1. Temporal and Spatial Variation of air-sea CO 2 Fluxes in the West Coast of Baja California, Mexico J. Martín Hernández-Ayón 1,Ruben Lara-Lara 2, Francisco Chavez 3, Taro Takahashi 4, Gilberto Gaxiola 2. 1 Instituto de Investigaciones Oceanológicas-UABC, Ensenada, B.C., Mexico; 2 CICESE-Ensenada, B.C., Mexico; 3 Monterey Bay Aquarium Research Institute, 4 Lamont-Doherty Earth Observatory. Introduction The California Current System (CCS) extends along the coasts of California (USA) and Baja California (B.C.), and to a great extent controls the upwelling and oceanography of the region (Ibarra-Obando et al., 2001). Seasonal coastal upwelling and the biological activity which it instigates are responsible for most of the variability in the distribution of macro- nutrients (C, N, P) and dissolved gases (CO 2 and O 2 ) along the central California coast region (Pennington & Chavez, 2000), we believe that a similar situation can be occurring in most of B.C. But of course many work need to be done. Furthermore, Interannual variability is closely linked to changes in the mid-latitude atmospheric circulation associated with El Niño events or La Niña perturbing the normal seasonal cycle on these coast. In Mexico, a few information is available related with the CO 2 system. This work present the first carbon dioxide study for the west coast of B.C. Methods Characteristic from Baja California Coast Acknowledgments: We really appreciate to Dr. Taro Takahashi and Dr. Fco. Chavez for the helpful pCO 2 data set that help us to learn more about the CO 2 system in our Mexican Coast. Conacyt for the support to the project “ FLUCAR-IMECOCAL”. A clear seasonal CO 2 fluxes regimes has been identified in the zone of the Mexican Pacific coast off Baja California: Part of winter to spring season all the coastal water was a sink for atmospheric CO 2 with a flux average during this period of -0.51 ±0.2 mol m2 year -1. However, the rest of the year it was a stronger source of CO 2 to the atmosphere with a flux average of 1.52 ±0.74 mol m 2 year -1. The mean annual flux for the Baja Coast was 0.97 ±1.1 mol m 2 year -1 suggesting this zone as a source of CO 2 to the atmosphere. Spatial data show North-South CO 2 flux variation in baja with the dominance of the low CO 2 during early the year in the waters over the shelf area making the region a net sink during upwelling season. But all this area became in a strong source of CO 2 after summer with higher fluxes south of Punta Eugenia (latitude 28 o ). Upwelling was suppressed during the warm anomaly associated with El Niño in the West Coast of Baja (Durazo and Baumgartner et al. 2002) and the pCO 2 (20 o C) data presented an effect with lower values during the events. In 2005 higher total CO 2 concentrations was measured in west coast of B.C. as a result of more intense Upwelling. The pCO 2 (SST) also had a different trend when is compared with the values measured from 1993-2001. Conclusions pCO 2 and temperature in surface waters close to the coast from 1993-2001 were used from the data set from Dr. Taro Takahashi and Dr. Fco. Chavez. The pCO 2 data have been obtained by a method using an infrared gas analyzer or gas- chromatograph for the determination of CO 2 concentrations in a carrier gas equilibrated with seawater at a known temperature and total pressure. The precision of pCO 2 measurements has been estimated to be about ± 0.7% on average. The quality controlled data are archived at www.ldeo.columbia.edu/res/pi/CO2. pCO 2 data from 2005 were obtained during July from the IMECOCAL program survey also using a infrared gas analyzer. Seawater samples collected in the coast of Baja during upwelling in 2004 and 2005 for Total CO 2 analysis were measured by coulometry. From Espinosa-Carreon et al. 2004 More total CO 2 was measured in 2005 than 2004. But also more Chl-a was observed. Observations during el Niño 1997-98 Spatial and Temporal Flux Variations A delayed coastal upwelling were observed along the U.S. West Coast in 2005 but in Baja California upwelling were more intense. Two clear features in Baja: 1)Sink of CO 2 before summer 2)Source after summer Southwest of B.C.was the biggest source of CO 2 Northwest of B.C.was the biggest sink of CO 2 In B:C the event was observed from Oct-January From Durazo and Baumgartner et al. 2002 Chl 2004 2005