On Entrainment and Diffusion in a Gulf of Mexico Anticyclonic Ring

During October‐November 1986 the baroclinic circulation of the central and western Gulf of Mexico was dominated by an anticyclonic ring that was being bisected by two north and south flanking cyclonic rings. The baroclinic circulation revealed a well‐defined cyclonic‐anticyclonic‐cyclonic triad system. The anticyclone's collision against the western gulf continental slope at 22.5°N, 97°W originated the north and south flanking cyclonic rings. The weakening of the anticyclone's relative vorticity, during the collision, was compensated by along‐shelf north (26 cm s−1) and south (58 cm s−1) jet currents and by the anticyclone's flanking water mass's gain of cyclonic vorticity from lateral shear contributed by east (56 cm s−1) and west (42 cm s−1) current jets with individual mass transports of ∼18 Sv. Within the 0–1000 and 0–500 dbar layers and across 96°W the magnitudes of the colliding westward transports were 17.80 and 8.59 Sv, respectively. These corresponding transports were 85 and 94% balanced by along‐shelf jet currents north and south of the anticyclone's collision zone. This indicates that only minor amounts (<15%) of the anticyclone's colliding westward transports might have flowed into the western gulf's continental shelf water mass or else they sank into deeper water along the continental slope during the anticyclone's collision event. The resultant effect of the coupled interaction between the anticyclone and the cyclonic pair was the surging of the water mass in the cyclones and its sinking in the anticyclone. This mechanism controlled the magnitude, direction, location of vertical advection, and transfer of kinetic energy from the upper to the deeper water layers. Our vertical transport estimates through the 1000‐m‐depth surface revealed a net vertical descending transport of 0.4 Sv for the ring triad system. This mass flux occurred primordially within the south central gulf region and most likely constituted a principal mechanism that propelled the gulf's deep horizontal circulation. The volume renewal time is ∼5 years for the ring triad system within 0–1000 dbar. The volume renewal time for the gulf's deep water layer (2000–3000 dbar), estimated as a function of its horizontal outflowing mass flux (1.96 Sv), is of the same order of magnitude and reveals that the deeper layer of the Gulf of Mexico is as well ventilated as its upper layer (0–1000 dbar). The ring triad's surface kinematic properties were derived from the sea surface baroclinic circulation field referenced to 500 dbar. Within this layer, individual ring geometries were conserved. Maximum tangential ring velocities were 60 and 58 cm s−1, for the north and south cyclones respectively, and 30 cm s−1 for the anticyclone. The corresponding periods of revolution were 16, 19, and 26 days, and vertical velocities calculated at the rings' peripheries, where maximum horizontal divergence was encountered, were 1.5, 1.0, and −1.0 m d−1.

Section: Water Mass Residence Timesupporting

confidence: 88%

Baroclinic flows, transports, and kinematic properties in a cyclonic‐anticyclonic‐cyclonic ring triad in the Gulf of Mexico

Vidal¹,

Vidal²,

Hernández³

et al. 1994

“…We assume that warm-core rings are axisymmetric [Schmitz and Vastano, 1976], hence changes in variables around a ring are negligib!e compared to changes with distance from ring center. Except during episodes of ring-Gulf Stream interaction [Joyce et al, 1983], this is a reasonable approximation.…”

Section: Formulation Of Physical Dynamicsmentioning

confidence: 99%

“…The flow field generated by this stream function is shown in Figure 1. The strongest upward water motion is at ring center where the ring relaxation is most evident [e.g., Schmitz and Vastano, 1976;Flierl and Mied, 1985]. The greatest horizontal velocities are at the ring edge near the surface where water escapes the ring along isopycnals.…”

Section: Formulation Of Physical Dynamicsmentioning

confidence: 99%

Prediction of phytoplankton growth in response to the frictional decay of a warm‐core ring

Franks¹,

Wroblewski²,

Flierl³

1986

A modelling study was conducted to examine the question, Is the high phytoplankton biomass which often develops in warm‐core rings of the Gulf Stream a consequence of the circulation associated with the frictional decay of the ring? A time‐dependent, two‐dimensional (r, z, t) model of plankton dynamics in a hypothetical ring similar in features to warm‐core ring 82B generates a lens of high phytoplankton biomass at ring center. Phytoplankton grow on nutrients advected into the euphotic zone as the depressed warm water in the ring's core rebounds and spreads out at the surface. This vertical motion induced as the ring's rate of rotation slows may be an important process maintaining the high production in warm‐core rings.

“…Based on other hydrographic evidence and other rings, they later modified this picture to include inflow both at the surface and below the thermocline, with an outflow at intermediate depths [Schmitz and Vastano, 1977;Vastano and Hagan, 1977]. The opposite pattern of flows was obtained in the case of a warm-core Gulf of Mexico eddy [Schmitz and Vastano, 1976]. However, there was no attempt to balance the angular momentum changes, so that these diagnostic calculations are not complete pictures of the meridional flows and may not be consistent with the velocity data.…”

Section: Introductionmentioning

confidence: 99%

Frictionally induced circulations and spin down of a warm‐core ring

Flierl¹,

Mied²

1985

Meridional motion driven by kinematic viscosity v and thermal diffusivity k within a warm‐core ring is examined. An analytic quasi‐geostrophic model is constructed that shows that the vertical mixing of heat or momentum is ineffective in driving secondary motion. Single‐celled meridional flows will occur within the ring whenever the Prandtl number Pr = vh/κh is not equal to 1. When Pr is greater than 1, the flow is up in the ring core, while Pr less than 1 causes downward motion at ring center. A numerical primitive equation model leads to qualitatively identical results but shows that the secondary flow is not long‐lived when Pr is less than 1, decaying within 2 months, while in the Pr greater than 1 case the flows are still vigorous at the end of the experiment (90 days).