Presentation on theme: "1. surface energy balance disparities 2. mean sea-level structure"— Presentation transcript:
1 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 systemsThe images shown herein are based on NCEP/NCAR reanalysis dataset,accessed thru the Climate Diagnostics Center website.
2 We aim to answer these questions: Northern hemispherepolar stereographic viewWe 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?
3 Suggested further reading Holton Chapter 6.1Bluestein 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 The book is somewhat outdated, but it is very legible.Peixoto and Oort (1992)4.1 transient and stationary eddies7.2 mean temperature structure7.3 mean height structure7.4 mean atmospheric circulationA very introductory, descriptive overview of the atmospheric general circulation (Word file).
4 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+ LnR H + LEsurface energy terms:R : net radiationSn: net solar radiationLn : net terr. radiationH: sensible heat fluxLE: latent heat fluxandR = 51 –21= 30R == 30+30 net radiation-30
5 The net solar radiation varies considerably with latitude and season
6 En = Sn + Ln -H-LE En net surface energy flux En zonal mean net incoming solar radiation SnEn = Sn + Ln -H-LEnet 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 scaleEnSource: Trenberth and Caron 2001: Estimates of Meridional Atmosphere and Ocean Heat Transports. Journal of Climate, 14, 3433–3443proportional to Earth surface area
7 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-analysesatmosphericWhat about the shortfall??Answer: that heat is transported by oceansSeasonal variation of the zonal mean of the meridional heat transport by the atmosphereSource: Trenberth and Caron 2001: Estimates of Meridional Atmosphere and Ocean Heat Transports. Journal of Climate, 14, 3433–3443PW: petawatt or 1015 W
8 latitude Ocean heat transport solid: zonal annual mean; dashed: ±1s (standard deviation)-30+30latitudeSource: Trenberth and Caron 2001: Estimates of Meridional Atmosphere and Ocean Heat Transports. Journal of Climate, 14, 3433–3443
9 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)
10 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)
11 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”)
12 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
13 2. Structure of SLP, winds, temperature seasonal march of sea level pressure and sfc winds northern oceans:polar lows: Aleutian, Icelandicsubtropical highs: Pacific, Bermudanorthern continents:- winter highs: Siberian, Intramtn- summer lows: Pakistan, Sonoransouthern 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.
14 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) mean𝜑 ′ 𝑙𝑜𝑛,𝑙𝑎𝑡 =𝜑 𝑙𝑜𝑛,𝑙𝑎𝑡 − φ (𝑙𝑎𝑡)
18 keep the magnitude of the zonal anomalies in mind 1000 mb temperature, NH winter departure from zonal meankeep the magnitude of the zonal anomalies in mind
19 hydrostatic balance implies negative surface temperature anomaly high SLPpositive surface temperature anomaly low SLPProof this assuming a flat pressure surface above the cold (warm) anomaly. The depth of this anomaly typically is ~ 2km.Type equation here.800 mbheightwarm Z largecold Z smallground(sea level)1000 mblowhigh𝑍 1000→850 = 𝑅 𝑇 𝑔 ln
21 keep the magnitude of the zonal anomalies in mind 1000 mb temperature, NH summerdeparture from zonal meankeep the magnitude of the zonal anomalies in mind
22 1000 mb height, SHguess whennote how the zonally rather symmetric subtropical high is interrupted over land
23 1000 mb temperature, SH summer departure from zonal mean note the subtropical warm pools over land, coincident with a low SLP anomalynote the unusually low SSTs in the eastern subtropical ocean basins
53 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.
54 mean meridional circulation 500 mb vertical velocity 100 units ~ 1 cm s-1note 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 tracknote the sinking (rising) on the lee (upwind) side of mountain ranges
55 How strong are the meridional cells? (zonal mean) JanNH winterHadleyFerrelnote the broad belt of subsidence (12-52ºN) in winter and the broad belt of ITCZ ascent (0-30ºN) in summer.ITCZIn the NH winter, over continents, the northern Hadley cell rising branch crosses the Equator into the SH ITCZ,and its sinking branch extends between 12-50ºNJulyFerrelNH summerNHHadleySHHadleyEffectively ascent dominates in the summer hemisphere, and sinking in the winter hemisphere, and the Hadley cell that straddles the equator is the strongest.ITCZ
56 ageostrophic flow & secondary circulation near jet streak Does this synoptic pattern apply to the mean circulation?
57 Wind 300 hPa, NH winterlook for evidence of a secon-dary meridional circulation around Japan’s jet streakAB
59 Specific humidity, Section A Thermally direct!xJet stream
60 Precipitation rate Jet core ITCZ Note: the vertical velocity field in jet cores will be revisited later for synoptic jets.Jet coreITCZ
61 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
65 summary: 3-10 day variability there is a ‘baroclinic storm’ track between 40-60º latitudestorms appear vertically coupled1000 mb variability similar to 500 mb variabilitystorm 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 landthe NH storm track has larger seasonal variability and is less zonally symmetric than the SH storm track
66 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
67 meridional cross section (potential temperature q, zonal wind speed U) U500 >> U1000pressure (mb)ElatitudeHolton (2004) p.145
68 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 windmore 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.
69 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 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.
71 Question:Where do lows and highs tend to form?Where do they decay?a Lagrangian perspective …
72 NH baroclinic storm track & preferred regions of cyclogenesis / cyclolysis contour: standard deviation of GPH vector: phase propagation vector1000 mb500 mb
73 Sea level cyclone formation and decay around North America Winter cyclolysisWinter cyclogenesis(Bluestein 1993, p 20)
74 anticyclone formation and decay around North America Winter anticyclolysisWinter anticyclogenesis(Bluestein p. 25)
75 On the movement of trofs and ridges: two forecast rules 1. Kicking back cut-off lows into the main streamTrofs 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)
76 On the movement of trofs and ridges: two forecast rules 2. asymmetric trofs & meridional movementIf 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.
77 Hovmöller diagrams red: short-wave trofs 24 hr averagedeparture of the 24 hr average from the zonal meanWinter 02-03500 mb height (m)30-40ºNall longitudesquestion: how does the stationary long-wave pattern shown here match with the 500 mb anomalies from zonal mean, discussed earlier ?Alpine lee cyclogenesisRockies lee cyclogenesisred: short-wave trofsblue: stationary long-wave ridgeSource:
78 “blocking” flow aloft occurs most frequently in spring around 50°N results from an anomalous long-wave ridge that blocks the progression of short wavesoccurs during ‘low-index’ cycleshigh index: strong zonal flowlow index: zonal flow weak, meridional flow strong3 types, qualitatively separatedHigh-over-low blockOmega blockupper-level ridge
79 high over low blockupper-level ridgeomega block
80 ConclusionsA 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 contrastThe 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 (~60ºN) ocean lows and continental highs dominate in winterA jet stream exists in the upper mid-latitude troposphereIts climatological position and strength are in thermal wind balance, thus it is stronger in winter than in summerThe separation between STJ and PJ is not present at all longitudes, nor in the zonal mean, in both hemispheres & seasonsHighly zonally asymmetric in the NH, due to continents and topographyQuasi-stationary trofs along east coasts of N America & AsiaVery symmetric in the SHJet 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 wavesbaroclinic 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 strongestthis is consistent with QG theory (stronger PVA and WAA), to be discussed later