The Annual Evolution of Geostrophic Flow in the Gulf of Maine: 1986–1987

Wendell S. Brown Ocean Process Analysis Laboratory, Institute for the Study of Earth, Oceans and Space, University of New Hampshire, Durham, New Hampshire

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James D. Irish Ocean Process Analysis Laboratory, Institute for the Study of Earth, Oceans and Space, University of New Hampshire, Durham, New Hampshire

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Abstract

The annual evolution of the geostrophic flow structure in the Gulf of Maine was studied with a combined set of moored pressure time-series measurements and five hydrographic surveys from August 1986 through September 1987. A series of quasi-synoptic dynamic height maps depicts a gulf flow structure whose typical spatial scales decrease from order 100 km during the winter to about half that in the summer, when the evolution of surface, intermediate, and deep water masses is most rapid and complex. The unusually large amount of freshwater in the gulf during 1987 was partially responsible for the establishment of a north–south across-gulf front during the summer. Year-long time series of bottom pressure and internal pressure (derived from temperature and conductivity measurements in Georges and Jordan basins) have been differenced with coastal synthetic subsurface pressures (SSP) to yield a history of the basin-scale geostrophic flow variability. The basin-scale geostrophic transport was dominated by cyclonic flow (>0.5 × 106 m3 s−1) through the gulf during autumn 1986. During early January 1987, the flow around Jordan Basin became anticyclonic as relatively fresh Scotian shelf water flowed into the eastern gulf. Though temporarily disrupted during April and May, the Jordan Basin anticyclone persisted through July. Inflows of slope water to Georges Basin helped to establish a robust (0.5 × 106 m3 s−1) cyclonic flow around Georges Basin in June and July. Though somewhat weakened (∼0.3 × 106 m3 s−1), the cyclonic gyre migrated with the flow of slope and Maine bottom water to Jordan Basin in August. The delay in the establishment of the cyclonic gyre in Jordan Basin in 1987 appears to have been related in part to the effects of the anomalously large amount of freshwater in the gulf during 1987. A conceptual model of the annual evolution of gulf-scale flow, based on a hypothesized interplay of pressure gradient forcing produced by variable inflows (outflows), thermohaline forcing, and, to a lesser extent, wind forcing, is presented.

Abstract

The annual evolution of the geostrophic flow structure in the Gulf of Maine was studied with a combined set of moored pressure time-series measurements and five hydrographic surveys from August 1986 through September 1987. A series of quasi-synoptic dynamic height maps depicts a gulf flow structure whose typical spatial scales decrease from order 100 km during the winter to about half that in the summer, when the evolution of surface, intermediate, and deep water masses is most rapid and complex. The unusually large amount of freshwater in the gulf during 1987 was partially responsible for the establishment of a north–south across-gulf front during the summer. Year-long time series of bottom pressure and internal pressure (derived from temperature and conductivity measurements in Georges and Jordan basins) have been differenced with coastal synthetic subsurface pressures (SSP) to yield a history of the basin-scale geostrophic flow variability. The basin-scale geostrophic transport was dominated by cyclonic flow (>0.5 × 106 m3 s−1) through the gulf during autumn 1986. During early January 1987, the flow around Jordan Basin became anticyclonic as relatively fresh Scotian shelf water flowed into the eastern gulf. Though temporarily disrupted during April and May, the Jordan Basin anticyclone persisted through July. Inflows of slope water to Georges Basin helped to establish a robust (0.5 × 106 m3 s−1) cyclonic flow around Georges Basin in June and July. Though somewhat weakened (∼0.3 × 106 m3 s−1), the cyclonic gyre migrated with the flow of slope and Maine bottom water to Jordan Basin in August. The delay in the establishment of the cyclonic gyre in Jordan Basin in 1987 appears to have been related in part to the effects of the anomalously large amount of freshwater in the gulf during 1987. A conceptual model of the annual evolution of gulf-scale flow, based on a hypothesized interplay of pressure gradient forcing produced by variable inflows (outflows), thermohaline forcing, and, to a lesser extent, wind forcing, is presented.

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