James J. Bisagni

Feature-oriented regional modeling and simulations in the Gulf of Maine and Georges Bank

The multiscale synoptic circulation system in the Gulf of Maine and Georges Bank (GOMGB) region is presented using a feature-oriented approach. Prevalent synoptic circulation structures, or ‘features’, are identified from previous observational studies. These features include the buoyancy-driven Maine Coastal Current, the Georges Bank anticyclonic frontal circulation system, the basin-scale cyclonic gyres (Jordan, Georges and Wilkinson), the deep inflow through the Northeast Channel (NEC), the shallow outflow via the Great South Channel (GSC), and the shelf–slope front (SSF). Their synoptic water–mass (T2S) structures are characterized and parameterized in a generalized formulation to develop temperature–salinity feature models. A synoptic initialization scheme for feature-oriented regional modeling and simulation (FORMS) of the circulation in the coastal-to-deep region of the GOMGB system is then developed. First, the temperature and salinity featuremodel profiles are placed on a regional circulation template and then objectively analyzed with appropriate background climatology in the coastal region. Furthermore, these fields are melded with adjacent deep-ocean regional circulation (Gulf Stream Meander and Ring region) along and across the SSF. These initialization fields are then used for dynamical simulations via the primitive equation model. Simulation results are analyzed to calibrate the multiparameter feature-oriented modeling system. Experimental short-term synoptic simulations are presented for multiple resolutions in different regions with and without atmospheric forcing. The presented ‘generic and portable’ methodology demonstrates the potential of applying similar FORMS in many other regions of the Global Coastal Ocean. r 2003 Elsevier Science Ltd. All rights reserved.

Satellite observations of sea surface temperature variability on southern Georges Bank

Abstract Sea surface temperature (SST) residual time series, derived from declouded Advanced Very High Resolution Radiometer (AVHRR) satellite imagery for southern Georges Bank, were examined for the period April–October 1987. Significant negative correlations were computed between the low-pass filtered SST residuals and daily-averaged tidal current magnitude, suggesting that for periods of ∼15 and ∼28 days, SST variability was related to the spring-neap tidal cycle in both the stratified and unstratified portions of southern Georges Bank during the stratified season. Maximum negative correlations occurred at a lag of ∼3 days, indicating that negative (positive) SST residuals lagged the maximum spring (minimum neap) tidal current by this amount. This lag and the measured 3σ SST residual of ∼3.0°C are in agreement with the summer hydrography on southern Georges Bank and a one-dimensional model in which the depth-independent vertical eddy diffusivity varies with time. Because meteorological forcing at the sea surface occurs on time scales of ∼1–7days, the data suggest that the observed, negative SST residuals are caused by periodic, enhanced vertical mixing of colder, sub-thermocline water into surface waters as a result of spring tidal currents during the stratified season. Given the inverse temperature-nitrate correlation for the region during the stratified season, the data suggest the occurrence of increased nitrate flux into the stratified and well-mixed regions of southern Georges Bank during spring tides. Periods of spring tides may result in the transport of a large fraction of the new nitrogen needed to sustain high primary production observed in the chlorophyll maximum within the stratified and well-mixed regions of southern Georges Bank during the stratified season.

Contrasting Response of the Eastern and Western North Atlantic Circulation to an Episodic Climate Event

AbstractRegional observational studies in the North Atlantic have noted significant hydrographical shifts in 1997–98 because of the episodic drop in the North Atlantic oscillation (NAO) during 1996. Investigation using a basin-scale model finds that, although the western North Atlantic (WNA) witnessed unusually low-salinity water by 1997, the eastern North Atlantic (ENA) simultaneously evidenced intrusions of high-salinity water at intermediate depths. This study shows that a major source of high salinity in the ENA is from the northward penetration of Mediterranean Outflow Water (MOW) that occurred concurrently with a westward shift of the subpolar front. The authors confirm that the low-salinity intrusion in the WNA is from enhanced Labrador Current flow. Results from climatological high- and low-NAO simulations suggest that the NAO-induced circulation changes that occurred in 1997–98 are a characteristic North Atlantic basin response to different forcing conditions during characteristic high- and low-N...

Ocean surface topography measured by the Geosat radar altimeter during the Frontal Air-Sea Interaction Experiment

An objective analysis (OA) technique was used to analyze Geosat Geodetic Mission residual sea height anomalies (RSHA) for the Frontal Air-Sea Interaction Experiment (FASINEX), conducted in the western North Atlantic. Preprocessing of the Geosat data included removal of a mean sea surface and orbit error and corrections for tides, wave height, inverse barometer, and ionospheric and dry tropospheric effects. Geosat data were not corrected for water vapor. Central to the analysis was the computation of a spatially anisotropic, time-dependent autocorrelation function directly from the Geosat data. Results show a single autocorrelation function peak, having a diameter of ∼270 km at 0-day time lag. A 15-month time series of RSHA topography was constructed at 10-day intervals, forming a set of 47 OA maps. These maps show a field of topographic features propagating to the west at ∼4 km d−1. The features exhibited spatial and temporal scales of ∼200 km and ∼50 days, respectively. The root-mean-square (rms) of the OA-derived feature amplitudes fluctuated slowly between 0.05 and 0.09 m with a period of ∼200 days over the 15-month interval. These fluctuations appeared to be related to the passage of large-scale, northeast-southwest trending sea surface temperature (SST) anomaly features reported for FASINEX, which were surface manifestations of large-scale, first-mode baroclinic Rossby waves. The rms amplitude of the OA-derived features was maximal (minimal) during periods of negative (positive) SST anomaly, when the depths of the mixed layer and main thermocline in the upper ocean shoaled (deepened). The fluctuations also appeared to be correlated with the structure of the field. Periods of decreased feature amplitudes corresponded to a “closely packed” structure, while during times of increased amplitudes, the field structure became “zonal” with regions of alternating sign. During FASINEX (January–June 1986) the field was characterized by the closely packed structure and decreasing feature amplitudes. This corresponded to the period of positive SST anomaly and high upper ocean heat content reported for FASINEX.