1. MODULATING FACTORS OF THE CLIMATOLOGICAL VARIABILITY OF THE MEXICAN PACIFIC; MODEL AND DATA. ABSTRACT. Sea Surface Temperature and wind from the Comprehensive Ocean-Atmosphere Data Set (COADS), density climatological fields from LEVITUS, TOPEX/POSEIDON sea surface height altimeter observations and the Regional Ocean Modeling System (ROMS) are used to study the climatological variability and surface circulation of the Mexican Pacific. Several agents that drive the large scale variability; seasonal and interannual equatorial signals that propagate alone the coast as Kelvin waves; influence of the Gulf of California and Gulf of Tehuantepec; upwelling signals offshore of Cabo Corrientes, Baja California and Tehuantepec. ROMS was implemented for the Mexican Pacific and validated using COADS data. Comparison of both data sets are shown and discussed. DATA. Twenty years (1978-1997) of SST and wind data from COADS, density climatological fields from LEVITUS and nine years (1993-2002) of sea surface height (SSH) altimeter observations from TOPEX/POSEIDON. All data sets, COADS, LEVITUS and TOPEX/POSEIDON are a spatial resolution of 1°x1°. MODEL. We use the eddy-resolving, incompressible, hydrostatic, primitive equation ocean model called Regional Ocean Model System (ROMS). The model uses a generalized stretched- coordinate system in the vertical (20 levels) and a curvilinear horizontal grid (40x150 points grid). For the model initial condition and open boundaries we use temperature and salinity monthly climatologies from Levitus. At the surface, the model is forced with wind stress monthly climatologies derivated from COADS. The model bathymetry is obtained from the Smith and Sandwell data. CONCLUSIONS. ✔ Seasonal climatology from model currents reproduce the main circulation patterns in the Mexican Pacific. ✔ Both the data and the model show the advection of annual sea surface height (SSH) westward around 10°N to 15°N, formed by instabilities of the North Equatorial Countercurrent (NECC) that are subsequently advected westward by the North Equatorial Current (NEC). ✔ Two robust features of the SST (i.e. position and phase of the thermal equator and extension of the warm pool) are reproduced the model. Figure 4. Seasonal climatology of the surface currents (m/s) over SSH anomalies (colours, in meters), derivated from the model. This climatology show similar patterns to the circulation derived from data (Figure 3), Instabilities of the north equatorial current near Tehuantepec break into eddies that propagate westward at about 10°N and 20°N. This feature can be observed in the data as well and has been reported previously (Zamudio et al., 2001). Figure 3. Seasonal climatology of the geostrophic currents at the surface, derivated from heigth residuals of TOPEX/POSEIDON added to dynamic height relative to 500 m; based on the temperature and salinity climatology of LEVITUS (1994). This method was documented by Lagerloef et al., 1999; and reproduce the typicall surface circulation in the Mexican Pacific. For example, in spring the California Current (CC) is strong and southward, with velocities ~15cm/s, although in fall is weak and its velocities dimished. For Ecuatorial System currents in general terms, during fall the NECC is strong and the NEC is at highest position (15°N). While during spring the NECC is weak and the NEC is strong and begin to migrate southward. Flores-Morales A.L., Parés-Sierra A. Department of Physical Oceanography, CICESE, Ens., B.C., México. Figure 5. Hovmuller diagram of the SSH (in meters) from a) Model (ROMS) and from b) Data (T/P); along 12°N. This show the westward propagation of signal generated near shore; moving offshore at a speed of about 0.19 m/s. This signal is produced by high dynamic activity and the form of the bathymetry in Tehuantepec. All features of the wind for this region; the position of the ITCZ, the perturbations asociated with the circulation of equatorial currents and a pronounced reduction in the thermocline slope that supports the NECC, have been imprinted in the circulation near to Tehuantepec and in equatorial zone (Kessler, 2002). This region (10°N to 15°N) contributes very importantly to the Equatorial Current System circulation (Figure 3). a) b) Figure 2. Seasonal climatology of SST ( C, contours represent the 28°C isotherm.) derivated from COADS and from the model (ROMS). The upper four figures are the SST from COADS, the spatial resolution of this data is 1°x1°; and bottom frames represent the SST from ROMS. An important and robust factor of this variable is the thermic equator position (8°N to 15°N), wich is north of the geographic equator. Other characteristic is the eastern North Pacific warm pool (Wang and Enfield 2001), defined by the 28 °C isotherm, extending beyond 110°W, during the spring to summer. In winter the warm pool retracts eastward and the same time the ITCZ migrate southward. Figure 5. Longitudinal averaged ITCZ position from COADS winds, and maximum of SST from COADS and ROMS. It is evident the strong connection between the wind pattern and oceanic thermal equator. The position of the ITCZ is regulated by the circulation of the Equatorial System Currents. LATITUDE Winter. Spring. Summer. Fall. LATITUDE LONGITUDE Figure 1. Model domain and Smith&Sandwell bathymetry. LATITUDE LONGITUDE alflores@cicese.mx Fall. Summer. Spring. Winter. LATITUDE LONGITUDE 0.4m/s Winter. Spring. Summer. Fall. LATITUDE LONGITUDE 0.5m/s