CAFE60v1: A 60-year large ensemble climate reanalysis. Part I: System design, model configuration and data assimilation.

Abstract

We detail the system design, model configuration and data assimilation evaluation for the CSIRO Climate retrospective Analysis and Forecast Ensemble system: version 1. CAFE60v1 has been designed with the intention of simultaneously generating both initial conditions for multi-year climate forecasts and a large ensemble retrospective analysis of the global climate system from 1960 to present. Strongly coupled data assimilation (SCDA) is implemented via an ensemble transform Kalman filter in order to constrain a general circulation climate model to observations. Satellite (altimetry, sea surface temperature, sea ice concentration) and in-situ ocean temperature and salinity profiles are directly assimilated each month, whereas atmospheric observations are sub-sampled from the JRA-55 atmospheric reanalysis. Strong coupling is implemented via explicit cross domain covariances between ocean, atmosphere, sea ice and ocean biogeochemistry. Atmospheric and surface ocean fields are available at daily resolution and monthly resolution for the land, subsurface ocean and sea ice. The system produces 96 climate trajectories (state estimates) over the most recent six decades as well as a complete data archive of initial conditions potentially enabling individual forecasts for all members each month over the 60 year period. The size of the ensemble and application of strongly coupled data assimilation lead to new insights for future reanalyses.

GO-SHIP Easy Ocean: Gridded ship-based hydrographic section of temperature, salinity, and dissolved oxygen

“…Here we show a comparison of the Pacific Ocean density structure along 170°W (GO-SHIP P15 section) observed in May and June 2016 from the GO-SHIP Easy Ocean gridded data product (white contours) overlaid on density from the CAFE60 reanalysis product 15 averaged over the same period (Fig. 4 ).…”

“…4 Pacific Ocean density ( ) along 170°W (P15 between 70°S-0°S, see Fig. 5 ) observed in May and June 2016 from the GO-SHIP Easy Ocean gridded data product (white contours) overlaid on density ( ) from the CAFE60 15 reanalysis product averaged over the same period. Colormap is from ref.…”

On Oceanic Initial State Errors in the Ensemble Data Assimilation for a Coupled General Circulation Model

“…Tang et al, 2020), in which DA is applied to each individual model component, and strongly coupled DA (SCDA; Han et al, 2013;F. Lu et al, 2015;O'Kane et al, 2021;Sandery et al, 2020;Shen et al, 2021;Sluka et al, 2016;Yoshida & Kalnay, 2018), in which DA is applied to the coupled climate model as a whole. In particular, when only one model component is assimilated in a coupled model, the WCDA is further classified as the quasi-weakly coupled DA (Penny & Hamill, 2017).…”

CAFE60v1: A 60-year large ensemble climate reanalysis. Part II: Evaluation

“…ENSO variability underwent a noticeable shift in behavior during the late 1970s (Fedorov and Philander 2000) after which La Niña events declined and El Niño events increased in frequency. This climate shift to warmer tropical conditions occurred around 1976-78 and was shown to be forced by subsurface changes in both temperature and salinity in the ocean south of the equator as part of an oscillation intrinsic to the equatorial and southern tropical Pacific (Giese et al 2002;O'Kane et al 2014).…”