J1.21 SOUTHERN ICELAND DISCHARGE AND REGIONAL CLIMATE NORTH ATLANTIC ATMOSPHERIC CIRCULATION RELATIONSHIPS 1973 – 1992 Glenn R McGregor*, Damian M. Lawler, Ian D. Phillips The University of Birmingham, Birmingham, UK 1. INTRODUCTION Southern Iceland lies in a humid sub- arctic zone. The rivers here demonstrate a strong seasonality in their flow that is matched closely with the seasonal march of temperature and to a lesser extent precipitation. Analyses of river flow records for this region have revealed that not only do river flows demonstrate considerable inter-annual variability but, decreasing spring and autumn flows also appear to be a feature of this region’s hydrology over the period 1973 to 1992. Given that river flow patterns in Southern Iceland are generally related to seasonal climate changes which in turn are related to the seasonal march of the general atmospheric circulation (Lawler et al, 2001) the purpose of this paper is to explore the extent to which the observed inter-annual variability of river flows and the secular changes noted for spring and autumn are related to variability and change in the atmospheric circulation over the North Atlantic. Explicitly this paper attempts to shed light on the possible mechanisms that link the following hypothesised chain of causality atmospheric circulation ► regional climate ► river flow variations. The remainder of this paper is divided into 5 sections. General methodology is outlined in the following section while general river flow characteristics are described in section 3. The nature of the associations between atmospheric circulation, regional climate and river flow are presented in section 4. A discussion of these follows in section 5. Conclusions are drawn in section 6. 2. METHODOLOGY Southern Iceland river flow is represented by monthly flow data from the ______________________________________ Corresponding author address: G.R.McGregor, School of Geography, The University of Birmingham, B15 2TT, UK. G.R.McGregor@bham.ac.uk the Skógá river basin (63 o 32’N,19 o 32’W). This drains the Myrdalsjökull ice-cap in southern Iceland which lies over the active Katla volcanic system. Located to the south-west of Myrdalsjökull ice cap, the Skógá also receives meltwater from the Eyjafjallajökull ice cap to the west. The Skógá river was selected for this study because of its continuity of flow data and the fact that discontinuous measurements of river flow in adjacent catchments mimic those evident for the Skógá. Furthermore human impact is minimal in this basin. Regional climate for Southern Iceland is described by mean monthly temperature (Temp) and precipitation (Precip) for Vik (63 o 26’N,19 o 01’W), Iceland’s southern most regional climate station. Monthly values of the North Atlantic and Arctic Oscillation indexes (NAOI and AOI respectively) as well as meridional (MVF) and zonal (ZVF) atmospheric water vapour flux are used to characterise atmospheric circulation conditions over the study area. The North Atlantic Oscillation Index (NAOI) is the standardised mean MSLP difference between Ponta Delgada in the Azores and Reykjavik in Iceland. (Marshall et al., 2001). The NAO is characterised by two marked phases that have their maximum impacts on climate in the boreal winter. When positive, atmospheric pressure in the vicinity of the Azores (Iceland) is anomalously high (low) the result of which are strong zonal flows over the North Atlantic and western and Northern Europe which bring warm and wet conditions to these regions. A negative phase of the NAO is associated with anomalously high (low) pressure over Iceland (Azores); meridional flows of cold arctic air predominate. Consequently the North Atlantic and continental regions to the east experience cold dry conditions. The NAO is part of a hemispheric mode of variability that is captured by the Arctic Oscillation (AO) (Thompson and Wallace, 2000). The AO is a zonally symmetric or annular structure characterized by a barometric seesaw between the polar cap regions poleward of 60 o N and the surrounding