Coastal Trapped Wave Propagation along the Southwest African Shelf as Revealed by Moored Observations

Abstract

Coastal trapped waves (CTWs) that propagate poleward along the southwest African shelf potentially leak energy from lower latitudes into the Benguela Upwelling System (BUS). Thus, in addition to local winds, these waves provide an important remote forcing mechanism for the upwelling region. The present study aims at elucidating the nature of CTWs in the northern BUS. To this end, we make use of multisite velocity observations from the Namibian shelf (18°, 20°, 23°S) and examine the alongshore velocity signal for signatures of CTWs by means of wavelet methods. We found that a substantial amount of energy is concentrated within a submonthly to subseasonal frequency band (10–50 days). Based on the coherence and phase spectra of the alongshelf currents, we provide evidence for a predominantly southward phase propagation and establish typical time and length scales of CTWs in the region. It turns out that their properties differ significantly within a few hundred kilometers along the coast. A comparison of the results with theoretical dispersion curves shows that this difference may be explained by variations in the bottom topography. Finally, we investigate the coupling of the alongshore currents with the coastal and equatorial wind stress and highlight regions of potential wave generation.

Slope and Shelf Flow Anomalies Off Oregon Influenced by the El Niño Remote Oceanic Mechanism in 2014–2016

“…In limited‐area theoretical or realistic numerical models, CTW at a particular location can be forced by any perturbations “upstream” of that location (upstream in the sense of the wave propagation), including the along‐shore wind stress or boundary conditions. Given that existing observing systems do not have enough spatial and temporal resolution to describe the structure and evolution of CTW and that the CTW‐related signal is hard to separate from other observed modes of variability, for example, local wind‐driven response or surface and subsurface eddy variability over the shelf and slope (Pelland et al., 2013), the evidence of the CTW has been presented mostly using statistical methods, including time‐lag correlation or coherence analyses of available coastal tide gauge or shelf velocity profile data (Allen & Denbo, 1984; Battisti & Hickey, 1984; Halliwell & Allen, 1987; Junker et al., 2019). These observation‐based analyses are sometimes combined with the analyses of regional model outputs (Kurapov, Erofeeva, et al., 2017; Springer et al., 2009).…”

Reply on RC2

“…The sea surface elevation followed the S-SW wind fields and at 00h on 14/09/2016 the consequent impact of the storm surge was observed in the coastal waters of Balneário Rincão. The gradual advance of this elevation to the northeast with a coherent period suggests the formation of a coastal-trapped wave, as noted by Houghton & Beer (1976), Brink (1991, Battisti & Hickey (1984), Yao et al (1984), Kitade & Matsuyama (2000) and Junker et al (2019). on the local hydrodynamics, especially on shallow waters driven by winds that are associated with the frontal system.…”

An investigation of sea level and circulation response during a coastal trapped wave event on the Eastern Agulhas Bank, South Africa

“…The omission of local wind forcing in the no-wind model demonstrates that remote forcing can be a significant contributor to local circulation, particularly during strong wind events. This is predominantly governed by the passing CTW, and similar processes have been investigated and described both locally (e.g, Jury et al (1990), Schumann and Brink (1990)) and elsewhere (e.g., South-eastern Australian Coast (Church et al 1986), Gulf of Mexico (Rivas 2017), South-west African coast (Junker et al 2019), Fukushima coast (Kubota et al 1981), South China Sea northern coast (Ding et al 2012), West coast of India (Amol et al 2012(Amol et al , 2018, Northern portion of J o u r n a l P r e -p r o o f the California Current System (Engida et al 2016)). Apart from the Port Alfred (B4) station, full depth alongshore current reversals were recorded at all other SAnL3 model stations within two model time step (6 hours) of the change in wind direction (Figure 9), with the ADCP's showing a strong full depth current reversal with a time lag of less than 4 hours to the wind switch (Figure 8).…”