Hydrography and circulation in the Northern Gulf of California during winter of 1994–1995

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Abstract

Hydrographic and current measurements in the Northern Gulf of California from December 1994 through March 1995 show conditions that are markedly different from those accepted as typical for winter. A rapid subsurface warm water intrusion (∼0.5 °C warmer) was observed, which reduced the stratification and stopped, and eventually reversed the typical anticyclonic winter circulation. An empirical orthogonal function analysis of the mean vertical structure of temperature and salinity of historical hydrographic data reveals that similar conditions occurred during March 1973 (warm water intrusion and weakening of stratification). In this study, it is proposed that these anomalous conditions are due to a combination of local and remote forcings: extensive water-mass formation in the Upper Gulf of California and the arrival of an intraseasonal downwelling coastally trapped wave to the Northern Gulf of California.

Introduction

The hydrography and the circulation of the Gulf of California (Fig. 1) are strongly seasonal, and the main driving agent for this seasonality is the Pacific Ocean, followed in importance by air–sea interaction. The Pacific forcing accounts for about two-thirds of the seasonal variability of the heat and salt budgets, a fact that is reproduced by numerical models forced at the mouth either by a Kelvin wave of annual period (Ripa, 1997; Beier, 1997) or by the seasonal variability of the hydrography at the gulf entrance (Marinone, 2003). In addition, these models use a monsoonal (sinusoidal) homogeneous wind forcing, which blows along the gulf from the northwest in winter and from the southeast in summer. These models produce a surface circulation which is mostly cyclonic in summer and anticyclonic in winter. This pattern is especially clear in the Northern Gulf of California (NGC) where a seasonally reversing closed gyre covering Delfín Basin dominates the circulation; this circulation pattern in the NGC is well supported by direct observations (Lavín et al., 1997b; Carrillo et al., 2002; Palacios-Hernández et al., 2002). The fluxes of heat and moisture across the sea surface are also seasonal (Lavín and Organista, 1988; Castro et al., 1994; Beron-Vera and Ripa, 2000, Beron-Vera and Ripa, 2002), but have little effect on the seasonal circulation; they account for part of the heat and salt budgets, and are responsible for the thermohaline structure of the upper layers of the gulf (Bray, 1988b; Lavín et al., 1995). For a recent review of the oceanography of the Gulf of California, see Lavín and Marinone (2003).

The Pacific Ocean also drives the most important interannual anomalies in the Gulf of California, namely those due to El Niño and La Niña (Baumgartner and Christensen, 1985; Robles and Marinone, 1987; Marinone, 1988; Bray, 1988a; Ripa and Marinone, 1989; Romero-Centeno, 1995; Lavín et al., 1997a, Lavín et al., 2003; López et al., 2005). During El Niño the sea level and the sea surface temperature (SST) present positive anomalies (up to 0.2 m and 3 °C, respectively), as low-salinity Tropical Surface Water invades the upper layers (∼150 m) up to Guaymas Basin (Torres-Orozco, 1993; Lavín et al., 1997a). At higher frequencies, coastal trapped waves generated by storms in the Pacific Ocean have been shown to dominate the coastal circulation in the Southern Gulf of California, and to be capable of affecting the NGC (Merrifield and Winant, 1989; Romero-Centeno, 1995; Zamudio et al., 2002; Martínez and Allen, 2004a, Martínez and Allen, 2004b).

Analyzing the PODAAC AVHRR SST record (1984–2000) for the Gulf of California, Lavín et al. (2003) found that the largest positive anomalies are due to El Niño events (see also Soto-Mardones et al., 1999), but also identified significant positive anomalies, which are as large as in weak to moderate El Niños events, which are not related to that phenomenon. The largest of such anomalies in their record occurred during summer of 1990 and winter of 1994–1995; the former lasted several months, while the latter lasted only 2 months. The origin of these anomalies was not discussed in depth, although it was suggested that they could be associated to variability of the warm water pool of the Eastern Pacific or to the variability of the equatorial Pacific (Lavín et al., 2003).

The seasonally reversing circulation pattern in the NGC (with a cyclonic gyre in summer and an anticyclonic gyre in winter) is a feature of the regional oceanography that is now well established, and biological and ecological consequences have been proposed (e.g. Santamaría del Ángel et al., 1994; Marinone et al., 2004) or are being considered. But of course, the ocean is so variable that anomalies are bound to occur, and it is useful to have an estimate of their possible effects. As mentioned above, most of the numerical models of the Gulf of California use highly idealized forcings, but eventually the variability will be included and observations will be needed to assess their capability.

In this article, we use direct observations to report that the hydrography and circulation in the NGC in the winter of 1994–1995 exhibited an anomalous behavior as compared with the climatological seasonal cycle, and possible explanations for the anomaly are investigated.

Section snippets

Data

From December 1994 through March 1996 five campaigns were undertaken in the NGC from the R/V Francisco de Ulloa and one from the R/V El Puma; in addition to CTD surveys (Figs. 2a–g), current meter moorings were maintained throughout that period at the sites shown in Fig. 2p (also see Table 1). A factory-calibrated SeaBird CTD (model SBE-911 plus) was used and the data were processed following García-Córdova et al. (1996). The data from the first two cruises, FU9412 (Nov. 26–Dec. 18, 1994) and

Hydrography

Vertical sections of temperature, salinity and density in December 1994 along the longitudinal transect indicated in Fig. 2g is shown in Fig. 3. Temperature (Fig. 3a) ranges from ∼19 °C at the surface to ∼12 °C at the bottom of Delfín Basin, the surface mixed layer is ∼100 m throughout the NGC, and the thermocline has a slightly concave shape. In the Upper Gulf (depths <30 m) temperature is ∼16 °C, which is over 2 °C colder than the surface layer offshore; surface and bottom thermal fronts are

Discussion

Observations of the hydrographic conditions and circulation in the NGC during the 1994–1995 winter show a dramatic change of the typical winter conditions in both hydrography and circulation: (a) a reversing of the anticyclonic gyre-like winter circulation, (b) a gain of heat of the water column (contrary to the loss of heat reported for this area during winter) and (c) a large salty water invasion into the deeper basins. In this section, a discussion of this anomalous oceanographic condition

Conclusions

Anomalous conditions during December 1994 and January 1995 in hydrography and circulation observed from CTD casts and current meters were described in detail. The anomaly consisted of a rapid intrusion of warm water (∼0.5 °C) below 350 m depth, reducing stratification and inverting the typical winter circulation. Hydrographic data from the Gulf of California database were used to perform an EOFs analysis over the vertical mean temperature and salinity distribution. This analysis showed

Acknowledgments

This work was supported by CICESE and CONACyT (México) through contracts 3209-T9307 and D41881-F. We thank the crew of the R/V Francisco de Ulloa for their collaboration during the cruises. We acknowledge the generous support of the ICMYL of UNAM for the use of the R/V El Puma in one of the cruises. Technical support by Victor Godínez, Carlos Cabrera and Salvador Sánchez.

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