Two-layer gap-leaping oceanic boundary currents: experimental investigation

Abstract

The problem of oceanic gap-traversing boundary currents, such as the Kuroshio current crossing the Luzon Strait or the Gulf Stream traversing the mouth of the Gulf of Mexico, is considered. Systems such as these are known to admit two dominant states: leaping across the gap or penetrating into the gap forming a loop current. Which state the system will assume and when transitions between states will occur are open problems. Sheremet (J. Phys. Oceanogr., vol. 31, 2001, pp. 1247–1259) proposed, based on idealized barotropic numerical results, that variation in the current’s inertia is responsible for these transitions and that the system admits multiple states. Generalized versions of these results have been confirmed by barotropic rotating-table experiments (Sheremet & Kuehl, J. Phys. Oceanogr., vol. 37, 2007, 1488–1495; Kuehl & Sheremet,J. Mar. Res., vol. 67, 2009, pp. 25–42). However, the typical structure of oceanic boundary currents, such as the Gulf Stream or Kuroshio, consists of an upper-layer intensified flow riding atop a weakly circulating lower layer. To more accurately address this oceanic situation, the present work extends the above findings by considering two-layer rotating table experiments. The flow is driven by pumping water through sponges and vertical seals, creating a Sverdrup interior circulation in the upper layer which impinges on a ridge where a boundary current is formed. The $\beta $ effect is incorporated in both layers by a sloping rigid lid as well as a sloping bottom and the flow is visualized with the particle image velocimetry method. The experimental set-up is found to produce boundary currents consistent with theory. The existence of multiple states and hysteresis, characterized by a cusp topology of solutions, is found to be robust to stratification and various properties of the two-layer system are explored.

Dynamics of Mesoscale Eddies Interacting With a Western Boundary Current Flowing by a Gap

“…The nonlinear solutions of a WBC flowing by a wide meridional gap are given by Sheremet (2001). He showed bifurcation and hysteresis of the WBC path in the vicinity of the gap, results later confirmed by laboratory rotating table experiments (Kuehl & Sheremet, 2009;Kuehl & Sheremet, 2014;Sheremet & Kuehl, 2007). At critical states, the WBC is shown to shift regimes under the perturbations of eddies (Yuan & Li, 2008;Yuan & Wang, 2011) and wind forcing (Wang et al, 2010;Zhong et al, 2013), the dynamics of which are closely related to the multiple equilibra and hysteresis of the WBC.…”

On the Loop Current Penetration into the Gulf of Mexico

“…Sturges and Lugo‐Fernández () review such Loop Current and associated eddy behavior studies prior to 2005, and a compendium of recent studies is introduced by Lugo‐Fernández and Hamilton (). The control mechanisms for both the intrusion of the Loop Current into the Gulf of Mexico and the eddy shedding, while the subject of numerous theoretical investigations (e.g., Hurlburt & Thompson, ; Kuehl & Sheremet, ; Nof, ; Reid, ; Sheremet, ) remain to be more fully explored.…”

Brief communication: A nonlinear self-similar solution to barotropic flow over varying topography

“…The problem formulation considered in this work follows that of Sansón and van Heijst (2002), Kuehl (2014) and Kuehl and Sheremet (2014). A rotating, single fluid layer is considered which flows along a sloping bottom topography (i.e., along slope barotropic flow).…”