2008 report
Progress: 50%
The work under task 3.4 consists of 1) Constructing a database of historical data - Hydrobase, 2) observations in the freshwater arrays at 79, 74 and 63 N, and 3) numerical modelling and data assimilation. The Hydrobase database has been updated (delivery 3.4-05). The observational work has been severely hampered by extensive sea ice and mooring losses. All preparations for mooring exchanges has been made; instruments purchased and shipped, cruises planned and carried out. At 79 N extensive ice cover made it impossible to reach the moorings. There were hence no data catch in 2007. The scientific work continues, however, based on previous data. At 63 N one suffered instrument losses for the third year in row. There is therefore no surface (freshwater) data catch at this site. Due to the losses the observations are discontinued, and the instruments moved to the 74 N array. There were no news from the 74 N array before this report was written.
In the numerical modelling part, the Model framework has been updated to the latest version of NEMO2. A 40 year control simulation at 1 degree resolution is showing good agreement with the flow through many straits and channels with the much higher resolution ¼ degree NEMO model. The first 40-year ocean synthesis with data assimilation was performed in September 2007. Results show good reproduction of water masses in most ocean basins. Some problems in the Northern N Atlantic and with the MOC are being studied now before further runs are performed. Results suggest that global and basin wide heat budgets can also be inferred from the synthesis.
Figure 1 shows a set of mass and heat transports from free runs of the NEMO model at 3 different resolutions 1 degree (green); ½ degree (red), ¼ degree (black). It can be seen the extent to which resolution affects the transport estimates. In general the 1 degree model is still remarkably good.
Figure 2 shows the time series of Arctic sea ice distributions from these 3 free runs of the NEMO model at different resolutions. The colour coding is the same as is figure 1, with the blue line representing validating observations of ice covered areas and extent.
The next step will be to assess the impact of data assimilation on these estimates.
An isotherm based version of the assimilation code has also been implemented and tested for period up to 3 years with some useful improvements. We have begun runs with data assimilation into the ¼ degree resolution model and expect to have ¼ degree assimilation for the period 1990-2004 by the end of the year. Coding has also begun for an isopycnal assimilation approach in the 1 degree model, which should show improved results at high latitudes. The availability of Damocles data and earlier data from ASOF array for comparing and testing the model will become vital in the coming 6 months.
