Assessment of the ECCO2 reanalysis on the representation of Antarctic Bottom Water properties
Ocean Science Discussions
We analyzed the ability of the Estimating the Circulation and Climate of the Ocean – Phase II (ECCO2) reanalysis to represent the hydrographic properties and variability of the Antarctic Bottom Water (AABW) in the Southern Ocean. We used a twenty-year observational database to perform comparisons of hydrographic properties and reanalysis data for the same time period (1992–2011). In addition, we evaluated four <i>case studies</i> based on current meter data and the AABW volume transport
... e transport estimates previously reported in the literature. The main Southern Ocean oceanographic features, as well as the characteristic shape of the regional potential temperature–salinity (θ–<i>S</i>) diagrams, are adequately represented by the reanalysis. However, the opening of an oceanic polynya in the Weddell Sea Sector, which has been clearly visible since 2005, contributed to an unrealistic representation of the hydrographic properties of the Southern Ocean primarily after 2004. In this sense, our analyses focused on the period that was identified as more reliable (1992–2004). In general, the reanalysis data showed surface waters that were warmer, saltier, and denser than observations, which may have resulted from the absence of Ice Shelf Water and from the overestimation of sea ice concentrations that limit oceanic heat loss during austral winters. Intermediate waters were generally colder, fresher, and denser than observations, whereas deep waters were warmer and less dense. These differences in deep water properties were partially a result of the inability to reproduce the densest AABW variety by reanalysis for most of the analyzed period and also because of the model's relatively coarse vertical resolution. Despite differences in absolute values, the upper AABW limit (γ<sup><i>n</i></sup> ≥ 28.27 kg m<sup>−3</sup>) and AABW occupied area were well represented in the WOCE repeat sections SR2 and SR4 for the studied periods. In section WOCE SR3, however, the estimates from the differences were not as well correlated, and the AABW layer thickness was underrepresented. The <i>case studies</i> showed a good representation of the AABW volume export and current velocity variability in the most important region of dense water export (i.e., the Weddell Sea). The exception is the AABW volume transport near the Kerguelen Plateau, in which the rugged local bathymetry and the relatively coarse model resolution hampered a fair representation of the transport variability by the reanalysis. Despite the consistency in terms of variability, absolute volume transport, and velocity, estimates were underrepresented in all cases. Moreover, the reanalysis was capable of reproducing the general variability pattern and trends of the AABW hydrographic properties reported by previous studies. Therefore, the ECCO2 data from the 1992–2004 period was considered adequate for investigating the circulation of the AABW and variability of the hydrographic properties, whereas data from the latter period (2005–2011) must be given careful attention.