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A westerly circulation index based on marine data between 1685 and 2008: Implications for the atmospheric variability in the Mediterranean. Gallego, D.

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Presentation on theme: "A westerly circulation index based on marine data between 1685 and 2008: Implications for the atmospheric variability in the Mediterranean. Gallego, D."— Presentation transcript:

1 A westerly circulation index based on marine data between 1685 and 2008: Implications for the atmospheric variability in the Mediterranean. Gallego, D (1)., García-Herrera, R. (2) and Wheeler, D. (3) 1.Universidad Pablo de Olavide, Seville, Spain 2.Universidad Complutense, Madrid, Spain 3.University of Sunderland, SR1 3PZ, UK

2 Antecedents… Luterbacher et al., Clim. Dyn, (2002) Up to date, the Mediterranean SLP reconstructions rely in land observatories.

3 Antecedents… Recently, this reconstruction has been largely improved by using marine data (Kuettel et al., Clim. Dyn., 2009)

4 Antecedents…

5 Incorporation of Atlantic marine data largely improves the reconstruction skill over the Mediterranean.

6 Motivation ¿Could marine data be used to construct a long and reliable series correlated with the Mediterranean climate? Premises: –We need an area with abundant marine data already digitalised (not for the moment in the Mediterranean!) –We need some variable that can be quantifyed reliably (so not temperature, pressure or wind force…)

7 Westerly index Recently, Wheeler et al. (2009) Has been developing a westerly index based on marine data. The index is the proportion of days per month with winds blowing from the west (4-point compass) around the English Channel (Lots of ships passed trought this Channel!) S EW N

8 Westerly index Westerly index (Wheeler et al., 2009) for the period 1685-1750 This index only needs wind direction measures, an standard over the sea and free of subjectivity

9 Expanding the westerly index We choose every ICOADS (V2.4) and CLIWOC (V 1.5) data inside the shaded region between 1750 and 2008 CLIWOC covers 1750-1850 ICOADS covers 1784-2008 This area is essentially the same chosen by Wheller et al.

10 Study area In order to be included, a CLIWOC or ICOADS record inside the shaded region must have: Complete date (day,month,year) Wind direction (not missing and not “variable”) Wind force Some figures… CLIWOC (v1.5) has 5.397 records in the period 1750-1850 (about 53 records / year) ICOADS (v2.1) has 3.393.080 records in the period 1784-2008 (about 15.080 records / year)

11 Number of raw data This graph shows the total number of wind observation used to compute the index (it includes both CLIWOC and ICOADS observations). Beware of the logarithmic scale of the y-axis!

12 Index definition Originally, the westerly index defined by Wheeler et al. (2009) was defined as the proportion of days in a given month with winds coming from the west (1 measure per day was abstracted). Using CLIWOC or ICOADS opens the possibility of having more than 1 wind measure per day (with different wind directions) so some criteria must be adopted to define a day with “westerly wind”. In this work, a day is classified as “westerly wind day” if it has more than 25% of the total number of observations with wind coming from the west. A 4-point compass have been used to compute the percentages. S EW N

13 Index definition Treatment of missing data: In order to avoid too much noise due to the absence of wind data, the following conventions were adopted: For a day, it is considered enough if we have at least 1 valid wind record inside the study area. In a month, we must have at least 10 days with at least 1 wind record to compute the monthly index. The seasonal values are computed when there are at least 2 months with defined index. The annual values are computed when there are at least 8 months with defined index.

14 Monthly index The monthly index is rather noisy (the thicker line is a 24-month gaussian filter. Prior to 1850 there are only some months represented Apparent trend toward more westerly flux during the period between 1850-1950.

15 Seasonal index

16 Annual index The annual graph clearly shows the trend toward a more westerly flux between 1850 and 1950, and a stabilization thereafter.

17 NAO correlation AnnualWinter (JFM) Spring (AMJ) Summer (JAS) Autumn (OND) 0.16 (p<0.05) 0.54 (p<0.01) 0.29 (p<0.01) 0.20 (p<0.01) 0.39 (p<0.01) Period 1823-2008 NAO index from Jones et al. (1997) The westerly index is significantly correlated with the NAO during the whole year, but as expected, the best correlation is observed in winter. Same level of correlation than in Wheeler et al. (r=0.475, p<0.01)

18 Running NAO correlation Running correlation between the NAO and the westerly index for a 31-year window. Significance level = 0.05 (significant correlations are marked in solid line) It appears that prior to 1900 the relationship NAO-index was somewhat weaker! Change in the zonal circulation?

19 Correlation with the geopotential Correlation between the 1000-hPa geopotential and the westerly index for the period 1948-2008 (NCEP/NCAR reanalysis). Shaded areas display significant correlation (p<0.05) It is evident that (in winter) the westerly index is related to the atmospheric circulation all over the Mediterranean at the same level that Northern Europe (with opposite sign).

20 Correlation with temperature These maps shows the correlation between the 1000-hPa air temperature and the westerly index for the period 1948-2008 (NCEP/NCAR reanalysis). Shaded areas display significant correlation (p<0.05) For temperature, the greater relationship is found for central and northern Europe (winter) but some areas of Spain, Italy and Greece shows significant levels of correlation as well. In summer, the relationship between the index and the temperature over England and Central Europe changes its sign.

21 Summary For the moment it is not possible to compute the westerly index between 1750 and 1850 (approx) because shortage of CLIWOC data. Working in progress… The large number of ICOADS data allows to compute a westerly index from 1850 onwards. The index shows trend (increasing westerly flux between 1850 and 1950) and probably oscillations approximately decadal (this is suggested in the figure of the monthly values). The index computed in this work is correlated with the NAO at the same levels that in Wheller et al. The running correlation with the NAO suggest a change in the relationship prior to 1900. We can be relatively confident in this result because from 1870 there are enough wind observations (about 5-10 per day) in the area. The westerly index is definitively correlated with the surface circulation all over Europe, including the Mediterranean Basin (winter). The westerly index is correlated with the surface temperature over England and Central Europe. The relation for the Mediterranean is still significant but weaker.

22 Work to do… Search for data between 1750 and 1850. (work in progress) Merge and homogeinize the complete series 1685-2008 Extract data over the Mediterranean to search for connections (work in progress)

23 Thank you!


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