Barrier Jets in the Gulf of Alaska - A satellite climatology

This study used spaceborne synthetic aperture radar to develop a climatology of the temporal (monthly resolution) and spatial (5 km resolution) distribution of existence, width, strength, flow enhancement, and separation from the coast of barrier jets and gap-flow hybrids in the Gulf of Alaska. The most striking feature of the temporal climatology is the sharp minimum of occurrence of both barrier jets and hybrids during the summer. Hybrids have a particularly strong seasonal minimum because of their origin of gap flows draining cold air out of the Alaskan interior. The spatial climatology of barrier jets shows strong variations in frequency of occurrence along the Gulf of Alaska coast. Barrier jets showed a strong preference for locations with high near shore terrain. Interestingly, the climatological frequency of barrier jets is most strongly correlated with the highest elevation within 100 km of the coast rather than the maximum or average across some smaller distance. Thus, coastal barrier jets reflect not just the terrain in the immediate vicinity of the coast, but that for a mesoscale distance inland comparable to the Rossby radius of deformation. Hybrids exhibit a similarly large spatial variability in frequency of occurrence. Each local maximum in the spatial climatology of hybrids begins just west of a major gap in the coastal terrain, rises quickly and then falls off gradually as one goes further west. This pattern reflects the origin of the hybrids as gap flow from the continental interior. The vast majority of hybrid jets originate in one of five gaps: Cross Sound, Yakutat Bay, Icy Bay, Copper River Delta, and Monatgue Strait.

The typical barrier jet extended 50 km out from the coast with a roughly log-normal distribution. There was little seasonal variation in jet width. The spatial variation was also small except in the strait between Kodiak Island and the mainland where the flow was constricted by land on both sides. A small fraction of the barrier jets were observed to detach from the coast for part of their length, a behavior not explained by existing theory. The typical separation was 10 km with values above 20 km being rare. Thus, most jets hug the coast making the near shore waters no safer for vessels seeking shelter.

Maximum wind speed for both barrier jets and hybrids ranges from 15 to in excess of 25 m/s with a median of 20 m/s. The barrier jets exhibit wind speeds of 2 to 3 times that observed on the synoptic scale further off shore. Thus, barrier jets pose both a hazard and a forecasting challenge in the Gulf of Alaska. The structural climatology for hybrid cases is similar to that for barrier jets so they pose similar problems.

The most intense barrier jets occur where barrier jets are most frequent, compounding the problem for vessels operating in this region. This climatologically favored region starts just to the west of Yakutat Bay and extends westward toward Prince William Sound, encompassing the approaches to the oil port of Valdez and the fishing port of Cordova.

These findings support the hypothesis that barrier jets are caused by hydrostatic blocking of onshore flow. They occur preferentially in regions with a climatology of onshore flow and in seasons of maximum wind speed. Likewise, their occurrence is closely tied to the elevation of the near shore terrain. The finding that "near shore" is best defined as within 100 km is a bit surprising, suggesting that the cold air damming and resultant barrier jet may be caused by mountains some distance inland and still extend past the coast.

The findings on hybrids suggest that drainage of cold air from the continental interior through coastal gaps plays a role in at least one fourth of the events (i.e. hybrids make up over one fourth of the total of hybrids plus barrier jets). Mesoscale modeling will be required to determine the extent to whether less obvious drainage flows contribute to the cases classified here as barrier jets.