Physical forcing of the shelf seas: what drives the motion of ocean? - Introduction to the Physical and Biological Oceanography of Shelf Seas - page 37

Figure 2.9 Average monthly discharge
of the Mekong River based on data
for 1933-53 and 1961-1966. The
corresponding monthly discharge of
the Rhine is shown as the dashed line.
Data supplied by the Centre for Ecology
and Hydrology, Wallingford, UK.

40

30

Mekong

20

10

Rhine

0

0 2 4 6 8 10 12

Month

2.3.2 Seasonal cycles of freshwater input

Unlikethesurfaceheatexchange, which follows rather predictable seasonal patterns
depending on latitude, rivers vary widely in their annual discharge cycles. While many
large rivers in temperate latitudes, such as the Rhine, maintain a substantial flow through-
out the year, smaller rivers often have a highly variable discharge. Rivers in tropical and
subtropical areas tend to exhibit particularly strong seasonal variation due to the mon-
soonal rain patterns. For example, Fig. 2.9 illustrates the seasonal cycle of the Mekong,
a large tropical river in Southeast Asia, which discharges near the entrance to the Gulf
of Thailand. The dry season flow of this river is of the same order as the mean flow
rate of the River Rhine but increases

40-fold during the peak runoff in the wet season.

∼

2.3.3 Global distribution of freshwater input

From a global perspective, the tropical oceans receive a disproportionately high
fraction of the total freshwater runoff entering the oceans. As can be seen in
Fig. 2.10 , the large discharges into the ocean from the tropical regions of South
America, Africa and Southeast Asia greatly exceed those from the rivers in mid
latitudes. Moreover, much of this freshwater buoyancy input to tropical seas is subject
to strong seasonal variation, as in our example of the Mekong. This pronounced
modulation of the buoyancy input as freshwater is in contrast to the buoyancy input
by the heat flux in the tropics which, as we noted earlier (Section 2.2.5 ) , tends to exhibit
minimal seasonal variation in the tropics. Not all tropical rivers exhibit strong sea-
sonal cycles; a notable exception is the Amazon, which carries a high proportion
( 20%) of all the freshwater entering the oceans. Its discharge is relatively stable,

varying by

∼

30% from the mean flow rate of 219 000m 3 s 1 .
It is apparent from Fig. 2.10 that, after the tropics, the next largest discharges
come from rivers debouching into the Arctic Ocean. Several very large rivers including
the Yenisey, the Lena and the Ob discharging from the north coast of Eurasia combine
with flow from the Mackenzie and other Canadian Arctic rivers to produce a com-
bined mean input of

∼

150 000m 3 s 1 , which is equivalent to more than two-thirds of

∼