class #15 friday, october 1, 2010 class #15: friday, october 1 chapter 7 upper-level winds chapter 8...

Class #15 Friday, October 1, 2010

Class #15: Friday, October 1

Chapter 7 Upper-level winds

Chapter 8 Oceanography

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Class #15 Friday, October 1, 2010 2

Winds and pressures (heights) well above the surface

• Pressures and heights are on average high in the tropics and decrease to lows close to the poles.

• Upper-level (500mb and above) winds are generally easterlies (blowing east to west) in the tropics and westerlies (blowing west to east) in higher latitudes.


Patterns in the upper-level westerlies

• The pattern of upper-level westerlies has waves, with axes of high height (ridges and highs) and axes of low height (troughs and lows). These appear on day to day weather maps.

• These wavelike patterns help transport energy poleward (advection) to balance the energy budget of the Earth and atmosphere.


Names for upper-level wind patterns

• When the waves are small, and the ridges and troughs are weak, the pattern is called zonal, or high index, meaning roughly west to east at constant latitude.

• When the waves have greater amplitude (north-south dimension), the pattern is called meridional, or low index, meaning that there is a lot of north/south motion.


More about upper-level patterns

• Sometimes there is zonal flow at high latitudes and meridional flow at low latitudes. This is a split-flow pattern.

• Sometimes persistent closed highs and lows form in a split-flow pattern when the meridional pattern is extremely meridional. This is called a blocking pattern, because it can be extremely persistent.


Implications of upper-level winds

• Blocking highs can lead to drought conditions and prolonged heat waves.

• Meridional flow accomplishes poleward energy transport that helps balance the energy balance of the Earth and atmosphere.


More implications of upper-level wind patterns

• Surface low-pressure centers, fronts, cloudiness and precipitation in midlatitude cyclones occur beneath the region east of the trough of waves in the upper-level winds near the jet stream and above the polar front.

• Hurricanes, in contrast, require weak upper-level winds.


More about waves in the westerlies aloft

• Shorter waves move eastward faster than the longer Rossby waves.

• Waves of different length can add and subtract to/from one another’s amplitude.

• Forecasting waves in the westerlies aloft is vital for everyday weather forecasting.

• El Niño/La Niña affect the westerlies aloft.


Complications of the real Earth

• Earth has seasons– The ITCZ (sometimes called the thermal

equator) shifts latitude with the seasons.– The ITCZ shifts north of the equator in NH

summer, and south of the equator in SH summer (NH winter)

• Earth has large land masses– Continents and oceans set up thermal

circulations


Observed surface pressures

• Vary with the seasons, requiring both a January and a July depiction

• Are on average high in the sub-tropics (near 30°) and near the pole

• Are on average low in the ITCZ and along the polar front (near 60°)

• In summer are high over the oceans and low over the continents (thermal lows).

• In winter are high over the continents and low over the oceans.


Seasonal shifts

• The ITCZ, the subtropical highs, and the polar front all shift southward in NH winter and northward in NH summer.

• Seasonal shifts are most intense over Asia, which has the largest continental air mass.

• The summer monsoon is wet, with low pressure over land; the winter monsoon is dry, with high pressure over land.


Other monsoons

• Africa, North America, and Australia have monsoon-like wind patterns, particularly in the warm season.


The oceans

• Cover about 70% of the earth’s surface.• Cover an even higher percentage in the tropics.• Exchange large amounts of energy and water with the

atmosphere at the surface.– Give water vapor to the atmosphere.– Give energy to the atmosphere in the form of latent heating.– Exchange energy with the atmosphere by convection and

conduction.

• Advect large amounts of energy from the equator towards the poles


Energy gains and losses from the point of view of the oceans

• When the oceans give up energy (negative, blue, purple) the atmosphere gains energy from the oceans.

• When the oceans gain energy (positive, red, yellow) the atmosphere loses energy to the oceans.

• Oceans supply energy to storms in middle latitudes.

• Oceans warm in summer and cool in winter.


The rate of energy transfer between atmosphere and oceans

• Is greater when the temperature difference is greater.

• Is in the direction from the higher to the lower temperature.

• Is greater when the wind speed is greater

• For latent heating, rates are greatest when saturation vapor pressure in the atmosphere is largest (high temperatures).


The sea surface

• Is the part of the ocean that interacts directly with the atmosphere.

• Has a temperature called the sea surface temperature (SST) actually measured a few feet below the surface at the intake level of a ship. Cold water is used by ships for air conditioning.

• The skin temperature is the temperature right at the surface.


Temperature in the oceans

• Generally there are three layers– The surface zone has the

highest temperatures. This is sometimes called the well-mixed (by waves and convection) zone.

– The thermocline is a zone of rapidly decreasing temperature as depth increases

– In the deep zone the temperature is slightly above freezing. Salt water freezes at a lower temperature than fresh water.


Temperature profiles for different latitudes

• The tropics have the steepest thermocline, because sea surface temperatures are greatest there.

• The middle latitudes have the deepest surface layer.

• Polar regions have surface temperatures near freezing.


Water, salt water, and air

• Salt water is denser than fresh water.– Icebergs, made of fresh water, float.

• Colder water is only very slightly denser than warmer water.

• Pressure in the oceans increases downward by 1 atmosphere about every 35 feet.

• Density is nearly constant in the oceans, whatever the depth.


Sea Surface Temperatures

• Are highest in the tropics, lowest at the poles.

• In middle latitudes and subtropics, are higher on east coasts than west coasts.

• In polar regions, lowest temperatures on east coasts.

• In tropical regions, highest temperatures on west coasts.

• Are highest in the equatorial western Pacific in the “warm pool” and the Indian Ocean.

• Correspond to warm and cold surface ocean currents.


Major Ocean Currents


Surface pressure, surface wind, and ocean currents

• The subtropical highs are persistent enough to create persistent anti-cyclonic wind flow.

• These winds create gyres of anti-cyclonic ocean surface currents.

• Ocean currents are bounded by land.


Wind and ocean current direction

• Friction between the air and the sea surface forces the water to move.

• The Coriolis force turns the water to the right (NH) or left (SH).

• Moving water influences the layer of water beneath.

• The entire pattern is called the Ekman spiral.

• On average, water moves to right (NH) or left (SH) in Ekman transport.


Cold currents, west coast, and upwelling

• Ekman transport moves water away from the shore, but that water must be replaced.

• Replacement water rises from below the thermocline. Mixing and cold water brings nutrients close to the surface and favors sea life.


El Niño

• Named “The (boy) child” for the season of most common occurrence.

• Is a common but short-lived feature, but occasional episodes last for months or a year or more. The episodes are what we call El Niño today.

• El Niño is a phenomenon that affects the entire Pacific Ocean and weather around the globe.

class #15 friday, october 1, 2010 class #15: friday, october 1 chapter 7 upper-level winds chapter 8...

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