June, and then dropped quickly in early July, implying the eddy shedding occurred.

The zonal wind stress in the Caribbean Sea is strengthening to its minimum about -10-4 m2 s-2 from the mid May to the late June, and weakening to -2 ×10-5 m2 s-2 towards the mid August (Fig. 3, top). Meantime, the model estimated transport through Yucatan Channel shows an increase from the mid May to the late June, and decreases towards the August (Fig. 3, middle). The correlation coefficient of the zonal wind stress and the transport is about -0.6(exceeding the 95% significance). This implies the zonal Caribbean wind mainly drives the Yucatan transport. This is consistent with the findings by Chang and Oey (2012). They analyzed a long-term dataset of wind and the Yucatan transport, and found out that the Yucatan transport peaks corresponding well to the easterly peaks in the Caribbean, especially in summer.The decrease of the Yucatan transport at the end of June in combination with the maximum Ci provides a favorable condition for LC eddy to separate.

Now we examine the upper and lower layer coupling. As CO2011, we define a semi-enclosed volume in the eastern Gulf, bounded by 900W (west), Yucatan Channel (South), and the Straits of Florida (east). The transports through west, south, and east boundaries in the upper and lower (shallower and deep than 800m) layers were calculated (not shown). Note that the east boundary of the lower layer is closed due to the shallow sill of the Straits of Florida. The divergence/convergence of the upper and lower layer is calculated as transport out of the layer (Trout) subtracts transport into the layer (Trin). Therefore, the positive value indicates divergent, and the negative is convergent. The time series of 10-day running average of Trout-Trinof upper (solid) and lower (dash) layers are shown in Fig. 4. From the end of March to mid June (before Jun/18), the upper layer was mainly convergent and the lower layer was divergent, indicating the growth of the LC. This corresponds to the Loop reforming phase described in Fig. 5a of CO2011. The mean SSH in the eastern Gulf (grey line in Fig. 4) also shows the higher sea level before Jun/18, consistent with the convergent. After Jun/18, the sea level dropped, the lower layer is convergent, and upper layer is divergent, corresponding to the incipient shedding phase in Fig. 5b of CO2011. An upward motion induced by the two layer coupling contributed to the shedding.

Considering the eddy size = 250km and the Rossby wave vecolityCi=0.05 m s-1, the time required for the eddy to move over its own size is about 2 months. If Yucatan inflow and Rossby wave dynamics alone, the eddy shedding would occur in mid August instead of Jul/08. There must have other processes accelerating the eddy shedding.

Baroclinic instability was calculated over two 30-day periods, May/16 - Jun/15 and Jun/01 - Jun/30 at depth 750m (Fig. 5). During the first period, a dominant instability (positive; shown in red; right panel) BC was seen north of the Campeche Bank around 24.80N, 88.30W where the LC flows over the deeper regions of the Gulf. The dominant instability was getting larger and spreading northwestward in the second period. These results are consistent with the 10-yr ensemble average of baroclinic instability in Oey (2008). The corresponding growth rate is about 1/40 ~ 1/10per day. Therefore, the instability accelerated the eddy shedding for about 10-40 days after Jun/18. This is consistent with the model estimated shedding on Jul/08.

The cyclonic vorticity ζ on the western (~50km) portion of the Yucatan Channel divided by f gives a good predictor of the LC northern boundary (Chang and Oey 2012). A high correlation, R2 = 0.83, was obtained (see Fig.2c of Chang and Oey 2012). This relation is in good agreement with the Reid’s formula (Reid 1972). The larger the ζ/f is, the more northward the LC extends. The Loop retracts as ζ weakens. The 10-day average time series of the ζ/f and northern boundary of the LC represented by latitude was shown in Fig.6 for the free model run May/15. From the end of the June to mid July, the ζ/f (red) decreases from 0.475 to 0.41, and meantime the northern boundary of the LC retracts from 26.80 N to 25.50 N, coincide with the eddy shedding.

Summary

This paper studies an eddy shedding event of the Loop Current in Jul, 2011 using ampi-version of the Princeton Ocean Model. A real-time forecast was conducted starting from Jul/01, 2011. The model predicted the eddy shedding on Jul/22/2011, consistent with the shedding time during Jul 22-29 from the satellites. To study theunderlying dynamics of the eddy shedding, we conducted a free-model run starting from May/15 to eliminate the influence of data assimilation on the ocean state in late May and June for the Jul/01 case. The May/15 run created an eddy shedding on Jul/08. We found that the underlying dynamics of the eddy shedding are:

1)The primary dynamics is the βeffectas discussed by Nof (2005). The growing LC was fed by the Yucatan influx, and ultimately reached a large size where the Rossby wave velocity(- βRo2) was about -0.05 m s-1. Meantime, the easterly wind stress in the Caribbean Sea decreasedand consequentlythe Yucatan inflow dropped. The Rossby wave speed exceeds the LC growth fed by Yucatan inflow. This provided a favorable condition for the eddy shedding;

2)The baroclinic instability north of the Campeche Bank accelerated the eddy shedding;

3)The upwelling induced by the upper-lower layer coupling in the eastern Gulf contributed to the eddy shedding too.

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