Sea Surface Turbidity 

 September 1979 



The northeastern Gulf of Alaska is bordered by mountainous coasthne 

 containing numerous glaciers, rivers, and streams which deliver large 

 quantities of suspended material to the gulf during the summer when 

 maximum discharge occurs. A CZCS image of the diffuse attenuation 

 coefficient of the area from late summer (9-2) illustrates various processes 

 that transport this coastally derived material offshore. (The diffuse 

 attenuation coefficient image is used instead of the pigment image due to 

 the preponderance of inorganic material in Alaskan coastal waters.) The 

 image also illustrates what appear to be streamlines of the Alaska Current, 

 the northern portion of the subarctic gyre that sweeps through the gulf 

 with peak surface speeds of 100 cm s"'. 



Inspection of the CZCS image of the Alaskan coast in the region around 

 Kayak Island (see locator map 9-1) reveals a prominent plume of turbid 

 coastal water deflected offshore in an anticyclonic gyre created by 

 impingement of the Alaska Current on Kayak Island. Similar offshore 

 transport of suspended particles occurs on the south side of Montague 

 Island, although in this image clouds obscure much of that area. These 

 coastal processes provide large quantities of material to the Alaska 

 Current for westward transport. 



West of Montague Island, the continental shelf widens and forces the 

 northern border of the Alaska Current southward. West of Kodiak Island, 

 the broad sweep of the Alaska Current in the eastern gulf coalesces into a 

 relatively narrow, fast-flowing current, with sharply defined borders, 

 called the Alaskan Stream. Although no synchronous hydrographic data 

 is available for this image, historical observations suggest that the sharp 

 ocean color gradient (dashed line-G) may indicate the southern limit of the 

 Alaska Current/ Alaskan Stream. 



The CZCS image of the westward flowing Alaska Current/ Alaskan 

 Stream presents markedly different flow characteristics than eastward 

 flowing boundary currents such as the Gulf Stream or the Kuroshio. 

 Streamlines of the Alaskan currents (not shown) are relatively organized 

 and show little indication of the multiple meanders and extensive eddy 

 formation observed in the Gulf Stream. Theoretical considerations based 

 on vorticity conservation predict multiple wavelike features in the flow 

 pattern of prograde flows (depth decreases to the left looking downstream) 

 such as the Gulf Stream but not in retrograde flows (depth decreases to the 

 right looking downstream) such as the Alaskan currents; CZCS imagery 

 of these two current systems support those predictions. 



In the southeastern Bering Sea, a change in the degree of vertical 

 mixing, occurring at roughly the 50-m isobath (9-1), controls the areal 

 distribution of both hydrographic properties and suspended particles. 

 Inshore of the 50-m isobath, the water column is typically well-mixed and 

 alongshore distribution of local sediment sources produces high turbidity 

 levels and sharp offshore gradients. In the CZCS image, the- diffuse 

 attenuation coefficient decreases abruptly seaward of this frontal zone, 

 particularly along the Alaska Peninsula. Unlike the coast of the Gulf of 

 Alaska, water flow on the north shore of the Alaska Peninsula is tidally 

 dominated and mean flow is very weak. Offshore transport of suspended 

 particles by advection rarely occurs. Loss of suspended particles from the 

 nearshore zone is largely by seaward diffusion; therefore, offshore 

 gradients are steep. 



