Switching from CORE to JRA-55 forcing Adele Morrison, Kial Stewart, Andy Hogg, Paul Spence, Aidan Heerdegen

Why JRA-55? Second global reanalysis conducted by the Japan Meteorological Agency. Utilises four-dimensional variational analysis (4D-Var). Generally improved representations of phenomena on wide range of space-time scales. Endorsed by CLIVAR Ocean Model Development Panel, used by other groups (e.g. NCAR). 1958-2012 (55 years) + ongoing (2013-present). 0.5° resolution, 3-hourly. Frequency of spatial correlation of daily precip with obs over tropics (22°S-22°N) Harada et al. 2016

Why JRA-55? The second global reanalysis conducted by the Japan Meteorological Agency. Utilises four-dimensional variational analysis (4D-Var). Generally improved representations of phenomena on wide range of space-time scales. Endorsed by CLIVAR Ocean Model Development Panel, used by other groups (e.g. NCAR). 1958-2012 (55 years) + ongoing (2013-present). 0.5° resolution, 3-hourly RMSE 0.099 Surface air temperature trend (K/dec), 1958-2001, compared with obs (HadCRUT4) with 80% data availability. RMSE 0.129 RMSE 0.151 Harada et al. 2016

Ocean forcing dataset from JRA-55 Bias adjustment needed for ocean/sea-ice modelling (as done for CORE and DRAKKAR projects) - led by Hiroyuki Tsujino (JMA-MRI) as part of OMDP-JRA55 collaborative effort. Version 1.1 recently available including runoff; documentation will be complete by end of year. Intended for use in OMIP – version 2 of CMIP6.

Selection of Repeat Year Forcing (RYF) CORE used a Normal Year Forcing (NYF) produced by a spectral averaging technique, but had many drawbacks (spatially noisy, lacking weather variance etc). Proposal is to replace with RYF – repeated single year forcing. Years are cut from 1 May to 30 April to avoid large discontinuities mid-season. JRA-55 Our uber index = 4*SOI + 2*SAM + 1*NAO Proposed years for testing: 1984-85, 1990-91, 2003-04. NCAR decided on same 3 years using uber index = 1*NAO + 1*SOI + 0.8*SAM + 0.5*NPI

Selection of Repeat Year Forcing (RYF) We have yet to test in our models – will begin asap. Comparison of RYF years in 1° NCAR model: AMOC NCAR prefers 1990-91 RYF, due to reduced global cooling, larger AMOC. Suggested that different groups may select different RYF years, due to strong sensitivity of model climate state to model choices and salinity restoring. NCAR is awaiting our RYF evaluation before making public recommendations and data releases of RYF. What diagnostics do we care about?

Other changes to model configuration Initial conditions –WOA13v2 temperature and salinity averaged over 1955-2012, – 0.25°, January in top 1500 m, ‘winter’ average below 1500 m (same as NCAR). WOA13v2 data used for sea surface salinity restoring – 0.25°, average of 1.25 m and 10 m levels (following NCAR protocol). Will test restoring timescale / salinity cap for JRA-55 forced models.

Other changes to model configuration Initial conditions –WOA13v2 temperature and salinity averaged over 1955-2012, – 0.25°, January in top 1500 m, ‘winter’ average below 1500 m (same as NCAR). WOA13v2 data used for sea surface salinity restoring – 0.25°, average of 1.25 m and 10 m levels (following NCAR protocol). Will test restoring timescale / salinity cap for JRA-55 forced models. Splitting of JRA-55 Antarctic runoff into basal melt and iceberg melt (distributed spatially according to Merino et al. 2016 climatology).