Remotely Sensed Seasonal Shoreward Intrusion of the East Australian Current: Implications for Coastal Ocean Dynamics

For decades, the presence of a seasonal intrusion of the East Australian Current (EAC) has been disputed. In this study, with a Topographic Position Index (TPI)-based image processing technique, we use a 26-year satellite Sea Surface Temperature (SST) dataset to quantitatively map the EAC off northern New South Wales (NSW, Australia, 28–32°S and ~154°E). Our mapping products have enabled direct measurement (“distance” and “area”) of the EAC’s shoreward intrusion, and the results show that the EAC intrusion exhibits seasonal cycles, moving closer to the coast in austral summer than in winter. The maximum EAC-to-coast distance usually occurs during winter, ranging from 30 to 40 km. In contrast, the minimum distance usually occurs during summer, ranging from 15 to 25 km. Further spatial analyses indicate that the EAC undergoes a seasonal shift upstream of 29°40′S and seasonal widening downstream. This is the first time that the seasonality of the EAC intrusion has been confirmed by long-term remote-sensing observation. The findings provide new insights into seasonal upwelling and shelf circulation previously observed off the NSW coast.

Climate Variability of the Northern Argentinean Shelf Circulation: Impact on Engraulis Anchoita

A high-resolution ocean model and hydrographic observations are used to characterize the shelf circulation of the northern Argentinean shelf during the study period (1993–2008) and ultimately to explore possible linkages among atmospheric, oceanic, and biological climatic variability. Abundance of larvae and eggs of the local anchovy species, Engraulis anchoita, exhibit a spatial and temporal variability similar to those stocks found in other parts of the world and that we interpret in the context of the particularities of the local circulation and hydrography. Two (statistically) coupled modes of wind stress-surface velocity are described and interpreted in terms of historical and new information. A complex picture emerges in which the intensity of both a thermal shelf front, the alongshore flow, and larvae abundance would be connected and forced by local wind stresses. For all areas examined on the shelf, the larvae/egg abundance would not be very sensitive to short-lived climatic fluctuations (e.g., year-to-year) but they would be indeed to regime shifts. The shallow shelf area bounded by the 39°S and 41°S parallels would expose a clearer linkage between physical and biological variables than that north of 39°S. We attribute this fact to the particular physical conditions found in the southernmost area, which would favor an increased habitat quality for Engraulis anchoita.

Beta-plane arrested topographic wave as a linkage of open ocean forcing and mean shelf circulation

Pressure anomaly set by the open ocean affects the dynamic topography and associated circulation over the continental shelf, which is explored here on a linearized β-plane arrested topographic wave framework that considers the variation in Coriolis parameter with latitude. It was found that on a meridional shelf, a nondimensional parameter Peβ, termed as the β Péclet number, signifies the characteristics of open ocean-shelf interaction. Peβ ≡ Dβ/α is determined by the ratio of long-wave-limit planetary to topographic Rossby wave speeds, i.e., the β drift Dβ, and the linear Ekman number α. On the western boundary shelf, due to the westward planetary Rossby wave, open ocean pressure propagates shoreward as Peβ > 1, and shelf circulation peaks where Peβ drops to 1. At this location, the planetary β effect is balanced by the bottom friction. Peβ = 1 must occur either on the shelf or on the coastal wall when Peβ > 1 is observed at the shelf edge. Whereas, on the eastern boundary shelf, Peβ < 0, the pressure anomaly is removed from the shelf, and hence the inductive circulation decays rapidly from the shelf edge. This β effect is robust on gently sloping meridional shelves. For zonal shelves, the planetary β increases the effective bottom slope on the northern boundary shelf but decreases it on the southern one, in a sense of potential vorticity conservation. However, this effect could be less significant in reality, given the complex dynamics involved. The above mechanism can explain the dynamics driving the Taiwan Warm Current in the East China Sea and its bifurcation around 28°N.

Transport variability over the Hawkesbury Shelf (31.5–34.5°S) driven by the East Australian Current

The Hawkesbury Bioregion located off southeastern Australia (31.5–34.5oS) is a region of highly variable circulation. The region spans the typical separation point of the East Australian Current (EAC), the western boundary current that dominates the flow along the coast of SE Australia. It lies adjacent to a known ocean warming hotspot in the Tasman Sea, and is a region of high productivity. However, we have limited understanding of the circulation, temperature regimes and shelf transport in this region, and the drivers of variability. We configure a high resolution (750m) numerical model for the Hawkesbury Shelf region nested inside 2 data assimilating models of decreasing resolution, to obtain the best estimate of the shelf circulation and transport over a 2-yr period (2012–2013). Here we show that the transport is driven by the mesoscale EAC circulation that strengthens in summer and is related to the separation of the EAC jet from the coast. Transport estimates show strong offshore export is a maximum between 32-33oS. Median offshore transports range 2.5–8.4Sv seasonally and are a maximum during in summer driven by the separation of the EAC jet from the coast. The transport is more variable downstream of the EAC separation, driven by the EAC eddy field. Onshore transport occurs more frequently off Sydney 33.5–34.5oS; seasonal medians range -1.7 to 2.3Sv, with an onshore maximum in winter. The region is biologically productive, and it is a known white shark nursery area despite the dominance of the oligotrophic western boundary current. Hence an understanding of the drivers of circulation and cross-shelf exchange is important.

Tidal processes and their spatial and temporal variability in the mid-field Guadalquivir ROFI

&lt;p&gt;In the Region Of Freshwater Influence (ROFI), located between sea exposure and estuary, characteristic physical processes of both estuarine and shelf seas overlap and impact on shelf circulation, sediment transport and ecosystem (Simpson, 1997). Although freshwater discharge typically exhibits the highest variability (Horner-Devine et al., 2015), this work focus on the tidal variability within the ROFI, which is often overlooked. This work addresses the spatial and temporal variability of tidal elevations and currents at the mid-field Guadalquivir ROFI (SW Spain), which is semidiurnal in character. Observations from five current-meter profiles, which were moored pointing upwards from June 2008 to December 2009 are analyzed. These instruments were placed along an arc, from south to north, and closing the estuary mouth. &amp;#160;&lt;/p&gt;&lt;p&gt;&amp;#160;&lt;/p&gt;&lt;p&gt;The analysis of the observations indicates that tides in the Guadalquivir ROFI have a close-to standing wave behavior. This is induced by the reflection at the continental margin of the northward-propagating tidal Kelvin wave. Regarding the M2 and M4 tidal constituents, which are relevant for residual sediment transport, their relative phase difference shows that, although the inner estuary is flood-dominant, in the mid-field ROFI zone ebb currents are slightly stronger than flood currents. Fortnightly variations are observed in the ratio of the M4 and M2 tidal amplitudes. Remarkably, the minimumvalues occur during the transition periods from neap to spring tides, whereas the maximum values are observed during neap tides. These results might suggest that there is still some influence of the tidal jet in this region.&lt;/p&gt;&lt;p&gt;&amp;#160;&lt;/p&gt;&lt;p&gt;Regarding the vertical structure of tidal currents, the M2 inclination varies with depth, being maximum near the bottom at all moorings. Tidal currents inclination also varies with time in the southern part of the ROFI. Moreover, in the southern part of the ROFI, current ellipses in the upper layer of the water column rotate clockwise whereas near the bottom currents rotate anti-clockwise, as revealed by the sign of the eccentricity. However, the eccentricity of tidal ellipses is uniform throughout the water column in the northern part of the ROFI. This along-coast variability of the vertical structure of the tidal ellipses suggests that the buoyant outflow circulates preferentially southwards, most likely driven by the prevailing winds.&lt;/p&gt;