Download Atmospheric Sciences 101, Summer 2003 Homework #5 – Solutions

Atmospheric Sciences 101, Summer 2003
Homework #5 – Solutions
1. Jet streams
a. During which season is the polar jet stream weakest? Explain your answer.
Summer – The polar jet stream is a result of the temperature difference between lower and
higher latitudes, which result in pressure gradients that cause the winds. In summer the
smaller temperature difference leads to weaker pressure gradients and thus a slower polar
jet stream.
b. Why do the jet streams (not to mention the Hadley cells and the Intertropical Convergence
Zone) move north and south during the course of each year?
The polar jet streams occur over the polar front, the boundary between warmer, lower
latitude air and colder, higher latitude air. That cold air expands farther equatorward
during winter and contracts farther poleward during summer, thus the polar jet stream
follows in those directions. The subtropical jet streams occur at the most poleward extent
of the Hadley cells, which also move north and south with the seasons because their rising
branches occur at the ITCZ, which in general moves slightly into the summer hemisphere
where sunlight is more direct and heating greater.
2. Ocean circulations
a. Explain why the surface waters just off the coast of California are so cold. (Hint: Begin the
description with the large-scale surface pressure pattern and the resulting surface winds.)
Particularly in summer there is higher pressure centered offshore to the west of that
region. Therefore the winds circulating around the persistent high at the coast are
generally northerly. The net transport of water due to the northerly winds is toward the
west, deflected to the right of the wind by the Coriolis force. To compensate for the water
being transported offshore, colder water from below is brought to the surface making the
surface water colder there.
b. What is the cause of most of the surface currents of the ocean?
Most surface currents are simply the result of the water being dragged in the same
direction as the prevailing winds.
3. Intertropical Convergence Zone (ITCZ)
a. What at the surface near the Equator causes the ITCZ?
The ITCZ is the result of the convergence of the trade winds, which are the surface branch
of the Hadley circulation, which means that though they have and easterly component, they
blow also toward the Equator.
b. Why is heavy precipitation typical at the ITCZ?
Besides being where the trade winds converge, the ITCZ is also close to the location of most
direct sunlight coming to the surface. It is very warm there and the air is quite moist. The
heating plus the convergence of the trade winds provide the lift to get air rising. The air
cools to saturation where rising resulting in clouds and rain.
4. Hot Seattle summer weather
Seattle almost never experiences extended heat waves (i.e., several consecutive days with
temperatures above 90°F). What specifically often happens to break a heat wave when it does
occur here during the summer? (Hint: In your answer describe the pressure patterns and local
circulation that develop in association with very hot temperatures around Puget Sound.)
When temperatures become very warm in western Washington a shallow thermal low
pressure region develops at and near the surface because of the heat making the air less
dense. The temperatures offshore are not nearly as warm, and the surface pressure there
remains consistently relatively high there during the summer because of the cool surface
and the persistent Pacific high. As a result of this pressure pattern, onshore flow from
higher pressure toward lower pressure develops. The onshore flow brings cooler air from
out over the ocean into the Seattle area and thus makes the region cooler.
5. El Niño Southern Oscillation (ENSO)
a. Fill in the table below describing how El Niño and La Niña events typically affect winter
weather in the Pacific Northwest. (Hint: see http://www.wrcc.dri.edu/enso/ensofaq.html)
El Niño
La Niña
Temperature
warmer than average
colder than average
Precipitation
drier than average
wetter than average
less than average
Greater than average
Snowpack
b. Which years during 1980-2003 had the 3 strongest El Niño events of that period?
1983, 1998, 1992
Similarly, which 3 years out of the period 1980-2003 had the strongest La Niña events?
1989, 2000, 1999
c. View the tables of average winter temperature and precipitation for the state of Washington
over the winters of 1980-2003 at:
http://www.atmos.washington.edu/2003Q3/101/wx/WAWinterTemp19802003NCDC.html
http://www.atmos.washington.edu/2003Q3/101/wx/WAWinterPrcp19802003NCDC.html
Focus on the 6 years you selected to answer part (b) above. Describe how well the ranks of
those years agree with the answers to part (a) about how winter temperature and precipitation
is likely to be affected in Washington by El Niño and La Niña events and tell how much
influence you think ENSO has on the overall winter weather in Washington. (Note: A bigger
number in the rank column means a warmer or wetter winter compared to the other winters.)
The ranks for the strong El Niño winter temperatures agree well with part (a) as those
three winters were all among the warmest 5-6 of the period 1980-2003. However, for the
strong La Niña winter temperatures only one (1989) was among the coldest of the period
while the other two were more middle-of-the-road. Precipitation was not very consistent
for either the strong El Niño or La Niña cases. One winter rank in each case agrees well
with part (a), but the other were average to more the opposite disagreeing with part (a).
Thus the ENSO can tell you whether a winter is more likely to be warmer/cooler or
wetter/drier than average, but it cannot tell you for certain what weather to expect.