SIO 217B Atmospheric and Climate Sciences II
Winter 2012
Homework #2
Due February 2 at the beginning of class
1. Two dominant features of wintertime near-surface atmospheric circulation over the
midlatitude NH are large semi-permanent low pressure centers located over the North
Pacific and North Atlantic. They are commonly called the Aleutian Low and the
Icelandic Low as a result of where the lowest SLP is located. The continents are
contrastingly dominated by the semi-permanent Siberian High and North American
High. The wintertime pattern of low SLP over ocean and high SLP over land reverses
during summer, when near-surface circulation over the ocean is dominated by the
North Pacific High and the Bermuda/Azores High. The center of the former is located
over the North Pacific at the latitude of California, and the center of the latter is
located over the Azores and/or Bermuda. These large and SLP low and high centers
affect the meridional direction of the near-surface wind, but the zonal component is
generally always westerly at middle latitudes.
(a) Explain why NH midlatitude oceans have low SLP during winter but high SLP
during summer and NH midlatitude continents have high SLP during winter but low
SLP during summer. Hint: consider that the ocean has much greater thermal inertia
than land.
During winter the continents cool down much more than the ocean because they have
little thermal inertia. The cold air over the continents becomes dense and the thickness
between pressure levels is reduced. The geopotential height of pressure surfaces in the
upper troposphere decreases, and an upper-level low develops. Air converges into the
upper-level low, thus increasing column mass. Greater column mass means higher
pressure at the surface. The transfer of mass from over the oceans to over the continents
means lower pressure at the surface over oceans.
During summer the continents warm up much more than the ocean because they have
little thermal inertia. The warm air over the continents becomes less dense and the
thickness between pressure levels is enhanced. The geopotential height of pressure
surfaces in the upper troposphere increases, and an upper-level high develops. Air
diverges out of the upper-level high, thus decreasing column mass. Less column mass
means lower pressure at the surface. The transfer of mass from over the continents to
over the oceans means higher pressure at the surface over oceans.
(b) Assuming surface friction is negligible, what will be the direction of the
meridional component of the near-surface wind on the eastern and western sides of
the ocean basins during winter and summer? Why?
If surface friction is negligible, wind will be nearly geostrophic. Geostrophic wind in the
NH has low pressure to the left and high pressure to the right. During winter there is low
pressure over the central ocean basin, so the meridional wind component will be
northerly on the western side and southerly on the eastern side. During summer there is
high pressure over the central ocean basin, so the meridional wind component will be
southerly on the western side and northerly on the eastern side.
(c) At the same latitude, the western coasts of continents are typically warmer than
the eastern coasts during winter but cooler during summer. Provide one reason for
this in terms of meridional temperature advection. Provide another reason for this in
terms of zonal temperature advection.
During winter the meridional wind component is southerly near the western coast and
northerly near the eastern coast of continents. Southerly winds in the NH come from the
warmer subtropics and thus warm the western coast. Northerly winds come from the
colder subarctic and thus cool the eastern coast. During summer the meridional wind
component is northerly near the western coast and southerly near the eastern coast of
continents, thus cooling the western coast and warming the eastern coast.
Zonal winds are westerly during winter and summer, but the ocean is warmer than the
continent during winter and cooler during summer. On the western coast during winter,
zonal winds come from the relatively warm ocean and thus have a warming effect. On the
eastern coast during winter, zonal winds come from the relatively cold continent and have
a cooling effect. On the western coast during summer, zonal winds come from the
relatively cooling ocean and thus have a cooling effect. On the eastern coast during
summer, zonal winds come from the relatively warm continent and cause warming.
2. Midlatitude winds have a much stronger westerly component in the middle and upper
troposphere than is the case near the surface. There is also much less land/ocean
difference in the meridional wind component in the middle and upper troposphere
than is the case near the surface.
(a) Explain why the wind is strongly westerly in the middle and upper troposphere at
middle latitudes.
Temperature decreases toward the pole so thickness between pressure surfaces also
decreases toward the pole. Pressure surfaces in the upper troposphere will have lower
height toward the pole and higher height toward the equator, so wind will be in the
westerly direction. The thermal wind relationship indicates that westerly wind will
increase with height when cold is on the left (poleward side).
(b) Describe whether winds will turn cyclonically or anticyclonically with height on
the eastern and western sides of the NH midlatitude ocean basins during summer and
winter. Describe whether warm or cold advection occurs on the eastern and western
sides of ocean basins during summer and winter.
Near-surface wind has a northerly component on the western side of ocean basins during
winter and on the eastern side of ocean basins during summer. This turns cyclonically
with height to westerly wind, indicating cold advection. Near-surface wind has a
southerly component on the eastern side of ocean basins during winter and on the western
side of ocean basins during summer. This turns anticyclonically with height to westerly
wind, indicating warm advection.