Estimates of vertical heat flux and stratification from Southern Georges Bank, interannual variability, 1985–1995

Abstract

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 11 yr (1985–1995) period for three depth zones which span water depths from less than 60 to 100 m on southern Georges Bank. Comparison between five-day averaged model Q estimates from this study and Q measurements from US-GLOBEC mooring ST1 during 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 ( Q e+ Q s+ Q lw) from each data set. Furthermore, there is virtually no IAV in residual five-day averaged Q e+ Q s+ Q lw 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 to 3. Long-term (1985–1995) monthly mean model-derived V computed for zones 1–3 shows 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 11 yr 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.