CORRECTION FACTORS FOR BATHYTHERMOGRAPH TEMPERATURES IN THE WESTERN MEDITERRANEAN

1. CORRECTION FACTORS FOR BATHYTHERMOGRAPH TEMPERATURES IN THE WESTERN MEDITERRANEAN Zunino P. (1), Tsimplis M.(2), Vargas-Yáñez M. (1) , Moya F. (1), García-Martínez M. (1) (1) Instituto Español de Oceanografía. Centro Oceanográfico de Málaga. Puerto Pesquero de Fuengirola s/n, 29640 Fuengirola, Málaga, Spain (2) National Oceanographic Centre. Unversity of Southampton. United Kingdon e-mail: pzunino@ma.ieo.es. www.ma.ieo.es/gcc INTRODUCTION Ocean temperature has been measured since the second half of the XIX century. Thenceforth, the methodology has been changing from reversible thermometers coupled to oceanographic bottle, bathythermographs (BT), CTD, satellites and autonomous profilers. More than one of theses devices have been employed at the same period of time. So, the different oceanographic datasets are a mix of data measured by the former instruments. It is known that BT temperatures are biased due to errors in the recorded pressure level which is related to the fall rate equation provided by the BT manufactures, and due to the temperature sensor itself. Therefore, when long-tern temperature studies have been done, biased data have been used, resulting in possible wrong estimation of ocean heat content and/or temperature trends and variability. Different ways for correcting BT data have been proposed. Some examples are the depth corrections proposed by Hanawa et al. (1995), Wijffels et al. (2008) or Ishii and Kimoto (2009); the temperature correction put forward by Levitus et al. (2009) or the depth and temperature correction proposed by Gouretski and Reseghetti, (2010). Our objective in the present work is to detected BT temperature biases in the Western Mediterranean using the MEDATLAS database and to design the best BT correction factors for the Western Mediterranean. DATA, METHODS AND RESULTS Used database: MEDATLAS. Period of time analyzed: 1945-2000. All the analyses have been carried out on each section of fig. 1. Figures 2 and 3 show that T monthly means calculated with bottle and CTD dataare not statistically different. The differences between both monthly means are due to the wide confidence intervals. The large uncertainties are caused by the low number of available monthly data even for the “large” study areas (see fig. 4). Accepting there are no differences between bottle and CTD data, we have corrected the Western Mediterranean BT data with the ensemble bottle + CTD data. Figure 1. Studied areas in the Western Mediterranean. Figure 3. T monthly means and its confidence intervals (95% confidence) estimated with bottle data (blue) and CTD data (red) in some selected pressure levels in the Balearic sea. Figure2. a, b and c display 3 different T monthly mean profiles calculated with bottle data (blue lines) and with CTD data (red lines). Cyan and pink lines are their corresponding confidence intervals (T-Student 95%). SUMMARY AND CONCLUSIONS We have estimated depth and seasonal dependent BT correction factors for each studied section (fig. 1) in the Western Mediterranean. Monthly T means calculated with BT data were corrected with our new correction factors. Differences between monthly T estimated with BT (original and corrected) data and bottle + CTD data have been calculated. Means and confidence intervals (T-Student at 95%) of these T differences have been also estimated (colored values shown in fig. 9). Figure 9 displays these T differences in Alboran and Ligurian Sea at four different pressure levels. We can see in figure 9 that in most of the plots the bias between TBT and Tbot+CTD is smaller when BT data are corrected with our new correction factors (red points and red values), including the 0 value in the confidence interval, which mean that the differences between BT and bottle+CTD data have been reduced if our new correction is applied. Monthly T means estimated with bathythermograph data and with bottle+CTD data are statistically different (see figures 5 and 6) Figure 6. Time evolution of monthly means obtained with BT data (in blue) and with bottle+CTD data (in red). Mean values for the whole period are included for both time series (solid lines). Figure 5. T monthly mean profiles estimated with BT data (in blue) and with bottle+CTD data (in red) in three different areas of fig. 1. Cyan and pink lines are the corresponding confidence intervals for BT and bot+CTD respectively. Figure 4. Number of data used to estimate each monthly mean: a) bottle data in Alboran Sea; b) CTD data in Alboran Sea; c) BT data in Alboran Sea, d) bottle data in Tyrrhenian Sea, e) CTD data in Tyrrhenian sea and f) BT data in Tyrrhenian Sea. The color bar for BT data is different than for the other instrumets, the one for BT data is on the right of BT plots. Previous BT corrections are depth and time dependent. It is known that the pressure error would be related to the water viscosity and/or the water column stratification which is associated to the seasonal cycle. However, no published BT correction has considered the seasonal influence. In figure 7, the seasonal cycle of monthly T estimated with BT and with bottle+CTD data is shown. The differences between monthly T means are seasonal dependent and consequently, the biases between BT and bottle+CTD temperatures are seasonal dependent. We have also evaluated these biases time dependence, and, as it can be seen in figure 8, there are no significant trends on the BT - bottle+CTD T biases, so, we have considered that the bias is not time dependent. Note: corrected detrendyBT is refering to BT data corrected with other correction factors calculated in the present work, but no shown, which take into account the T bias trends. It has not been explained because the one explained reduce the most the T differences between BT and bottle + CTD data. Figure 9. T differences in some different pressure levels in a) Alboran Sea, and b) Ligurian Sea. The differences are between monthly means estimated with: original BT data – (bottle + CTD) data in blue; corrected BT data – (bottle + CTD) data in red; and corrected (detrended )BT data – (bottle + CTD) data in green. Values on the right of each plot are the T bias mean for the whole period ± the confidence intervals (T-Student 95%). Figure 8. T bias trends estimated in 3 different areas indicated in fig. 1 Figure 7. Seasonal cycle of montly T means estimated with BT data (in blue) and with bottle + CTD data (in red) at 10, 50, 200 and 400 dbar. Algerian Basin 2. ACKNOWLEDGEMENTS This research has been supported by European Science Foundation (ESF) for the activity entitled 'Mediterranean Climate Variability and Predictability' (grant 3090) REFERENCES Gouretski, V. and F. Reseghetti, 2010, On depth and temperature biases in bathythermograph data: Development of a new correction scheme based on analysis of a global ocean database. Deep-Sea Research I, Vol. 57(6), pp. 812-834, doi:10.1016/j.dsr.2010.03.011. Hanawa, K., P. Raul, R. Bailey, A. Sy and M. Szabados (1995): A new depth-time equation for Sippican or TSK T-7, T-6, and T-4 expandable bathythermographs (XBTs). Deep-Sea Res., 42, 1423-1451. Ishii, M. and M. 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