Physical and biogeochemical variability from hours to years at the Bermuda Testbed Mooring site: June 1994–March 1998

Abstract

Abstract The Bermuda Testbed Mooring (BTM), initiated in 1994, provides important information concerning periodic and episodic processes. The BTM enables collection of virtually continuous data during periods of inclement weather when traditional sampling is not possible, and provides otherwise inaccessible data in the important spectral range of minutes to a month. New methodologies and analyses are evolving for interdisciplinary mooring measurements. For example, BTM and shipboard temperature and spectral downwelling irradiance data sets have been compared and are in good agreement. Several meteorological parameters determined using operational analyses and climatological estimates also compare favorably with BTM calculations. Kinetic energy (by frequency domain), de-correlation time scales, and spectra were computed using the BTM data sets. The ratio of kinetic energy attributed to periods less than 2 days to that in periods greater than 2 days is generally greater at 45 m than at 71 m (the winter season being the exception). The upper layer has more kinetic energy due to inertial motions, while the lower layer is more influenced by mesoscale eddies. It is inferred that important interactions occur between these layers. It is hypothesized that the specific timing of the onset of spring stratification and fall breakdown of stratification, as well as accompanying phytoplankton blooms, are controlled by the specific synoptic wind forcing and mesoscale eddy conditions. An example of this scenario is described using unique BTM observations of a fall bloom event. Specifically, (1) a mode eddy with a shallow (roughly 80 m) nutricline passed the site, (2) deepening of the mixed layer (from about 50–175 m) caused entrainment of nutrients into the euphotic layer, and (3) a three-fold increase in depth-integrated chlorophyll was observed. Finally, de-correlation time scales were found to lie between 5 and 15 days for the key physical and biological variables. These de-correlation scales set upper limits on sampling intervals necessary to resolve the important physical and biogeochemical processes.