Impact of Ocean Observations on Ocean (Re)Analysis and Coupled Forecasts Zuo, H, Balmaseda, M A, Balan-Sarojini, Roberts, C, Mayer, M, Tietsche, S, De Rosnay, P

Impact of ocean observations on ocean (re)analysis and coupled forecasts Zuo, H, Balmaseda, M A, Balan-Sarojini, Roberts, C, Mayer, M, Tietsche, S, de Rosnay, P This study is trying to address the following proposed topics in NWP-7 • S11. Ocean: Impact in ocean coupled assimilation • S16. Impact Assessment for seasonal And Climate Applications • S19. Impact Assessment for Arctic observation Configurations of the system: Ocean and sea-ice OSEs have been carried out with the ECMWF operational Ocean and sea-ice ensemble Reanalysis system – OCEAN5, with details described in Zuo et al., (2018). The ocean model is the NEMO with a horizontal resolution of ¼ and 75 levels in the vertical (and a low resolution variant of ~1 and 42 levels). The LIM2 sea-ice has been coupled to NEMO with the same horizontal resolution. The seasonal coupled reforecasts were carried out with a low resolution (Tco199 + 1 Ocean) version of the operational ECMWF SEAS5 system (Johnson et al., 2018). The Ocean Data Assimilation (ODA) system is NEMOVAR (Weaver et al., 2005; Mogensen et al., 2012) in its 3D-Var FGAT (first guess at appropriate time) configuration. Observations assimilated in our ODA system include in-situ temperature and salinity, SST, sea-ice concentration (SIC), sea-ice thickness (SIT) and sea-level anomalies (SLAs). Summary of Results Assessment of ocean and sea-ice observation impacts on ocean reanalysis, and coupled reforecasts have also been carried out using the operational ECMWF system, by initializing the ocean and sea- ice states from different OSE reanalyses. Overall assimilation of ocean observations has a strong positive impact on the performance of ocean reanalysis, with almost 2/3 of the error reduction comes from assimilation of in-situ observations (Argo, XBT/MBT, CTD, Mooring, Mammals, gliders …). Removal of Argo floats degrades the ocean state almost everywhere except for the tropical Pacific and Indian Oceans. Generally, the tropical Atlantic seems to be more sensitive to the removal of in situ observations than the other tropical ocean basins. Some remote effects are also visible when only the in-situ observations in the Atlantic Ocean are removed. Removal of all ocean observations leads to significant degradation in forecasted ocean states from week 1 to week 4, and has a negative impact on coupled forecasts SST and ENSO prediction (loss of ~2 months skill). Introducing sea-ice thickness assimilation has greatly reduced the biases in the sea-ice initial conditions, which then leads to improvement on predictive skill of pan-Arctic sea-ice for lead times of up to 7 months. Verification data sets include the EN4 objective analysis for Temperature and salinity, oceanic transports from the RAPID arrays and at several other locations, ESA CCI2 SST data set, SIC from OSI-SAF 409b, SIT from the AWI CS2MOS data set. Coordinated efforts on developing a experimental framework and analysis methodology for assessing observation impact in ODA and coupled forecasts are needed. A consistent, homogenous and deep reaching global ocean observing network is absolutely essential for both operational NWP and climate monitoring services. Since many operational NWP Centers are moving towards coupled DA in their future systems, it becomes even more important to maintain/enhance the current GOOS network, especially for the mixed layer depth, at the Western Boundary Currents and in the polar regions. .

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