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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El Niño 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.