Subject: Equatorial Biology Effects

The biggest ecological effect of an El Ni+/-o is the collapse of the
nutrient supply off the coast  of  Peru.  In 1970, the coastal strip
along Peru and Ecuador supplied 1/12th of the world's landing of all
fish, by weight.  This was the largest fishery in the world, and was
dominated by the anchovy in astonishing numbers,  which  is  largely
used  for  fertilizer.  Stocks declined slightly in 1971 due to over
fishing.  They declined enormously in 1972  due to a major El Ni+/-o
and due to over fishing.  Then as a result of another El Ni+/-o only
2 years later, the stocks fell below the level at which  they  could
reproduce  themselves.   In  1975,  the  Fisheries  Minister of Peru
declared a moratorium on  fishing.   Despite  the dramatic action of
the Fisheries Minister, the damage appears to  have  been  permanent
and the stocks have never recovered and will never recover.  Rather,
sardines  have  moved  into the area and replaced the anchovy in the
ecosystem.

High Latitude Biological Effects

Off the coast of  North  America,  we  see  a warming of the coastal
ocean.  The warming is large in years of  major  El  Ni+/-os,  small
during  moderate  El Ni+/-os and non-existent during minor El Ni+/-o
episodes.   However,  when   a   significant   warming  does  occur,
biological effects also occur.

1) The warm water allows unusual species to migrate into our  waters
that do not normally belong here.  In 1982/83, 1991/92, and 1992/93,
mackerel  ranged  farther  north  than  usual.   These are voracious
feeders and prey on  juvenile  salmon.   The  effect was very large.
Mackerel caught in Barkley Sound in 1992 and 1993 were discovered to
have 6 to 8 juvenile salmon in their stomach contents.

2) It was suggested by Lawrence Mysak,  about  10  years  ago,  that
during  warm  years  the  inbound  migration  paths  of  salmon  are
displaced northwards.  Since that time, we have better monitoring of
the  "northern  diversion"  rate which is defined as the fraction of
the Fraser  River  salmon  stocks  that  choose  to  return  via the
Johnstone Strait rather than the usual Juan  de  Fuca  Strait.   The
northern  diversion  rate varies between about 10% and 90%, and over
17  years  of   observations,   is   correlated  with  June  average
temperature at Kains Island at a  correlation  coefficient  of  r  =
0.86.   This  is  surprisingly high and suggests that we have a very
useful tool for predicting the migration paths of salmon.  The large
warm water events  are  all  El  Ni+/-o  events,  thus the migration
routes of salmon are determined to a high degree by processes taking
place at the equator.

3) There are periodic reports of other biological effects,  such  as
reproductive  failure  in  bird populations at the Farallon Islands,
but these are of little practical importance off the coast of B.C.

I  realise  that  this  biological  discussion  is  very  brief, the
scientific staff  at  the  Pacific  Biological  Station  are  better
trained   and   equipped   to   discuss   this   topic  than  I  am.

Last   modified   on   Wednesday,    25-Jun-97   13:33:02   PDT   by
hjfree@ios.bc.ca.
....
eof

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Forwarded message follows:

El NiƱo Induced Ocean Eddies in the Gulf of Alaska

Arne Melsom(1), Harley E.  Hurlburt(2), E. Joseph Metzger(2), Steven
D. Meyers(3) and James J. O'Brien(3)

(1) Department of Geophysics, UniversIty of Oslo,  P.O.   Box  1022,
Blindern,  N-0315,  Oslo,  Norway.   (2)  Naval Research Laboratory,
Stennis  Space  Center,  MS,   39529-5004,   USA.   (3)  Center  for
Ocean-Atmospheric Prediction Studies, The Florida State  University,
Tallahassee, FL 32306-3041, USA.

ABSTRACT

Observations reveal substantial eddy activity in the Gulf of Alaska,
with the Sitka eddy being a frequently observed anticyclonic feature
near  57 N. A unique high-resolution numerical model that accurately
reproduces eddy formation, size,  and  lifetime is able to duplicate
the observations.  The  decadal  simulation  allows  examination  of
interannual   variations   in   the   eddy   activity.   Interannual
variability in the upper  ocean  coastal  circulation in the Gulf of
Alaska is due to the El Ni+/-o/Southern  Oscillation  phenomenon  in
the  tropical  Pacific.   El  Ni+/-o  events  destabilize the Alaska
Current, creating  multiple  strong  anticyclonic  eddies  along the
coast.  These eddies then slowly propagate into the Gulf  of  Alaska
and  live for years.  El Viejo (La Ni+/-a) events generally suppress
eddy formation.  This high latitude  El Ni+/-o phenomena must have a
major effect on local fisheries.

The continental slope of the northeast Pacific is  a  major  habitat
for  a number of commercially harvested fish such as salmon, halibut
and herring (1).  It has been suggested that year-to-year changes in
fish  stock  recruitment  and  return  migration  routes  of certain
species in the Gulf of Alaska are affected by  low  frequency  ocean
variability.   A  theory  which has been advocated in several recent
studies  is  that  this   variability   may   be  attributed  to  El
Ni+/-o/Southern Oscillation (ENSO) events which trigger  propagating
coastal   Kelvin  waves  and  also  cause  changes  in  the  general
atmospheric circulation (2, 3).   A number of observational programs
have been performed (4, 5),  as  well  as  several  numerical  model
simulations (6, 7).

High  resolution  numerical  models  of  the ocean circulation allow
study of mesoscale (~100  km) motion.  Recent technological advances
in computinghave made it possible to apply these  models  to  global
and  basin-scale  domains.  The effects of remote tropical events on
mid-latitude    mesoscale    circulation    through    Kelvin   wave
teleconnections are now accurately represented.  Moreover, the local
response to the atmospheric circulation  is  well-described.   These
new  high  resolution  global  and  basin-scale models realistically
simulate  the  temporal   and   spatial   variability  of  mesoscale
circulation  features.   This  class  of  numerical  models  is   an
important  tool,  since the limited quantity of in situ observations
is  a  general,  reoccurring   problem   for  examination  of  ocean
circulation on the mesoscale.  Major features of  the  Navy  Layered
Ocean Model include isopycnal layers, nonlinear primitive equations,
free surface and a semi-implicit time scheme (8,9,10).

We present results based on a simulation of the ocean circulation in
the  northern  and  tropical  Pacific  Ocean, from 20 S to 62 N. The
model consists of six isopycnal layers and has a resolution of 1/8 x
45/256 (latitude x longitude)  which  is  approximately 13 km in the
Alaska Gyre  region.   The  model  was  spun  up  to  a  statistical
equilibrium  using  the  Hellerman  and Rosenstein (HR) monthly mean
wind stress climatology  (11).   Subsequently,  the model was forced
for a 14 year period (1981-1994) starting January 1, 1981 with daily
1000 mb winds from the  European  Centre  for  Medium-Range  Weather
Forecasts  (ECMWF)  (10).   The ECMWF 1981-1994 mean was replaced by
the annual mean from HR.  Previous numerical simulations of the Gulf
of Alaska  circulation  used  climatological  or  monthly winds, the
present simulation incorporates a more accurate description  of  the
temporal variability of the wind forcing.

We  restrict  our study to a subdomain in the eastern Gulf of Alaska
bounded by 46 N to  the  south  and  145  W  to the west.  We mostly
discuss the results obtained for  the  two  uppermost  layers  since
these contain the essential dynamics of the upper ocean circulation.

The  model accurately reproduces much of the observed variability in
the region of  interest,  as  illustrated  by model comparisons with
coastal stations.  The results  in  Figure  1  exemplify  this  good
agreement  and  validates the numerical representation of the annual
and interannual variability.  The  dominance  of  ENSO events in the
interannual variability along the coast of the  Gulf  of  Alaska  is
apparent in Figure 1b .  This variability has a strong effect on the
local circulation.

A  coastal current, corresponding to the Alaska Current, is observed
in the simulation results.  This current is strongest during winter,
when the flow is  northward.   In  spring the current weakens, moves
slightly offshore,  and  breaks  into  eddies.   During  summer  its
direction  is  frequently  southward.   A  similar seasonal cycle is
found in observational data (12), where  it is attributed to a shift
in the atmospheric circulation from a surface pressure low in winter
to a summer high (13).  (The mean annual cycle of  the  upper  layer
thickness  is  displayed in Movie 1.) The transport and the vertical
velocity shear associated  with  the  coastal  current reach maximum
values in December or January.  At this time, the current starts  to
meander.   The  alongshore  wavelength and offshore amplitude of the
meanders are typically 200 km and 40 km, respectively.  However, the
amplitude may become ~100 km, after which the current usually breaks
into eddies.  The meandering is observed in both of the layers under
consideration.

As the  meanders  break  up,  anticyclonic  and  cyclonic eddies are
formed by  baroclinic  instability.   The  anticyclonic  eddies  are
generally  larger  than  the  cyclonic  eddies.  The cyclonic eddies
dissipate quite rapidly,  whereas  the anticyclonic eddies sometimes
survive for well over a year.  Typically, the stronger  anticyclonic
eddies  are  generated  during  winter.   The  eddies  are  seen  to
propagate  slowly  southwestward (i.e., offshore), with an estimated
propagation speed of  0.5-1  cm/s,  or  ~200  km/year.  However, the
direction of propagation may temporarily be reversed,  possibly  due
to the local wind forcing.

Following   conventional  theory  for  baroclinic  instability,  the
wavelength of the most rapidly  growing disturbance is a function of
the buoyancy, layer thickness and latitude  (14).   In  the  coastal
regions  of  the Gulf of Alaska this scale varies from ~50 km to ~75
km for upwelling and downwelling perturbations, respectively.  Thus,
downwelling instabilities are well resolved, whereas the description
of unstable disturbances in the upwelling situation is less precise.

Upwelling perturbations stabilize the flow  and do not yield eddies.
Additionally, the generation of cyclonic eddies may be  artificially
suppressed  due  to  their relatively small horizontal extent, since
horizontal gradients  are  dampened  by  a  Laplacian friction which
selectively dampens small scales.  (This is necessary for  numerical
stability).

In  an  extensive examination of oceanographic data collected during
1927-77  in  the  Gulf   of   Alaska,  Tabata  (15)  concluded  that
``baroclinic eddies occur frequently in this region.  Among these is
the recurring, well developed, anticyclonic eddy situated  within  a
few  hundred  kilometers  of  Sitka'', now commonly called the Sitka
eddy (13).

The diameter of the Sitka eddy  is  observed  to be 200 km - 300 km,
with a vertical isopycnal deflection of 20 m - 100  m.  Furthermore,
the  initial  location  of the Sitka eddy is 150 km - 200 km off the
coast of Baranof Island, and  it  can  persist  for 10 to 17 months,
drifting southwestward.  All of these aspects of the Sitka eddy  are
reproduced  qualitatively  and quantitatively by the numerical model
(diameter 150 km -  300  km,  isopycnal  deflection  of up to 100 m,
initial location ~200 km off Baranof Is., persisting for longer than
one year).

Tabata (15) indicates that one or more anticyclonic eddies exist off
Sitka during most years, if  not  every  year.   A  Sitka  eddy  was
definitely present in 1958, 1960, 1961 and 1977.  The data also give
some  evidence of the Sitka eddy in 1954, 1956, 1959, 1962 and 1967.
For many years the  data  are  insufficient  to draw any conclusion.
The numerical model produces at  least  one  anticyclonic  eddy  off
Sitka  in  six  of  the  nine eddy generation seasons.  In 1984, the
circulation in the region is dominated  by the lasting effect of the
strong eddy that was generated the year  before;  and  although  the
isopycnal interfaces do not clearly indicate the presence of an eddy
in  the  winter and spring of 1985, the circulation in the region is
distinctly anticyclonic.  The 1989 season  follows an El Viejo event
and eddies are not generated.  (El Viejo means cold SST anomalies in
the tropical Pacific and is the opposite phase from El Ni+/-o.)

During the model run, there are several years that are classified as
El Ni+/-o years in the tropical  Pacific.   The  largest  El  Ni+/-o
event   in   recent  history  occurred  during  the  1982/83  boreal
(northern) winter.  (Model eddy generation and off-shore propagation
are  animated  in  Movie  2  and  Movie  3,  respectively.  Vertical
velocity shear is demonstrated in Movie 4.) Between  1981  and  1991
there  is  only  one recognized El Viejo event -- during the 1988/89
boreal winter.  (Model upper  layer  thickness during this event can
be viewed in Movie 5.)

Figure 2 shows the deflection of the interface  between  the  second
and  third  layer  in the model during an El Ni+/-o and an El Viejo.
This depiction indicates an ENSO influence on eddy generation in the
Gulf of Alaska.  Furthermore, keeping in mind that the mean depth of
the upper two layers is ~170  m  in coastal regions, the ENSO effect
is substantial in this area.  Extreme events in the tropical Pacific
Ocean affects both the atmospheric and oceanic circulation.   It  is
impossible  to conclude from Figure 2 alone whether the changes that
are seen in the area are due  to remote forcing in the ocean through
a coastal Kelvin wave teleconnection (16) or due to changes  in  the
local  atmospheric  forcing.  Both forcings are present in the model
dynamics and it is difficult  to separate their independent effects.
However, coastal Kelvin waves are clearly observed in the  model  to
propagate  from  the  equatorial  Pacific  into  the Gulf of Alaska.
Their  arrival  in  the  Gulf   is  immediately  antecedent  to  the
disruption of the Al aska

Previous studies have hypothesized that the interannual  variability
in the Gulf of Alaska may be affected by ENSO events in the tropical
Pacific  Ocean  (2,  3).   Although  such a link is not discussed by
Tabata, the  observations  of  the  Sitka  eddy indicate interannual
variability, yielding clues to any relationship  with  the  tropical
ocean.   In his study, Tabata refers to the possible presence of the
Sitka eddy with various degrees of  certainty (``a hint of it'', ``a
suggestion of it'', ``definite indication'' and so on).  Though  the
various degrees of certainty in the observational information may be
attributed  to  the  sparsity  of  available data, it is likely this
reflects  the  interannual   variability   of  the  mesoscale  ocean
circulation off Sitka as well.  If the Sitka eddy is weak,  it  will
be correspondingly hard to detect in the observational data.  As can
be seen from Figure 3 , the simulation results contain a significant
amount of mesoscale variability in this region.

In  the  datasets  that  are  surveyed by Tabata (15), the spring of
1958, the summers of 1960 and  1961,  and the spring of 1977 are the
times when the Sitka eddy was undoubtedly present.  Both 1957/58 and
1976/77 were seasons with an El Ni+/-o event in the tropical Pacific
Ocean.  This corresponds  well  to  the  results  of  the  numerical
simulation:  the  1982/83  El  Ni+/-o  was  the  largest in the 20th
century and appears to have  produced  eddies in the Gulf of Alaska.
On the other hand, the 1986/87 El  Ni+/-o,  which  was  of  moderate
size,  does not spawn a large number of eddies in the Gulf of Alaska
in the numerical model, so  the  evidence is inconclusive.  The lack
of a significant ENSO response in 1986/87 may be due  to  inadequate
ECMWF winds over the equatorial Pacific during this time.

Repression  of  eddy  activity following El Viejo tropical events is
supported by the measurements off  Sitka.  There are two such events
reviewed in Tabata (15), which occur in 1955/56 and 1956/57 El Viejo
events.  In the spring and summer of 1956 and 1957, Tabata concludes
that the Sitka eddy is either weak or non-existent.   Following  the
1988/89  El  Viejo  tropical event, the eddy activity in the Gulf of
Alaska is strongly suppressed in the numerical model.

Both the observational data  and  the  model result suggest that the
interannual variability in the Gulf of Alaska is correlated with  El
Ni+/-o  and  El  Viejo  events  in  the tropical Pacific Ocean.  The
long-term  effect  of  such  events   in  the  North  Pacific  Ocean
circulation was recently reported  based  on  results  from  another
version   of  the  present  numerical  model  (17).   There  it  was
demonstrated that the  1982/83  El  Ni+/-o triggered planetary waves
that crossed the North Pacific basin and caused a partial  northward
re-routing  of  the  Kuroshio  Extension  in  1992-93.   This  paper
demonstrates  that the high-latitude ocean also responds strongly to
ENSO events, having  memory  over  thousands  of kilometers and many
years.  The discovery of  remotely  forced  interannual  changes  in
upper  ocean  circulation  heralds  a  new  era in understanding the
behavior of the upper ocean on short-term climate scales.  How these
oceanic changes effect marine life  or local weather patterns is not
fully understood.

REFERENCES

1.  R. K. Reed, R. D. Muench, J. D. Schumacher, Deep-Sea  Res.   27,
509 (1980).

2.  R. K. Reed, Deep-Sea Res.  31, 169 (1984).

3.  P. F. Cummins, J. Phys.  Oceanogr.  19, 1649 (1989).

4.   P.  K.  Heim,  M.  A. Johnson, J. J. O'Brien, J. Geophys.  Res.
97C, 17,765 (1992).

5.  P. F. Cummins and  L.  A.  Mysak,  J. Phys.  Oceanogr.  18, 1261
(1988).

6.  W. J. Emery and K. Hamilton, J. Geophys.  Res.  90C, 857 (1985).

7.  L. A. Mysak, in El Ni+/-o north: Ni+/-o effects in  the  eastern
subarctic  Pacific  Ocean, W. S. Wooster, Ed.  (Univ.  of Washington
Sea Grant Publ., 1985) p.97.

8.  H.E.  Hurlburt  and  J.O.   Thompson,  J.  Phys.  Oceanogr.  10,
1611, (1980).

9.  A. J. Wallcraft, NOARL Report no 35, Naval Res.   Lab.,  Stennis
Space Center, MS, 21 pp (1991).

10.   H.E.   Hurlburt,  A.J.   Wallcraft,  W.J.   Schmitz, Jr., P.J.
Hogan and E.J.  Metzger, J. Geophys.  Res., (in press), (1995).

11.  S. Hellerman and M.  Rosenstein,  J. Phys.  Oceanogr.  13, 1093
(1983).

12.  G. S. E. Lagerloef, R. D. Muench, J. D.  Schumacher,  J.  Phys.
Oceanogr.  11, 627 (1981).

13.  T. C. Royer, Deep Sea Res.  22, 403 (1975).

14.  E. A. Eady, Tellus 1, 33 (1949).

15.  S. Tabata, J. Phys.  Oceanogr., 12, 1260 (1982).

16.   D.B.   Chelton  and  R.E.  Davis, J. Phys.  Oceanogr., 12, 757
(1982).

17.  G. A. Jacobs, H. E.  Hurlburt,  J. C. Kindle, E. J. Metzger, J.
L.  Mitchell, W. J.  Teague,  A.  J.  Wallcraft,  Nature,  370,  360
(1994).

18.   Financial  supported is provided by SERDP and by the Norwegian
Science Council (Norges  Forskningsrad).   The Physical Oceanography
Branch of the Office of Naval Research provides the base support for
Center for Ocean-Atmosphere Prediction Studies.  Funding for NRL  is
provided  by  the Office of Naval Research and the Advanced Research
Projects  Agency  grant  SC25046  and  the  Strategic  Environmental
Research Development Program (SERDP)  by Dr.  John Harrison.  Thanks
to D. Muller for many helpful discussions.