CSIR NET June 2016 Q62: Physical Oceanography
Ocean circulation (deep thermohaline flow) is ultimately driven by horizontal pressure gradients generated by spatial differences in water density (temperature/salinity).
Water density differences created by heating/cooling and evaporation/precipitation set up horizontal pressure gradients within the ocean (isobaric surfaces are tilted). It is this horizontal pressure gradient force that continuously drives water to flow from high to low pressure, sustaining circulation. Without persistent horizontal pressure gradients, large-scale flow would stall even if density differences existed. Option 4 is correct.
1) Incorrect — density gradients are the cause, but circulation is mechanistically driven by the pressure gradients they create, not density itself. 2) Incorrect — vertical pressure gradients are balanced by gravity (hydrostatic balance) and do not drive horizontal flow. 3) Incorrect — deep water mass formation is a consequence/source region process, not the sustaining mechanism. 4) Correct — horizontal pressure gradients are what keep the circulation perpetually operating.