Observed - PowerPoint Presentation

Slide1

Observed structure of the mean planetary-scale extratropical circulation

1. surface energy balance disparities2. mean sea-level structure3. mean structure aloft4. baroclinic variability5. the movement of synoptic-scale systems

The images shown herein are based on NCEP/NCAR reanalysis dataset,

accessed thru the

Climate Diagnostics Center

website

.Slide2

We aim to answer these questions:

What explains the observed climatological SLP (sea level pressure) distribution and its seasonal variation?What explains the “upper-level” GP height and flow pattern?

How do the GP height patterns relate to temperature?

How

does the upper-level structure relate to the surface lows and highs?How do upper-level trofs move in the baroclinic storm track?

Northern hemisphere

polar stereographic viewSlide3

Suggested further readingHolton Chapter 6.1

Bluestein 1993, esp. section 1.1.8 [Climatology of cyclogenesis and anticyclogenesis]section 1.2.5 [Climatology of lows and highs]Palmen and Newton (1969) various chapters are useful. The book is somewhat outdated, but it is very legible.

Peixoto

and

Oort (1992)4.1 transient and stationary eddies7.2 mean temperature structure7.3 mean height structure7.4 mean atmospheric circulationA very introductory, descriptive overview of the atmospheric general circulation (Word file). Slide4

1. background basics: the Earth’s energy budget (global, annual mean)

-30

+30

net radiation

the units are in % of the TOA incoming solar radiation, i.e. S/4 (S=solar constant = 1380

W/m

2

)

R = S

n

+ L

n

and

R



H

+

LE

R = 51 –21 = 30

R = 7 + 23 = 30

surface energy terms:

R : net radiation

S

n

: net solar radiation

L

n

: net terr. radiation

H: sensible heat flux

LE: latent heat fluxSlide5

The net solar radiation varies considerably with latitude and seasonSlide6

E

n

= S

n

+ Ln -H-LE

net surface energy flux En

Source: Trenberth and Caron 2001: Estimates of Meridional Atmosphere and Ocean Heat Transports.

Journal of Climate,

14, 3433–3443

note:

if the x-axis was plotted linearly in terms of surface area (R

2

cos

f

dl df, with R=earth radius, f=latitude and l=longitude), then the green-shaded area would equal the orange one.

zonal mean

note the linear scale

net incoming solar radiation Sn

net outgoing terrestrial radiation (-Ln)

proportional to Earth surface area

E

nSlide7

The required total heat transport in order to maintain an annual-mean steady state (RT), and estimates of the total atmospheric transport AT from NCEP and ECMWF re-analyses

What about the shortfall??

Seasonal variation of the zonal mean of the meridional heat transport by the atmosphere

PW: petawatt or 10

15

W

Source: Trenberth and Caron 2001: Estimates of Meridional Atmosphere and Ocean Heat Transports.

Journal of Climate,

14, 3433–3443

Answer

: that heat is transported by oceans

total

atmosphericSlide8

O

cean heat transport

solid:

zonal annual mean

; dashed: ±1s (standard deviation)

Source: Trenberth and Caron 2001: Estimates of Meridional Atmosphere and Ocean Heat Transports.

Journal of Climate,

14, 3433–3443

0

+30

-30

latitudeSlide9

The

poleward

energy transfer that is needed to offset the pole-to-equator net radiation imbalance is accomplished partly by the troposphere, partly by the oceans.

(

ºN)

annual mean,northern hemisphere

this graph seems to overestimate the heat transport by oceans (Source: Ackerman and Knox 2003)Slide10

Seasonal march of S

n, Ln and R

Note:

- the latitudinal variation of

Sn is far larger than that of Ln and dominates that of R- the zonal asymmetry of R (land-sea contrast) is rather small- the desert areas over land are radiatively deficient (anomalously low R for their latitude, on account of the large Ln

loss)Slide11

Seasonal march of surface energy fluxes

Note how LE and H vary tremendously with season, between land and ocean, and even over land and over ocean. H and LE tend to compensate each other. Their variation can be explained in terms of forested regions

vs

deserts, warm

vs cold ocean currents, the sea ice edge, continental airmasses advected over water, etc. Note that oceans absorb and release far more heat than land (“storage change”) Slide12

seasonal march of surface air temperature

note that the amplitude of the annual temperature range is higher at:

- higher latitudes

- over land rather than over water [this does NOT occur in terms of net radiation R

n]- over large land masses, especially their eastern sideSlide13

2. Structure of SLP, winds, temperatureseasonal march of sea level pressure and sfc winds

observations:

- A see-saw SLP variation dominates over the

northern

continents, with highs in winter and lows in summer. The seasonal variation of the polar lows and subtropical highs over the northern oceans is also large, and is in opposition to SLP variations over land at corresponding latitudes.- The southern hemisphere is far more zonally symmetric.- Note the extremely low SLP around the Antarctic ice dome.

northern oceans: polar lows: Aleutian, Icelandic subtropical highs: Pacific, Bermuda northern continents: - winter highs: Siberian, Intramtn - summer lows: Pakistan, Sonoran

southern oceans:

- circumpolar (southern) low

- subtropical highs (3 oceans)Slide14

polar perspectiveThe

following plots are all polar stereographic.Either winter or summer is shown, either the NH or SH.

S

ome

maps display ‘zonal anomalies’, i.e. the departures from the zonal (constant latitude) mean

 Slide15

SL pressure

NH winterSlide16

1000 mb height

NH winter

What SLP is that?

Z

1000 = 280 m

answer:

p

SL

~1035

mb

 Slide17

1000 mb temperature

NH winterSlide18

1000 mb temperature, NH winter departure from zonal mean

keep the magnitude of the zonal anomalies in mindSlide19

hydrostatic balance implies

negative surface temperature anomaly

 high SLP

positive surface temperature anomaly

 low SLPProof this assuming a flat pressure surface above the cold (warm) anomaly. The depth of this anomaly typically is ~ 2km.

800 mb

ground

(sea level)

1000 mb

warm

 Z

1000-850

large

cold

 Z

1000-850

small

low

high

height

Type equation here.

 Slide20

guess when

1000 mb heightSlide21

1000 mb temperature, NH summer

departure from zonal mean

keep the magnitude of the zonal anomalies in mindSlide22

guess when

1000 mb height, SH

note how the zonally rather symmetric subtropical high is interrupted over landSlide23

1000 mb temperature, SH summer

departure from zonal mean

note the subtropical warm pools over land, coincident with a low SLP anomaly

note the unusually low SSTs in the eastern subtropical

ocean basinsSlide24

guess when

1000 mb height, SHSlide25

1000 mb height, SH winter!

400 m ~1050 mbSlide26

1000 mb height, SH

Antarctic high removed

ignoredSlide27

1000 mb temperature

SH winterSlide28

1000 mb temperature, SH winter

departure from zonal mean

ignored

note that these zonal anomalies are relatively smallSlide29

3. upper-level climatological structurefocus on winter in the northern hemisphere (NH)Slide30

seasonal march of the 500 mb height

wind speedSlide31

500 mb height

NH winter

note the seasonal-mean trofs, coincident with the cold anomalies at low levels

1000 hPa temperatureSlide32

500 mb height (zonal anomaly)

NH winter

two quasi-stationary trofs

 wavenumber 2 patternSlide33

Temperature 850 mb, NH winterSlide34

Wind @300 mb, NH winter

m s

-1

m s

-1Slide35

Verify qualitatively that climatological fields are roughly in thermal wind balance. For instance, look at the meridional variation of temperature with height (in Jan)Slide36

Around 30-45 ºN, temperature drops northward, therefore westerly winds increase in strength with heightSlide37

The meridional temperature gradient is large between 30-50ºN and 1000-300 mb

thermal wind

Therefore the zonal wind

increases rapidly from

1000 mb up to 300 mbSlide38

Question:Why, if it is colder at higher latitude, doesn’t the wind continue to get stronger with altitude ?Slide39

There is definitively a jet ...Slide40

Answer: above 300 mb, it is no longer colder at higher latitudes...

tropopauseSlide41

Tropopause pressure (hPa), NH winterSlide42

Wind 300 mb, NH winter

80 WSlide43

Zonal-mean wind, 80

ºW, troposphere and lower stratosphereSlide44

Wind @300 hPa, NH winter

A

BSlide45

section B, West Coast

temperature

West Coast of N America

tropopauseSlide46

West Coast of N America

section B, West Coast

zonal wind speed

STJ

PFJSlide47

section A, Japan

temperature

Japan

tropopauseSlide48

Japan

section A, Japan

zonal wind speed

the polar-front jet (PFJ) has merged with the subtropical jet (STJ)Slide49

Wind 300 mb, NH winter

STJ

PFJ

STJ

PFJ

note that at most longitudes (esp. Asia and the Pacific), a single jet is presentSlide50

GP height @ 300 mb, NH winterSlide51

Potential vorticity @345 K

stratospheric air

tropospheric

airSlide52

Schematic zonal-mean cross section (after Palmen & Newton)

 ITCZSlide53

The Palmen-Newton model has three meridional circulation cells in each hemisphere

Note that the three-cell pattern ignores seasonal variation and land-sea contrast.Slide54

mean meridional circulation

500 mb vertical velocity

note that blue is upward motion (

w

<0)note the rising motion near the ITCZ and subtropical sinking, the latter mainly in the winter hemispherenote the seasonal march of the ITCZ (monsoon)note the rising motion in the baroclinic storm track

note the sinking (rising) on the lee (upwind) side of mountain ranges 100 units ~ 1 cm s-1Slide55

How strong are the meridional cells? (zonal mean)

NH winter

NH summer

Jan

July

Hadley

Ferrel

NH

Hadley

Ferrel

SH

Hadley

note the broad belt of subsidence (12-52

º

N) in winter and the broad belt of ITCZ ascent (0-30

º

N) in summer.

In the NH winter, over continents, the northern Hadley cell rising branch crosses the Equator into the SH ITCZ,

and its sinking branch extends between 12-50ºN

Effectively ascent dominates in the summer hemisphere, and sinking in the winter hemisphere, and the Hadley cell that straddles the equator is the strongest.

ITCZ

ITCZSlide56

ageostrophic flow & secondary circulation near jet streak

Does this synoptic pattern apply to the mean circulation?Slide57

Wind 300 hPa, NH winter

A

B

look for evidence of a

secon-dary

meridional

circulation around

Japan’s

jet streakSlide58

Circulation, Section ASlide59

x

Jet stream

Specific humidity, Section A

Thermally direct!Slide60

Precipitation rate

ITCZ

Jet core

Note: the vertical velocity field in jet cores will be revisited later for synoptic jets.Slide61

4. SLP and 500 m height intraseasonal variability

A 3-10 day bandpass filter is used to highlight ‘synoptic’ disturbances.

This filter will highlight storms

due to

baroclinic instability*(the midlatitude ‘storm track’)

*In theory this may include tropical cyclones, but they are rareSlide62

hPa

SLP variability (3-10 day)

m

m

hPa

hPa

Northern hemisphere storm track, SLP, winter

Northern hemisphere storm track, 500 mb, summer

Northern hemisphere storm track, 500 mb, winter

Northern hemisphere storm track, SLP, summerSlide63

hPa

Southern hemisphere storm track, SLP, winterSlide64

hPa

Southern hemisphere storm track, SLP, summerSlide65

summary: 3-10 day variabilitythere is a ‘

baroclinic storm’ track between 40-60º latitudestorms appear vertically coupled 1000 mb variability similar to 500 mb

variability

storm track intensity (synoptic SLP variability) relates to

meridional T gradientstronger in winter than in summerstrongest east of the continentsstorm track intensity also is stronger over ocean than land the NH storm track has larger seasonal variability and is less zonally symmetric than the SH storm trackSlide66

Relationship between surface cyclone and UL wave

trofduring the lifecycle of a frontal disturbance

500

mb

height (thick lines)

SLP isobars (thin lines)layer-mean temperature (dashed)

The deflection of the upper-level wave contributes to deepening of the surface low.

food for thought: how is the UL-LL coupling possible if U

500

>> U

1000

?Slide67

meridional cross section (potential temperature q

, zonal wind speed U)

pressure (mb)

latitude

Holton (2004) p.145

W

E

U

500

>> U

1000Slide68

first answer: SLP is not material, so surface lows & highs can move much faster than the zonal wind

more in-depth answer: UL (Rossby) waves move upstream, against the current. LL disturbances tend to propagate with the current. More on this when we broach PV thinking. Question:

how is the UL-LL coupling possible if U

500

>> U1000?Slide69

The movement of UL

trofs and ridgesRossby waves result from the conservation of vorticity. The restoring force is b or, more generally, the meridional gradient of the absolute vorticity. The resulting circulation causes the wave to propagate westward. c is the

Rossby

wave propagation speed, u zonal wind, k and l are zonal and

meridional wavenumbersIn short waves, the advection of relative vorticity dominates. The wave propagation speed is slow and they move with the prevailing westerly flow.In long waves, the advection of planetary vorticity dominates. Their speed is large and they are generally stationary, or may retrograde.Slide70

slp, fronts, precip

850 mb temperature,

& LL jets

Shapiro

1990

fracture

seclusion

incipient

frontal fracture

bent-back warm front

T-bone

seclusion

Wave cyclone

evolutionSlide71

Question: Where do lows and highs

tend to form?Where do they decay? a Lagrangian perspective …Slide72

1000 mb

500

mb

NH

baroclinic storm track & preferred regions of cyclogenesis /

cyclolysiscontour: standard deviation of GPHvector: phase propagation vectorSlide73

Winter cyclogenesis

Winter cyclolysis

(Bluestein 1993, p 20)

Sea level cyclone formation and decay around North AmericaSlide74

Winter anticyclogenesis

Winter anticyclolysis

(Bluestein p. 25)

anticyclone formation and decay around North AmericaSlide75

1. Kicking back cut-off lows into the main stream

Trofs may be cut-off from the westerly flow and they become stationary at lower latitudes. Cut-off lows may dissipate or they may be ‘kicked back’ into the main currentThe kicker trof needs to approach the cut-off to within ~2000 km (Henry’s rule)

On the movement of

trofs

and ridges:

two forecast rules Slide76

If the max cyclonic shear is on the upstream side of the trof, the trof

will tend to move equatorward, and may deepen.

If the max cyclonic shear is on the downstream side of the

trof

, the trof will tend to move poleward.

On the movement of trofs and ridges:

two forecast rules

2. asymmetric

trofs

&

meridional

movementSlide77

Hovmöller diagrams

red: short-wave trofs

blue: stationary long-wave ridge

Winter 02-03

500 mb height (m)

30-40

º

N

all longitudes

24 hr average

Alpine lee cyclogenesis

Source: http://www.cdc.noaa.gov/map/clim/glbcir.shtml

Rockies lee cyclogenesis

question: how does the stationary long-wave pattern shown here

match

with the 500

mb

anomalies from zonal mean, discussed earlier ?

departure of the 24 hr average from the zonal meanSlide78

“blocking” flow aloft

occurs most frequently in spring around 50°Nresults from an anomalous long-wave ridge that blocks the progression of short wavesoccurs during ‘low-index’

cycles

high index: strong zonal flow

low index: zonal flow weak, meridional flow strong 3 types, qualitatively separatedHigh-over-low blockOmega blockupper-level ridgeSlide79

upper-level ridge

high over low block

omega blockSlide80

Conclusions

A seasonally-variable net surface energy imbalance exists, with En>0 at low latitudes and En<0 at high latitudes.The atmospheric component of the meridional

heat transfer is achieved in part by the

meridional

mean circulation (Hadley), but mainly by the mid-latitude eddy circulation associated with the high variability observed on the 3-10 day timescale (baroclinic eddies).The Palmen-Newton model of the general circulation of the atmosphereHighly simplified (seasonal variations/ land-sea contrast are very important) - applies better in the SHThe only strong meridional cell in the zonal mean circulation is the Hadley cellThe seasonal variation of SLP in the NH is dominated by the land-sea contrast

The annual temperature range over land, esp the eastern end of the land masses, is far larger than that over the oceans. This is not explained by the annual range of net radiation, which shows weak zonal anomalies.Subtropical (~30ºN) ocean highs and continental lows dominate in summerPolar (~60

º

N) ocean lows and continental highs dominate in winter

A jet stream exists in the upper mid-latitude troposphere

Its climatological position and strength are in thermal wind balance, thus it is stronger in winter than in summer

The separation between STJ and PJ is not present at all longitudes, nor in the zonal mean, in both hemispheres & seasons

Highly zonally asymmetric in the NH, due to continents and topography

Quasi-stationary

trofs

along east coasts of N America & Asia

Very symmetric in the SH

Jet stream separates reservoirs of very different air (in terms of potential vorticity)The jet stream carries quasi-stationary long waves and eastward-moving, baroclinic short waves

baroclinic storm track most apparent in the 3-10 band-pass filtered data, at LL and ULstronger in winter than summer, storm track most ‘intense’ near east coasts, where the UL jet and LL baroclinicity are strongest this is consistent with QG theory (stronger PVA and WAA), to be discussed later