Seasonal Evolution of the Geostrophic Circulation in the Northern Gulf of California

Abstract

An improved description of the seasonal variability of the geostrophic circulation in the northern Gulf of California is obtained from a historical hydrographic database larger than in previous studies. The evolution of the geostrophic circulation consists of: a cyclonic (summer) period, an anticyclonic (winter) period and two transition periods. The cyclonic (summer) period lasts from June to September (four months) and is characterized by a strongly baroclinic cyclonic gyre which dominates the circulation. The anticyclonic (winter) period starts in November and persists until April (six months); the anticyclonic geostrophic circulation is less robust than that in summer. The transition periods take of the order of one month, occurring around October and April–May, and are characterized by the simultaneous presence of both types of gyres. Empirical Orthogonal Function analysis shows a strong seasonal signal in the dynamic height for the first mode (97·2% of the total variance), whose spatial structure is a closed gyre, which oscillates between cyclonic during the summer months and anticyclonic the rest of the year. The results strengthen and extend those from previous work based on analysis of hydrographic data, on limited direct observations, on satellite image analysis, and on numerical modelling.