Stratification Summary Talk: Physical Oceanographic Results

Robert Beardsley

I. Introduction

The first intensive U.S. GLOBEC/Georges Bank process-oriented study was conducted during winter-summer, 1995. Called the Stratification Study, the field effort included a coordinated set of physical and biological measurements designed to test the central hypothesis that the spring development of vertical stratification and reduced wind-driven mixing over the southern flank of Georges Bank strongly influences the distribution of the zooplankton food supply, thus influencing the vital rates and survival of cod and haddock during their planktonic early life stages.

The main objectives of this summary talk were (a) to describe the physical processes which influence water column stratification and subtidal circulation over the southern flank of Georges Bank during the January-August, 1995 Stratification Study, and (b) to describe the biological response of the target species (zooplankton and larval fish) to the seasonal changes in stratification and circulation. The presentation was split into two parts corresponding to the above objectives, with B. Beardsley and J. Runge dicussing the physical oceanographic and biological results respectively.

II. Physical Oceanographic Results

The physical oceanographic components of the Stratification Study included: a moored array of meteorological and oceanographic instrumentation to monitor the surface forcing (wind stress and heat flux) and oceanographic response over the southern flank, microstructure profiling to estimate turbulent mixing on both sides of the tidal mixing front, repeat cross-bank hydrographic/ADCP sections and additional hydrographic/ADCP profiling at biological stations to monitor the seasonal change in water stratification and other water properties, the deployment of satellite-tracked drifters to measure near-surface Lagrangian currents, and the processing of AVHRR data to monitor frontal variability during the study. A prime objective of the moored array was to examine the role of surface heating/cooling in the vertically-integrated heat budget at the mid-shelf site ST1 (40.86N, 67.56W; water depth 76~m) located between the shallower tidal mixing front and deeper shelf-slope front.

The surface heat flux at ST1 was dominated by large latent and sensible heat losses during February and early March, associated with the passage of winter cyclones. By mid-March, the spring increase in insolation and decrease in both frequency and intensity of wind-driven cooling events combined to produce a positive net heat flux into the ocean. From mid-March through July, the surface heat flux was dominated by insolation and long-wave radiation and increased to a seasonal peak of about 300 W/m2 in early July. Hurricane Felix caused a net heat loss at ST1 in early August just prior to the mooring recovery.

The water column at ST1 was cold (4-7 degC) and exhibited little thermal stratification from February to May, when warm Slope Water intruded along the bottom over the southern flank. As the Slope Water retreated, a shallow thermocline was left which tended to deepen and intensify through the rest of the deployment. The vertical salinity difference at ST1 was generally weak except during the May intrusion of salty Slope Water and a second intrusian of Slope Water at mid-depth in August after Hurricane Felix. Except during these intrusion events, the pynocline was set by the thermocline.

Currents at ST1 were dominated by the semidiurnal M2 (12.42 hr) tidal component, with depth-averaged velocity amplitudes of about 45 cm/s (30 cm/s) along the major (minor) axis of the tidal ellipse which was aligned in the on-bank (along-bank) direction. The subtidal currents were generally weaker and strongly polarized in the along-bank direction. During the unstratified February-March period, the subtidal currents were coherent in the vertical and appear to be a wind-driven response to the winter cyclones. As the surface winds relax in spring and the water column stratification develops, the subtidal along-bank currents towards the southwest increased in strength and steadiness.

The local heat budget at ST1 was examined in terms of the time evolution of the depth-averaged temperature. After an initial cooling during February-March and some warming in April-May, the depth-averaged temperature increased steadily at a rate of 1.7 degC per month as the thermocline strengthened and deepened. The integrated surface heat flux accounts for most of the seasonal variation in the heat content (depth-averaged temperature) at ST1. Advective heat flux contributions include off-bank (wind-driven) cooling in February-March, along-bank cooling during May-July, and the Slope Water intrusion events in May and August.

Significant Slope Water intrusions were observed in 1995 and in 1997. These intrusions can push the shelf/slope front 10's of km on-bank over the southern flank, however, these intrusions appear to cause little net mixing with shelf waters based on the local heat budget. In both 1995 and 1997, the intrusions appear to be associated with advancing warm-core rings, and may contribute to the formation of the shelf-break jet with surface-intensified velocities up to 50 cm/s.

Principal PO Contributors:

R. Beardsley, K. Brink, J. Churchill, J. Irish, S. Lentz, R. Limeburner, R. Weller, S. Werner, A. Williams, W. Williams, C. Alessi (WHOI); J. Bisagni, J. Manning, D. Mountain, (NMFS); N. Oakey, P. Smith (BIO); D. Hebert, R. Burgett (URI).