Inherent optical properties imagery of the western North Atlantic Ocean: Horizontal spatial variability of the upper mixed layer

Abstract

Until now, no satellite methods were available for the study of inherent optical properties (IOPs) over wide areas of the world's oceans. Herein the coefficients of phytoplankton absorption, chromophoric dissolved organic matter (CDOM)‐detritus absorption, and total backscattering have been retrieved from Sea‐viewing Wide Field‐of‐view Sensor (SeaWiFS) 412, 490, and 555 nm reflectances by linear matrix inversion of an oceanic radiance model. The retrieved phytoplankton absorption coefficient from an October 6, 1997, image of the Middle Atlantic Bight (MAB) reveals (1) characteristic patchiness over the shelf, slope, Gulf Stream, and Sargasso Sea, (2) a warm‐core ring being encircled by a spiral streamer of phytoplankton‐containing cooler shelf water, and (3) phytoplankton prominence at South Atlantic Bight capes and remarkable minima between the capes. The retrieved CDOM‐detritus absorption coefficient image readily shows (1) the characteristic offshore 20× decline from ∼0.4 to 0.02−1, (2) a sharp drop at the Gulf Stream NW boundary, (3) striations of CDOM‐detritus within the Gulf Stream, and (4) that during the fall, non‐bloom season, the dominant absorbing constituent over the MAB continental shelf is not phytoplankton but CDOM‐detritus. The retrieved total backscattering coefficient image shows a patchy depression over the middle to outer shelf (compared to the Gulf Stream and inner shelf). A feature common to all three inherent optical property images is the northeasterly extrusion of phytoplankton, CDOM‐detritus, and other constituents from Cape Hatteras coastal/shelf water along a streamer adjacent to the Gulf Stream. No similar evidence of shelf water export was observed over the Diamond Shoals area south of Cape Hatteras where previous researchers have reported episodic offshore advection of shelf water. A second mechanism for carbon export can be seen in the New York Bight where a tongue of outflow from the Hudson River can be seen to cross the shelf and interact with the northern wall of a warm‐core ring. A concurrent 150 m altitude airborne underflight with a calibrated ocean‐viewing 256‐channel spectroradiometer provided supplementary atmospheric correction to the SeaWiFS remote sensing reflectances and validation of the resulting SeaWiFS IOP retrievals. Simultaneous airborne lidar fluorescence was used to confirm further the SeaWiFS CDOM‐detritus and phytoplankton absorption coefficient IOP retrievals. The results strongly suggest that if satisfactory atmospheric correction can eventually be achieved without airborne underflights, then the SeaWiFS data will readily allow global‐scale IOP variability studies.