is both seasonal and interannual. Early in the summer each year 

 water transits the basin most rapidly, typical residence times 

 being about two weeks. In 1985, this condition maintained 

 until almost the end of August, in 1987 until the middle of 

 August, and in 1988 the beginning of the month. Nineteen 

 eighty-six was different; beginning the second week in July, 

 residence times increased to about one month and remained so 

 over the summer. 



RESIDENCE TIMES OF ANADYR WATER IN THE CHIRIKOV BASIN 



(PLOTTED BT OflV OF INGRESS THROUGH ONADTR STRAlTl 



^1988 





ol I J . . . I 



I 1 1 



, I .... I .... I . 



, I . . . . I . . . 



90 195 200 205 210 215 220 225 2iO 235 240 245 



JULIAN OAy 



r T T 



ILV e AUG 1 SEPT 1 



Fig. 12. Residencetimesof Anadyr Currenl water in iheChirikov basin for the 

 lour yeans of ISHTAR study. Times are plotted by day of ingress 

 through Anadyr Strait. Notice typical residence times in summer are 

 about 2-weeks through July, then a switch to much longer times 

 somctiine during August. Nineteen eighty-si.\ was different, with 

 residence times of 3 to 4 weeks over the summer. 



The residence times are closely related to the wind regime 

 and its variations. Over most of the year, the strength of north 

 flow through the system, and variations in this flow (e.g., from 

 north to south), are well correlated with the local winds; for 

 example, Bering Strait north transport and the north-south 

 wind component correlation is r > 0.7. In early summer each 

 year, though, the correlation breaks down. Regional winds 

 become light, without strong variations, the flow becomes 

 decoupled from the wind, and the currents are stronger and 

 directed more steadily to the north (Coachman & Aagaard, 

 1988). These are the conditions for short residence times, and 

 were observed to obtain at the beginning of July each year. 



Over the remainder of the year, winds are both stronger 

 and more variable, and the flow is driven into variations that are 

 reasonably correlated with those of the wind. Thus the periods 

 of slow and reversed (southward) How become more frequent, 

 and residence times become markedly longer. This changeover 

 from "summer" to normal wind regime occured at different 

 timesbetween the beginning and end of August in 1985, 1987, 

 and 1988. Nineteen eighty-six, however, was anomolous; the 

 typical "suinmer" flow condition, decoupled from the wind, 

 never really developed. The resulting longer residence times 

 over the production season were undoubtedly responsible for 

 the greater accumulation of biomass in the Chirikov than in the 

 southeastern Chukchi basin in 1986, as opposed to more 

 "normal" years when more accuinulates in the Chukchi ( Walsh 

 £'/«/., 1989). 



To provide more insight as to specific wind conditions 

 causing longer residence times. Fig. 13 was prepared. First, the 

 north-south component of wind at Bering Strait was examined 

 by itself, but no relationship with residence tiines was apparent. 

 The forces driving the flow field variations are obviously more 

 complex than just the local wind in Bering Strait. So the wind 

 at Anadyr Strait was added, and a qualitative picture emerges. 

 A primary condition for long residence times seems to be a 

 sustained trend of change in the winds to northerlies 

 (i.e., directed to the south) combined with a sustained, strong 

 divergence of the wind field over the Chirikov basin. The 

 divergence is where the winds at Anadyr are either less strong 

 to the north, or stronger to the south, than those at Bering Strait. 

 Under these conditions, the normal sea surface slope down to 

 the north is negated and readily reversed. Without a "push" 

 from the south, water parcels can hang around in the Chirikov 

 basin for very long times (as long as two months). 



Fig. 13. North-south component of winds at Bermg Strait and Anadyr Strait 

 over the summers of 1985-87 (data smoothed with S-point runnning 

 means). Long residence times seem to be associated with changes 

 toward strong south-directed winds and a sustained, strong divergence 

 of the wind field over the Chirikov basin (hachured). 



Chukchi Sea 



The third, downstream production center of the northern 

 Bering Sea ecosystem is in the Chukchi Sea. ISHTAR has 

 studied the southeast comer of the region. Southwest from 

 Pt. Hope lies a production center where huge chlorophyll 

 biomass has been measured (cf. Fig. 3) and also some of the 



24 



