The Upper Ocean ( 10 M) Has a Complex and Variable Vertical Temperature Structure That

Intra- and inter-annual subsurface hydrologichydrographic variations in the vicinity of the Tropic of Cancer (Mazatlán, Mexico).

David Serrano and Arnoldo Valle-Levinson*

*Corresponding author (Si estás de acuerdo)

Abstract…

1. INTRODUCTION

Sea temperature is one of the most relevant oceanographic parameters associated with climate and weather systems. Salinity, together with temperature and pressure, determine the density of sea water; changes in density over time and space are responsible for important movements in the ocean. Sea surface temperature (SST) is widely used in studies of atmospheric and oceanic problems, including the environmental management of oceans and fisheries. Subsurface temperature has a complex and variable vertical structure that is associated with atmospheric forcing, ocean turbulence, advective processes, internal waves and changes in sea surface level, among other things. Changes in the mixed layer depth, in the thermocline and in the depth of a specific isotherm are usually analyzed to describe the spatiotemporal variations of subsurface seawater temperature. Variations in sea surface height are also related to vertical fluctuations of the thermocline. Despite their relevance to climate studies, in situ oceanographic observations are not sufficient to fully characterize the kinematics and thermal states of the oceans.

Seasonal, inter-seasonal and longer time-scale variations in sea temperature and salinity (surface and subsurface) have been described and analyzed in different regions of the world. In the South China Sea, Zhou and Gao (2002) investigated the intraseasonal variability of the subsurface temperature; they concluded that the intraseasonal variability of subsurface temperature is determined by the vertical displacement of the thermocline, which produces subsurface temperature variations of large amplitude. In addition, Ishii et al. (2003) determined that dynamic height anomalies estimated from temperature and climatological salinity were highly correlated with sea level height observations in the tropical Pacific. Moreover, based on conductivity-temperature-depth (CTD) measurements from the western Pacific Ocean (along 165º E ) for the 1993–1998 period, Maes and Behringer (2000) showed that altimetry observations such as those from the TOPEX/Poseidon satellite are representative of thermal and haline variability, and that altimetry signals may be projected downward into the upper oceanic layers. In the North Atlantic, Hughes et al. (2009) used time series of SST and sea subsurface temperature to demonstrate that variations in surface temperature are reflected in sea subsurface conditions; they concluded that surface temperature in situ may or may not have a close relationship with subsurface temperature; therefore, it is important to consider the local hydrography using SST data as an approximation of subsurface conditions.

In the Gulf of California, several studies reported relevant information on the subject; for example, in an ecological study, Carballo and Nava (2007) described the sea surface temperature of the Bay of Mazatlán for the period 2001-2003, and determined that temperature varies over a wide range along the seasonal cycle, reaching 12 °C (January-August 2001). In Bahía Concepción, a bay on the west coast of the Gulf of California, López-Cortés et al. (2003) found that subsurface water temperature in the bay for the year 1998 was affected by El Niño, recording higher temperatures compared to the years 1997 and 1999, along with a 2 to 3 month delay of the stratification period. In a subsequent study conducted in 2005 in the same bay, Cheng et al. (2010) studied the variations of water temperature along a seasonal cycle, and found that the temperature in the water column remained almost homogeneous during the winter, whereas in the summer it was stratified; they also determined that the advection of cold water from the Gulf of California has an important role in the thermal structure of the water column.

Moreover, to analyze the spatiotemporal variation of the subsurface thermal structure of the ocean, several authors have used time series analysis, empirical orthogonal functions, the estimation of the heat fluxes at the ocean-atmosphere interface, as well as the estimation of sea level surface variations with respect to fluctuations in time and space of the thermocline. For example, in the Canary and Iberian Basins of the North Atlantic, Müller and Siedler (1992) used an array of 22 subsurface moorings to determine that the vertical structure of the water column can be well approximated by the barotropic and first-order baroclinic dynamical modes as well as with an empirical orthogonal function. Along the Atlantic Ocean, at about 26ºN, Szuts et al. (2012) found that subsurface fluctuations in the center of the basin are large and were well described by a first baroclinic mode; they also found that these signals have long periods and were accurately described by sea surface height. In the South China Sea, Liu et al. (2001) determined that at three oceanographic stations the amplitudes of the time series of the depth of the 22 ºC isotherm were two orders of magnitude greater than the sea surface height. They also concluded that the first baroclinic mode, represented by the ratio between sea surface height and thermocline depth anomaly, is dynamically important in the central South China Sea. In the Tropical Pacific Ocean, Smith and Chelliah (1995) used harmonic analysis to show that, in general, subsurface temperature variations are much larger than surface variations, and that most of the subsurface variations are associated with changes in the depth of the thermocline. Finally, Cheng et al. (2010) determined the horizontal heat advection in Conception Bay by calculating the difference between the rate of change of heat content in the water column and the net surface heat flux. Similarly, Castro et al. (1994) estimated the horizontal heat advection along the Gulf of California by integrating the difference between the rate of change of heat content at a depth of 400 m and the net surface heat flux.

The present study was conducted over four years of water column observations in the vicinity of Mazatlán, Sinaloa, (Fig. 1), located in the Mexican Pacific coast, at the mouth of the Gulf of California. The main objectives of this study were to analyze changes of temperature, salinity and density in time and space, and to determine the role of sea surface height fluctuations in the thermocline, as well as the role of the water masses and advective processes that are reflected in the heat content of the water column. Section 2 briefly describes the area of study and data collection methods. Section 3 describes the changes in time and space of temperature and salinity in the water column from the surface to 127 m depth; it also describes and identifies the most important harmonics of the time series, as well as the dominant modes of variability in temperature and salinity. The relationship between the sea surface height and the isotherm of 18 °C is also addressed in this section, as well as the heat content in the water column and the net heat flux. Finally, the discussion and the conclusions are presented in sections 4 and 5 respectively.

2. STUDY AREA AND DATA COLLECTION

Mazatlán Bay is in northwestern Mexico in the coastal state of Sinaloa, located at 23° 12’ N and 106º 25’ W in the southeastern Gulf of California (Fig. 1). This region is influenced by the water masses of the Gulf of California, by Subtropical Subsurface water, Tropical Surface water and California Current water (Castro et al., 2000; Hendrickx and Serrano, 2010). The tide in the bay is mixed, mainly semidiurnal, with a form number F = 0.575. The maximum air temperature can reach 40 ºC (August-September) and the minimum temperature 15 °C (January-February; data from the weather station at the Mazatlán Airport). The region is under the influence of tropical cyclones from May to November; winds from the northwest have been recorded at the entrance of the Gulf of California, from December to March, reaching magnitudes of 12 m s-1 (Douglas et al., 1993).

Hydrographic data were obtained from 49 oceanographic surveys carried out from August 2005 to August 2009. Water temperature and salinity were measured by CTD (SEABIRD-9) at one station located at 23º 05’ N and 106º 36’ W, approximately 20 km of coastline (Fig. 1). The samplings were carried out on the boat Miztli of ICMyL (UNAM). The sampling interval was about a month; the station depth was 127 m; the vertical sampling frequency was 2 Hz; the temperature and salinity were interpolated for every meter depth and all the samples were collected between 9 and 11 a.m. The data were analyzed every meter depth (127 time series constructed from 49 surveys of the water column performed with the CTD). The time series were constructed using piecewise cubic Hermite interpolation, with Δt = 5 days; the interpolation generated matrices of 127 rows by 292 columns of temperature, salinity and. In order to relate the surface changes of sea level with the subsurface temperature structure, we used a regional time series (23º N 106º 30’ W) of sea surface height (SSH) from September 2005 to September 2009 constructed with readings from the TOPEX/Poseidon satellite (http://coastwatch.pfeg.noaa.gov/erddap/griddap/erdTAssh1day.html). In order to assess the horizontal heat advection, we calculated the difference between the rate of change of heat content in the water column and the net surface heat flux (Castro et al., 1994; Cheng et al., 2010). To approximate the net heat flux of the area of study, we used daily meteorological data from the Mazatlán Airport (23º 10’ N and 106º 16’ W) from August 2005 to August 2009. (http://www.wunderground.com/global/stations/76459.html). In addition, the time series of sea surface temperature was constructed using data from NOAA (http://coastwatch.pfeg.noaa.gov/erddap/griddap/erdAAssta1day.html). The heat content (HC, in J m-2) of the water column was calculated in accordance with Cheng et al. (2010),

Where is the specific heat of seawater, is water density, is water temperature, is a reference temperature arbitrarily set to zero ºC, andis the depth of the water column (127 m); was calculated in agreement with Millero et al. (1973).

The net heat flux at the surface (W m-2) was calculated as the sum of: .

Where is the net heat flux, or the difference between heat gains and losses across the air-sea interface, is the net radiation flux calculated as the difference between the net downward flux of solar radiation and the net upward flux of long-wave radiation from the ocean (Gill, 1982), is the sensible heat flux due to air-sea temperature differences, and is the latent heat flux due to water vapor transport.

The sensible and latent heat fluxes were estimated using the standard bulk formulae from the tropical ocean global atmosphere/coupled ocean atmosphere response experiment (Fairall et al., 1996)

and

Whereis the air density, is the specific heat of the air, is the sensible heat transfer coefficient (Stanton number), is the wind speed, is the air temperature, is the latent heat of evaporation of seawater, is the latent heat transfer coefficient, is the specific humidity at air temperature, and is the specific humidity at sea surface.

Moreover, we used Fourier analysis to estimate the amplitude and phase of the six main harmonics of the temperature and salinity fields. Power spectra were calculated for the time series of temperature and salinity at depths of 1, 30, 60, 90 and 120 m. In addition, the relative influence of conservative temperature and absolute salinity on the stratification of the water column and on the double diffusive convection was calculated using the TEOS-10 (Thermodynamic Equation of Seawater-2010), in which the Turner angle (Tu) is defined as: , where is the coefficient of thermal expansion, is the haline contraction coefficient and is the density of seawater; represents the vertical gradient. Finally, with the purpose of identifying the water masses in the water column, a TS-diagram was constructed using data from the 49 surveys.

3. RESULTS

3.1. Temperature, salinity and

Figure 2a shows the time series of temperature for a four year period (2005-2009) from the surface to 127 m depth. Four cycles are evident in the time series. The maximum surface temperature of 31.47 °C was recorded in August 2009 and the minimum temperature of 12.25 °C in January 2009 at 127 m depth. At the surface, the minimum temperature was 18.32 ºC, recorded in December 2007. The average surface temperature throughout the observation period was 26 °C. Subsurface (50-80 m depth) intrusions of cold water without apparent external thermal forcing (atmospheric heat fluxes) were recorded between September 2006 and February 2007, as well as between August 2008 and February 2009. The isotherm of 18 °C can be regarded as the lower limit of the thermocline (< -0.08 ºC m-1).

The time series of salinity shows four seasonal cycles that are still evident in the first 40 m depth (Figure 2b). Highest salinities were recorded in May and June between 40 m and the surface, with a maximum of 35.35 g kg-1. The lowest salinities occurred between July and November (the wet or tropical season). The minimum value was 31.08 gr kg-1, recorded at the surface in October 2008. The variance of salinity over time decreased with increasing depth, with 0.393, 0.041, 0.023, 0.008 and 0.003 g kg-1 for 1, 30, 60, 90 and 120 m, respectively. The fact that the highest temperatures coincided with the lowest salinities suggested that these relatively warm and fresh waters were associated with river plumes advected from shore. This is the first time that freshwater influence is reported in this part of the entrance to the Gulf of California.