The Alaskan Stream and Gulf of Alaska Circulation 

 July and September 1979 



The waters of the northern Gulf of Alaska contain very large across- 

 shelf gradients of salinity caused by the high influx of fresh water along the 

 British Columbia and Alaska coasts. This influx alters the vertical density 

 gradients, resulting in contrast in surface temperature and phytoplankton 

 concentration. Near the coasts the suspended sediments brought in by the 

 fresh water also result in strong but smaller scale ocean color differences. 



Persistent cloud cover in this area makes successful satellite imaging a 

 rare event. For thermal infrared images the horizontal temperature 

 contrasts are low, usually covering a range of 1° to 2° C, making it difficult 

 to trace ocean features. The CZCS ocean color (pigment) images, by 

 contrast, show dramatic flow features traced by the varying 

 phytoplankton concentrations in the stratified Gulf of Alaska water. 



The CZCS pigment image on 10 July (4-2) covers a small strip of the 

 northeast Pacific Ocean along the southern edge of the Aleutian Island 

 chain (4-1 ). The image shows the surface ocean color structure associated 

 with the Alaskan Stream in terms of pigment level variations in the range 

 of 0.4 to 1.0 mg m"\ Some of the Aleutian Islands appear as black areas in 

 the image; others are covered by clouds and cannot be identified. 

 However, most of the southeastern coasts of the Aleutian Island chain in 

 the right half of the image are cloud-free. 



The Alaskan Stream is the major current by which water leaves the Gulf 

 of Alaska. It flows westward as a narrow jet along the continental slope, 

 between 200 m and 2,000 m (4-1). The stream structure visible in the image 

 (4-2) confirms the narrow (60 km) width deduced by Royer (1981) from 

 hydrographic observations, and shows the start of a recirculating eddy 

 near the bottom center of the image, south of Unalaska Island. Such 

 recirculation, in which Gulf of Alaska water mixes into the Pacific Ocean, 

 has been deduced to occur within a range of longitudes as discussed by 

 Thomson (1972). The position of this eddy is at the start of this range and 

 near the position for the start of recirculation reported by Wright (1981), 

 for March 1980. Eddies of this type are of great interest and were, for 

 example, the subject of a 30-day cruise in 1981. Such images, if taken 

 immediately before and during such a cruise, would be very valuable both 

 for the planning of the cruise and for the subsequent data analysis. 



The CZCS pigment image on 6 September (5-2), shows turbulent 

 patterns traced out by varying phytoplankton pigment concentrations 

 over a large area of the Gulf of Alaska. The patterns are distorted by the 

 convergent flow of the surface water into the Alaska Current. It is this 

 current which moves across the middle of the image from right to left 

 following the 2,000-m contour on the shelf break (5-1), and converges into 

 the Alaskan Stream to the west. The indicated pigment levels and the flow 

 pattern on the left side of the image are very similar to those in 4-2, which 

 shows the same area two months earlier. 



A solid cloud layer covers the far left side of the image, and there are 

 cloud bands and cloud lines across the right side (compare with 5-1). 

 Patches of clouds confuse the coastline, especially near the upper right 

 corner. Streaks of thinner clouds are not perfectly masked to black north 

 of the Alaska Peninsula in this example, and some of the more linear light 

 features in the center of the image are of atmospheric origin. 



Light tones in the upper right area of image 5-2 indicate the areas of high 

 chlorophyll content on the continental shelf southwest and northeast of 

 Kodiak Island and near Middleton Island. Shoals in those areas cause 

 vertical mixing, allowing nutrient-rich water to be brought to the surface. 



4-1. Locator Map tor 4-2. 



iJPBIT i:C?89 10JUL79 220858 TO 221008 GMT Gtniri 1-4=2 THRESHOLD OFF TILT hUGLE 0.0 

 GRID CENTER DATA: 220933 GMT 54.84M 16T-8TW SUM EL 56 hZ 152 ROLL -0-2 RITCH -0-0 YhH 0-2 



.19 .27 .39 .57 



ALGORITHMS; 01 00 00 00 00 02 01 00 00 00 



567 SCAN LINES PROCESSED SCAN LINES MISSING 



1.28 ;.ll :-t6 ^-e^ 9-:? U'58 25-25 32-l 



CIP VER CRT NO DPS VER SPEC # FRAME # 

 V810528 ZE46201- V820501 FT3204O ZG046202h 



4-2. Nimbus-7, Orbit 3589. 10 July 1979, CZCS Pigment Image. 



