The formation of a cold-core eddy in the East Australian Current

Macdonald, Helen; Roughan, Moninya; Baird, Mark E.; Wilkin, John

doi:10.1016/j.csr.2016.01.002

Cited by 26 publications

(38 citation statements)

References 41 publications

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“…Another possible explanation is that two distinct zooplankton communities characterize CE and ACE, with higher grazing pressure in CE environments. Such behavior is consistent with the water in CEs from this region tending to have more coastal organisms than ACEs (Macdonald et al, 2016). However, the interpretation of the difference in zooplankton behavior requires some caution because it may be related to eddy dynamics not directly considered in our simulations (i.e., sub-mesoscale interactions).…”

Section: Zooplanktonsupporting

confidence: 60%

Key Drivers of Seasonal Plankton Dynamics in Cyclonic and Anticyclonic Eddies off East Australia

et al. 2016

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Mesoscale eddies in the south west Pacific region are prominent ocean features that represent distinctive environments for phytoplankton. Here, we examine the seasonal plankton dynamics associated with averaged cyclonic and anticyclonic eddies (CE and ACE, respectively) off eastern Australia. We do this through building seasonal climatologies of mixed layer depth (MLD) and surface chlorophyll-a for both CE and ACE by combining remotely sensed sea surface height (TOPEX/Poseidon, Envisat, Jason-1, and OSTM/Jason-2), remotely sensed ocean color (GlobColour) and in situ profiles of temperature, salinity and pressure from Argo floats. Using the CE and ACE seasonal climatologies, we assimilate the surface chlorophyll-a data into both a single (WOMBAT), and multi-phytoplankton class (EMS) biogeochemical model to investigate the level of complexity required to simulate the phytoplankton chlorophyll-a. For the two eddy types, the data assimilation showed both biogeochemical models only needed one set of parameters to represent phytoplankton but needed different parameters for zooplankton. To assess the simulated phytoplankton behavior we compared EMS model simulations with a ship-based experiment that involved incubating a winter phytoplankton community sampled from below the mixed layer under ambient and two higher light intensities with and without nutrient enrichment. By the end of the 5-day field experiment, large diatom abundance was four times greater in all treatments compared to the initial community, with a corresponding decline in pico-cyanobacteria. The experimental results were consistent with the simulated behavior in CE and ACE, where the seasonal deepening of the mixed layer during winter produced a rapid increase in large phytoplankton. Our model simulations suggest that CE off East Australia are not only characterized by a higher chlorophyll-a concentration compared to ACE, but also by a higher concentration of large phytoplankton (i.e., diatoms) due to the shallower CE mixed layer. The model simulations also suggest the zooplankton community is different in the two eddy types and this behavior needs further investigation.

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Section: Zooplanktonsupporting

confidence: 60%

Key Drivers of Seasonal Plankton Dynamics in Cyclonic and Anticyclonic Eddies off East Australia

et al. 2016

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“…Eddy formation and shedding mechanisms have been openly debated; however, there is a consensus that mesoscale eddies in WBCs form initially from meanders in the flow, becoming increasingly unstable through barotropic and baroclinic instabilities that propagate in WBCs [e.g., Bowen et al ., ; Stammer , ; Mata et al ., ]. Eddies form and shed due to the transfer of mean energy from the jet to eddy kinetic energy [e.g., Rubio et al ., ; Macdonald et al ., ]. In baroclinic flows, mean potential energy is transferred due to density‐driven differences while mean kinetic energy from horizontal shear is transferred in barotropic flows.…”

Section: Introductionmentioning

confidence: 99%

“…[] and Hetland [] where submesoscale flows are characterized by high Rossby number or low Richardson number. Frontal eddies can grow through a combination of wind forcing, where a sudden reversal in the wind direction from poleward to equatorward provides surface momentum along the western edge of the billow (A. Schaeffer et al, submitted manuscript) and a subsequent transfer of energy from the jet to the billow [ Macdonald et al ., ]. In an idealized numerical modeling study, Macdonald et al .…”

Section: Introductionmentioning

confidence: 99%

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A tale of two eddies: The biophysical characteristics of two contrasting cyclonic eddies in the East Australian Current System

et al. 2017

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Mesoscale cyclonic eddies are known to be highly productive. Less well‐known are the dynamics and productivity of smaller cyclonic eddies, known as frontal eddies, that form on the landward side of western boundary currents. In this study, we investigate the physical and biogeochemical properties of two contrasting cyclonic eddies in the East Australian Current (EAC). The first (“Murphy”), a mesoscale cyclonic eddy that formed at ∼28°S with a diameter of ∼160 km and high surface chlorophyll‐a concentrations, which lived ∼47 days. The second (“Freddy”), a smaller frontal eddy (∼35 km diameter) that formed from a shelf water billow ∼7 days prior to sampling at ∼31.5°S and was advected off the shelf along the EAC front (from ∼200 m to 4000 m of water). Both eddies were at least 1000 m deep with a similar steric height anomaly. We introduce and employ “the method of closest approach” using shipboard ADCP velocities to estimate the eddy centers, which reveals significant tilting through the water column. We estimate rotation rates of 4–10 days and 1–9 days and Rossby numbers 0.25–0.1 and 0.6–0.1, from the surface to 600 m for Murphy and Freddy, respectively. High‐resolution altimetry measurements from the SARAL/AltiKA satellite provide estimates of the ageostrophic component of rotation. Our results show that the frontal eddy is significantly more ageostrophic, energetic, and productive than the mesoscale cyclone, despite its small size and short life (∼4 weeks). We suggest that frontal eddies have potential to contribute significantly to the net productivity of the Tasman Sea region.

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“…This data assimilative model represents a significant improvement on previous modelling work in the EAC for these purposes: e.g. Roughan et al (2003), which was based on climatology; Macdonald et al (2013Macdonald et al ( , 2016, which focused on process studies of warm core and cold core eddies; and Zavala-Garay et al (2012), which used 4D-Var with a much coarser resolution. The reanalysis development and evaluation is presented as follows.…”

Section: Introductionmentioning

confidence: 99%

Development and evaluation of a high-resolution reanalysis of the East Australian Current region using the Regional Ocean Modelling System (ROMS 3.4) and Incremental Strong-Constraint 4-Dimensional Variational (IS4D-Var) data assimilation

et al. 2016

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Abstract. As with other Western Boundary Currents globally, the East Australian Current (EAC) is highly variable making it a challenge to model and predict. For the EAC region, we combine a high-resolution state-of-the-art numerical ocean model with a variety of traditional and newly available observations using an advanced variational data assimilation scheme. The numerical model is configured using the Regional Ocean Modelling System (ROMS 3.4) and takes boundary forcing from the BlueLink ReANalysis (BRAN3). For the data assimilation, we use an Incremental StrongConstraint 4-Dimensional Variational (IS4D-Var) scheme, which uses the model dynamics to perturb the initial conditions, atmospheric forcing, and boundary conditions, such that the modelled ocean state better fits and is in balance with the observations. This paper describes the data assimilative model configuration that achieves a significant reduction of the difference between the modelled solution and the observations to give a dynamically consistent "best estimate" of the ocean state over a 2-year period. The reanalysis is shown to represent both assimilated and non-assimilated observations well. It achieves mean spatially averaged root mean squared (rms) residuals with the observations of 7.6 cm for sea surface height (SSH) and 0.4 • C for sea surface temperature (SST) over the assimilation period. The time-mean rms residual for subsurface temperature measured by Argo floats is a maximum of 0.9 • C between water depths of 100 and 300 m and smaller throughout the rest of the water column. Velocities at several offshore and continental shelf moorings are well represented in the reanalysis with complex correlations between 0.8 and 1 for all observations in the upper 500 m. Surface radial velocities from a high-frequency radar array are assimilated and the reanalysis provides surface velocity estimates with complex correlations with observed velocities of 0.8-1 across the radar footprint. A comparison with independent (non-assimilated) shipboard conductivity temperature depth (CTD) cast observations shows a marked improvement in the representation of the subsurface ocean in the reanalysis, with the rms residual in potential density reduced to about half of the residual with the free-running model in the upper eddy-influenced part of the water column. This shows that information is successfully propagated from observed variables to unobserved regions as the assimilation system uses the model dynamics to adjust the model state estimate. This is the first study to generate a reanalysis of the region at such a high resolution, making use of an unprecedented observational data set and using an assimilation method that uses the time-evolving model physics to adjust the model in a dynamically consistent way. As such, the reanalysis potentially represents a marked improvement in our ability to capture important circulation dynamics in the EAC. The reanalysis is being used to study EAC dynamics, observation impact in state-estimation, and as forcing for a vari...

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The formation of a cold-core eddy in the East Australian Current

Cited by 26 publications

References 41 publications

Key Drivers of Seasonal Plankton Dynamics in Cyclonic and Anticyclonic Eddies off East Australia

Key Drivers of Seasonal Plankton Dynamics in Cyclonic and Anticyclonic Eddies off East Australia

A tale of two eddies: The biophysical characteristics of two contrasting cyclonic eddies in the East Australian Current System

Development and evaluation of a high-resolution reanalysis of the East Australian Current region using the Regional Ocean Modelling System (ROMS 3.4) and Incremental Strong-Constraint 4-Dimensional Variational (IS4D-Var) data assimilation

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