Effects of cyclonic mesoscale eddies on the marine ecosystem in the Kuroshio Extension region using an eddy-resolving coupled physical-biological model

Abstract

Effects of mesoscale eddies on the marine ecosystem in the Kuroshio Extension (KE) region are investigated using an eddy-resolving coupled physical-biological model. The model captures the seasonal and intra-seasonal variability of chlorophyll distribution associated with the mesoscale eddies, front variability, Kuroshio meanders, and upwelling. The model also reproduces the observed interannual variability of sea surface height anomaly (SSHA) in the KE region along a zonal band of 32–34°N from 2002 to 2006. The distribution of high surface chlorophyll corresponds to low SSHA. Cyclonic eddies are found to detach from the KE jet near 150°E and 158°E and propagate westward. The westward propagating cyclonic eddies lift the nutrient-rich thermocline into the euphotic zone and maintain high levels of chlorophyll in summer. In the subsurface layer, the pattern in chlorophyll is influenced by both lateral and vertical advection. In winter, convection inside the eddy entrains high levels of nutrients into the mixed layer, increasing production, and resulting in high chlorophyll concentration throughout the surface mixed layer. There is significant interannual variability in both the cyclonic eddy activity and the surface phytoplankton bloom south of the KE jet, although whether or not there is a causal link is unclear.

Appendix

1.1 Ecosystem model

The marine ecosystem model is a simple nitrogen-based Nitrate, Phytoplankton, Zooplankton, and Detritus (NPZD) pelagic model (Oschlies 2001). The evolution of any biological tracer concentration C _i in the OFES is governed by an advective-diffusive-reaction equation

$$ \frac{{\partial {C_i}}}{{\partial t}} = - \nabla \cdot \left( {u{C_i}} \right) + \nabla \cdot \left( {A\nabla C} \right) + sms\left( {{C_i}} \right) $$

(1)

where the first and second terms on the right-hand side represent advection and diffusion, respectively. The velocity vector, u, is given by OFES, and the lateral and vertical diffusion coefficients, Ah and Az, are the same as used for tracer fields in OFES. The last term is the source-minus-sink term due to biological activity. For the individual biological tracers (Phytoplankton, P; Zooplankton, Z; Detritus, D; and Nitrate, N), the source-minus-sink terms are given by

$$ sms(P) = \overline J \left( {z,t,N} \right)P - G(P)Z - {\mu_P}P - {\mu_{pp}}{P^2} $$

(2)

$$ sms(Z) = {\gamma_1}G(P)Z - {\gamma_2}Z - {\mu_Z}{Z^2} $$

(3)

$$ sms(D) = \left( {1 - {\gamma_1}} \right)G(P)Z + {\mu_{pp}}{P^2} + {\mu_z}{Z^2} - {\mu_D}D - {w_S}\frac{{\partial D}}{{\partial z}} $$

(4)

$$ sms(N) = {\mu_D}D + {\gamma_2}Z + {\mu_P}P - \overline J \left( {z,t,N} \right)P $$

(5)

where $\overline J$ is the daily averaged phytoplankton growth rate as a function of depth z, time t, and nitrate concentration, N. G is the grazing function. Following Hurtt and Armstrong (1996), the phytoplankton growth rate is taken to be the minimum of light- and nutrient-limited growth,

$$ \overline J \left( {z,t,N} \right) = \min \left( {\overline J \left( {z,t} \right),{J_{\max }}\frac{N}{{{k_1} + N}}} \right) $$

(6)

where $\overline J \left( {z,t} \right)$ denotes the purely light-limited growth rate averaged over 24 h, and J _max is the light-saturated growth. $\overline J \left( {z,t} \right)$ is computed using the analytical method of Evans and Parslow (1985).

$$ \overline J \left( {z,t} \right) = \frac{1}{{{\tau_{24h}}}}\int\limits_0^{24h} {\frac{1}{{{z_k} - {z_{k + 1}}}}} \int\limits_{{z_{k + 1}}}^{{z_k}} {J\left( {z,t} \right)dzdt} $$

(7)

where

$$ J\left( {z,t} \right) = \frac{{{V_p}\alpha I\left( {z,t} \right)}}{{{{\left[ {V_p^2 + {{\left( {\alpha I\left( {z,t} \right)} \right)}^2}} \right]}^{1/2}}}} $$

(8)

$$ I\left( {z,t} \right) = I{(t)_{z = 0}}{e^{\left( { - \int\limits_{\overline z }^0 {{k_c}Pdz} } \right)}} $$

(9)

$$ I{(t)_{z = 0}} = PAR\tau (t)2\frac{{{\tau_{24h}}}}{{{\tau_{sun}}}}{\hbox{shotwave}}(t) $$

(10)

$$ {J_{\max }} = {V_p} = a{b^{cT}} $$

(11)

Following Fasham (1995), the grazing of phytoplankton by zooplankton is given by

$$ G(P) = \frac{{g\varepsilon {P^2}}}{{g + \varepsilon {P^2}}} $$

(12)

The individual biological parameters are listed in Table 1.

Table 1 Parameters of ecosystem model