Sea Surface Temperatures, October 19, 2004

Easterly Waves

Long waves occur in bands of geostrophic wind flowing above the friction layer. Long waves may flow toward the west or toward the east depending on which of the major global wind belts they occur in. Easterly waves are "long waves" that occur within the trade wind belt, start over north western Africa, and propagate toward the west in the lower tropospheric tradewind flow across the Atlantic Ocean. They are first seen usually in April or May and continue until October or November. They occur between 5-15 degrees N. They have a wavelength of about 2000 to 2500 km, a period of ~3-4 days, and move at approximately 18 - 36 km/h. Approximately two easterly waves per week travel from Africa to North America during hurricane season. Passing from the African continent onto the cool Eastern Atlantic, the waves generally decay, but remnants mostly survive to the Western Atlantic and Caribbean where they regenerate. Only 9 out of 100 easterly waves survive to develop into gale-force tropical storms, or full-fledged hurricanes.

About 60% of the Atlantic tropical storms and minor hurricanes (Saffir-Simpson Scale categories 1 and 2) originate from easterly waves. However, nearly 85% of the intense (or major) hurricanes have their origins as easterly waves. The majority of synoptic scale systems from Africa propagate beyond the Caribbean and the Central American Isthmus into the Eastern Pacific, where some intensify into Tropical Storms. It has been suggested that nearly all of the tropical cyclones that occur in the Eastern Pacific Ocean can also be traced back to Africa. Many Typhoons in the Western Pacific are also believed to develop from Easterly Waves, although more work is needed on the relationship of Easterly Waves in the Western and Eastern Pacific.

Fig.1. Approximate location, amplitude and wavelength of easterly waves.

At first, an easterly wave has a small amplitude, and produces mild rain showers. Powerful thunderstorms and the force of high-altitude winds amplify the wave when atmospheric conditions are favourable. Several severe thunderstorms begin to form, and eventually a tropical storm may develop.

Fig. 2. The development of easterly waves off the west coast of Africa.

Troughs and Ridges in Easterly Waves

In order to understand why an easterly wave generates convection, we need to understand what ridges and troughs are in longwaves, and how they affect the behavior of geostrophic wind. In the figure below, we see the formation of a curve in the geostrophic wind, concave toward lower pressure. This is termed a trough in the wave. As the wind crosses a reference latitude toward the south, we observe a curve in the wind concave toward high pressure. This is called a ridge in the wave.

Fig. 3. Relationship between troughs, ridges and atmospheric pressure in easterly waves.

 

Regions of Convergence and Divergence in Easterly Waves.

Easterly waves influence the movement and pressure of air in the tradewind flow. This is because at certain locations in the long wave, wind speeds up or slows down. These changes cause stretching (divergence), or piling up (convergence), respectively, of the air parcel in the wave. Let’s start by seeing why air changes speed as it moves through a long wave.

Remember that gradient wind is geostrophic wind that flows parallel to curved isobars, and occurs in the absence of friction. When gradient wind is moving through a curve, centrifugal force acts on the parcel of air toward the outside of the curve (see black arrow below). When the wind is curving around low pressure (i.e. moving through a trough), the centrifugal force is acting opposite to the pressure gradient force, or PGF (green arrow below). If the Coriolis force (red arrow), and the wind speed (yellow arrow), were to remain the same, there would now be an imbalance of force acting contrary to the PGF. In order to balance the forces, and maintain an unaccelerated wind, the wind slows down. This automatically reduces the Coriolis force, and rebalances the gradient wind. Because the wind has temporarily slowed down, we call this subgeostrophic wind.

Fig. 4. Forces acting to produce subgeostrophic wind moving through a trough.

Conversely, when the wind is curving around high pressure (i.e. moving through a ridge), the centrifugal force is acting in the same direction as the PGF. If the Coriolis force, and the wind speed, were to remain the same, there would now be an imbalance of force acting in concert with the PGF. In order to balance the forces, and maintain an unaccelerated wind, the wind speeds up in the ridge. This automatically increases the Coriolis force, and rebalances the gradient wind. Because the wind has sped up (with respect to geostrophic wind along straight isobars), we call this supergeostrophic wind.

Fig. 5. Forces acting to produce supergeostrophic wind moving through a ridge.

Completing the picture of a long wave, we see in the figure below that an air parcel moving along through a series of troughs and ridges will alternate between subgeostrophic (slower) gradient wind in troughs, and supergeostrophic (faster) gradient wind in ridges. When wind slows down during its approach to a trough, the air "piles up", causing convergence. When wind speeds up during its approach to a ridge, the air parcel "stretches", causing divergence.

Fig. 6. Regions of convergence and divergence in an easterly wave.

Ahead of a trough, where the air in the wave is slowing down and converging, some air gets "pushed up" away from the surface, producing lower pressure near the surface. Conversely, ahead of an upper level ridge, where the air is speeding up and diverging, air gets "sucked down" into the long wave, producing subsidence and higher pressure near the surface. In this way, regions of subsidence and ascent at the surface are related to the position of troughs and ridges in the easterly wave. Lowerlayer divergence, subsidence, and fair weather are found ahead (upwind, or to the west) of the trough axis. Convergence, ascending motion and heavy weather (showers and towering cumulus) are concentrated to its rear (to the east).

Fig. 7. Locations of ascent and subsidence in an easterly wave in relation to the trough axis.

 

Fig. 8. Location of convergence and divergence in an easterly wave in relation to the trough axis.

The horizontal structure of an easterly wave is clearest between 700 and 500 mb, and the wave seldom affects the air above the 100 mb level. The figure below shows how the easterly wave (best seen at the 700 mb level) is causing cyclonic circulation at 850 mb, and convergence at the surface (SFC).

Fig. 9. Streamlines at different heights through an easterly wave. Streamlines and wind barbs of total flow field for composite African wave at (A) surface, (B) 850 mb,(C) 700 mb, and (D) 200 mb.

(Adapted from Hastenrath, 1991)

The convection and cloud formation associated with easterly waves is often observed from satellites as an inverted V pattern in clouds over the Atlantic Ocean.

Fig. 10. Inverted V cloud pattern caused by convection along an easterly wave.

Atmospheric Circulation

Microscale

Size:  meters Time: seconds

Mesoscale Size: kilometers Time: minutes to hours

Macroscale     Synoptic

Size: 100s to 1000s kilometers Time: days

    Global (planetary) Size: Global! Time:  Days to weeks

Macroscale Circulation

To begin, imagine the earth as a non-rotating sphere with uniform smooth surface characteristics. Assume that the sun heats the equatorial regions much more than the polar regions. In response to this, two huge convection cells develop.

Simple, single cell atmospheric convection in a non-rotating Earth.  "Single cell" being either a single cell north or south of the equator.

Figure 7.5 in The Atmosphere, 8th edition, Lutgens and Tarbuck, 8th edition, 2001.

An intermediate model: We now allow the earth to rotate.  As expected, air traveling southward from the north pole will be deflected to the right. Air traveling northward from the south pole will be deflected to the left.

However, by looking at the actual winds, even after averaging them over a long period of time, we find that we do not observe this type of motion.  In the 1920ís a new conceptual model was devised that had three cells instead of the single Hadley cell.  These three cells better represent the typical wind flow around the globe.

Idealized, three cell atmospheric convection in a rotating Earth.  "Three cell" being either three cells north or south of the equator.  The deflections of the

winds within each cell is caused by the Coriolis Force. Figure 7.5 in The Atmosphere, 8th edition, Lutgens and Tarbuck, 8th edition, 2001.

Horse Latitudes Around 30°N we see a region of subsiding (sinking) air.  Sinking air is typically dry and free of substantial precipitation. Many of the major desert regions of the northern hemisphere are found near 30° latitude.  E.g., Sahara, Middle East, SW United States.

Doldrums Located near the equator, the doldrums are where the trade winds meet and where the pressure gradient decreases creating very little winds.  That's why sailors find it difficult to cross the equator and why weather systems in the one hemisphere rarely cross into the other hemisphere.  The doldrums are also called the intertropical convergence zone (ITCZ).

A) Idealized winds generated by pressure gradient and Coriolis Force.  B) Actual wind patterns owing to land mass distribution..

Figure 7.7 in The Atmosphere, 8th edition, Lutgens and Tarbuck, 8th edition, 2001.

Owing to the tilt of the Earth's axis in orbit, the ITCZ will shift north and south.  It will shift to the south in January and north in July.

A) Southern shift of ITCZ in January.  B) Northern shift of ITCZ in July. Figure 7.9 in The Atmosphere, 8th edition, Lutgens and Tarbuck, 8th edition, 2001.

This shift in the wind directions owing to a northward or southward shift in the ITCZ results in the monsoons.  Monsoons are wind systems that exhibit a pronounced seasonal reversal in direction.  The best known monsoon is found in India and southeast Asia.

Winter -- Flow is predominantly off the continent keeping the continent dry.

Summer -- Flow is predominantly off the oceans keeping the continent wet.

Monsoons happen not only in southeast Asia and India, but also in North America.  They are responsible for the increased rainfall in the southwest US during the summer months and the very dry conditions during the winter months.

Jet Streams (revisited)

Remember, we have already talked about why the jet stream forms.  well, jet streams will form at the approximate boundaries between the cells we'v e just discussed.  So we should have a subtropical jet stream as well as a midlatitude jet stream.

Location in map view (left) and cross-sectional view (right) of the jet streams.

Figure 7.14 in The Atmosphere, 8th edition, Lutgens and Tarbuck, 8th edition, 2001.    

Since the locations of the midlatitude and subtropical jet streams are close to the cell boundaries, the jets will migrate with the seasons, like the ITCZ.

Figure 7.15 in The Atmosphere, 8th edition, Lutgens and Tarbuck, 8th edition, 2001.

Next time, we will talk of how oceanic circulation works with atmospheric circulation.      

GLOBAL ATMOSPHERIC CIRCULATIONEarth is a spinning sphere.More energy received at equator than at poles

Poleward of 40° more energy lost to space than received from the sun

Moving energy poleward produces the world's climate patterns

1. HADLEY CIRCULATION

At equator, warm raising air produces a belt of low pressure and easterly winds. Known as the ITCZ, shifts north and south with earth's orbit.

N-S circulation. Hot air raises at equator, Movement poleward at tropopause, descent at 30o, movement equator-ward at Earth's surface.

WALKER CIRCULATION:

E-W circulation, air raising in Indonesia, descending in Galapagos

Major Semi-Permanent Features of Atmospheric Circulation: Earth is a spinning sphere.

2. THE WESTERLIES a belt of strong westerly winds at ca. 40°. Between the sub-tropical highs and sub-polar lows

3. THE ITCZ inter-tropical convergence zone (Hadley Cells)

a. Coriolis Effect:: deflection toward right (left) in northern (southern) hemisphereb. Flux density of energy is greatest at equator (2 cal cm-2 yr-1) top of atm.

Equatorward of 4Oo, more energy received than emitted.Poleward of 4Oo, more energy radiated back into space than received.Redistribution of this energy is the driving force for climate.

Global Geography (Ocean Basins) produce more Semi-Permanent Features of Atmospheric Circulation

4. SUB-TROPICAL HIGHS: Descending air at 30o produces clockwise circulating cells, strongest in summer; e.g., Pacific & Bermuda - Azores High,.

5. SUB-POLAR LOWS at 60o circulate counterclockwise, and are strongest in winter. e.g., Aleutian Low, Greenland Low.

USE OF SEMI-PERMANENT FEATURES TO EXPLAIN CLIMATE CHANGE Jet Stream (CLIMAP) Pacific High (Graham, 2004) Early Holocene Xerothermic / Pluvial (Davis, Sellers 1987, 1989)

Circulation of the westerlies is in the form of waves Rossby Waves long-period, planetary-scale waves that are the major source of meridional transport of energy.

Zonal Flow: (e.g., winter - greater temperature gradient) low amplitude waves, stronger winds, less heat and moisture transport poleward

Meridional Flow: high amplitude waves, greater heat and moisture transport poleward

WEATHER PATTERNS ASSOCIATED WITH SEMIPERMANENT FEATURES

Teleconnections: precipitation and temperature anomalies associated with atmospheric circulation patterns.

ENSO: El Niño - Southern Oscillation

SOI Southern Oscillation Index Atmospheric Pressure of Tahiti - Darwin (for example)

High SOI (La Niña): strong E-W equatorial circulation, strong upwelling (cold) east Pacific coast, reduced stream flow and increased forest fires in Southwest

Low SOI (El Niño): slow E-W circulation, weak upwelling (fishery failure in Chile), warm eastern Pacific, wet SouthwestTeleconnectionsinstrumental historyEl Niño and other patterns Holocene history of El Niño 2003 El El Niño Comparison of ENSO's

PNA - Pacific North American Pattern(Leathers et al., 1991, 1992; Cyan, 1996)

High PNA strong meridional transport by westerlies, strong Aleutian LowLow PNA zonal transport by jet stream

PDO - Pacific Decadal Oscillation 20-30 year period(Mantua et al., 1997)

High PDO (warm) strong Aleutian Low, weak transport of moisture by westerlies into NorthwestLow PDO cooler wetter Pacific Northwest

Other weather-pattern acronymshttp://ggweather.com/enso/mjo.htm

AO: Arctic Oscillation http://www.washington.edu/AAO: Antarctic Oscillation http://www.cpc.ncep.noaa.gov/AMO: Atlantic Multidecadal Oscillation www.agu.org/journals/gl/gl0412/MJO: Madden-Julian Oscillation http://www.cpc.ncep.noaa.gov/NAO: North American Oscillation / North Atlantic Oscillation

MONSOON CLIMATOLOGY

1. LAND-SEA CONTRAST o differential heating: land warms faster than ocean o warm air on land raises and is replaced by moist air over ocean

2. PLATEAUS o land at high elevation (thinner atmosphere) absorbs more

insolationo land (and air) heat faster, creating greater land-sea contrast

3. CROSS-EQUATORIAL FLOW o air moving from winter to summer hemisphere strengthens flow o picks up moisture crossing ocean

4. TOPOGRAPHIC BARRIERS o Coriolis effect deflects moving air right (left) in northern

(southern) hemisphere o African (Tethyan) mountains deflect air and enhance cross-

equitorial flow

5. GEOLOGIC HISTORY o Monsoon Maxima coincide with coincidence of perihelion and

summer solstice o Pleiatocene Pluvials o Cretaceous (coincidence of perihelion and equinox)