Variability and change of arctic energy and fresh water budgets

A research project to assess the intra-annual up to decadal variability of the coupled energy and freshwater budgets in the Arctic is proposed. Accurate estimates of the coupling of atmosphere, hydrosphere and cryosphere through vertical fluxes as well as of the lateral flow of energy and freshwater into and out of the arctic region are instrumental for understanding climate variability and climate change in the Arctic but also in the surrounding extratropics. They are considered a key scientific question by several international research panels. Existing diagnostic estimates of parts of the arctic energy budgets have large uncertainties and mostly cover too short periods for robust assessments of interannual up to decadal variability. Also many quantities could be estimated only indirectly or as a mean over the whole arctic ocean. To overcome these limitations we propose to estimate the oceanic energy and freshwater budgets using several diagnostic approaches involving atmospheric and oceanic reanalyses as well as direct oceanic measurements and sea ice reconstructions or reanalyses. In particular it is planned to directly estimate the oceanic horizontal flux divergence terms which are key to get overspecified budgets in the ocean. No such estimates exist yet although they are highly desirable for understanding arctic climate variability and for verification of climate models. The proposed efforts will draw on important recent improvements of the Arctic observing system, such as the availability of moored measurement sites at all arctic gateways or the digitization of upper air data back to the early 1950s. Equally importantly ocean reanalysis data sets calculated with dynamic data assimilation systems that include a sea ice model are now being produced. The working group of the PI has already developed significantly better estimates of arctic atmospheric budgets compared to recent publications and will now extend its expertise to the ocean domain. There are also good prospects for refining the group`s data homogenization methods to address the relatively high number of outliers and data gaps in early arctic upper air and surface data. The research is planned in collaboration with international research centers and national research networks. The proposed efforts are expected to lead to statistically significantly more accurate assessments of the arctic energy and freshwater budgets, spatially resolved to scales on the order 50\,km back to the beginning of the satellite era (ca. 1979). For the period 1950-1979 a new assessment of the atmospheric energy budget that builds upon new digitized and homogenized upper air and surface input data is proposed as well. The value of the new estimates will be demonstrated by studies on climate variability and by intercomparison with independent data sets such as flux measurements and climate models. Funds of EUR 230.895 for two years are requested.

The accurate estimation of regional energy and water transports in atmosphere, ocean and sea ice, particularly in the rapidly changing Arctic, is central for the understanding and quantification of Climate Change. Those transports cannot be measured directly. They require a sophisticated combination of different observations from space, the atmosphere, the Earth's surface and the deep sea. At the start of this research project, the existing estimates of the coupled Arctic energy budget showed large discrepancies. The budgets, i.e. the sum of energy transports and energy storage, which should be zero, showed an unexplained deficit of 25% of the single transports. In addition the observed periods were too short for reliable estimates of interannual or decadal variations. In the project it was possible to reduce this discrepancy in the multiannual energy budgets to less than 2% of the single transports. It was achieved through using the best available atmospheric, oceanic and sea ice data and throrugh significant improvements in the numeric computation of the budgets. The accuracy of estimated seasonal variations of the arctic budgets could be improved by 50%, although there are still uncertainties, particularly in the observations of ice melt. Also the estimates of the arctic freshwater budget could be improved significantly. Not least, the project results revealed that the climate change related energy storage in the arctic is not larger than in outside the arctic. This may seem surprising since the temperatures at the arctic earth surface increase much faster than in most other regions, The reason is that the additional energy in the arctic climate system is stored near the surface or in used for sea ice melt, whereas the global oceans mix down the additional energy into great depths. The new budget estimates seems suitable as reference for comparisons with climate model simulations. Uncertainties remain, however, due to insufficient observations below the arctic sea ice and near the arctic water straits. The proposed extension of budget estimates back to the 1950s could not be realized, since the necessary input data had not been available until the end of the project. The project did not only use observation data but strived to improve them as well. Particularly the radiosonde data at high latitudes have been homogenized, This means that artificial shifts in the time series due to changes in the instrumentation or observation practice have been removed. Only then these data can be reliably used for the observation of climate change. The homogenized data went into high resolution global analyses of the climate state, most notably ERA5, an area where the European climate and environmental monitoring Copernicus is worldwide leader.