Section 1: Examples of simple diagnostic models of ecosystem response to climate forcing
Example #2: Northwest Pacific Zooplankton
Sanae Chiba (JAMSTEC), Andrew Davis (Geogea Tech)
Background
It has been well reported that dynamics of the Aleutian Low, that is closely related to the Pacific Decadal
Oscillation, affected the lower trophic levels in the western North Pacific through the wind-driven,
bottom-up process that changes nutrient and light availability within mixed layer. The other PDO-related,
yet less studied process in LTL variation is influence of dynamics of Kurosho-Oyashio Currents. Finding
the low-frequency variation in biogeographical shift in zooplankton community in the Kuroshio/Oyashio
transition region based on analysis of the Odate Collection zooplankton data set, Chiba et al. (2009)
hypothesized that spin-up of the Kuroshio and Oyashio after the climatic RS in the late 1970s enhanced
northward transport of warm water species and southward transport of cold water species, that resulted in
high zooplankton concentration in the narrow area between the two currents.
Passive Tracer Experiment
To test the hypothesis and to assess the effect of low-frequency transport on variations in zooplankton
populations in the Kuroshio/Oyashio transition region, we conducted the passive tracer experiment using
the output of the Ocean Model for Earth Simulator (OFES) global eddy-resolving ocean model (1/10°)
developed at JAMSTEC (Masumoto et al 2004). Our hindcast provided current and sea level data from
1950 to 2000, employing NCEP/NCAR reanalysis climatology forcing (Kalnay et al 2006). We chose a
region including the Odate sampling area and major features of the Kuroshio/Oyashio extension (Fig. 1).
We used this model-derived current data to drive a Lagrangian transport algorithm. The algorithm
simulates both the advection and diffusion of passive tracers-particles with negligible mass and neutral
buoyancy that represent zooplankton in this study. Particles were released from both areas in March, then
we counted the number of particles from both regions ending up in the sampling area in September. This
gave us yearly indices of transport that we could correlate with zooplankton observations. Model output
was compared to the observed Odate zooplankton distribution for 1960-1999.
We found that the observed number of warm-water species was significantly and strongly correlated
(r = .58) with northward transport by the Kuroshio (fig. 2). This means that a large portion of warm-water
zooplankton is being carried into the Kuroshio/Oyashio transition zone from the south, and that the
amount transported varies interannually and interdecadally. While this underlying physical variability
remains difficult to attribute, our results further reinforce the role of climate as a driver of ecosystem
variability. We failed to find the significant match between model output and the observed distribution of
cold water species. That suggests that zooplankton population in the northern area might be determined
by the wind-driven, mixed layer process rather than ocean current dynamics.
Further Subject: Seasonality
This study area is know as nursery and feeding ground of many commercially important fish species, such
as sardine and saury, thus lower frequency variation in zooplankton population would have a significant
influence on recruitment success of those fish. Fish species migrate to the area in the various seasonal
timing and life stages. Therefore, the seasonality in zooplankton advection should be considered to
assess the LTL and HTL link. Temporal and spatial resolution of the Odate Collection data is not sufficient
to assess the monthly level analysis of zooplankton distribution. However, May-September average
abundance of warm water species showed the highest correlation with tracer counts in August, indicating
that interannual variation in seasonal dynamics of currents should be investigated.
References
Chiba S, et al (2009) Geographical shift of zooplankton communities and decadal dynamics of the
Kuroshio–Oyashio currents in the western North Pacific. Global Change Biology, 15,1846–1858
Kalnay E, et al (1996) The NCEP/NCAR 40-year reanalysis project, Bulletin of the American
Meteorological Society, 77, 437-471
Masumoto Y (2004) Generation of small meanders of the Kuroshio south of Kyushu in a highresolution
ocean general circulation model, Journal of Oceanography, 60, 313-320
Fig. 1 Target area for passive tracer
experiment. Blue boxes: Tracer release
area; Red boxes: zooplankton data
Fig. 2 Time-series of warm water zooplankton
abundance (black line) and tracer particle
counts in the south box (red line) and both
north and south boxes (green) in the Fig. 1.