Observational gaps revealed by model sensitivity to obser-vations

To understand the quality of the existing observing system in the Arctic to capture important elements of change over the Arctic we performed a gap analysis with respect to the Arctic Ocean, the Arctic atmosphere and the high-latitude carbon-monitoring network. The main points of the findings are:
1) The ocean observing system: The satellite altimeter system is a critical system to monitor the high-frequency variability. Due to the presence of sea ice in winter time, most of the area can be observed only every 5-10 days, leading to large observing gaps. Closing the gap can be done with new arrays of bottom pressure sensors such as tide gauges or moorings in the ocean bottom. In addition, high-frequency transport measurements are required in the Fram, Davis Straights, the Barents Sea Opening, and north of the Laptev Sea. On the seasonal cycle, bottom pressure observations from GRACE are required to monitor the mass related variabil-ity and sea-ice observations are crucial for monitoring the halosteric related variability. On decadal time scales, it is important to have a sufficient hydrographic observing component capable of capturing temperature and salinity changes over the entire Arctic Ocean from the surface to the bottom. New algorithms that can recover sea level from sea ice covered areas may help to improve current satellite altimeter systems, and to improve the ability to monitor the Beaufort Gyre.
2) The atmosphere observing system: The density of the existing radiosonde observation network is not the most critical factor for the quality of T850 forecast. Instead, the results pointed out that stations on small islands in the middle of the Atlantic Ocean are critical for the quality of analysis. The Central Arctic Ocean and the Northern North-Atlantic would prob-ably benefit most from new sounding stations. Efforts to improve the quality of radiosonde observations, especially in Russia, would be very beneficial for the quality of T850 forecasts in the Arctic and sub-Arctic. Current data assimilation systems are probably not adequate to op-timally exploit the information from the existing observational network.
3) GHG fluxes observing system: The existing network of pan-Arctic atmospheric monitoring sites provides continuous, well-calibrated observations on atmospheric greenhouse gas mixing ratios, generating basic information to quantify surface-atmosphere greenhouse gas exchange processes for most regions in Canada, Europe, and Western Russia; also the Arctic Ocean re-ceives good overall data coverage. Regions showing limited data coverage include the Russian Far East, Western Alaska, and the Eastern Canadian Provinces. Areas where footprint coverage gaps exist seasonally include parts of Western Russia and Central Siberia. Investments in ob-servational infrastructure in any of these areas would be beneficial to increase the overall coverage of the pan-Arctic atmospheric network for greenhouse gases.