Influence of intraseasonal eastern boundary circulation variability on hydrography and biogeochemistry off Peru

The intraseasonal evolution of physical and biogeochemical properties during a coastal trapped wave event off central Peru is analysed using data from an extensive shipboard observational programme conducted between April and June 2017, and remote sensing data. The poleward velocities in the Peru–Chile Undercurrent were highly variable and strongly intensified to above 0.5 m s−1 between the middle and end of May. This intensification was likely caused by a first-baroclinic-mode downwelling coastal trapped wave, excited by a westerly wind anomaly at the Equator and originating at about 95∘ W. Local winds along the South American coast did not impact the wave. Although there is general agreement between the observed cross-shore-depth velocity structure of the coastal trapped wave and the velocity structure of first vertical mode solution of a linear wave model, there are differences in the details of the two flow distributions. The enhanced poleward flow increased water mass advection from the equatorial current system to the study site. The resulting shorter alongshore transit times between the Equator and the coast off central Peru led to a strong increase in nitrate concentrations, less anoxic water, likely less fixed nitrogen loss to N2 and a decrease of the nitrogen deficit compared to the situation before the poleward flow intensification. This study highlights the role of changes in the alongshore advection due to coastal trapped waves for the nutrient budget and the cumulative strength of N cycling in the Peruvian oxygen minimum zone. Enhanced availability of nitrate may impact a range of pelagic and benthic elemental cycles, as it represents a major electron acceptor for organic carbon degradation during denitrification and is involved in sulfide oxidation in sediments.

Why do Benguela Niños lead Atlantic Niños?

<p>We investigate the lag between warm interannual Sea Surface Temperature (SST) events in the eastern equatorial Atlantic, the Atlantic Niños, and the occurrence of Benguela Niños along the southwestern Angolan coast. It is commonly agreed that both events are associated with equatorial and subsequent coastal-trapped wave propagations driven remotely by a relaxation of the trade-winds. Yet, we observe that coastal SST anomalies off Angola tend to precede the ones in the equatorial cold tongue region by ~1 month.</p><p>We explain this counter-intuitive behavior using experimentation with a tropical Atlantic Ocean model. Using idealized wind-stress perturbations from a composite analysis, we simulate warm equatorial and coastal events over a stationary and then, seasonally-varying ocean mean-state. Results show that when wind-stress perturbations are confined to the western central equatorial Atlantic, the model yields equatorial events leading the coastal variability, consistent with the propagation path of the waves. This implies that neither the differences in the ocean stratification between the two regions (thermocline depths or modal wave contributions) nor its seasonal variability controls the timing between events. Only if wind-stress anomalies are prescribed in the coastal fringe, the coastal warming precedes the eastern equatorial SST anomaly peak, emphasizing the role of the local forcing in the phenology of Benguela Niños.</p><p>Both warmings originate from a reduction in the strength of the South-Atlantic Anticyclone. Nevertheless, local processes initiate the coastal warming before the remotely-forced equatorial waves impact the eastern equatorial SST. Then, equatorward coastal wind anomalies, driven by a convergent anomalous circulation located on the warm Atlantic Niño, stop the remotely-forced coastal warming prematurely.</p><p>In conclusion, this study shows evidence that Atlantic and Benguela Niños are connected via an ocean teleconnection associated with equatorial and coastal wave propagations, but they are also tied by a large-scale atmospheric circulation and ocean-atmosphere interactions.</p>

Energy Fluxes in Coastal Trapped Waves

The calculation of energy flux in coastal trapped wave modes is reviewed in the context of tidal energy pathways near the coast. The significant barotropic pressures and currents associated with coastal trapped wave modes mean that large errors in estimating the wave flux are incurred if only the baroclinic component is considered. A specific example is given showing that baroclinic flux constitutes only 10% of the flux in a mode-1 wave for a reasonable choice of stratification and bathymetry. The interpretation of baroclinic energy flux and barotropic-to-baroclinic conversion at the coast is discussed: in contrast to the open ocean, estimates of baroclinic energy flux do not represent a wave energy flux; neither does conversion represent the scattering of energy from the tidal Kelvin wave to higher modes.

Application of Classical Coastal Trapped Wave Theory to High-Scattering Regions

Since the 1970s, analytical models of coastal trapped waves (CTWs) have been developed using a first-order wave equation in the long-wave limit. Formulations of this kind require assumptions of a straight coastline with similar shelf bathymetry. These assumptions prevent the models from capturing the scattering and backscattering behavior of propagating CTWs that encounter changing coastlines, bathymetry, or shelf width. CTW modes from two different analytical models, one of which includes friction and stratification, are compared with CTW observations of velocity and pressure from a study region near the Outer Banks off the North Carolina coast in the United States. The coastline in the study region is relatively straight locally but is bounded by an estuary to the north and shelf narrowing to the south, both of which induce scattering. The models suggest that the CTWs in this region are insensitive to changes in stratification, implying that observed seasonal differences in wave magnitude are due to seasonal wind forcing. Furthermore, friction is found to be important, particularly for mode-1 propagation, but higher-order modes are prevalent despite the importance of friction. There is very poor agreement between the observed and modeled free and forced CTWs because of scattering. This lack of agreement indicates that this is not a globally applicable theoretical formulation because many global coastlines violate the basic assumptions.

Influence of intraseasonal eastern boundary circulation variability on hydrography and biogeochemistry off Peru

The Peruvian Upwelling System is characterized by high primary productivity fuelled by the supply of nutrients in a highly dynamic boundary circulation. The intraseasonal evolution of the physical and biogeochemical properties is analysed based on shipboard observations and remote sensing conducted between April and June 2017 off central Peru. The poleward transport in the subsurface Peru Chile Undercurrent was highly variable and strongly intensified between mid and end of May. This intensification was likely caused by a first baroclinic mode downwelling coastal trapped wave excited at the equator at about 95° W that propagated poleward along the South American coast. The intensified poleward flow shortens the time of water mass advection from the equatorial current system to the study site. The impact of the anomalous advection is mostly noticed in the nitrogen cycle because during the shorter time needed for poleward advection less fixed nitrogen loss occurs within the waters. This causes a strong increase of nitrate concentrations and a decrease in the nitrogen deficit. These changes suggest that the advection caused by the coastal trapped wave supersedes the simultaneous effect of anomalous downwelling in terms of nutrient response.

Observations of Diurnal Coastal-Trapped Waves with a Thermocline-Intensified Velocity Field

Using 18 days of field observations, we investigate the diurnal (D1) frequency wave dynamics on the Tasmanian eastern continental shelf. At this latitude, the D1 frequency is subinertial and separable from the highly energetic near-inertial motion. We use a linear coastal-trapped wave (CTW) solution with the observed background current, stratification, and shelf bathymetry to determine the modal structure of the first three resonant CTWs. We associate the observed D1 velocity with a superimposed mode-zero and mode-one CTW, with mode one dominating mode zero. Both the observed and mode-one D1 velocity was intensified near the thermocline, with stronger velocities occurring when the thermocline stratification was stronger and/or the thermocline was deeper (up to the shelfbreak depth). The CTW modal structure and amplitude varied with the background stratification and alongshore current, with no spring–neap relationship evident for the observed 18 days. Within the surface and bottom Ekman layers on the shelf, the observed velocity phase changed in the cross-shelf and/or vertical directions, inconsistent with an alongshore propagating CTW. In the near-surface and near-bottom regions, the linear CTW solution also did not match the observed velocity, particularly within the bottom Ekman layer. Boundary layer processes were likely causing this observed inconsistency with linear CTW theory. As linear CTW solutions have an idealized representation of boundary dynamics, they should be cautiously applied on the shelf.

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

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.