Regional climate and atmospheric circulation controls on the discharge of two British rivers

The dependence of the discharge (Q) of two contrasting UK rivers (Itchen, Ewe) on concurrent and lagged regional climate (RC) and atmospheric circulation (AC) variations was assessed over the period 1974-97. RC variables used were temperature and precipitation; the AC indicators used were 850 hPa water vapour flux anomalies (VF) at five western European stations, and the Arctic (AOI) and North Atlantic Oscillation (NAOI) indices. Correlation analyses were performed to assess Q-RC and Q-AC relationships before two sets of multiple linear regression models were developed to specify monthly Q values from RC and AC. Q-RC associations were generally stronger and more seasonally consistent than Q-AC relationships, with the flow of the Itchen (southern England) and Ewe (northern Scotland) being most sensitive to temperature (TEMP) and precipitation (PPT) respectively. In most months, discharge values of both rivers were positively associated to zonal and vector VF anomalies over the British Isles and northern France, but inversely related to vector VF over Iceland. The AOI and NAOI were significantly related to the Ewe's flow only; relationships were strongest in the winter half-year. Monthly AC regression models explained 14.8-81.0% (25.0-90.9%) of the discharge variability of the lichen (Ewe). Strong AC forcing of the lichen's discharge is confined to the winter (DJF), since the lichen's direct meteorological signal is attenuated by groundwater dynamics in other seasons. Analysis of anomalous flow periods (e.g. 1988-92 and 1995-7) revealed that discharge does not always respond in the same manner to a given RC/AC forcing, as the relationships themselves vary interannually as well as between the two rivers.