Blue Maps: a new ocean analysis

July 7th, 2021

Blue Maps is a new system, developed by the Bluelink Team, that produces gridded estimates of ocean temperature, salinity, and sea-level by exploiting the data assimilation system used for BRAN and OceanMAPS, to deliver maps with the accuracy of an observation-based product.

The data consists of near-global weekly maps of ocean properties at 0.1 ° resolution. It combines all Argo data and satellite ocean observations using Ensemble Optimal Interpolation (EnOI) implemented with an ensemble that represents multiple time- and space-scales.

The Blue Maps system is described by Oke et al 2021. Blue Maps is a relatively lightweight system: it is able to produce analyses for several years in just a couple of days. This means that the developers could explore the sensitivity of the system to many different factors. By contrast, the Bluelink ReANalysis is so expensive that only a few different configurations can sensibly be tested. The experience gained during the development of Blue Maps has cast a clear light on the trade-offs between choices of ensemble, localisation radii, and ensemble size when creating a data assimilation scheme. The Blue Maps paper described a series of trial experiments that were conducted to optimise these choices, and provides a frank commentary of some of the pitfalls of ocean data assimilation. The final Blue Maps system uses the EnOI method to project observations onto a range of different spatio-temporal scales (mesoscale, intraseasonal, seasonal, and interannual). This diversity of anomalies in the ensemble is shown to be an important factor in the accuracy of Blue Maps. The ensemble size used is larger than most EnOI applications and uses longer length-scales than most.

The development of Blue Maps uncovered many issues with our ocean data assimilation system. We’re now really well positioned to deliver better, more accurate forecasts and reanalyses in the future. – Peter Oke, CSIRO

The results are comparable to other observation based products. Blue Maps produces analyses of sea-level with accuracy of about 4 cm; and analyses of upper-ocean (deep) temperature and salinity with accuracy of about 0.45 (0.15) degrees and 0.1 (0.015) practical salinity units, respectively. The advantage of Blue Maps is its gridded nature at an eddy-resolving resolution, which opens it to a large range of possible applications.

Blue Maps data is publicly available. For details, see the Data Access page.