Quasigeostrophic Forecasting and Physical Processes of Iceland-Faroe Frontal Variability

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  • 1 SACLANT Undersea Research Centre, La Spezia, Italy
  • | 2 Division of Applied Sciences and Department of Earth and Planetary Sciences, Harvard University, Cambridge, Massachusetts
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Abstract

Using a hydrocast survey of the Iceland-Faroe Front (IFF) from October 1992, quasigeostrophic forecasts are studied to validate their efficacy and to diagnose the physical processes involved in the rapid growth of a cold tongue intrusion. Explorations of 1) the choice of initial objective analysis parameters, 2) the depth of the unknown level of no motion, 3) the effects of surrounding mesoscale activity, 4) variations in the boundary conditions, and 5) simple assimilation of newly acquired data into the forecasts are carried out.

Using a feature validation technique, which incorporates a 1) validating hydrocast survey, 2) satellite SST images, and 3) surface drifter observations, most of the forecasts are found to perform well in capturing the key events of the validation strategy, particularly the development of the cold tongue intrusion (though it tends to develop somewhat more weakly and slightly farther downstream than observed). Sharp resolution of frontal structure (to capture seed anomalies in the IFF, which later can grow to large amplitude) and smooth representation of far-field boundary conditions (to eliminate spurious persistent inflow/outflow at the boundaries, which can corrupt developing interior flows) are found to be crucial in generating good forecasts.

An analysis of the potential and kinetic energy equations in the region of the developing cold tongue intrusion reveals a clear signature of baroclinic instability. Topography has little influence on this particular instability event because it tends to be surface intensified and occurs rapidly over a timescale of 3–5 days.

Abstract

Using a hydrocast survey of the Iceland-Faroe Front (IFF) from October 1992, quasigeostrophic forecasts are studied to validate their efficacy and to diagnose the physical processes involved in the rapid growth of a cold tongue intrusion. Explorations of 1) the choice of initial objective analysis parameters, 2) the depth of the unknown level of no motion, 3) the effects of surrounding mesoscale activity, 4) variations in the boundary conditions, and 5) simple assimilation of newly acquired data into the forecasts are carried out.

Using a feature validation technique, which incorporates a 1) validating hydrocast survey, 2) satellite SST images, and 3) surface drifter observations, most of the forecasts are found to perform well in capturing the key events of the validation strategy, particularly the development of the cold tongue intrusion (though it tends to develop somewhat more weakly and slightly farther downstream than observed). Sharp resolution of frontal structure (to capture seed anomalies in the IFF, which later can grow to large amplitude) and smooth representation of far-field boundary conditions (to eliminate spurious persistent inflow/outflow at the boundaries, which can corrupt developing interior flows) are found to be crucial in generating good forecasts.

An analysis of the potential and kinetic energy equations in the region of the developing cold tongue intrusion reveals a clear signature of baroclinic instability. Topography has little influence on this particular instability event because it tends to be surface intensified and occurs rapidly over a timescale of 3–5 days.

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