Rossby waves, propagation, breaking, climatic effects

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Lecture 12

• Rossby waves, propagation, breaking, climatic
  effects
• Marine stratocumulus regime
• The ocean, its role in the climate system

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Rossby wave mechanism
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Rossby wave conceptual model

• Even though the term wave conjures up linear
  wave propagation, this concept can be extended to
  apply to nonlinear waves or breaking waves.
• The largest scales in the extratropics can be
  described as quasi-linear and quasi-stationary.
• Define linear, stationary

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Rossby waves (continued)

• Planetary scale is quasi-stationary, quasi-linear
• Synoptic scale can range from linear waves to
  strongly nonlinear breaking waves
• Storm tracks are manifestations of these. In the
  entrance region have developing systems (or
  waves) at the exit occluding low pressure systems
  or breaking waves

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Midlatitude Stormtracks
From lecture 11

• In the NH they are concentrated over the two
  ocean basins, Pacific and Atlantic, guided by the
  jet maxima over the two ocean basins
• In the SH form an almost continuous band in
  midlatitudes
• Collection of storms in various stages of
  development, usually cyclogenesis at the entrance
  and mature storms further on

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From lecture 11
Rossby Waves
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From lecture 11
Meridional and Zonal Flow
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• Linear, dissipative and time dependent theory
• ? absorption.

• Linear theory appears to work well in explaining
  observed longitudinal asymmetries, however.

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PV on 350K surface on 4, 5 and 6 July 1979
PV on 350K surface on 16, 17 and 18 Dec 1993
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Nonlinear theory

• Linear propagation from midlatitudes to lower
  latitudes
• Waves break as they approach their critical
  latitude (u0 stationary waves)
• Rearrangement of PV field in the critical layer
  (advection around closed streamlines)

Wave propagation
Wave breaking
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Linear vs nonlinear behavior near critical line
Linear absorption
Undulating PV contours
Once wave breaking takes place, wave activity can
pile up in the wave breaking region
Wave activity may still be absorbed in the wave
breaking region given enough dissipation -- or
else, given suitable background flow..
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Wave breaking can result in reflection
Nonlinear reflection. Wave activity is flushed
out of the wave breaking region
Once wave breaking takes place there is the
possibility of nonlinear reflection
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Planetary Wave Breaking (PWB) rapid and
irreversible large-scale overturning of
PV.
Example of a PWB event, 4 Feb. 1996. Animation
of Daily PV on 340K surface (2-8 Feb).
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Criteria for detecting PWB

• Reversal in the latitudinal PV gradient in the
  tropopause region.
• Localized eastward PV gradient about the break
  (anticyclonic breaking).
• High (low) PV must be part of a tongue of PV
  originating in the extratropics (tropics).
• Identify breaking point the point farthest
  west equatorward that satisfies the above
  criteria the earliest.

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Both cyclonic and anticyclonic breaking

• Have extended the work to look cyclonic as well
  as anticyclonic breaking
• Have found forcing of climate patterns from
  breaking
• NAO (direct)
• NAO (due to breaking over E Pacific a few days
  earlier)
• PDO (direct)

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NCEP / NCAR Reanalysis
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The marine stratocumulus

• In the area of the subtropical high (descending
  branch of the Hadley cell)
• Cold underlying ocean (east ocean basins)
• Sets up a temperature inversion at the top of the
  planetary boundary layer (PBL)
• The cloud layer exists at the top of PBL

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Stratus/stratocumulus regime as seen in a visible
satellite image
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Effects of subsidence on lapse rate
Upper region descends over greater distance than
lower region. Therefore, warms more.
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Remember flow over mountain. On the way up
latent heat release mitigates some of the
cooling. On the way down, warms at the dry
adiabatic lapse rate. Very hot and dry.
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Oceanography the study of oceans

• They are a source of atmospheric water vapor
• They exchange energy and trace gases with the
  atmosphere
• They transport heat poleward
• It takes approximately two weeks for all the
  water in the atmosphere to recycle. The oceans
  provide the majority of water for precipitation.

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Net energy gains/losses at the surface of the
ocean --- Surface heat flux
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Surface fluxes of energy and trace gases

• The rate of heat and moisture transfer depends on
  temperature/moisture difference as well as wind
  speed.
• Warm SST and high wind are favorable to large
  heat exchanges between atmosphere and ocean
• US west coast vs. coast of N. Europe, cool SST
  vs. warm SST Affects climate profoundly

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Annual mean poleward transport of energy by
atmosphere and ocean
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More on poleward heat transport

• There is a certain compensation between heat
  transport in the atmosphere and ocean. If the
  atmosphere transports less, the ocean will step
  in and transport more and vice versa.

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Ocean typical vertical temperature structure
Upside version of the atmosphere
Thermocline is the transition zone between mixed
and deep layer
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Just like tropopause height in the atmosphere,
the depth of the mixed layer depends on latitude
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Sea surface temperature (SST)
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Things to note about distribution of SSTs

• SSTs off west coasts in subtropics (to
  midlatitudes) are cool
• SSTs off east coasts in midlatitudes are warm
• SSTs off east coasts in high latitudes are cold

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The major surface ocean currents (wind driven so
they resemble the atmospheric wind patterns)