Discovering the Azores Front/Current system with SeaWiFS imagery Ana Martins 1 , Igor Bashmachnikov, Virginie Lafon, Ana Mendonça, Felix José, Miguel Figueiredo, Luis Macedo Department of Oceanography and Fisheries, University of the Azores, 9901-862 Horta, Faial, Azores, Portugal ABSTRACT Ocean Colour (OC) sensors have been primarily used in biological studies. More recently, OC information has been attracting the attention of oceanographers, as a potential method for revealing physical structures in the ocean. In this study, OC data obtained from SeaWiFS imagery is used, for the first time, to detect the weak Azores Current (AzC) and the associated Azores Front (AzF). Previous studies show that the frontal interface is well seen on SST imagery only during the cold season, while it is disguised during the warm season through the formation of a strong seasonal thermo- cline. With SeaWiFS imagery, the frontal interface is well identified around 34 o N as an asymmetric zonally stretched band of higher near-surface chlorophyll a (Chl a) values north of the AzF, accompanied by a sharp decrease to the south. Quasi-stationary meanders, previously derived from SST fields for the same region, are also well observed in OC imagery. Monthly-averaged Chl a along a meridional cross-section shows that, from spring to autumn, the front is clearly visible. In winter, differences across the front are less pronounced, and the front is more easily identified on SST fields. OC gradients weaken to the east, corresponding to the general weakening of the AzC. In situ CTD data reveal a sharp and meandering thermohaline and dissolved oxygen front located at 33-34.5 o N and 31 o W. This study suggests that OC imagery, combined with other sensors, provide an important tool to investigate ocean dynamic variability, by helping to detect frontal zones with great precision. Keywords Azores Current, Azores Front, Ocean Colour 1. INTRODUCTION The Azores Current (AzC) is located at the eastern edge of the North Atlantic Subtropical Gyre. The origin of the AzC is found to be at about 50 o W, near the southeastern Newfoundland Rise, where the Gulf Stream separates on several branches one of which is the AzC. The latter is observed between 33-36 o N and displays a strong seasonal variability, providing the major portion of the upper ocean transport to the eastern basin circulation ( 1 ) (Fig.1). Measurements re- vealed the current to be a weak baroclinic flow, reaching its maximum transport in spring of about 20 Sv (1 Sv = 10 6 m 3 s -1 ), and having 13 Sv of annual average ( 2,3 ). The current is only about 100 kilometres wide with a surface average speed of 10-30 cm s -1 ( 4,5 ). It is very close to zonal, slightly deviating to the north and then, east of 20 o W, sharply turn- ing south towards the Madeira archipelago ( 6,7 ). The Azores current is a region of high dynamic instability, resulting in intensive meandering with wavelengths of several hundred kilometres and dominating periods from 200 to 490 days ( 3,7,8,9,10 ). The meanders periodically detach from the current, forming on both sides of the front, series of intensive cy- clones and anticyclones ( 3,6,10,11 ). To the north of the AzC, a weak westward flowing Azores Counter Current (AzCC) is observed ( 12 ). The AzCC is thought to be a time average of regular series of westward propagating anticyclones, origi- nated in the eastern part of the AzC ( 3 ). The Azores Current is associated with a pronounced subtropical frontal zone  the Azores front separating colder and fresher Eastern North Atlantic Central waters from warmer and more saline Subtropical waters ( 13 ). The cross-frontal temperature gradients reach 1 o C per 50 km ( 8 ) and play leading role in the formation of cross-frontal horizontal density variations in the upper 600 m layer ( 4 ). The frontal interface is traditionally determined by an outcrop of 18 o C isotherm ( 3,8 ). During cold season it is well seen in Sea Surface Temperature (SST) satellite images ( 13 ). During warm season the frontal interface at the sea surface is effectively disguised by the formation of a strong seasonal thermocline, implying 1 anamartins@notes.horta.uac.pt; phone +351 292 200 408; fax: +351 292 200 411; www.horta.uac.pt