196
Chapter 9: The Atlantic Ocean
Senegambia. This, the Guinea Dome, is a permanent feature that has been compared to
“a mountain sitting on the end of the thermocline ridge between the NEC and NECC.”
Its relative development appears not to be related to variance in the local Ekman vertical
velocity fields responding to local wind, and it is one of the principal distantly forced
features of the regional oceanography of the tropical Atlantic (Siedler et al., 1992).
Vertical flux and upwelling along the equator are forced by divergence in the easterly
wind field caused by the change in sign of the Coriolis force at the equator. Unrelated to
upwelling, current shear between the fast eastward flow of the equatorial undercurrent
(EUC), and the slower, broader westward surface flow of the SEC produces exchange
between deep, cool EUC water and the warmer SEC. These two dynamic processes
contribute to significant vertical nutrient flux at, and close to, the equator. Chlorophyll
images show that, just as in the Pacific, the consequences of equatorial divergence take
the form of waves associated with Rossby or tropical instability waves (TIWs) that are
more perfectly developed in the Pacific; nevertheless, they also occur in the Atlantic
both north and south of the equator—and not only in summer, when they are most
readily observed in surface fields of SST and chlorophyll (see Color plate 7). They are
generated by barotropic instability in the shear between the equatorial undercurrent and
the SEC and are particularly energetic in the central part of the WTRA province where,
at 15–35
W, annual mean eddy kinetic energy is >200 cm
2 sec
−1 in the 20- to 50-day
band (Jochem et al., 2004). The region of 1
N 15
W concentrates the largest variability in
TIWs when a long time series (1998–2001) is averaged over years (Catabiano et al., 2005).
TIWs are observed in both SST and chlorophyll images as cusp-shaped, or lunate,
features of wavelength 1000–2000 km and period of 20–40 days, that reveal both their
anticylonicity and their westward progression at velocities of ∼ 05 m sec
−1 . Thus, they
propagate across the WTRA province and into the NBC in periods of only several
months. The distribution of convergence and divergence within a TIW is discussed later,
in relation to their effects on biota.
Response of the Pelagic Ecosystems
Generally, the chlorophyll field of the equatorial Atlantic responds to the westward tilt
of the equatorial thermocline in boreal summer by showing a demarcation near the
15–20
W pivot line: to the west, near-surface chlorophyll values are consistently lower
than those to the east, a fact recently confirmed at sea by Pérez et al. (2005). This line is
close to the boundary between WTRA and ETRA.
The pelagic ecology of the WTRA responds to this and to the other physical processes
discussed in the previous section; indeed, the readily available sea surface chlorophyll
images, both 7-day and monthly, are the most effective tool for characterizing instantaneous circulation patterns simply and effectively. In these images, two seasonally changing
patterns of surface chlorophyll capture our attention: (i) along and to the north of the
equator, especially east of the BCC retroflection area where the TIWs, discussed earlier,
are most strongly expressed (ii) along the axis of the NECC and over the Guinea Dome.
I shall discuss these features individually next, based on examination of 64 consecutive
monthly SeaWiFS images from September 1997 to December 2002.
The Equatorial Divergence, Tropical Instability Waves, and the NBC Retroflection
During most of the year, the consequences of equatorial divergence are evident in the
chlorophyll field as a band of chlorophyll enhancement symmetrically aligned with the
equator. This feature is absent only briefly each year, for 2–4 months, usually from
January to April. For some months before and after this, it may be weak and diffuse. In
later boreal summer, usually in the period June–September when westward zonal wind
stress is maximal in the western ocean, chlorophyll enhancement is also maximal. Of
Chapter 9: The Atlantic Ocean
Senegambia. This, the Guinea Dome, is a permanent feature that has been compared to
“a mountain sitting on the end of the thermocline ridge between the NEC and NECC.”
Its relative development appears not to be related to variance in the local Ekman vertical
velocity fields responding to local wind, and it is one of the principal distantly forced
features of the regional oceanography of the tropical Atlantic (Siedler et al., 1992).
Vertical flux and upwelling along the equator are forced by divergence in the easterly
wind field caused by the change in sign of the Coriolis force at the equator. Unrelated to
upwelling, current shear between the fast eastward flow of the equatorial undercurrent
(EUC), and the slower, broader westward surface flow of the SEC produces exchange
between deep, cool EUC water and the warmer SEC. These two dynamic processes
contribute to significant vertical nutrient flux at, and close to, the equator. Chlorophyll
images show that, just as in the Pacific, the consequences of equatorial divergence take
the form of waves associated with Rossby or tropical instability waves (TIWs) that are
more perfectly developed in the Pacific; nevertheless, they also occur in the Atlantic
both north and south of the equator—and not only in summer, when they are most
readily observed in surface fields of SST and chlorophyll (see Color plate 7). They are
generated by barotropic instability in the shear between the equatorial undercurrent and
the SEC and are particularly energetic in the central part of the WTRA province where,
at 15–35
W, annual mean eddy kinetic energy is >200 cm
2 sec
−1 in the 20- to 50-day
band (Jochem et al., 2004). The region of 1
N 15
W concentrates the largest variability in
TIWs when a long time series (1998–2001) is averaged over years (Catabiano et al., 2005).
TIWs are observed in both SST and chlorophyll images as cusp-shaped, or lunate,
features of wavelength 1000–2000 km and period of 20–40 days, that reveal both their
anticylonicity and their westward progression at velocities of ∼ 05 m sec
−1 . Thus, they
propagate across the WTRA province and into the NBC in periods of only several
months. The distribution of convergence and divergence within a TIW is discussed later,
in relation to their effects on biota.
Response of the Pelagic Ecosystems
Generally, the chlorophyll field of the equatorial Atlantic responds to the westward tilt
of the equatorial thermocline in boreal summer by showing a demarcation near the
15–20
W pivot line: to the west, near-surface chlorophyll values are consistently lower
than those to the east, a fact recently confirmed at sea by Pérez et al. (2005). This line is
close to the boundary between WTRA and ETRA.
The pelagic ecology of the WTRA responds to this and to the other physical processes
discussed in the previous section; indeed, the readily available sea surface chlorophyll
images, both 7-day and monthly, are the most effective tool for characterizing instantaneous circulation patterns simply and effectively. In these images, two seasonally changing
patterns of surface chlorophyll capture our attention: (i) along and to the north of the
equator, especially east of the BCC retroflection area where the TIWs, discussed earlier,
are most strongly expressed (ii) along the axis of the NECC and over the Guinea Dome.
I shall discuss these features individually next, based on examination of 64 consecutive
monthly SeaWiFS images from September 1997 to December 2002.
The Equatorial Divergence, Tropical Instability Waves, and the NBC Retroflection
During most of the year, the consequences of equatorial divergence are evident in the
chlorophyll field as a band of chlorophyll enhancement symmetrically aligned with the
equator. This feature is absent only briefly each year, for 2–4 months, usually from
January to April. For some months before and after this, it may be weak and diffuse. In
later boreal summer, usually in the period June–September when westward zonal wind
stress is maximal in the western ocean, chlorophyll enhancement is also maximal. Of