Interannual stratification variability over southern Georges Bank

J. Bisagni

Interannual variability (IAV) of net vertical heat flux (Q ), potential energy anomaly or “stratification index” (V ) and date of onset for water column stratification were studied using five-day averaged model output over an eleven-year (1985-1995) period for three depth zones which span water depths from less than 60 m to 100 m on southern Georges Bank. Comparison between 5-day averaged model Q estimates from this study with Q measurements from US-GLOBEC mooring ST1 from winter-summer 1995 shows that nearly all of the difference between uncalibrated model estimates and calibrated ST1 measurements is attributable to the difference between the sum of the latent, sensible and long-wave components (Qe + Qs + Qlw) from each data set. Furthermore, there is virtually no IAV in residual 5-day averaged Qe + Qs + Qlw values relative to the long-term (1985-1995) mean seasonal cycle, suggesting strongly that the calibration of model Q estimates using measurements from mooring ST1 during 1995 may be applied to other years (1985-1994). Calibrated long-term (1985-1995) monthly mean Q values from this study show a similar seasonal cycle to climatological values determined from ship of opportunity data. However, values from this study are higher relative to the ship of opportunity long-term monthly mean climatology, with largest differences occurring during winter. IAV of monthly mean Q for zones 1-3 on southern Georges Bank is significant with the magnitude of residuals increasing from zones 1-3. Long-term (1985-1995) monthly mean model-derived V computed for zones 1-3 show zone 1 (< 60-m depth) remaining well-mixed throughout the year with no stratification development and therefore no detectable IAV for V . However, within zone 2 (60-80-m depth) and zone 3 (80-100-m depth) significant permanent stratification begins to develop during May and April, respectively, increasing almost linearly from May-July and reaching a maximum during July and August, respectively. IAV of V is large for zones 2 and 3 with typical standard deviations of 1000-5000 J m-2 occurring from late spring-summer with the largest coefficient of variation during spring and fall. Results from linear regression for zones 2-3 show that IAV of V is controlled largely by IAV of Q , explaining up to ~80% of the variance, with IAV of wind-mixing being of secondary importance. Results from model-derived five-day averaged V show that temporary or “transient” stratification nearly always occurs in early May and mid-April for zones 2 and 3, respectively, over the eleven-year study period and is associated with reduced tidal stirring during periods of neap tide. The mean date of first transient stratification for zone 3 coincides with the historical maximum abundance of early-stage (< 6 mm) cod larvae and copepod nauplii for Georges Bank. In addition, the historical maximum abundance of haddock larvae occurs on Georges Bank at the same time that permanent or “seasonal” stratification is established in zone 3.