Long-term observations of transport, eddies, and Rossby waves in the Mozambique Channel

Data from an array of current meter moorings covering a period of two and a half years are
used to estimate the varying transport through the Mozambique Channel.
The total transport during this period is small.
Below 1200 m the transport is weak but a prominent deep western boundary undercurrent
with cores at 1700 and 2200 m is found that transports 1.5 Sv to the north. The transport
shows a large temporal variability, and neither a continuous upper layer western
boundary current nor a continuous deep undercurrent is found. The variability in the upper
layer is dominated by a period of 68 days and results mainly from eddies that migrate
southward through the Mozambique Channel. In addition to this southward propagation, a
westward-propagating signal is evident from a space-time diagram of the throughflow.
The signal is interpreted as a Mozambique Channel Rossby normal mode. This
interpretation is consistent with results from a Principal Oscillation Pattern Analysis
(that estimates normal modes from the data) and a quasi-geostrophic channel model. A
detailed inspection of a single ‘‘eddy event’’ shows that a precursor of an anticyclone is a
strong southward current along the Madagascar coast that propagates westward to the
center of the Channel. During the westward propagation, the current becomes unstable
inducing an anticyclone. This scenario connects the westward-propagating mode with the
eddy growth and explains the coincidence of the eddy and Rossby mode frequency.