Observed structure of the mean planetary-scale extratropical circulation 1. surface energy balance disparities 2. mean sea-level structure 3. mean structure aloft 4. baroclinic variability 5. the movement of synoptic-scale systems The images shown herein are based on NCEP/NCAR reanalysis dataset, accessed thru the Climate Diagnostics Center website.

We aim to answer these questions: Northern hemisphere polar stereographic view 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?

Suggested further reading Holton 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 The book is somewhat outdated, but it is very legible.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 eddies 7.2 mean temperature structure 7.3 mean height structure 7.4 mean atmospheric circulation A very introductory, descriptive overview of the atmospheric general circulation (Word file).

1. background basics: the Earth’s energy budget (global, annual mean) the units are in % of the TOA incoming solar radiation, i.e. S/4 (S=solar constant = 1380 W/m2) R = Sn+ Ln R  H + LE surface energy terms: R : net radiation Sn: net solar radiation Ln : net terr. radiation H: sensible heat flux LE: latent heat flux and R = 51 –21 = 30 R = 7 + 23 = 30 +30 net radiation -30

The net solar radiation varies considerably with latitude and season

En = Sn + Ln -H-LE En net surface energy flux En zonal mean net incoming solar radiation Sn En = Sn + Ln -H-LE net outgoing terrestrial radiation (-Ln) note: if the x-axis was plotted linearly in terms of surface area (R2 cosf dl df, with R=earth radius, f=latitude and l=longitude), then the green-shaded area would equal the orange one. note the linear scale En Source: Trenberth and Caron 2001: Estimates of Meridional Atmosphere and Ocean Heat Transports. Journal of Climate, 14, 3433–3443 proportional to Earth surface area

PW: petawatt or 1015 W total atmospheric What about the shortfall?? 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 atmospheric What about the shortfall?? Answer: that heat is transported by oceans Seasonal variation of the zonal mean of the meridional heat transport by the atmosphere Source: Trenberth and Caron 2001: Estimates of Meridional Atmosphere and Ocean Heat Transports. Journal of Climate, 14, 3433–3443 PW: petawatt or 1015 W

latitude Ocean heat transport solid: zonal annual mean; dashed: ±1s (standard deviation) -30 +30 latitude Source: Trenberth and Caron 2001: Estimates of Meridional Atmosphere and Ocean Heat Transports. Journal of Climate, 14, 3433–3443

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. annual mean, northern hemisphere (ºN) this graph seems to overestimate the heat transport by oceans (Source: Ackerman and Knox 2003)

Seasonal march of Sn, 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)

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”)

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 Rn] - over large land masses, especially their eastern side

2. Structure of SLP, winds, temperature seasonal march of sea level pressure and sfc winds 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) 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.

polar perspective The following plots are all polar stereographic. Either winter or summer is shown, either the NH or SH. Some maps display ‘zonal anomalies’, i.e. the departures from the zonal (constant latitude) mean 𝜑 ′ 𝑙𝑜𝑛,𝑙𝑎𝑡 =𝜑 𝑙𝑜𝑛,𝑙𝑎𝑡 − φ (𝑙𝑎𝑡)

SL pressure NH winter

1000 mb height NH winter 𝑝= 𝑝 𝑆𝐿 𝑒 − 𝑍 1000 /𝐻 =1000 𝑚𝑏 What SLP is that? Z1000 = 280 m 𝑝= 𝑝 𝑆𝐿 𝑒 − 𝑍 1000 /𝐻 =1000 𝑚𝑏 answer: pSL ~1035 mb

1000 mb temperature NH winter

keep the magnitude of the zonal anomalies in mind 1000 mb temperature, NH winter departure from zonal mean keep the magnitude of the zonal anomalies in mind

hydrostatic balance implies negative surface temperature anomaly  high SLP positive surface temperature anomaly  low SLP Proof this assuming a flat pressure surface above the cold (warm) anomaly. The depth of this anomaly typically is ~ 2km. Type equation here. 800 mb height warm Z1000-850 large cold Z1000-850 small ground (sea level) 1000 mb low high 𝑍 1000→850 = 𝑅 𝑇 𝑔 ln 1000 850

1000 mb height guess when

keep the magnitude of the zonal anomalies in mind 1000 mb temperature, NH summer departure from zonal mean keep the magnitude of the zonal anomalies in mind

1000 mb height, SH guess when note how the zonally rather symmetric subtropical high is interrupted over land

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 basins

1000 mb height, SH guess when

1000 mb height, SH winter! 400 m ~1050 mb

Antarctic high removed 1000 mb height, SH Antarctic high removed ignored

1000 mb temperature SH winter

note that these zonal anomalies are relatively small 1000 mb temperature, SH winter departure from zonal mean ignored note that these zonal anomalies are relatively small

3. upper-level climatological structure focus on winter in the northern hemisphere (NH)

seasonal march of the 500 mb height wind speed

500 mb height NH winter 1000 hPa temperature note the seasonal-mean trofs, coincident with the cold anomalies at low levels 1000 hPa temperature

500 mb height (zonal anomaly) NH winter two quasi-stationary trofs  wavenumber 2 pattern

Temperature 850 mb, NH winter

Wind @300 mb, NH winter m s-1 m s-1

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

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

thermal wind The meridional temperature gradient is large between 30-50ºN and 1000-300 mb Therefore the zonal wind increases rapidly from 1000 mb up to 300 mb

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

There is definitively a jet ...

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

Tropopause pressure (hPa), NH winter

Wind 300 mb, NH winter 80 W

Zonal-mean wind, 80ºW, troposphere and lower stratosphere

Wind @300 hPa, NH winter B A

section B, West Coast temperature West Coast of N America tropopause

section B, West Coast zonal wind speed West Coast of N America STJ PFJ

section A, Japan temperature Japan tropopause

the polar-front jet (PFJ) has merged with the subtropical jet (STJ) section A, Japan zonal wind speed Japan the polar-front jet (PFJ) has merged with the subtropical jet (STJ)

Wind 300 mb, NH winter STJ PFJ PFJ STJ note that at most longitudes (esp. Asia and the Pacific), a single jet is present STJ PFJ PFJ STJ

GP height @ 300 mb, NH winter

Potential vorticity @345 K stratospheric air tropospheric air

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

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.

mean meridional circulation 500 mb vertical velocity 100 units ~ 1 cm s-1 note that blue is upward motion (w<0) note the rising motion near the ITCZ and subtropical sinking, the latter mainly in the winter hemisphere note 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

How strong are the meridional cells? (zonal mean) Jan NH winter Hadley Ferrel note the broad belt of subsidence (12-52ºN) in winter and the broad belt of ITCZ ascent (0-30ºN) in summer. ITCZ 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 July Ferrel NH summer NH Hadley SH Hadley 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

ageostrophic flow & secondary circulation near jet streak Does this synoptic pattern apply to the mean circulation?

Wind 300 hPa, NH winter look for evidence of a secon-dary meridional circulation around Japan’s jet streak A B

Circulation, Section A

Specific humidity, Section A Thermally direct! x Jet stream

Precipitation rate Jet core ITCZ Note: the vertical velocity field in jet cores will be revisited later for synoptic jets. Jet core ITCZ

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 rare

SLP variability (3-10 day) Northern hemisphere storm track, SLP, winter Northern hemisphere storm track, SLP, summer hPa hPa Northern hemisphere storm track, 500 mb, winter Northern hemisphere storm track, 500 mb, summer hPa m m

Southern hemisphere storm track, SLP, winter hPa

Southern hemisphere storm track, SLP, summer hPa

summary: 3-10 day variability there is a ‘baroclinic storm’ track between 40-60º latitude storms appear vertically coupled 1000 mb variability similar to 500 mb variability storm track intensity (synoptic SLP variability) relates to meridional T gradient stronger in winter than in summer strongest east of the continents storm 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 track

Relationship between surface cyclone and UL wave trof during 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 U500 >> U1000? 66

meridional cross section (potential temperature q, zonal wind speed U) U500 >> U1000 pressure (mb) E latitude Holton (2004) p.145

Question: how is the UL-LL coupling possible if U500 >> U1000? 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.

The movement of UL trofs and ridges Rossby 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 wavenumbers In 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.

Wave cyclone evolution fracture seclusion slp, fronts, precip T-bone slp, fronts, precip frontal fracture incipient bent-back warm front fracture seclusion 850 mb temperature, & LL jets Shapiro 1990

Question: Where do lows and highs tend to form? Where do they decay? a Lagrangian perspective …

NH baroclinic storm track & preferred regions of cyclogenesis / cyclolysis contour: standard deviation of GPH vector: phase propagation vector 1000 mb 500 mb

Sea level cyclone formation and decay around North America Winter cyclolysis Winter cyclogenesis (Bluestein 1993, p 20)

anticyclone formation and decay around North America Winter anticyclolysis Winter anticyclogenesis (Bluestein p. 25)

On the movement of trofs and ridges: two forecast rules 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 current The 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 2. asymmetric trofs & meridional movement 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.

Hovmöller diagrams red: short-wave trofs 24 hr average departure of the 24 hr average from the zonal mean Winter 02-03 500 mb height (m) 30-40ºN all longitudes question: how does the stationary long-wave pattern shown here match with the 500 mb anomalies from zonal mean, discussed earlier ? Alpine lee cyclogenesis Rockies lee cyclogenesis red: short-wave trofs blue: stationary long-wave ridge Source: http://www.cdc.noaa.gov/map/clim/glbcir.shtml

“blocking” flow aloft occurs most frequently in spring around 50°N results from an anomalous long-wave ridge that blocks the progression of short waves occurs during ‘low-index’ cycles high index: strong zonal flow low index: zonal flow weak, meridional flow strong 3 types, qualitatively separated High-over-low block Omega block upper-level ridge

high over low block upper-level ridge omega block

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 atmosphere Highly simplified (seasonal variations/ land-sea contrast are very important) - applies better in the SH The only strong meridional cell in the zonal mean circulation is the Hadley cell The 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 summer Polar (~60ºN) 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 UL stronger 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