Cook Inlet marine ice data

(2500 + 32 R)0.5 - 50

Ig =

(2)

period. Anchorage closely follows Kenai, with ap-

proximately 14% fewer FDDs and 7% more TDDs

during the winter season. The other two sites illus-

At the beginning of the ice season, the mean air

trate the moderating effect that proximity to the Gulf

temperature usually fluctuates above and below freez-

of Alaska has on air temperature. Homer, near the

ing for several days, if not weeks, before remaining

southern end of Cook Inlet, has significantly milder

below freezing. During this time the ice cover may

air temperatures in winter than Kenai and Anchorage,

oscillate between freezing and melting several times

with about half the FDDs and about 60% more TDDs.

before becoming permanent. Bowditch stated that

Kodiak, located south of Cook Inlet in the Gulf of

Equation 2 is to be applied after that initial fluctuation

Alaska, has about one-fifth the FDDs and about two

period. According to LaBelle et al. (1983), the first

and a half times the TDDs of Anchorage and Kenai.

significant ice does not form in Cook Inlet until after

The season totals of FDDs and TDDs for the pe-

an initial accumulation of 33.3 FDDs, probably to ac-

riod of record (POR) are shown in Figure 12. Year-to-

count for this fluctuation period. This stipulation

year variability is substantial at all four stations. There

causes Equation 2 to become

was no statistically significant long-term trend of warm-

ing or cooling at any of the sites based on these data.

[2500 + 32( R 33.3)]0.5 - 50

The FDD record for Anchorage compiled by

(3)

LaBelle et al. (1983), together with the more recent

data of this report, is shown in Figure 13. The data

sets match closely during the period that they overlap

which we used to calculate the ice thickness values in

(19741982). Least-squares linear fits through the re-

Table 6.

spective data sets show opposite trends. The earlier

Also in the manner of LaBelle et al., we calculated

data exhibit a marked warming trend of 11 FDD/yr,

theoretical ice melt for Cook Inlet, Im, using the rela-

whereas the later time frame shows a slight cooling

tionship of Bilello (1980):

trend of +4 FDD/yr.

Im = 0.93 (TDD)

(4)

3.4 Cook Inlet marine ice data

Ice conditions in Cook Inlet have been routinely

where Im is in centimeters and TDD is the accumu-

reported since the 1969-70 winter season as an aid to

lated thawing degree-days using a temperature base

navigation. Annual reports were published through-

of 1.8C. However, Bilello empirically derived Equa-

out the 1970s by the Alaska Region Headquarters of

tion 4 using data from 12 Canadian sites and only 1

the NWS in Anchorage (Hutcheon 1972a, b, 1973;

Alaskan site to give the thickness decrease of a fast

Schulz 1977a, b, c, 1978; Eaton 1980; Poole 1980,

ice sheet from its maximum thickness.

1981a, b). These were post-season reports that typi-

The calculated monthly values for Im in Table 6

cally provided a qualitative description of the ice con-

show that above-freezing mean air temperatures can

ditions as they related to specific weather phenom-

occur at any time during the winter season and that

ena, air temperature, and FDDs recorded at Anchorage.

TDDs will cause departure from the theoretical ice

They also provided the historical dates of the first sig-

growth relationship (eq 3). Of course, the reverse is

nificant ice formation and the end-of-the-season ice-

also true--FDDs occurring at any time during the thaw

out dating back to the 1969-70 season. Although simi-

period will result in a departure from the melt calcu-

lar ice reports were apparently not published after the

lated by Bilello's equation. The reader is therefore cau-

1979-80 season, the dates of ice formation and ice-

tioned to consider the Ig and Im values in the table only

out continued to be compiled by the NWS up through

as indicative of the relative severity of ice conditions

the 1985-86 season (Table 3).

in Cook Inlet throughout the ice season. This is espe-

In December 1984 the NWS in Anchorage began

cially true for FDDs during October and April and for

sporadic analysis and forecasting of the extent, con-

TDDs during the rest of the season, as shown by their

centration, and stage of development of Cook Inlet

higher standard deviations.

ice. With the importance of commercial navigation,

In Table 6 we summed FDDs with respect to 0C

fishing, and tourism access to remote sites around

and TDDs with respect to 1.8C to be consistent with

Cook Inlet, the practice continues today with even

the stated method of LaBelle et al. (1983). Figure 11

greater scientific discipline and regularity. The NWS

shows the mean monthly FDD and TDD totals for

has provided regular ice reports since 1989 for Alas-

19731997 for four locations on Cook Inlet (Fig. 1).

kan coastal and offshore waters. Since 1994 Russell

In general, Kenai is the coldest site--it accumulates

Page has held the position of ice forecaster for that

the most FDDs and the fewest TDDs over the winter

region and has developed numerous tools to aid in the