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It is expected that the assimilation of data in an ocean model disrupts the model’s dynamical balance. This disruption occurs because the analysis fields are not a precise solution to the equations of the model. Therefore, the model adjusts to the applied increments. With this limitation in mind, we assess the impact of sequential data assimilation (DA) on vertical velocity - chosen because of its sensitivity to model dynamical balances. We investigate vertical velocity within mesoscale eddies in the Tasman Sea and off Tasmania, Australia in an ocean reanalysis that assimilates observations every four days. In a free-running model, with no DA, ocean eddies often show a coherent pattern of alternating upward and downward cells. These cells are induced by eddy distortion (i.e., the eddy becoming more or less isotropic). In this study, we aim to first determine if a data-assimilating model can reproduce these alternating upward and downward cells; and then to investigate how the cells are affected by sequential DA. We view this study as a step towards better understanding the suitability of data-assimilating ocean models for detailed analysis of ocean circulation and ocean dynamics. We show that the vertical velocities are typically two times larger in magnitude on the day after assimilation, compared to subsequent days. Despite the model adjustment, the alternating upward and downward cells are present in the data-assimilating model. In fact, we find that the model adjustment acts like an artificial eddy distortion on the first day after assimilation - when the model is adjusting to the applied increments - amplifying the vertical velocities within the eddies. This artificial distortion impacts temperature and salinity, through vertical advection. The vertical advection terms are about 1.5 times higher on the day after assimilation, compared to other days. The temperature and salinity changes resulting from the stronger vertical advection are mostly 2–10 times smaller than the increments of these properties applied during assimilation. Our findings suggest that a data-assimilating model may be useful for investigating the detailed dynamics of the ocean, but care should be taken when interpreting results. Moreover, we suggest that it is prudent to disregard the model’s fields within the first day after assimilation - particularly in analyses of dynamical balances.

The traditional view of the circulation in the Tasman Sea includes a coherent, quasi-zonal, eastward flow towards the northern tip of New Zealand that is widely referred to as the Tasman Front. This flow was first suggested in the 1960s by oceanographers reasoning that the source of volume transport “feeding” the East Auckland Current, around northern New Zealand and extending down the east coast, must be off eastern Australia. The first reported in situ measurements of the Tasman Front included hand-drawn temperature sections that highlighted features that are not supported by observations. Here, we objectively map the original data, showing that the published reports of strong, sub-surface, north-south temperature gradients were unjustified. In the absence of additional observations, we can’t be sure of the context of those original measurements - so we have objectively “searched” fields in a recent 25-year ocean reanalysis to identify scenarios that are consistent with the observations. We suggest that the most likely interpretation of the original data, is that they measured an eddy field - or a series of discontinuous streams across the central Tasman Sea. We also analyse data from surface drifting buoys to show that water traversing the Tasman Sea may take several different paths. We conclude that a continuous, zonal, eastward flow across the Tasman Sea is less common than widely-accepted conceptual models imply. Instead, we suggest that the eastward flow between Australia and northern New Zealand is perhaps better described as an “eastern extension of the EAC” - since this doesn’t imply the presence of a front, and doesn’t preclude a broad flow. For clarity, we also suggest that the southern branch of the EAC, known as the EAC extension, should be referred to as the “southern extension of the EAC”.

The traditional view of the East Australian Current (EAC), as depicted in many schematics, is of a continuous boundary current that flows along the shelf off eastern Australia, between approximately 18°S and about 32.5°S, where it separates from the coast and continues either towards New Zealand, along the Tasman Front; or towards Tasmania, as the EAC Extension. Additionally, it is widely recognised that eddies are prevalent in the EAC region – particularly south of the EAC separation. We revisit this long-standing paradigm and suggest that the EAC is perhaps better viewed as a continuous, meandering stream, flowing adjacent to the coast that “feeds” a field of mesoscale eddies. Observations show that EAC eddies are prevalent over a broad region of the western Tasman Sea, as far north as 25°S, typically with maximum intensity between 30 and 35°S. At any instant in time the EAC jet is usually evident as a poleward stream adjacent to the continental shelf edge. Other commonly accepted components of the EAC System, including the Tasman Front and EAC Extension, are rarely seen as distinct, identifiable features. Rather, these features are evident only in time-mean fields, when the eddy-variability is filtered out. It is also common for the EAC to be spatially discontinuous – due to the presence of eddies – often with multiple short streams that sometimes separate and re-attach to the coast. Recognition of the EAC as an eddy-dominated current system has seen many recent studies focus on various aspects of eddies in the EAC System, providing new insights into mesoscale ocean dynamics. Recent studies of individual eddies have shown that the circulation within eddies, including tilting and vertical motion, is more complex than previously understood. A summary of these studies, along with a review of the EAC System, particularly its path, separation, and eddy field is presented here.