Observations of a major warm water intrusion onto Georges Bank, summer 1997

C. M. Lee and K. H. Brink

During June 27 - July 5, 1997, a sequence of SeaSoar surveys was made along the southern flank of Georges Bank in an attempt to quantify exchanges across the shelfbreak front. The Seasoar is a towed, undulating profiler equipped with sensors for pressure, temperature, conductivity (salinity), chlorophyll fluorescence, and zooplankton (Video Plankton Recorder: VPR and Tracor Acoustic Plankton Sampler: TAPS). Shipboard ADCP and IMET sensors made measurements of currents and meteorological variables. Conditions were generally well stratified except over the center of the Bank, where tidal mixing prevailed. Here we present an overview of the observations obtained in these surveys.

A plume-like feature just south of the Bank, evidently of Gulf Stream origin, dominated our sampling region (Fig. 1). Warm, salty, low chlorophyll waters associated with the feature extended to depths greater than 125 m (Figs. 2, 3 and 4). In contrast, waters over the Bank were relatively cool, fresh and higher in chlorophyll albeit confined to a subsurface maximum). Early observations of the plume revealed a narrow streamer of cold, fresh water being drawn off-bank following the downstream edge of the warm feature. Later surveys documented a tendril of 'plume' water penetrating through the shelfbreak front to the 60 m isobath, well onto the Bank (Fig. 5). The tendril was about 20km wide and confined to the upper 50m (or less) of the water column. Lighter waters associated with the tendril moved in over denser Bank waters, forming pronounced fronts separating the two water masses.

Figure 1. AVHRR sea surface temperature reveals a sharp front separating cold Bank waters from slightly warmer Shelf waters to the south. Black lines mark the survey track- the South Flank Survey stretched to cover much of the Southern Flank. The large, warm hammerhead structure just south of the 100 m isobath is a plume-like extension of what appears to be Gulf Stream Water. ADCP currents suggest that the plume splits when it nears the Bank, flowing both to the west and to the east. Note the narrow streamer of cold water running off-bank along the northeastern edge of the plume.

Figure 2 and 3. Cross-bank sections of temperature and chlorophyll fluorescence along the southern flank, running between the 70 m and 2000 m isobath (shallow water to the north) as viewed looking down on the Bank from the south.

• Waters directly over the Bank show only weak salinity variability, with a shallow cap of warm, light water overlaying a weakly stratified interior.

• Further off-bank, a tongue of Bank water lies above slightly warmer, saltier Shelf water and beneath the warm, saline Gulf Stream waters associated with the plume.

• Bank waters exhibit a strong subsurface maximum in chlorophyll fluorescence beneath the stratified cap. In contrast, plume waters show only weak enhancement beneath the (deeper) surface layer.

• Compare the AVHRR image (Fig. 1) with the northeastern-most section, which runs along-bank (SW-NE) across the cold streamer extending off-bank along the plume edge. Waters on the SW end of the section have Gulf Stream characteristics, while those to the NE are more Slope-like. Neither carries an enhanced chlorophyll fluorescence signature. Cold, fresh water with a strong subsurface chlorophyll fluorescence maximum similar to that found over the Bank (Fig. 4), occupies the middle of the section, suggesting that the streamer seen in Fig. 1 draws water off the Bank.

Figure 4. THETA-S diagram colored by chlorophyll fluorescence from the eastern four cross-slope sections illustrate the primary water types. Waters over and near the Bank cluster on the left (fresh) side of the plot while Gulf Stream waters lie on the right (saline), with a clear mixing line extending between them. Bank waters lie on the near-vertical line (8 C < THETA < 15 C, S ~ 32.5 psu) and have elevated chlorophyll fluorescence values. A hint of saltier Shelf water lies beneath the Bank Water, forming a near-horizontal line 7.5 C < THETA < 10 C, 32.5 psu < S < 34.5 psu). Gulf Stream waters associated with the plume cluster in the upper right corner (20 C < THETA < 26 C, 36 psu < S < 36.5 psu) and have low chlorophyll fluorescence.

Figure 5. AVHRR imagery now shows the plume intruding further on-bank, with a prominent tendril extending well inside the 100 m isobath. Note that the sections were run farther on-Bank, following the feature as it moved into shallower water.