An observation of rapid thermocline formation in the Middle-Atlantic Bight

Abstract

A time-series temperature profile has been constructed from XBT drops made during the spring of 1973 at an industrial waste disposal site 60 km off the coast of Delaware. Isothermal conditions were found throughout the water column until mid-April. Surface warming was sudden and accomplished without detectable return to isothermy. Thermocline formation is attributed to a combination of vernal warming and influx of a warm water mass from the southeast. The temperature at 3 meters rose from 6.7 to 9.5 °C in 98 hours, an average of 0.7 deg./day. A distinct thermocline formed between 18 and 23 meters (ΔT = 1.8 °C) within 7 days of the cessation of isothermy. No appreciable deepening of the thermocline was observed in the month following its formation. The rate of near-surface warming was greatly reduced after thermocline formation. Temperature at 3 meters rose from 9.6 to 13.3 °C in the 30-day period from 21 April to 20 May, an average of 0.1 deg./day. Short-term incursions of warm bottom water were observed on two occasions, one before themocline formation and one after thermal stratification. Density stratification in the area is primarily thermally induced in the period following thermocline formation.