The Four Primary Biomes of the Upper Ocean
97
though spawning in freshwater, spend almost their whole lives in the open ocean. Some
species of tuna perform meridional migrations that enable them to exploit the westerlies
biome even to quite high latitudes during summer in all oceans. Here, too, we encounter
the poleward limits of the small, vertically migrant bathypelagic fish that are more
characteristic of the Trades biome.
Trades Biome
The Case 4 model is appropriate to that part of the ocean at low latitudes (forming
22% of the whole ocean) that lies under the influence of the trade winds, between the
STC zones that lie across the subtropical gyres of each hemisphere, where seasonality
is weakest and where changes in depth of discontinuity layers is imposed principally
by geostrophic adjustment of the zonal equatorial current systems. Mixed-layer depth is
therefore relatively constant, so that primary production rate and phytoplankton biomass
take characteristically low values; a deep chlorophyll maximum is almost always present,
the “Typical Tropical Profile” being a paradigm for the vertical arrangement of the
planktonic food web.
The nutricline is perennially shoaler than the photic depth, except during exceptional
events, so vertically integrated production rate is not light-limited, although productivity
is nutrient or trace element limited. This situation obtains essentially year-round, and
the pattern breaks down with deepening of the mixed layer only briefly and only in some
regions. Minor rate increases are forced by geostrophic response of the pycnocline to
seasonality in trade wind stress, and by open-ocean Ekman suction, or by divergence,
particularly at the equator.
Because of the different shape of ocean basins, and the different arrangement of high
terrain on the continents, the pattern of trades and westerly winds is not identical over
each central gyre. As noted earlier, the STC zones of the southern hemisphere lie much
farther poleward than in the north, so that the partition between their characteristic
biomes differs significantly. Thus, there is an important distinction between the proposal
for an ecological partition discussed here, and previous proposals in which conditions
within central gyres were assumed to be essentially uniform (e.g., Miller, 2004). On the
contrary, here it is thought to be significant that almost the whole South Atlantic gyre
lies below trade winds, while the demarcation between trades and westerlies lies across
the North Atlantic central gyre at about the latitude of Bermuda: a similar partition of
wind regimes occurs in the North Pacific.
The Case 5 model describes the dynamics of those parts of the trade-wind zone where
strong seasonality is a feature of local wind forcing; these are the monsoon regions where,
in the extreme case of the northwest Indian Ocean, seasonal wind reversal results in
seasonal reversal of the monsoon currents, and seasonal alternation between eutrophic
and oligotrophic biological systems. The small zonal dimension of the tropical Atlantic
permits a strong seasonal response to the surge of southerly trades across the equator
that is analogous to the effect of monsoon reversal in the Arabian Sea.
Though the seasonal variance in frictional wind stress at the sea surface here is
equivalent to that in the latitudes of winter westerlies, the consequences are quite different,
as was discussed in Chapter 4. It is, of course, the seasonal meridional migration of
the north and south trade-wind belts and of the intertropical convergence zone (ITCZ),
marking the doldrum region of calm winds between the trades, that is the source of the
variability in wind stress that forces seasonality in the tropical ocean. The migration of
the ITCZ modifies the sign and intensity of Ekman vertical motion over large regions
(Isemer and Hasse, 1987).
97
though spawning in freshwater, spend almost their whole lives in the open ocean. Some
species of tuna perform meridional migrations that enable them to exploit the westerlies
biome even to quite high latitudes during summer in all oceans. Here, too, we encounter
the poleward limits of the small, vertically migrant bathypelagic fish that are more
characteristic of the Trades biome.
Trades Biome
The Case 4 model is appropriate to that part of the ocean at low latitudes (forming
22% of the whole ocean) that lies under the influence of the trade winds, between the
STC zones that lie across the subtropical gyres of each hemisphere, where seasonality
is weakest and where changes in depth of discontinuity layers is imposed principally
by geostrophic adjustment of the zonal equatorial current systems. Mixed-layer depth is
therefore relatively constant, so that primary production rate and phytoplankton biomass
take characteristically low values; a deep chlorophyll maximum is almost always present,
the “Typical Tropical Profile” being a paradigm for the vertical arrangement of the
planktonic food web.
The nutricline is perennially shoaler than the photic depth, except during exceptional
events, so vertically integrated production rate is not light-limited, although productivity
is nutrient or trace element limited. This situation obtains essentially year-round, and
the pattern breaks down with deepening of the mixed layer only briefly and only in some
regions. Minor rate increases are forced by geostrophic response of the pycnocline to
seasonality in trade wind stress, and by open-ocean Ekman suction, or by divergence,
particularly at the equator.
Because of the different shape of ocean basins, and the different arrangement of high
terrain on the continents, the pattern of trades and westerly winds is not identical over
each central gyre. As noted earlier, the STC zones of the southern hemisphere lie much
farther poleward than in the north, so that the partition between their characteristic
biomes differs significantly. Thus, there is an important distinction between the proposal
for an ecological partition discussed here, and previous proposals in which conditions
within central gyres were assumed to be essentially uniform (e.g., Miller, 2004). On the
contrary, here it is thought to be significant that almost the whole South Atlantic gyre
lies below trade winds, while the demarcation between trades and westerlies lies across
the North Atlantic central gyre at about the latitude of Bermuda: a similar partition of
wind regimes occurs in the North Pacific.
The Case 5 model describes the dynamics of those parts of the trade-wind zone where
strong seasonality is a feature of local wind forcing; these are the monsoon regions where,
in the extreme case of the northwest Indian Ocean, seasonal wind reversal results in
seasonal reversal of the monsoon currents, and seasonal alternation between eutrophic
and oligotrophic biological systems. The small zonal dimension of the tropical Atlantic
permits a strong seasonal response to the surge of southerly trades across the equator
that is analogous to the effect of monsoon reversal in the Arabian Sea.
Though the seasonal variance in frictional wind stress at the sea surface here is
equivalent to that in the latitudes of winter westerlies, the consequences are quite different,
as was discussed in Chapter 4. It is, of course, the seasonal meridional migration of
the north and south trade-wind belts and of the intertropical convergence zone (ITCZ),
marking the doldrum region of calm winds between the trades, that is the source of the
variability in wind stress that forces seasonality in the tropical ocean. The migration of
the ITCZ modifies the sign and intensity of Ekman vertical motion over large regions
(Isemer and Hasse, 1987).