On the influence of local and north Atlantic wind forcing on the seasonal variation of sea level on the Newfoundland and Labrador Shelf

Abstract

Sea level data from St. John's, Newfoundland, and from Nain, Labrador, are compared with results from a 1/4 ø x 1/4 ø resolution numerical model of the Newfoundland and Labrador shelf and the neighboring Labrador Sea. The model is barotropic, employs dynamics linearized about a state of rest, and uses linear bottom friction. The model is driven by the seasonal part of the Hellerman and Rosenstein wind stress field and by inflow from the rest of the North Atlantic specified along the eastern boundary. The latter is taken from a 1 ø x 1 ø version of the model applied to the North Atlantic between 10øS and 80øN. The results indicate that our simple model can account for that part of the annual cycle of sea level on the shelf directly attributable to wind forcing. In particular, the model can account for the difference between observed monthly mean sea level at St. John's, corrected for atmospheric pressure variations, and the seasonal steric height anomalies above 150 dbar on the neighboring shelf. We also compare our model results with the linear regression analysis carried out by Thompson et al. on monthly mean sea level data from Nain for the period 1964 to 1972. The model confirms the gains calculated for both the local wind driven signal and also that part of the signal due to wind forcing over the North Atlantic beyond the Labrador Sea. Comparing St. John's with Nain, we find that at St. John's, the model response is accounted for primarily by wind set-up on the shelf, whereas at Nain, the influence of North Atlantic wind forcing is a larger part of the total signal. These results suggest that data from the tide gauge at Nain could be useful for verifying model-predicted changes in the large-scale ocean circulation. The results also seem to be confirming the presence of the offshore transport variations predicted by the model.

Summer surface circulation on the Newfoundland shelf and grand banks: The roles of local density gradients and remote forcing

“…Close to shore the wind can maintain sea level gradients that drive flow below the Ekman layer. From numerical results presented by Greatbatch et al (1990), we estimated that the above summer mean wind stress sets up sea level by about 3 cm within about 100 km of the southeast coast of Newfoundland, leading to a southward geostrophic flow of about 3 cm s"…”

“…Smith, 1937;Petrie and Anderson, 1983;Peterson, 1987;Greenberg and Petrie, 1988;Hukuda et al, 1989;Greatbatch et al, 1990;Lazier and Wright, 1993;Narayanan et al, 1995;Tang et al, unpublished manuscript). The Labrador Current system is believed to have three branches at Hamilton Bank ( Fig.…”

Annual sea level variations off Atlantic Canada from satellite altimetry

“…Han et al (2002) studied annual sea level variation over the Scotian Shelf and Newfoundland Shelf using TOPEX/Poseidon along-track data from 1992-1998. They found that the annual cycle over the outer Scotian Shelf and the Scotian Slope could mostly be accounted for by steric height, while the coastal sea level off Eastern Newfoundland could be attributed to both local steric height from Petrie et al (1983) and a largescale wind forcing (Greatbatch, deYoung, Goulding et al, 1990). Han (2004) derived the annual sea level cycle in the Gulf of St. Lawrence using TOPEX/Poseidon along-track data from 1992-1999 and showed approximate agreement between the altimetric results and tide-gauge data.…”

Basin‐scale variability in the Labrador Sea from TOPEX/POSEIDON and Geosat altimeter data

“…The wind-driven Labrador Sea circulation from a barotropic model was found to be enhanced in January and February and weakened in July [Greatb.atch and Gouding, 1989; Greatbatch et al, 1990]. Greatbatch et al [1990] pointed out that the annual variability of the model gyre circulation was related to the wind forcing over the entire North Atlantic.…”

“…The wind-driven Labrador Sea circulation from a barotropic model was found to be enhanced in January and February and weakened in July [Greatb.atch and Gouding, 1989; Greatbatch et al, 1990]. Greatbatch et al [1990] pointed out that the annual variability of the model gyre circulation was related to the wind forcing over the entire North Atlantic. The observed winter gyre in the western Labrador Sea, first discussed by Clarke and Gascard [1983], was generated by convection after intense surface cooling [Seung, 1987].…”