DERECHA IZQUIERDA MIEMBRO

The discrepancies of the wind speed trends from each reanalysis are difficult to grasp from the individual maps. For this reason a simple method to summarise the information and convey a

coherent message is shown in Figure24for DJF.

Coloured regions correspond to those locations where the reanalyses agree in the increasing or decreasing long-term behaviour of the wind speed over the last decades.

A positive and significant increase of the 10-m wind speed in boreal winter is reproduced by the three reanalyses over the tropical oceans (Figure

24a). These positive wind speed trends could be

linked to the changes in the global circulation, in particular, the recent strengthening of the Walker circulation (England et al. 2014). Over land, the reanalyses show a robust negative trend over Eurasia, India, the Sahel and southern Africa. Wind increases are not robust and show

patchy patterns. The decreasing wind speeds

in South Africa have already been noticed in reanalysis products and attributed to the changes in the large-scale circulation (Nchaba et al.

2016). Although the trends in the Northern

Hemisphere continents have been described in

section 3.5.1 for the ERA-Interim reanalysis,

the agreement between the three reanalyses indicates that some of the mechanisms proposed previously as potential drivers of the wind speed trends, such as changes in land use or aerosol concentrations, cannot be the only explanation of the negative trends because they are dealt with in

different ways by each reanalysis. An alternative explanation of the decreasing trends might lay in changes in the large-scale circulation. If the large-scale circulation play a role, similar trends would be observed in the free troposphere. This

is illustrated in Figure24b, where the agreement

of the 850 hPa wind speed trends among the

different reanalyses is displayed. Over land,

the three reanalyses provide similar results at both levels. The main differences between the

trends in the two levels (Figure 24a,b) are the

negative trends in the South Atlantic at 850 hPa. Most of the white areas correspond to regions with complex topography because it is where the MERRA-2 wind speed data are usually not available at 850 hPa since the assimilation system does not extrapolate data to levels with pressure larger than the surface pressure.

Figure 24b also shows that there are several

regions (e.g. northern South America or

Australia) where the three reanalyses do not agree in the sign of the trends in winter. These disagreements can be due to low and non- significant trends, hence, either a positive or a negative small trend is possible but also there are different sources of uncertainty affecting the trends. Among the uncertainty sources that can produce such discrepancies can be considered the different ways in which low-level wind speeds are derived, the observational sources included, or the corrections for the instrumental drifts that can generate inconsistencies in the observations.

The discrepancies between the reanalysis

on one side and ERA-Interim and MERRA-2 on the other. The large trend generally found for the JRA-55 reanalysis over land (Figure

B11) has been suggested to be attributable to

deficiencies in deriving wind speed for that particular reanalysis (Japanese Meteorological Agency, personal communication) as it has

been explained in section 3.1.2. In the

JRA-55 there is a negative wind speed bias near-surface that is not fully corrected with

the data assimilation. Besides, changes in

the availability of observations can have an impact on the data used for the correction, resulting in large differences in wind speed

Figure 24 – Coherence maps between the ERA-Interim, JRA-55 and MERRA-2 reanalyses about the

a) 10-m and b) 850 hPa c) 10th _{percentile and d) 90}th _{percentile wind speed trends. Blues (Reds)}

indicate agreement between the three reanalyses about the negative (positive) trends of 10-m wind speed for DJF in the 1981-2015 period. Asterisk indicates that the trends are significant (t-test at a 95% confidence level): no asterisk indicates that the trends are not significant, (*) only one of the reanalysis has significant trends, (**) that any two reanalyses have significant trends, and (***) that the three reanalyses have significant trends. Grey areas indicate where the surface level is higher than 850 hPa.

3.5. Long-term variability

trends. These limitations are not affecting the 2-m air temperature, as this variable shows a higher number of coherent regions among the

three reanalyses used here (Figure B9) than

those illustrated for the 10-m wind speed. This important information is typically not reported in the reanalysis documentation available to wind energy users, who might misuse the reanalysis data to characterise long-term variability of wind

speed. The trends of wind speed at 850 hPa

for JRA-55 and MERRA-2 (Figure B12) show

that the JRA-55 trends at 850 hPa are similar in magnitude to those of both ERA-Interim and MERRA-2, supporting the hypothesis that the overestimation of the JRA-55 trends is a feature due to the treatment of the winds near the surface.

The consistency among the three reanalyses in

the representation of the trends for the 10th and

90th _{percentiles of the wind speed has been}

shown in Figure 24(c,d). Differences between

the 10th and 90th percentiles can be identified

over Eurasia where the three reanalyses agree in

the negative trends, but for the 10th percentile,

these trends are not significant. By contrast

for the 90th percentile, the negative trends are

significant in the three reanalyses. In most of the regions, coherence maps for the extremes are very similar to that obtained for the 10-

m wind speed (Figure 24a), which indicates

that the uncertainty affecting the reanalyses is similar for the high and low wind speed to that for the mean wind speed. The analysis of the uncertainty from the different observational sources for the 2-m air temperature trends have

been also carried out (FigureB9). The coherence

among the renalyses is particularly high over land areas in most of the seasons, but particularly in SON, when the three reanalyses agree in the positive 2-m air temperature trend in most

of the regions over land. The discrepancies

are mostly over the tropical oceans, particularly in the tropical Atlantic, where the MERRA-2 reanalysis displays positive temperature trends, while in the ERA-Interim and JRA-55 these trends are very low and non-significant. Over land, white areas are identified in South Africa in DJF, MAM and JJA, and also in Australia in DJF and MAM. However, the number of regions where the reanalyses show a disagreement in the temperature trends is lower than for the 10-m wind speed trends.