A study of oceanic responses to mesoscale processes in the Middle Atlantic Bight
Date
2015
Authors
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Publisher
University of Delaware
Abstract
The responses of the ocean to several mesoscale processes in the Middle Atlantic Bight (MAB), including the phytoplankton blooms, the cold pool and the horizontal heat advection, are investigated on different time scales using multi-sensor data including in-situ measurements, satellite observations and reanalysis data.
Taking the 2011 summer bloom as a study case, we investigated the transport and nutrient delivery mechanisms are investigated for large phytoplankton blooms of the New Jersey coast. The horizontal advection of the chlorophyll is approximately one order larger than the horizontal diffusion averaging the whole study area. And the sea surface currents are found to correlate with the wind stress but only for part of the middle shelf. Based on the horizontal advection simulation driven by HF radar current, the bloom is divided into three sub-regions. The northern coastal section of the bloom shows high-low-high pattern in chlorophyll time series, which is related to coastal upwelling and an anticyclonic eddy. Coastal currents play an important role in the eddy formation and the southward transport of the bloom. The southern coastal section of the bloom is supported by the nutrients from coastal upwelling and driven offshore by wind. The northeast section of the bloom is fueled by nutrient source upwelled through Ekman pumping. The magnitude of Ekman pumping is of the same order as the magnitude of the upwelling along the coast. These identified mechanisms could also be applied to more than half of bloom events during 2002 to 2013. In the summer of 2011 off the coast of New Jersey, processes in different sub-regions worked together, leading to this large phytoplankton bloom.
The influence of the cold pool in the Middle Atlantic Bight (MAB) to cross-shelf sea surface slope is explored by fitting an annual harmonic to temperature and salinity profiles from 1993 to 2012, and comparing to the 20-year-averaged altimetry sea level anomaly (SLA). The consistency within bottom temperature, thermal steric height, total steric height and altimetry observation validates that the cold pool induces depressed sea level in the middle shelf overlapping with the dominant surface seasonal cycles. Temporally, the cold pool pattern is most apparent in July and August as a result of magnitude competition between the thermal and haline steric height. In addition, Ensemble Empirical Mode Decomposition (EEMD) is employed to reconstruct the altimetry SLA and reveals the middle-shelf depression pattern from single year’s SLA data. The locations of the SLA depression from 1993 to 2012 agree with the cold pool locations identified from in-situ measurements, suggesting a promising application of altimetry SLA in the cold pool study.
Conclusively the modulation of the cross-shelf sea level variation by the cold pool is revealed, which contributes to the understanding of the sea level response to water masses on the continental shelf. Continuous satellite observations from 1992 to 2012 allow to investigate the contribution of the horizontal heat flux to the upper layer heat budget on different time scales over the middle and outer shelf of MAB. On the seasonal scale, the shelf averaged temperature variations are controlled by the vertical heat flux. On longer time scales, the temporal temperature variations are determined by the competition between the vertical heat flux and the horizontal heat advection. The horizontal heat diffusion is relatively small comparing to other terms in the heat budget. In the upper mixed layer, the heat advection due to the sloping sea surface is comparable to the magnitude of the wind-induced Ekman transport, each of which has different spatial distribution over the shelf. The interactions among different time scales (the sub-seasonal, seasonal, interannual, long-term scales and the mean) of the heat advection are quantitatively evaluated. For the advection related to the geostrophic currents, the largest components are the seasonal temperature by the mean currents, the mean temperature by the sub-seasonal currents, and the seasonal temperature by the sub-seasonal currents. They together contributing 80% to the total energy of the heat advection. For the advection associated with the Ekman currents, the largest components are the seasonal temperature by the mean Ekman currents, and the seasonal temperature by the seasonal Ekman currents. The relative magnitudes of temporal components are determined by the spectral distribution of the temperature and currents. Spectral analysis on the temporal components also suggests that the variations of the horizontal heat advection on specific time scales are contributed from multiply time scale variations. Through the detailed investigations on the mesoscale processes in the MAB, including the chlorophyll concentrations, the sea level and the heat advections, my work improves the understanding of the coastal ocean variations on different time scales, facilitates better interpretation of the observations, and clarifies the linkage among different variables. Moreover, climate change and the varying ocean are placing new challenges to coastal researchers. This dissertation enables further investigations of the coastal ocean responses to the climate change.