ANNUAL REPORT - 1998

TITLE OF PROJECT:

 
PRINCIPLE INVESTIGATOR NAMES AND ADDRESSES:

H. J. Niebauer, Atmospheric and Ocean Science, University of Wisconsin, Madison, WI. 53706 (608) 265-5181. email: niebauer@sunset.meteor.wisc.edu

 Tina Wyllie-Echeverria, Joint Institute for the Study of Atmosphere and Oceans, University of Washington, Seattle, WA 98195 (206) 463- 5514 email:tinawe@u.washington.edu

 Nicholas A. Bond, Joint Institute for the Study of Atmosphere and Oceans, University of Washington, Seattle WA 98195. (206) 526-6459; email:bond@pmel.noaa.gov.

 
OTHER PARTICIPATING RESEARCHERS:

James Schumacher, Pacific Marine Environmental Laboratory, 7600 Sand Point Way NE, Seattle, WA 98115. (206) 526-6197, email: jdschu@pmel.noaa.gov.

 Phyllis Stabeno, Pacific Marine Environmental Laboratory, 7600 Sand Point Way NE, Seattle, WA 98115. (206) 526-6453, email: stabeno@pmel.noaa.gov.

 James Overland, Pacific Marine Environmental Laboratory, 7600 Sand Point Way NE, Seattle, WA 98115. email: overland@pmel.noaa.gov.
 

PROGRESS:

We have hypothesized that short-term (interannual) climate/weather fluctuations over the eastern Bering Sea shelf strongly affect oceanographic conditions, which in turn, strongly affect pollock abundance, as well as other fisheries. We have focused on the changes in atmospheric forcing and resulting changes in the cool shelf bottom water that have occurred and are occurring over the period ~1947-present. We have put special emphasis on the atmospheric "regime shift" in the mid-late 1970s. We are concentrating on the more recent El Nino of 1997-98 as it has appeared to have caused very strong fluctuations in the Bering Sea ecosystem (e.g., coccolithophore blooms which here to fore have seldom occurred in the Bering Sea). In this next year, we will especially monitor the conditions during the predicted and developing La Nina. We are examining the influence of these changes on the biology including primary productivity and fish populations.

Our approach consists of gathering, organizing and updating time series of atmospheric, oceanic (including the cold pool and sea ice) and biological (especially primary production and pollock) parameters, and then performing analyses to identify linkages between the atmosphere, ice, ocean and biology. To this point, all of the time series of ~50 years length of monthly mean wind, air and sea-surface temperature, and % ice cover from the Bering Sea, monthly mean sea-level atmospheric pressure over the north Pacific and northern hemisphere, and finally the Southern Oscillation Index from the south Pacific have been collected, collated, processed into 1998. These are available through niebauer@sunset.meteor.wisc.edu. To address the "cold pool" hypotheses in our project (i.e., fluctuations in the temperature of the pool of cold water on the shelf bottom results from the severity of the previous winters weather), ~30 years (1966-1996) of CTD profiles for the Bering Sea shelf have been collected, collated, and are available (chu@ims.alaska.edu), at the U of Alaska at Fairbanks. These are being updated through 1998. Also available at the UAF web site are annual maps of bottom temperature (cold pool).

Data sets of primary production and productivity have occurred only intermittently over the past 30 years. These data are still being located, collated and summarized and analyses is beginning. Work that is farther along is a comparison of satellite derived productivity and production data with environmental factors for the past five years by L.J. Miller and Dave Eslinger of the University of Alaska Fairbanks. L.J. Miller's master's thesis of this analysis should be available shortly.

Time series have been collected, collated and processed for walleye pollock (ages 1, 2 and 3 and older), yellowfin sole, arrowtooth flounder, and Pacific cod. Data are from benthic trawl surveys covering the eastern and central Bering Sea shelf since 1972 and each species contains 7,426 stations sampled during that time period (contact: tinawe@u.washington.edu). Analyses are underway including comparison with environmental data (e.g., Figs. 1 & 2).

 
SCIENTIFIC ACCOMPLISHMENTS:

 The atmospheric forcing of the cold pool of the SE Bering Sea shelf has been examined for the period 1958-1997. Daily output fields from the NCAR/NCEP Reanalysis include the surface fluxes of momentum, sensible and latent heat, and shortwave and longwave radiation and their relationships to the background, large-scale atmospheric flow. The atmospheric forcing has been evaluated during both the winter and spring/summer periods. During the former, the focus has been on sea-ice formation and transport over the SE Bering Sea shelf and their relationships to the surface heat fluxes and meridional wind stresses. During the spring through summer, the focus has been on the processes important to the oceanic mixed layer and pycnocline, namely the surface heating, particularly due to shortwave radiation, and the wind mixing and Ekman pumping.

Our results show that the interannual variations in the wintertime atmospheric forcing are considerable (e.g., the standard deviation in the average net surface heat fluxes is 40 W m^-2). These variations are well correlated with the duration of sea ice, and in particular, the southernmost extent of the sea ice (Fig. 1). The interannual variations in the latter have correlation coefficients of 0.64 and 0.72 with the winter-average net surface heat fluxes and meridional wind stresses, respectively. Striking interannual variations have also been documented in the atmospheric forcing during the spring and summer. Interannual variations in SST tendency during May through July are correlated about equally with the net surface heat fluxes (largely due to variations in low cloud cover) and a combination of the wind mixing and Ekman pumping. The latter are important because of their impacts on the depth of the mixed layer. Based on the net heat fluxes at the surface, and the fraction of these fluxes that go towards heating the mixed layer, estimates have been made in the rate of heating below the pycnocline. The years with a persistent cold pool (e.g., the early 1970's, 1995) typically had about 30 W m^-2 less heating below the mixed layer than those without a cold pool. This result indicates the importance of the dynamics of the mixed layer and pycnocline for establishing the stratification that can insulate the cold pool from the summer heating.

We have completed most of the milestones and over the next year will continue to concentrate on finishing the milestone of developing and refining conceptual and statistical models of atmosphere, ocean, primary production, pollock links. For example, a hypothesis thus far reasonably well supported by our results is that oceanographic conditions are controlled by climate/weather fluctuations. This is how we think it works: During winters with a strong and eastward displaced Aleutian low (most often accompanying an El Nino), the Bering Sea shelf is warm and the cold pool is small. During winters with a weak and westward displaced Aleutian low (most often accompanying La Nina), the Bering Sea shelf is cool and the cold pool is large. However, a climate-driven physical regime shift occurred on the Bering Sea shelf in the mid-late 1970's. Since the regime shift, El Nino driven Aleutian lows have moved so far to the east that winds actually come off Alaska and cause the Bering Sea ice to advance as in 1998 (Fig. 1). The regime shift in the physical environment was largely responsible for the changes in fish populations on the Bering Sea shelf that occurred in the mid-late 1970's (see below). The physical conditions occurring after the regime shift have persisted to some extent, but the biological response was short lived (~one year class was strongly effected although its effect on the pollock fishery has been felt for years).

A second hypothesis that is not so well tested out yet is that oceanographic conditions, particularly temperatures, limit the growth of some populations. In cold years with an extensive cold pool, pollock are forced off the shelf on to the outer shelf (e.g., Fig. 2). Pollock recruitment is reduced because of lower egg hatch, and concentration of adult pollock result in increased cannibalism. In warm years with a small cold pool, pollock recruitment is enhanced because of higher egg hatch, and the dispersal of pollock results in decreased cannibalism. Within a year or so, the population catches up so that cannibalism balances egg hatch. This may have been the mechanism that caused the pollock fluctuations over the regime shift.

The seasonal sea ice index was updated through 1997 and 1998 (Fig. 1). Sea ice extended across the shelf beyond 57^o 30'. The unusual occurrence of a coccolithophore bloom during 1997 led to further analysis of sea ice characteristics that may help us understand the physical conditions that exist during, and preceding, such an unusual bloom. We found that in spring, sea ice retreated 2.5 degrees of latitude within one week. Sea ice retreat of at least 2.5 degrees in one week has occurred 10 times in the previous 26 years but never before coupled with the late timing of May 22. In 1998 we are again observing a large coccolithophore bloom on the shelf and again this spring we witnessed a rapid retreat of ice of 3 degrees latitude. However, the timing was earlier, falling within the average time of mid-April. Perhaps the previous years bloom influenced the dominance of coccolithophores on the shelf.

The distribution of both age-1 and pollock age 2 and older is predicted to be concentrated on the outer shelf during cool conditions (Wyllie-Echeverria and Wooster 1998). The far extent of sea ice in winter 1997 leads us to predict cool summertime conditions. Fig. 2 depicts the distribution of age 3 and older pollock relative to the cold pool during the summer of 1997. Pollock sampled at stations with a concentrations of at least 100 fish per hectare were distributed outside the cold pool and primarily on the outer shelf. Similar results occurred for age 1 and age 2 pollock. The southernmost extension of the sea ice as detected by the NASA Scatterometer is also shown to give an idea of the extent of winter ice conditions preceding cool summertime shelf conditions and resultant distribution patterns of pollock.

 
APPLICATIONS:

 Wyllie-Echeverria, T. and W. S. Wooster. 1998. Year to year variations in Bering Sea ice cover and some consequences for fish distributions. Fisheries Oceanography 7(1). (in press)

 Niebauer, H.J., 1998. Variability in Bering Sea Ice Cover as affected by a regime shift in the north Pacific in the Period 1947-96. Journal of Geophysical Research (in press).

 Niebauer, H.J., N. Bond, L.P. Yakunin, and V.V. Plotnikov. 1998. On the climatology and ice of the Bering Sea. In: The Bering Sea: Physical, Chemical and Biological Dynamics, Loughlin, T.R. and K. Ohtani (eds.), Alaska Sea Grant Press (in press).