Synoptic Flow Patterns that Influence Wind-Induced Mixing and the Temporal Evolution of Hypoxia over Western Long Island Sound
Bratton, Sean Dennis
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Bottom dissolved oxygen (DOb) concentrations in wLIS have exhibited both a long-term decline as well as marked inter-annual variability over the last several decades (1950-2009). This behavior continues despite New York City having reduced the quantity of nitrogen in upper East River Water pollution Control Plants (WPCPs) over the last two decades (Wilson t al. 2008). Processes controlling the inter-annual variability, intensity, spatial extend, and duration of hypoxia is still not fully understood. A number of studies have focused on anthropogenic factors, but air-sea interactions have also been documented to be important. The objective of this study is to identify physical factors including wind direction and speed in the form of `mixing events', to help explain inter-annual and inter-decadal DOb variability. NYC DEP and EXRX LISCOS buoy DOb and temperature data were used in conjunction with La Guardia (LGA) and NBDC buoy 44022 meteorological data to identify wind speed and directional criteria preferred for water column mixing. It was found that for winds 30??-110?? with magnitudes >= 4 m s-1 are the most efficient for mixing. Using these wind criteria resulted in a 1.5 mg L-1 increase in DOb on average within the water column. A synoptic climatology was created, using NCEP/NCAR Reanalysis data, based on the above wind criteria. These events were then categorized based on relevant location and type of the synoptic system into one of three groups; high pressure patterns, low pressure patterns, or hybrid patterns. Synoptic average and anomaly patterns, including 500-hPa geopotential heights and mea sea-level pressure (mSLP), were used for daily lag periods -4 to 0 days prior to the start of a mixing event to determine the synoptic evolution of low and high pressure patterns that produced mixing events. It was determined that high pressure patterns, including Pre-High and Extended High patterns, result in the highest percentage of total mixing events (76.3%) and display a significant increasing trend suggesting the frequency of anticyclones is a significant indicator for the frequency and trend of mixing events. Lastly, a connection between seasonal DOb/hypoxia data and mixing event metrics including; frequency, spacing, and duration are used to determine the extent of the relationship and if mixing events are able to help explain the recent (20-30 year) trends in seasonal DOb. It was found that mixing events are a strong indicator of the inter-annual fluctuations in DOb and the duration of hypoxia, however are not able to explain the 20-30 year DOb and hypoxia trends represented in these datasets.