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(For further details on the methodology and results for this project at AOML, please refer to the Atlantic Meridional Heat Transport Background & FAQ.)
Refining uncertainty estimates is a critical component of ongoing heat transport research at AOML. Uncertainties in the heat transport estimates may arise due to methodology, and due to data accuracy and representation issues. Uncertainties can include:
In order to test the validity of the methodology in the North Atlantic, historical data along the 24N line are used in the simple geostrophic method to compare heat transports estimates. The four historical sections can be systematically reduced to temperature data in the upper 850 m to simulate XBT data and the resultant heat transport suggests what sort of errors can be found from using real XBT transects. These errors estimates are ongoing and more information will be posted here shortly.
In order to test the validity of the methodology in the South Atlantic, data from the WOCE A10 line (Siedler et al., 1996) is used in the simple geostrophic method to compare heat transports estimates. A10 data can be systematically reduced to temperature data in the upper 850 m to simulate XBT data and the resultant heat transport suggests what sort of uncertainties can be found from using real XBT transects. The POCM model heat transport is also analyzed to examine the possible errors from uncertainty in ageostrophic motions.
We found that the largest errors were introduced by using the Levitus Climatology to simulate the subsurface density field (about 0.2 PW) and the uncertainty in the absolute flow field (0.2 PW). A summary of the various uncertainties follows:
Several projects are underway or are planned that will attempt to constrain these uncertainty estimates further, to provide overall uncertainties in the heat transport values presented:
Reasonable estimates of the heat transport uncertainty will be derived (from models and CTD/ARGO data), that includes updated estimates from the improved methodology so that recommendations for future observing system improvements can be made, e.g., XBTs that extend to 1000m might improve our heat transport estimate by some amount. The ultimate goal of these uncertainty estimates is to quantify our confidence in the heat transport results and provide the scientific basis to improve their accuracy.
The following table shows estimates of total meridional heat transport in PW(†) along the AX18 line. Heat transport estimates are shown using two different estimation methods, as outlined in the Methodology section of the AX18 FAQ:
|Month, Year||Heat Transport using Levitus S (PW)||Heat Transport using Thacker S (PW)||Difference|
(†): 1 PW = 1 petawatt = 1015 Watts
The effect of these two different methods of salinity estimation leads to a ~0.1 PW difference in heat transport estimates. That suggests an additional uncertainty in heat transport due to the uncertainty in salinity. Improvements to the heat transport estimates may be made through observations of salinity either with expendable CTDs or profiling ARGO floats.