al. 2010; Kaupp et al. 2011). The shears between the westward-flowing South
Equatorial Current, the North and South Countercurrents and the eastward-flowing
Equatorial Undercurrent produce tropical instability waves (TIWs) that propagate
from west to east at 50 km per day. These waves distort the upwelling tongue in a
wave-like pattern visible in satellite images of sea-surface temperature (Plate 11.4).
Animations of modeled TIWs are posted at: http://www.atmos.washington.edu/
∼robwood/images/1999_2000_ct15.avi.
(^) The three-dimensional circulation of TIWs is best visualized as a vortex in the water
column (Kennan & Flament 2000). Downwelling and northern transport occur at the
west edge, with upwelling and westward transport at the northern edge. Strong TIWs
dilute nutrients and chlorophyll by advecting nutrient-depleted water toward the
equator, whereas weak TIWs enhance local upwelling and result in higher
productivity and biomass accumulation than in the core of upwelled water (Evans et
al. 2009).
(^) Waters in the eastern equatorial Pacific resemble the subarctic Pacific in that they
are never depleted of nitrate, and phytoplankton are always present in modest, non-
bloom quantities. Together with the subarctic Pacific and much of the Southern
Ocean, it is an HNLC region. As in the cold HNLC regions, iron limitation constrains
the phytoplankton to small size; protozoan grazing regulates the phytoplankton stock
to near constancy; and surface-layer recycling of fixed nitrogen as ammonia
suppresses nitrate utilization so that it is never completely used up. These ideas about
the cascade of iron effects have been tested in two so-called IRONEX studies
conducted to the southeast of the Galapagos (on the equator west of Ecuador). The
results are described by Martin et al. (1994) and Coale et al. (1996). The second
experiment benefited from the experience of the first. Coale et al. added 225 kg of
iron (FeIII) as acidic (pH 2.0) iron sulfate solution to the prop wash of their ship as it
traced out a grid of 72 km^2 around a central buoy. This raised the iron concentration in
the water from less than 0.2 nM to greater than 1.0 nM. The enriched “patch” could
be followed by analysis of seawater for minute quantities of the inert chemical tracer
SF 6 (sulfur hexafluoride) added at the same time. Iron concentration in the patch was
sustained for over a week by subsequent 110 kg additions after 3 and 7 days. The
result is most clearly shown in the color plots of the patch (Plate 11.5). They rapidly
got a big bloom of phytoplankton, eventually reaching 3 μg Chl liter−1, a level never
seen under natural conditions in that region. It was large cells, particularly diatoms,
which showed the largest increases (85-fold). There was corresponding nitrate and
carbonate draw-down.
(^) The IRONEX-I team (Martin et al. 1994) also compared phytoplankton processes
upstream (east) and downstream (west) of the Galapagos, showing differences
corresponding to a plume of phytoplankton that is usually present downstream of the
islands. They claim that this is attributable to iron washing off the Galapagos