Abstract
Daily images collected with the MODIS sensor on the NASA Aqua satellite were used to describe chlorophyll-a (chl-a) concentrations before, during, and after synoptic-scale meteorological pulses in the Yucatan Basin, Caribbean Sea. The relative influence of vertical diffusive and vertical advective transport on mixing of the near surface layer was quantified with wind data from the QuikSCAT satellite. Computation of vertical density eddy diffusivity and gradient Richardson numbers was done with data from the three-dimensional MERCATOR model. The model evidenced the importance of vertical shear (i.e., vertical diffusive processes) in generating mixing during autumn and winter (2007–2009). During moderate meteorological pulses (e.g., cold fronts, easterly tropical waves, and low-pressure systems with wind speeds of 9–15 m s–1 sustained over 2 days), mixing caused by diffusive transport (eddy viscosity of 10 × 10–3 m2 s–1) was at least one order of magnitude higher than upward advective mixing (0.3 × 10–3 m2 s–1). During the passage of hurricanes Ivan (September 2004) and Wilma (October 2005), upward advective mixing (5–7 m2 s–1) dominated mixing of the upper water column and was nearly four orders of magnitude higher than during moderate meteorological events. Background chl-a concentrations of 0.03–0.08 mg m–3 were observed during the July–October period. During synoptic weather pulses, three different patterns in the chl-a concentrations were observed: first, chl-a concentrations in the range of 0.05 to 0.12 mg m–3 followed moderate meteorological pulses; second, higher regional chl-a concentrations (0.5–2.0 mg m–3) followed the passage of hurricanes; and third, the formation of filaments showing apparent high chl-a (0.3–1.5 mg m–3) south of Cuba, possibly caused by freshwater discharge from land. The changes in chlorophyll patterns following meteorological events illustrate typical patterns of connectivity among different parts of the Yucatan Basin.
Highlights
Satellite-derived chlorophyll-a concentrations, computed based on the ratio of seawater-leaving radiances obtained by ocean color sensors, have been applied to study the temporal and spatial variability of highly productive upwelling systems (e.g., Peláez and McGowan 1986, McClain 2009), as well as the functioning of oligotrophic systems (e.g., Babin et al 2004)
The Yucatan Basin is affected by weather phenomena yearround, including moderate and extreme meteorological forcing (Pérez-Santos et al 2010). In this manuscript we describe chl-a distributions in the Yucatan Basin at synoptic scales and assess their response to winds and circulation regime
The Yucatan Basin is located in the area known as the Atlantic warm pool (Wang and Lee 2007), which strengthens during the boreal summer and early autumn
Summary
Satellite-derived chlorophyll-a (chl-a) concentrations, computed based on the ratio of seawater-leaving radiances obtained by ocean color sensors, have been applied to study the temporal and spatial variability of highly productive upwelling systems (e.g., Peláez and McGowan 1986, McClain 2009), as well as the functioning of oligotrophic systems (e.g., Babin et al 2004). High chl-a levels can occur when well-lit surface waters experience a break-down in thermal stratification and increased mixing due to wind, resulting in the availability of nutrients from the subsurface layer (Müller-Karger et al 1991, González et al 2000). By analyzing satellite-derived chl-a from the Coastal Zone Color Scanner (CZCS), Müller-Karger et al (1991) obtained the first multi-year time series of monthly mean surface chl-a concentrations for the oligotrophic Gulf of Mexico and the northwestern Caribbean Sea; they observed higher Chl-a concentrations from December to February (>0.18 mg m–3) and lower concentrations in May and June ( 0.06 mg m–3). González et al (2000) further examined these patterns of phytoplankton concentrations in the Caribbean Sea, the Gulf of Mexico, and the Sargasso Sea, and discovered that the highest chl-a concentrations were associated with higher vertical mixing after the passage of cold fronts
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