An Intercomparison of Global Reanalysis Products for Southern Africa’s Major Oceanographic Features

Abstract

A regional assessment of three global ocean reanalysis products is presented for southern Africa’s major oceanographic features. The reanalyses include Mercator Ocean’s Global Reanalysis (GLORYS), the Commonwealth Scientific and Industrial Research Organisation’s (CSIRO) Bluelink Reanalysis (BRAN) and the Fleet Numerical Meteorology and Oceanography Center’s (FNMOC) global Hybrid Coordinate Ocean Model (HYCOM) reanalysis. The aim is to provide modelers with sufficient information for selecting the appropriate product for use as boundary conditions to force their regional ocean models, as well as to provide marine industries, relevant government agencies and academics with insight into the optimal reanalysis product for their purposes. The reanalyses are compared to both assimilated and independent observational datasets spanning various regions within the southern African marine environment. While all reanalysis products reproduce the eastern and western boundary current systems surrounding southern Africa, limitations exist. BRAN outperforms the other reanalyses in its representation of the Mixed Layer Depth, contributing to its good representation of coastal SSTs in the Benguela upwelling system, whereas GLORYS and HYCOM’s misrepresented MLD result in significant warm biases in this region. The Angola-Benguela Frontal Zone and it’s variability is best reproduced by BRAN and HYCOM. The Agulhas Current system’s major components are well reproduced by both GLORYS and BRAN. HYCOM, however, simulates considerably more early retroflections than are observed which have resulted in its mean eastward location. While all the reanalyses overestimate the occurrence of Agulhas meanders, GLORYS and BRAN resolve the associated variability best. Agulhas Current transport is best resolved by GLORYS, unlike BRAN and HYCOM which largely overestimate the magnitude of its south-westward flow, linked to their misrepresentation of the Current’s vertical structure. The bay-scale and nearshore evaluations highlighted issues pertaining to the resolution of the reanalyses and their use at such a small scale. The reanalyses are limited by their resolution, as well as by their misrepresentation of submesoscale processes or lack thereof, prompting the need for the development of regional downscaled models in and around the southern African oceans based on the global ocean reanalysis products.

Robust estimates for the decadal evolution of Agulhas leakage from the 1960s to the 2010s

“…BRAN2020 is available at daily resolution from 1993 to present, but the vertical velocity field is only provided from 1998. The Agulhas Current system is realistically simulated by BRAN2020, but the representation of Agulhas leakage has not been analyzed in detail 67 . It is noteworthy that, in contrast to the non-assimilative ocean models, BRAN2020 features a mean leakage transport that falls within the lower range of the observational estimates (16.9 ± 1.2 Sv).…”

Bottom marine heatwaves along the continental shelves of North America

“…Using a 0.25v ersion of GLORYS, Chi et al 51 finds that this reanalysis stands out among other eddy-permitting products in its ability to reproduce the mean state and variability of the Gulf Stream along the Southeast US coastline. Additionally, in a comparison of global ocean reanalyses in the Benguela Current System, Russo et al 52 found that GLORYS was the most accurate tool among those compared. Similarly, de Souza et al 53 found that GLORYS had the smallest biases in water column structure in the nearshore environment of New Zealand.…”

Observations of Barrier Layer Seasonal Variation in the Banda Sea

“…It shall be noted that Argo float data we use for our analysis is also assimilated in the BRAN2020. A recently published study evaluating three global ocean reanalysis products demonstrates that BRAN2020 outperforms the Hybrid Coordinate Ocean Model and Mercator Ocean's Global Reanalysis in Southern Africa, especially for the MLD (Russo et al., 2022). Numerous researches in the Australasian region have been supported by BRAN2020 data, including analysis of the Fraser Gyre off southeast Queensland (Ismail et al., 2017), studies of intraseasonal variability of the Indonesian Throughflow (Schiller et al., 2010), and mixed layer heat and mass budget in the Banda Sea (Ismail et al., 2023).…”