Question

Background: As you know there is a high pressure region in the center of the Pacific called the North Pacific high. This high pressure region is responsible for the clockwise rotating geostrophic flow in the North Pacific called the North Pacific gyre. Locally, we call this current the California current.

Theory:

a) A typical atmospheric pressure difference between the center of the North Pacific high and its edge 1000 nautical miles away is 10-20 millibar.

- Assume that the pressure gradient is linear and estimate the horizontal pressure gradients caused by this pressure difference. Give you answer in Pascal/meter.

- Compute the geostrophic currents resulting from this pressure difference in m/s and knots.

b) Due to the Coriolis force once the North Pacific gyre starts moving, water curves towards the center of the gyre. This water accumulates and increases the sea surface height at the center of the gyer by as much as 1-2 meters above the exterior of the gyre which may be 1000 nautical miles from the center.

- Use the usual relationship between sea surface height and pressure to convert this height difference to a pressure difference.

- Assume that the pressure gradient is linear and estimate the horizontal pressure gradients caused by this pressure difference. Give you answer in Pascal/meter.

- Compute the geostrophic currents resulting from this pressure difference in m/s and knots.

Answer 1

1 bar = 10⁵ Pa gives 1 milibar = 100 Pa

thus 10 milibar = 1000 Pa and 20 milibar = 2000 Pa

1000 nautical mile = 1852000 m

Thus Linear Pressure Gradient to

i.e horizontal gradient of pressure is

,   

is the angle of latitude. may vary between 0 to 50 degrees

in m/s

1 m/s = 1.944 knots

gives knots

(b)