THE EASTERN-MEDITERRANEAN
3. RESULTS AND DISCUSSION 1. Site representativeness
3.3. Is there a longtime trend in the vapour isotopes?
Plotting our entire data set versus time show a clear trend of decrease in δ18Ο over the 9-year measurements period. A trend could be expected as over this time frame, strong warming was observed over the eastern-Mediterranean, considerably stronger than the mean global warming for this time frame [19].
Because vapour sampling for this study was carried only over several hours during the day, we checked for the possible effect of sampling duration.
There was no correlation between individual samples collection times and δ18Ο (r = –0.06). However annual mean δ18Ο showed strong negative correlation with the annual mean collection duration (r = –0.87). Even if we exclude the extreme sampling times (>8 hours and l> 6.5 hours; Fig. 8) a strong correlation with δ18Ο (r = –0.81) is observed. This effect cannot be the result of decreasing effi ciency in the collection system over time, since such effect would result with the opposite trend (heavy isotopes preferentially condensed with decreasing effi ciency over time).
Why an effect of sampling time is manifested only in the annual mean values? It is likely that other effects, such as synoptic variability (on the days to weeks time-scale) and seasonal variation (on the longer scale) dominates over the effect introduced by change in sampling time. Only when we average over all other effects this smaller sampling effect becomes evident. But this leads to the question of how can sampling time infl uence the isotopic composition of the collected vapour?
The observed trend can probably be explained by the daily cycle in surface turbulence. The collection started at 8AM, and ended between 12 PM and 4 PM when the boundary layer is turbulent. The morning hours (specially in winter) are characterized by lingering nocturnal inversion with typical low wind speeds and, in turn, slow mixing of surface vapour with that from aloft
(which have lighter isotopic composition, see discussion below). Warming of the ground by solar radiation during the day increases turbulence and mixing in the afternoon. In the summer, there is also a typical sea breeze front that reaches our sampling site in the late morning [20], and further increases turbulence.
The increase turbulence along the daily cycle would be expected to mix down vapour with lighter isotopic composition (see discussion below) and could create the apparent link between longer sampling time (higher proportion of afternoon vapour) and lower delta values. Indeed, such afternoon decrease in δ18Ο was observed in an earlier study in this region [21].
The signifi cance of the small variations in sampling time (even ½ hour) made it diffi cult to make conclusions regarding inter-annual variability or FIG. 8. Relationship between annual mean collection time (in hours) for all years of study and annual mean δ18O (a and b).
longtime trends in our 9-years dataset. Future studies should consider such effect when determining a sampling strategy (e.g. use full 24 hours cycle, or the newly available diode-laser systems for real-time measurements). Note, however, that there was no relationship between sampling duration and month of year, and as a result, the mean monthly values are not correlated (r = –0.12) with the mean monthly collection time. Thus, there was no effect of sampling duration on the seasonal cycle, which is the focus of the discussion above.
The results of the detailed trend analysis indicated that the apparent temporal trend in the vapour isotopes cannot be attributed to climatic changes and likely refl ect at sampling uncertainties. However, it helps demonstrate fi rst, the importance of considering the mixing patterns of the planetary boundary layer at the short time scales. And second, the potential of using short term effects studying the dynamics of boundary layer turbulence, and in studies of canopy evapotranspiration. In such studied the short term variations in vapour isotopes are used as tracers of surface water vapour fl uxes, such as the contributions of soil evaporation and plant transpiration [22].
ACKNOWLEDGEMENT
The fi nancial support from GLOW-JR, BSF and the MINERVA foundation is greatfully acknowledged.
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