Discussions and summary

In this chapter, we reviewed the status of research on the upper-ocean physical response to hurricanes and the hurricane-induced phytoplankton blooms observed by the satellite remote sensing. Using the phytoplankton bloom triggered by Hurricane Katrina as an example, a notable phytoplankton bloom triggered by hurricane wind-driven upwelling and vertical mixing is analyzed and quantified with combined data sets of the NCEP winds, AMSR-E microwave SST, and MODIS ocean color products. MODIS-Aqua ocean color observation shows that the hurricane-driven phytoplankton bloom lasted less than one week. By looking at both the blue and the NIR ocean contributions, we conclude that the phytoplankton bloom is the only source for the ocean surface optical property change after Hurricane Katrina. In addition, AMSR-E SST and MODIS-Aqua ocean color observations show that physical and biological responses to the hurricane are not synchronized with the hurricane winds. Indeed, it is found that the ocean physical response as represented by SST lags the hurricane peak wind speed by about one day, while the maximum biological response could be observed nearly four days later after the passing of the hurricane. Indeed, satellite observations (e.g., AMSR-E and MODIS-Aqua) provide us with effective tools to monitor the physical, optical, geochemical, and biological changes of the ocean environments following an extreme weather event such as a hurricane. They also extend further evidence that the tropical cyclone is an important mechanism to pump nutrients into the upper euphotic zone and result in significant phytoplankton blooms, thereby leading to an increase of the ocean's primary production.

Even though satellite observations provide insights on the phytoplankton and its relationship with the physical response and nutrient supply changes following a hurricane, the location of the phytoplankton bloom at (24°N, 84°W) cannot be explained only by upwelling and vertical diffusion associated with Hurricane Katrina. In fact, it has been well documented that the maximum SST drop usually occurs to the right of the hurricane track because of the asymmetry in turning direction of the wind-stress vector that drives a very strong asymmetry in the mixed-layer velocity (Price, 1981). On the contrary, the observed Katrina-induced maximum SST drop and phytoplankton bloom at (24°N, 84°W) occurred on the left of the hurricane track. According to a National Hurricane Center report (www.nhc.noaa.gov/pdf/TCR-AL122005_katrina.pdf), the center of Hurricane Katrina was at (24.5°N, 84°W) while moving towards the west on August 27 at 0600 UTC. Thus, the location of the observed maximum SST drop and the center of the phytoplankton bloom is 20-30 km left on the hurricane track. The co-location of the cyclone eddy and the phytoplankton bloom explains this contradiction to the classic rightward-bias response theories: the pre-existing cold-core eddy at (24°N, 84°W) played an important role in the bloom event.

The numerical simulations in this study have further supported this hypothesis. The simulation results show that the SST cooling is more significant in the presence of the cyclone eddy at the location of the observed phytoplankton bloom. Although the response of the surface nitrate, phosphate, and silicate concentrations is due to the same mechanism of upwelling and vertical diffusion that induces SST cooling, the enhancement of the three types of nutrient concentrations at the surface are quite different. The surface nitrate and phosphate concentrations increase by ~100% and 40%, respectively, while the surface silicate concentration increase only ~20%. Given the same strength of upwelling and vertical diffusion, the enhancement of the surface nutrient levels depends on the initial vertical gradient of the subsurface nutrient concentrations within the maximum depth that the upwelling and vertical diffusion processes can reach. Generally, the vertical gradient of the nitrate and phosphate concentrations within the top 70 m depth are larger than that of silicate, and thus lead to much more significant surface enhancement. In addition, the preexisting cyclonic eddy greatly enhances the vertical gradient of nitrate and phosphate in the top 70 m depth, and consequently leads to more significant surface nitrate and phosphate concentrations in Experiment I. However, because of deep nutricline in the silicate vertical profile, the eddy does little enhancement on its vertical gradient of the top 70 m depth. Thus, there is little difference in the surface silicate enhancement between the two simulated cases.

Satellite observations revealed that Hurricane Katrina led to a significant SST cooling and a phytoplankton bloom near (24°N, 84°W) in the Gulf of Mexico. The SSH anomaly derived from satellite altimetry also showed that a southward-propagating Loop Current frontal eddy co-located with the phytoplankton bloom, which suggested that the eddy was an important factor in the phytoplankton bloom event. In this study, HYCOM is used to study the process of upper ocean responses to Hurricane Katrina. Two different cases with and without the presence of a cyclone eddy at the location of the observed phytoplankton bloom are simulated. The model run demonstrates that the Katrina-induced phytoplankton bloom is attributed to the pre-existing eddy, as well as to a slow-moving hurricane. The cyclonic eddy plays a critical role in the development of the Katrina-induced bloom, and it significantly strengthens the upper ocean dynamics and nutrient responses. In the simulations, the Katrina-induced upwelling and vertical diffusion process can reach ~70 m depth. The cyclonic eddy in the phytoplankton bloom area uplifted the isotherms and increased vertical temperature, nitrate, and phosphate gradients in the upper 70 m depth, thus leading to more significant SST cooling and increases in nitrate and phosphate concentrations than in the non-eddy case. Our model results also suggest that the nitrate concentration plays a dominant role in the development of the phytoplankton bloom with over 100% increase in the surface concentration, while the phosphate increases ~40%. The silicate has a minimal effect on the bloom with the smallest increase for its surface concentration.

Was this article helpful?

0 0
Renewable Energy 101

Renewable Energy 101

Renewable energy is energy that is generated from sunlight, rain, tides, geothermal heat and wind. These sources are naturally and constantly replenished, which is why they are deemed as renewable. The usage of renewable energy sources is very important when considering the sustainability of the existing energy usage of the world. While there is currently an abundance of non-renewable energy sources, such as nuclear fuels, these energy sources are depleting. In addition to being a non-renewable supply, the non-renewable energy sources release emissions into the air, which has an adverse effect on the environment.

Get My Free Ebook

Post a comment