Lecture 12

• Rossby waves, propagation, breaking, climatic effects

• Marine stratocumulus regime

• The ocean, its role in the climate system

Rossby wave mechanism

“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

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

Midlatitude Stormtracks

• 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

From lecture 11

Rossby Waves

From lecture 11

Meridional and Zonal Flow

From lecture 11

• Linear, dissipative and time dependent theory

absorption.

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

PV on 350K surface on 4, 5 and 6 July 1979

PV on 350K surface on 16, 17 and 18 Dec 1993

Nonlinear theory

• Linear propagation from midlatitudes to lower latitudes

• Waves break as they approach their critical latitude (u=0 stationary waves)

• Rearrangement of PV field in the critical layer (advection around closed streamlines)

Wavepropagation

Wavebreaking

Linear vs nonlinear behavior near critical line

Once wave breaking takes place, wave activity can pile up in the wave breaking region

Linear absorption

Undulating PV contours

Wave activity may still be absorbed in the wave breaking region given enough dissipation -- or else, given suitable background flow…..

Wave breaking can result in reflection

Once wave breaking takes place there is the possibility of nonlinear reflection

Nonlinear reflection. Wave activity is flushed out of the wave breaking region

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).

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.

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)

NCEP / NCAR Reanalysis

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

Stratus/stratocumulus regime as seen in a visible satellite image

Effects of subsidence on lapse rate

Upper region descends over greater distance than lower region. Therefore, warms more.

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.

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.

Net energy gains/losses at the surface of the ocean --- Surface heat flux

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

Annual mean poleward transport of energy by atmosphere and ocean

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.

Ocean: typical vertical temperature structure Upside version of the atmosphere

Thermocline is the transition zone between mixed and deep layer

Just like tropopause height in the atmosphere, the depth of the mixed layer

depends on latitude

Sea surface temperature (SST)

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

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

wind patterns)