| dc.description |
Tropical instability waves (TIWs) are prominent seasonal features in both the equatorial Pacific and Atlantic Oceans. This work quantifies their role in modulating the distributions of nutrients and phytoplankton biomass. Using an eight year record of biannual ship observations along the Tropical Atmosphere Ocean (TAO) buoy array, cruise sections crossing TIWs were identified. Both a case study approach of individual TIWs and a first attempt at calculating their average effect on mixed layer properties were performed.
Examination of individual TIWs demonstrates that their effect on nutrient and chlorophyll distributions is a function of the TIW intensity. Both strong and weak TIWs drive elevated nutrient concentrations directly on the equator, but strong TIWs possess enhanced recirculation which advects nutrient- and chlorophyll-poor waters from adjacent to the upwelling zone equatorward. This decreases nutrient concentrations from approximately 2°N to 8°N. Weak TIWs retain elevated nutrient concentrations in this latitudinal band due to less recirculation in TIW vortices, permitting chlorophyll increases.
These differences between strong and weak TIWs were only observed north of the equator. Less recirculation was observed in TIW vortices south of the equator. This resulted in nutrient enhancements from TIWs along the southern portions of the cruise sections, especially in the eastern Pacific. The differences between northern and southern TIW dynamics suggest strong differences in TIW modulated carbon cycling between the two hemispheres.
Seasonal modification of the equatorial currents also influences the extent to which TIWs alter nutrient and chlorophyll distributions. TIWs observed during boreal winter demonstrated enhanced nutrient and chlorophyll concentrations north of the equator. This resulted from the water mass north of the upwelling zone containing elevated nutrient and chlorophyll concentrations due to a shallow thermocline. Thermocline shoaling in boreal winter is the result of a slowing in the South Equatorial Current and North Equatorial Countercurrent caused by the seasonal decrease in westward trade wind velocities. These results suggest that there is a synergistic effect from TIWs and the seasonal shoaling of the thermocline which may be important for carbon cycling north of the equator.
Composites of average mixed layer nutrient concentrations show TIW-induced nutrient enhancement on and south of the equator along most of the TAO lines, but no subsequent increase in mixed layer chlorophyll. This is likely due to the lag time between nutrient enhancement and biomass increase and/or chlorophyll increases in unsampled portions of the vortex. Regions of elevated chlorophyll concentrations were observed in SeaWiFS composites in unsampled portions of TIWs, which suggests that TIW induced lateral transport of nutrients may be driving important episodic export events south of the equator. |
|