C. A. Collins 1, R. Castro Valdez 2, A.S. Mascarenhas 2, and T. Margolina 1 Correspondence: Curtis A. Collins, Department of Oceanography, Naval Postgraduate School, Monterey, California, USA; 1 Naval Postgraduate School, Oceanography Department, Monterey, CA, USA; 2 Universidad Autόnoma de Baja California, Ensenada, B.C., Mexico. Acknowledgements. Financial support for this project was provided by NSF, US Navy, CONACyT, Mexican Navy. Thomas Rago, Marla Stone, Rafael Blanco, Eduardo Gil and Sergio Larios assisted with data collection. Figure 3. Mean geostrophic velocity (cm/s) superimposed on mean salinity across the entrance to the Gulf (Fig. 1). Mean was determined from eighteen sections occupied between 1992 and Baja California is on the left and Sinaloa on the right. The contour interval for geostrophic velocity is 5 cm/s. Dashed (positive) isotachs indicate flow into the Gulf and solid (negative) isotachs indicate flow out of the Gulf. Salinity values are given by the color bar at the right. Abstract Ocean transports between the Pacific Ocean and the Gulf of California contribute to the seasonal heating and cooling of the Gulf and add high salinity waters to the surface and upper thermocline waters of the Pacific. These transports have been measured by 1) shipboard hydrographic measurements across the Gulf along a section between Sinaloa and Baja California and 2) moored arrays of temperature, salinity and pressure instruments on either side of the entrance to the Gulf in water 130 m deep. The moored measurements extended from November 2003 to May 2006 and the hydrographic section was occupied eighteen times between 1992 and Overturning circulation within the Gulf was clearly indicated by the patterns of salinity along the hydrographic section in which inflows of fresher Pacific waters (S 34.8 for densities between 25 and 26 kg/m 3 ) along Baja California. Steric heights were calculated for each mooring and differences between moorings resolved baroclinic flow at 40 and 80 dbar relative to 120 dbar. Differences in steric heights between the mooring locations were compared to differences in satellite sea level height anomalies at the mooring locations. Agreement was good. The satellite data provided a more robust measure of the annual cycle of the mean surface geostrophic flow and transport because 18.3 years of continuous observations were available. Satellite data indicated that geostrophic surface flow was into the Gulf from mid-January to mid-September and was out of the Gulf for the remainder of the year. Maximum inflow (outflow), 0.07 m/s (0.04 m/s), occurred on July 1 (November 13). The mean surface flow was 0.01 m/s, directed into the Gulf. Shipboard data also showed a mean surface geostrophic current of 0.01 m/s. Subsurface transports were into the Gulf above 50 m at a rate of 0.06 Sv and out of the Gulf between 50 and 350 m at 0.24 Sv. Below 350 m, net transport was close to zero. Figure 1. Entrance to the Gulf of California. Red dots are CTD stations and yellow dots are mooring locations. Water depth is given by the color scale to the right of the chart. Figure 2. Schematic diagram of moorings at Cabo Pulmo and El Dorado (Fig. 1). Microcats are shown as blue rectangles at 120, 80, and 40 m EGU Summary The horizontal circulation (Fig. 3) at the entrance to the Gulf is cyclonic. High (low) salinity waters originate in the Gulf (Pacific). High salinity waters at depths >75 m along Baja California which flow out of the Gulf are evidence of a shallow meridional overturning circulation. Measurements of surface geostrophic flow into the Gulf were made by moorings (Fig. 4) and compared with altimetry (Fig. 4, 5). The correlation coefficient between the two time series was Seasonal variability dominated the 18.3 year time series of surface geostrophic flow (Fig. 6) although outflows were generally less than inflows and in some years (1995,1997, 2003, ) were weak (>-.02 m/s). Inflow dominated the seasonal cycle and occurred from yearday 15 to 260 with maximum inflow of 0.07 m/s on yearday 180. Outflow was maximum, m/s, on yearday 317. Largest SLA was observed during the El Niño, 0.3 m vs 0.2 m in other years. The observed surface geostrophic flow (Fig. 7) indicated that during fall and winter, inflow persisted and outflow was delayed until March and April. i The meridional overturning rate can estimated using Fig. 8. The rate of inflow, Vi, in the upper 50 m was 0.06 Sv and the rate of outflow, Vo, between 50 and 370 m was 0.24 Sv. The (evaporation-precipitation) for the Gulf is low, Sv, and cannot explain the observed difference between inflow and outflow. The mean surface geostrophic flow from SLA, m/s, agreed well with that obtained from CTD data (Fig 8). Figure 4. Time series of steric height (40/120 dbar) and AVISO SLA difference, El Dorado minus Cabo Pulmo. Positive values correspond to flow into the Gulf. Figure 6. Surface geostrophic velocity into the Gulf of California based upon AVISO SLA differences between El Dorado and Cabo Pulmo. Data were smoothed. Figure 5. Relationship between Steric Height (40/120 dbar) and AVISO SLA difference. Figure 7. Annual cycle of surface geostrophic velocity into the Gulf of California based upon AVISO SLA differences between El Dorado and Cabo Pulmo. (upper) Mean. Data for 1 July 1997 to 30 June 1998 are shown as a red line. (lower) Standard Deviation. Figure 8. Vertical profiles of geostrophic velocity across the entrance to the Gulf using reference layers of 400, 450, 500, 550, 600 dbars. Positive values indicate flow into the Gulf of California. This was calculated using hydrographic data shown in Fig. 3.