Dissertation Abstract

The variability of bottom dissolved oxygen (DO), temperature, and salinity in Long Island Sound (LIS) is examined using observations. During summer, stratification is intensified under weak wind and bottom DO starts to decrease. For the westernmost and shallow stations, bottom DO is correlated with stratification, but this relationship is not significant at deep stations. The bottom DO decreases until reaching its minimum, when bottom temperature is around 19-20 ℃, and then the recovery of minimum DO in early fall is fast. The results suggest spring bloom may be an important source of organic carbon and biological uptake plays a more crucial role in seasonal evolution of bottom DO than previously thought. There is a strong seasonal variation in temperature, and its interannual variability is characterized by a higher variance during winter than summer. Salinity shows seasonal cycles, but there is a long-term change. Empirical orthogonal function analysis indicates the first modes of principle components contain quasi-biennial periodicities. In addition, the salinity anomaly shows an additional signal at a decadal time-scale. The seasonal variations are primarily associated with heat flux and freshwater, but forcings other than local processes control the interannual variability, most probably through horizontal exchange. The significant correlation between the salinity and the Gulf Stream suggests the long-term variability of the salinity in LIS is possibly connected to shelf-slope water. This is also supported by findings in Chesapeake Bay. Using wavelet analysis, bottom salinity reveals significant powers centered at the periods of 2 and 7.5 years. This demonstrates the interannual variability is forced by freshwater, and the quasi-decadal variability is associated with shelf-slope water condition. The Regional Ocean Model System (ROMS) is used to examine the role of horizontal advection in seasonal heat budget. It is found the net surface heat flux dominates the fall/winter heat balance, acting to cool the water column throughout the sound. In contrast, the horizontal advection plays a significant role during spring/summer due to the heat loss over the shallow areas and the heat gain along the deep channel.