Effects of hurricanes on ambient noise in the Gulf of Mexico

Long-term omni-directional ambient noise was collected at several sites in the Gulf of Mexico during 2004 and 2005. The Naval Oceanographic Office deployed bottom-moored Environmental Acoustic Recording System (EARS) buoys approximately 159 nautical miles south of Panama City, Florida, in water depths of 3200 meters. The hydrophone of each buoy was 265 meters above the bottom. The buoys were located near a major shipping lane, with an estimated 1.5 to 4.5 ships per day passing nearby. The data duration was 14 months, and data were sampled at 2500 Hz with a bandwidth of 10-1000 Hz. Data were processed in eight 1/3-octave frequency bands, centered from 25 to 950 Hz. Monthly values of the following statistical quantities were computed from the resulting eight time series of noise spectral level: mean, median, standard deviation, skewness, kurtosis, and coherence time (the time for the autocorrelation function of each time series to fall to e"¹ of its central, zero-lag value). Four hurricanes were recorded during the summer of 2004, and they had a major impact on all of the noise statistics. Hurricane Charlie was recorded in August, followed by Hurricanes Frances, Ivan, and Jeanne in September. Nearby National Data Buoy Center (NDBC) weather buoys recorded wind speed and wave height data, which allowed for comparison of underwater noise levels with the wind speed and significant wave height data during extreme weather conditions. During hurricane conditions, the ambient noise levels at higher frequencies (400-950 Hz) are elevated, as expected, and are highly correlated with the wind and wave height data. The ambient noise levels at lower frequencies (25-100 Hz) are depressed, perhaps an indicator of less shipping activity during extreme wind conditions. The fewest number of peaks, as well as troughs, per day in the noise levels are observed from 200-950 Hz, yielding the smallest estimate of nearby ships per day. The average time between peaks, as well as trou- ghs, is maximum from 200-950 Hz. The variability of the data is high at higher frequencies (400-950 Hz) during hurricanes, as indicated by the standard deviation and the spread of the data (the difference between the 10^th and the 90^th percentiles). The skewness is positive from 25-400 Hz and negative from 630-950 Hz. The kurtosis is high from 50-100 Hz, peaking at 100 Hz. The coherence time peaks during hurricanes in the higher frequency bands. The coherence time is maximum from 200-950 Hz, ranging from 10 hours at 200 Hz to 33 hours at 950 Hz. The passage of Hurricane Ivan is analyzed in detail. Ivan was actually recorded twice. Ivan passed by the EARS buoys once, went ashore near the Alabama-Florida border, moved into the Atlantic Ocean, and then its remnants came back into the Gulf of Mexico a second time. During its first approach, Ivan was a Category 4 hurricane as its eye passed within 101 nmi of the EARS buoys. Its acoustic effects were evident for a 3-day period. Noise levels in higher frequency bands increased by approximately 11 dB per day during Ivan\´s first approach and decreased by approximately 11 dB per day as Ivan departed the EARS buoy\´s range.