ACCESS NUMBER: TO-85243 

STUDY TITLE: Measurements of Temperature, Salinity, and Circulation in Cook Inlet, Alaska 

REPORT TITLE: Measurements of Temperature, Salinity, and Circulation in Cook Inlet, Alaska 

CONTRACT NUMBER: 1435-01-98-CA 30909; TASK ORDER (TO): 85243 

SPONSORING OCS REGION: Alaska 

APPLICABLE PLANNING AREAS: Cook Inlet, Alaska 

FISCAL YEAR OF PROJECT FUNDING: 2002 

COMPLETION DATE OF REPORT: October 2003 

COST: FY 2002: $26,203; CUMULATIVE PROJECT COST: $26,203 

PROJECT MANAGER: Stephen R. Okkonen 

AFFILIATION: University of Alaska Fairbanks 

ADDRESS: Institute of Marine Science, University of Alaska Fairbanks, Fairbanks, AK 99775-7220 

PRINCIPAL INVESTIGATOR*: Stephen R. Okkonen 

KEY WORDS: Tide rip fronts, density-driven currents, Cook Inlet 

BACKGROUND: 

The Cook Inlet area is home to an extensive petrochemical industry with both marine and land-based 

components. This project investigates the spatial and temporal variability of the hydrographic structure 

and density-driven currents in the local marine environment. The results from this project contribute to 

understanding how these physical features and processes might influence the behavior and trajectory of 

spilled oil and, as such, are potentially useful for spill response planning and training. 

OBJECTIVES: 

The principal objectives for this project were to assess spatial and temporal variability in the hydrographic 

structure and density-driven circulation in central and lower Cook Inlet, Alaska. 

DESCRIPTION: 

Surface-to-bottom CTD (conductivity–temperature–depth) profiles of temperature and salinity were 

acquired along eight transects in central and lower Cook Inlet, Alaska during spring and fall 2002. Station 

spacing between successive CTD casts was nominally 3.7 km (2 nmi), although near the coast, station 

spacing was often 1.85 km (1 nmi). The spring and fall surveys were conducted during periods when 

freshwater inputs to the inlet were relatively low and high, respectively. Additionally, temperature and 

salinity profiles were acquired along a single transect that was crossed four times during an eight hour 

period. These sequential surveys monitored the flood-to-ebb evolution of the hydrographic structure. 

This project had an educational outreach component involving eight Kenai Peninsula Borough Schools. 

Students and teachers collectively fabricated ~7500 drift cards that were released along CTD transects 

during the spring hydrographic survey. About 50 students and teachers participated in the spring and fall 

hydrographic surveys of the Homer transect. 

SIGNIFICANT CONCLUSIONS: 

There is a spring-to-fall freshening of ~1.5 psu in the lower inlet and of ~3 psu in the central inlet. The 

west and middle tide rips in central Cook Inlet are frontal features associated with the freshwater plume 
ACCESS NUMBER: TO-85243 

that extends southward from the Forelands. Frontal gradients were observed to be stronger in the fall 

than in the spring. Multiple hydrographic observations along a fixed transect in central Cook Inlet showed 

that the inclination of the middle tide rip front decreased during the flood tide, which had the effect of 

diminishing the speed and vertical extent of the southward-flowing baroclinic jet. Also during the flood 

tide, the average temperature decreased ~0.5 °C and the average salinity increased ~1.3 psu. 

STUDY RESULTS:

Baroclinic currents associated with the west and east rip fronts locally augment the barotropic tidal current 

during the flood and act in opposition during the ebb. Conversely, the baroclinic current associated with 

the middle rip front will locally oppose the barotropic tidal current during the flood and locally augment the 

barotropic tidal current during the ebb. These interactions between the baroclinic and barotropic currents 

produce a current regime in which the onset of northward surface flow near the middle rip front lags the 

onset of northward surface flow near the eastside beaches. Additionally, during the ebb tide, the 

southward-flowing surface currents near the middle rip are typically faster than southward-flowing 

currents near the beach. These observations indicate that Cook Inlet numerical spill trajectory models 

need to be fully three-dimensional, incorporating buoyancy (temperature and salinity) effects at seasonal 

to semidiurnal time scales to improve their forecasting skill. 

STUDY PRODUCTS: 

Okkonen, S.R. 2003. Measurements of temperature, salinity, and circulation in Cook Inlet, Alaska. MMS 

Science Steering Committee Meeting, April 2003, Anchorage, Alaska. 

Okkonen, S.R. 2003. Measurements of temperature, salinity, and circulation in Cook Inlet, Alaska. MMS 

Information Transfer Meeting, March 2003, Anchorage, Alaska. 

Okkonen, S.R. and S.S. Howell. 2003. Measurements of temperature, salinity, and circulation in Cook 

Inlet, Alaska. University of Alaska Coastal Marine Institute Annual Research Review, February 2003, 

Fairbanks, Alaska. 

Okkonen., S.R. and S.S. Howell. 2003. Measurements of Temperature, Salinity, and Circulation in Cook 

Inlet, Alaska. Final Report. OCS Study MMS 2003-036, University of Alaska Coastal Marine Institute, 

University of Alaska Fairbanks and USDOI, MMS, Alaska OCS Region, 28 p. 

*PI’s affiliation may be different than that listed for Project Manager(s). 
ACCESS NUMBER: TO-85243 

Gulf of Alaska subregion base map. 

West Foreland

Kenai/Soldotna

Drift River

Terminal

Ninilchik

Anchor Point

Homer

Nanwalek

Chinitna

Point

Tyonek

Nikiski

Seldovia

Augustine I. Port Graham

KENAI PENINSULA

COOK INLET

61 N

60 N

59 N

154 W 153 W 152 W 151 W 150 W

Kalifornsky Beach

Humpy Point

East Foreland

Kalgin I.

Anchorage

Map of Cook Inlet showing station locations along transects and west, middle and east tide rip fronts.