Presentation on theme: "Observed structure of the mean planetary-scale extratropical circulation 1. surface energy balance disparities 2. mean sea-level structure 3. mean structure."— Presentation transcript:
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.Climate Diagnostics Center website
We aim to answer these questions: 1.What explains the observed climatological SLP (sea level pressure) distribution and its seasonal variation? 2.What explains the “upper-level” GP height and flow pattern? 3.How do the GP height patterns relate to temperature? 4.How does the upper-level structure relate to the surface lows and highs? 5.How do upper-level trofs move in the baroclinic storm track? Northern hemisphere polar stereographic view
Suggested further reading Holton Chapter 6.1 Bluestein 1993, esp. –section [Climatology of cyclogenesis and anticyclogenesis] –section [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 The book is somewhat outdated, but it is very legible.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).introductory, descriptive overview of the atmospheric general circulation
1. background basics: the Earth’s energy budget (global, annual mean) 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 = = 30 surface energy terms: R : net radiation S n : net solar radiation L n : net terr. radiation H: sensible heat flux LE: latent heat flux
The net solar radiation varies considerably with latitude and season
E n = S n + L n -H-LE net surface energy flux E n 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 , with R=earth radius, =latitude and =longitude), then the green-shaded area would equal the orange one. zonal mean note the linear scale net incoming solar radiation S n net outgoing terrestrial radiation (-L n ) proportional to Earth surface area EnEn
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 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 atmospheric
Ocean heat transport solid: zonal annual mean; dashed: ±1 (standard deviation) Source: Trenberth and Caron 2001: Estimates of Meridional Atmosphere and Ocean Heat Transports. Journal of Climate, 14, 3433– latitude
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)
Seasonal march of S n, L n and R Note: - the latitudinal variation of S n is far larger than that of L n 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 L n 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 R n ] - over large land masses, especially their eastern side
2. Structure of SLP, winds, temperature seasonal 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)
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
SL pressure NH winter
1000 mb height NH winter What SLP is that? Z 1000 = 280 m answer: p SL ~1035 mb
1000 mb temperature NH winter
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. 800 mb ground (sea level) 1000 mb warm Z largecold Z small low high height
guess when 1000 mb height
1000 mb temperature, NH summer departure from zonal mean keep the magnitude of the zonal anomalies in mind
guess when 1000 mb height, SH 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
guess when 1000 mb height, SH
1000 mb height, SH winter! 400 m ~1050 mb
1000 mb height, SH Antarctic high removed ignored
1000 mb temperature SH winter
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 note the seasonal-mean trofs, coincident with the cold anomalies at low levels 1000 hPa temperature
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 note that blue is upward motion ( <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 100 units ~ 1 cm s -1
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- 52ºN) in winter and the broad belt of ITCZ ascent (0-30ºN) 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
ageostrophic flow & secondary circulation near jet streak Does this synoptic pattern apply to the mean circulation?
Wind 300 hPa, NH winter A B look for evidence of a secon- dary meridional circulation around Japan’s jet streak
Circulation, Section A
x Jet stream Specific humidity, Section A Thermally direct!
Precipitation rate ITCZ Jet core Note: the vertical velocity field in jet cores will be revisited later for synoptic jets.
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
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 U 500 >> U 1000 ?
meridional cross section (potential temperature , zonal wind speed U) pressure (mb) latitude Holton (2004) p.145 W E U 500 >> U 1000
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 >> U 1000 ?
The movement of UL trofs and ridges Rossby waves result from the conservation of vorticity. The restoring force is 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.
Question: Where do lows and highs tend to form? Where do they decay? a Lagrangian perspective …
1000 mb 500 mb NH baroclinic storm track & preferred regions of cyclogenesis / cyclolysis contour: standard deviation of GPH vector: phase propagation vector
Winter cyclogenesis Winter cyclolysis (Bluestein 1993, p 20) Sea level cyclone formation and decay around North America
Winter anticyclogenesis Winter anticyclolysis (Bluestein p. 25) anticyclone formation and decay around North America
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
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 movement
Hovmöller diagrams red: short-wave trofs blue: stationary long-wave ridge Winter mb height (m) 30-40ºN all longitudes 24 hr average Alpine lee cyclogenesis Source: 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 mean
“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
upper-level ridge high over low block omega block
Conclusions A seasonally-variable net surface energy imbalance exists, with E n >0 at low latitudes and E n <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