E

D — -E x k, n b dk dxk ——, dx a b dk dyk —-:-, y dy

gaIo Cn

Here, we introduce the Ekman layer thickness dE, which represents a balance between Coriolis force and eddy friction, analogous to the way of geostrophic balance between Coriolis force and pressure gradient. This thickness increases with larger values of eddy viscosity, but decreases with large values of Coriolis parameter. It is reminded that the velocity in the Ekman layer thickness approaches the geostrophic velocity while oscillating slightly about that value in the vertical coordinate.

Fig. 2 displays the zonal component of phytoplankton-induced geostrophic current at 100 m depth, when surface chlorophyll concentration is given by equation (1). Note that the coordinate origin of the biomass distribution function, B(x, y), is located on the equator and the length is scaled by 1°. It is seen that the flows are westward in the northern hemisphere, corresponding to the anomalous pressure gradient due to the existence of phytoplankton. Since the chlorophyll concentration distribution function given by equation (1) is symmetric about the equator, phytoplankton-induced geostrophic currents in the southern hemisphere is also westward. Thus, the phytoplank-ton-induced geostrophic currents are westward in the equatorial region of several degrees north and south of the equator in both the hemispheres.

Fig. 3 displays meridional components of phytoplankton-induced geostrophic currents in the domain near the equator (0.3° N to 1.4° N). It shows the equatorward anomalous currents induced by the anomalous zonal pressure gradient due to the corresponding zonal gradient of phytoplan-kton concentration given by equation (1). Figs. 2 and 3 represent that phytoplankton-induced geostrophic currents converge toward the equator

Geostrophic Cycle

Figure 2: Zonal component of chlorophyll biomass induced southward geostrophic currents (m s-1). The horizontal coordinate origin is located at dateline on the equator and the lengths are scaled by 1°. y coordinate spans 0.5-1.5° N (For colour version, see Colour Plate Section).

Figure 2: Zonal component of chlorophyll biomass induced southward geostrophic currents (m s-1). The horizontal coordinate origin is located at dateline on the equator and the lengths are scaled by 1°. y coordinate spans 0.5-1.5° N (For colour version, see Colour Plate Section).

Geostrophic Cycle

Figure. 3: Meridional component of chlorophyll biomass-induced southward geostrophic currents (m s-1). The horizontal coordinate origin is located at dateline on the equator and the lengths are scaled by 1°. y coordinate spans 0.3-1.4° N (For colour version, see Colour Plate Section).

Figure 4: Analytical solutions for biologically generated horizontal current (m s-1) at the depth 100 m (longitude (183-190 E) and latitude (0.5-1 N)) (For colour version, see Colour Plate Section).

where the phytoplankton concentration generates a strong zonal gradient along the equator. Fig. 4 is the longitude-latitude map of the phytoplankton-induced geostrophic currents in (cm sec-2) at the depth of 150 m. It shows that the large water mass off the equator converge toward the equator east of the dateline along 0.6° N. The large amount of water mass flow back to the east in the equatorial ocean region between 0.6° N and the equator. Since these flows are in geostrophic balance, the flow never crosses the equator due to the singularity of Coriolis parameter on the equator.

Thus, these flows feed the EUCs with biomass-induced subsurface water mass (at the depth of 100 m in our example shown in Fig. 4). These subsurface anomalous flows induced by phytoplankton biomass merge into the EUCs, which are thus strengthened and eventually rise toward the ocean surface in the eastern equatorial Pacific. Thus, the colder waters with rich nutrients rise upward in the eastern equatorial Pacific to feed more nutrients to biomass-rich region, implying positive feedback mechanism between ocean ecosystem and geophysical fluid system in the ocean with life activity. This mechanism corresponds to the hypothesis of phytoplankton-induced EUCs and associated lower temperature surface anomalous spots seen in the previous ocean general circulation experiments (Nakamoto et al., 2001; Miller et al., 2003; Ueyoshi et al., 2003).

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