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The air-sea interface is a key gateway in the Earth system. It is where the atmosphere sets the ocean in motion, climate/weather-relevant air-sea processes occur, and pollutants (i.e., plastic, anthropogenic carbon dioxide, radioactive/chemical waste) enter the sea. Hence, accurate estimates and forecasts of physical and biogeochemical processes at this interface are critical for sustainable blue economy planning, growth, and disaster mitigation. Such estimates and forecasts rely on accurate and integrated in situ and satellite surface observations. High-impact uses of ocean surface observations of essential ocean/climate variables (EOVs/ECVs) include (1) assimilation into/validation of weather, ocean, and climate forecast models to improve their skill, impact, and value; (2) ocean physics studies (i.e., heat, momentum, freshwater, and biogeochemical air-sea fluxes) to further our understanding and parameterization of air-sea processes; and (3) calibration and validation of satellite ocean products (i.e., currents, temperature, salinity, sea level, ocean color, wind, and waves). We review strengths and limitations, impacts, and sustainability of in situ ocean surface observations of several ECVs and EOVs. We draw a 10-year vision

Abstract. There are numerous networks and initiatives concerned with the non-satellite-observing segment of Earth observation. These are owned and operated by various entities and organisations often with different practices, norms, data policies, etc. The Horizon 2020 project GAIA-CLIM is working to improve our collective ability to use an appropriate subset of these observations to rigorously characterise satellite observations. The first fundamental question is which observations from the mosaic of non-satellite observational capabilities are appropriate for such an application. This requires an assessment of the relevant, quantifiable aspects of the measurement series which are available. While fundamentally poor or incorrect measurements can be relatively easily identified, it is metrologically impossible to be sure that a measurement series is "correct". Certain assessable aspects of the measurement series can, however, build confidence in their scientific maturity and appropriateness for given applications. These are aspects such as that it is well documented, well understood, representative, updated, publicly available and maintains rich metadata. Entities such as the Global Climate Observing System have suggested a hierarchy of networks whereby different subsets of the observational capabilities are assigned to different layers based on such assessable aspects. Herein, we make a first attempt to formalise both such a system-of-systems networks concept and a means by which to, as objectively as possible, assess where in this framework different networks may reside. In this study, we concentrate on networks measuring primarily a subset of the atmospheric Essential Climate Variables of interest to GAIA-CLIM activities. We show assessment results from our application of the guidance and how we plan to use this in downstream example applications of the GAIA-CLIM project. However, the approach laid out should be more widely applicable across a broad range of application areas. If broadly adopted, the system-of-systems approach will have potential benefits in guiding users to the most appropriate set of observations for their needs and in highlighting to network owners and operators areas for potential improvement.

Ship and buoy reports of wind, air pressure, temperature, humidity, and sea temperature for 2007 and 2008 have been compared with values from the operational Met Office global numerical weather prediction (NWP) system. Ship reports have been categorized by vessel type, recruiting country, and manual or automatic reporting. Most estimated ship winds (except Dutch ones) are too strong and are better treated as measured winds. After height adjustment, the average ship wind speeds are reasonably consistent with buoy wind speeds, but the global model winds are 6%-12% weaker. Extra care is needed when comparing/assimilating values close to the coast. Ship air temperatures are too warm during the afternoon due to solar heating of the ship. Adjustment of the pressure to sea level is a problem for some larger ships. Passenger ship reports are relatively poor quality for several variables and their winds are rather strong. Automated ship reports and those from research and coast guard vessels tend to be of good quality but some of the winds are slightly too weak. In most respects buoy data have the best quality. A number of improvements to the Met Office observation processing system are being made, notably tightening of the quality checks and better height adjustment of winds.

Capsule Atmospheric River Reconnaissance is a multi-year research and operations partnership to evaluate the potential of targeted airborne observations over the Northeast Pacific to improve forecasts of atmospheric river impacts on the U.S. West Coast at lead times of less than five days.

Originally the only surface data assimilated in the Met Office global forecasting system were pressure and marine winds but now most temperatures, humidities and land winds are also used. Adjustments for differences between station and model height are essential for pressure and temperature; new height adjustments for humidity and wind were introduced. These changes brought the global and regional forecasting systems much closer in their use of surface data and forecast performance for surface variables. Winds from islands and headlands not resolved in the forecast model and tropical land winds are excluded. Extra reports (notably Metars) have been introduced into the system. The assimilation of land station temperature and humidity reports gave a clear improvement to short‐range forecasts of ‘screen’ temperature and humidity and small improvements to pressure forecasts. The assimilation of winds over land areas had little impact—wind speed biases, especially at night, are part of the problem. The surface pressure assimilation improves pressure and upper atmosphere forecasts but has little effect on other surface variables. The observation innovations reveal aspects of observation and model errors and other factors such as the proximity to the coast and the importance of the diurnal cycle.

Large-scale freshening of the northern Atlantic, and concurrent salinity increases in the low-latitude Atlantic upper layers, have been widely reported for the second half of the twentieth century. The role of anthropogenic and/or unforced variability processes in these changes, and the potential for the high-latitude freshening to slow the Atlantic meridional overturning circulation (MOC), have been the subject of debate. These issues are investigated by comparing observed and simulated changes, using the Third Hadley Centre Coupled Model (HadCM3). This analysis suggests that a substantial part of the observed trends could be related to multidecadal variability of the MOC. Using an SST-derived proxy for historical MOC changes, in conjunction with model internal variability relationships, suggests that much of the observed evolution of northern Atlantic freshwater content can be explained as being driven by unforced MOC variability.HadCM3 simulations with "external" historical time-varying forcings show anthropogenically forced increases in the main hydrological cycle over the Atlantic: an increase in net precipitation at high latitudes and in net evaporation in the subtropics. In the northern Atlantic the freshening from additional surface freshwater is counteracted by changes in ocean freshwater transport. A similar ocean compensation is absent at lower latitudes, where there is decreasing freshwater content. It is suggested that in the recent historical period this externally forced trend is likely to have led to anomalies exceeding the unforced variability range.

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