Transcript Warm path
The Gulf Stream – troposphere connection: the “warm path” A. Czaja, L. Sheldon, B. Vanniere & R. Parfitt Imperial College, London & Grantham Institute for Climate Change All papers mentioned in the talk can be found at: http://www.sp.ph.ic.ac.uk/~aczaja SST (deg C) SNAPSHOT SST (deg C) Gulf Stream “warm tongue” CLIMATOLOGY NB: ERA-interim data (DJF 2002-2012) Dynamic topography (cm) Time mean SST (contoured at 1K interval) and surface dynamic topography (color) NB: SST is ERA-interim (DJF 2002-2012), dyn. topo is Maximenko (yr mean, 1992-2002) Climate as opposed to weather focus (i.e., impact of SST on storm-track) Damping of temperature anomaly by turbulent air-sea heat fluxes SST contours Steering of temperature contours by synoptic waves Eastward “Climate” approach to this problem (i.e., impact of SST on storm-track) Damping of temperature anomaly by turbulent air-sea heat fluxes SST contours Steering of temperature contours by synoptic waves “Warm path”: impact of the Gulf ( t u x ) v y Stream warm tongue on the warm 2 of vsector cyclones / y “Climate” approach to this problem (i.e., impact of SST on storm-track) Damping of temperature anomaly by turbulent air-sea heat fluxes SST contours Steering of temperature contours by synoptic waves “Cold(path”: impact of the Gulf Stream t u x ) v y warm tongue on the cold sector of 2 v See Benoit / yVanniere’s talk! cyclones... Outline • Empirical evidence for the warm path in ERAinterim reanalysis data in the cold season • Mechanisms: simulations at various spatial resolutions with the Met Office Unified Model • New perspectives • Summary The warm path in ERAinterim reanalysis data (DJF 1979-2012) A dynamical diagnostic for the warm path • Focus on the WCB and treat the cold front as a 2D feature • Relate the strength of the ascent to the buoyancy contrast across the slanted front: Δb ~ s(B)-s(A) NB: s = specific moist entropy, M = absolute momentum Schematic from Catto et al. (2010) A simple diagnostic for surface fronts Snapshot on a given winter day : normalized F (shading) & SLP anomaly (ci=5mb) NB: ERA-interim data (DJF 1979-2012) Fronts with weak stability only Frequency (% of days) • Use the frontal index of Sheldon et al. (2015). Fronts are present about 10-15% of the time in winter. • Fronts with weaker stability (low Ri) are seen shifted towards, and more narrowly confined over, the GS warm tongue. All fronts Frequency (% of days) Evidence for the warm path: spatial distribution of fronts frequency Other measures of moist dynamics: without consideration of vertical extent • No localisation of the climatology over the Gulf Stream warm tongue SCAPE CAPE Courtesy of Michael Glinton Uni. Reading PhD thesis (2014) DJF mean CAPE or SCAPE (J/kg) Other measures of moist dynamics: with consideration of vertical extent • Localisation over the Gulf Stream warm tongue VRS: thickness (m) of layers (not necessarily contiguous) with: Courtesy of Michael Glinton Glinton (2014) Uni. Reading PhD thesis (2014) DJF mean vertical extent of realisable slantwise instability (VRS, in km) Composite sections of ω (Pa/s) in the transverse plane of the fronts • Two populations of fronts are considered: tropopause tropopause Weak stability Large stability • Comparable magnitude of ascent in the population of fronts with weak and large stability Composite sections of normalized ω in the transverse plane of the fronts • Two populations of fronts are considered: tropopause Weak stability Large stability • Deeper ascent in the population of fronts with weak stability Mechanisms “Warm path” physics: working hypotheses • Weak air-sea heat fluxes (warm air over warm water) • Deep, slanted and moist adiabatic ascent. • Low (moist) Richardson number • Midlatitudes proper (unlike “cold path”) GS weakens air-sea heat flux Nearly thermally adjusted air & water “Warm path” & spatial resolution NB: UPSCALE data used here is DJF 1985-2011 UPSCALE data % of winter days • UPSCALE data suggests that the “warm path” requires higher spatial resolution than current OAGCMs (at least 25km). This is primarily to resolve the GS warm tongue and to generate fronts with sufficiently low Ri. Weak stability fronts: change in frequency (25km – 135km) Nearly thermally adjusted air & water Simulations with the Met Office North Atl. domain Unified Model (res 12km) Realistic SSTs Global domain (res 40km) • Nested grid over a North Atlantic domain • One event: “bomb” storm passing over the Gulf Stream on Jan 14 2004 • Two experiments: Smoothed Smoothed SSTsSSTs • 3D backward trajectories from the core of the ascending region at t=1day (z=4km, 5, 6km) t = 0 (varying z) • Two “source regions”: low and midlevel streams. Height (m) Realistic SST Height (m) Smooth SST Height (m) Back trajectories from t=24h (core of ascent) with low level origin • More air parcels with low level origin with realistic SST • These reach higher up in the realistic SST case • Their ascent is also narrower The increased ascent can be related to changes in air-sea interactions Realistic SSTs Smoothed SSTs Narrower & stronger core of ascent Broader & weaker core of ascent Moist entropy (CTL-SMTH, along trajectories) = + mean max min net latent sensible Time (hours) = + net latent sensible New perspectives (i) “Existence” of storm track (ii) Response of AGCMs to extra-tropical SST anomalies Diabatic heating and the storm track: challenging Hoskins & Valdes’ (1990) view Total thermal forcing by “eddies” Mean diabatic heating CI=0.25 K/day CI=0.25 K/day A large degree of cancellation between the two. Is there really a “residual heating” available to drive Rossby waves? Thermal forcing of Rossby waves by “synoptic systems”? • Latent heat release is balanced by adiabatic expansion during ascent • Weak “residual heating” available to drive slower forms of motions as a result weak thermal forcing “Environment” grid box Weak residual heating of the environment …and there is no cancellation between upward and downward motion in a cyclone • Dry isentropic upglide and downglide component of ω are large and cancel out • The asymmetry comes from the component of ω across dry isentropes • suggests a vorticity rather than a thermal forcing by the Gulf Stream See Parfitt & Czaja (submitted to QJRMS) Hoskins et al. (2003) Qrad ~ -1K/day Qlat ~ +5K/day Green et al. (1966) Atmospheric response to SST anomalies HR = ¼ deg LR = 1 deg • The warm path is not resolved in coarse AGCMs and this might explain the dependence of atmospheric sensitivity on SST on spatial resolution. Smirnov et al. (J. Clim. 2015) Atmospheric response to SST anomalies • The warm path is not resolved in coarse AGCMs and this might explain the dependence of atmospheric sensitivity on SST on spatial resolution. HR = ¼ deg LR = 1 deg Excited by vorticity source associated with time mean and deep upward motion Smirnov et al. (J. Clim. 2015) Summary • The Gulf Stream impacts on the warm sectors of cyclones through a weakening of air-sea interactions (alignment of air and sea isotherms), resulting in enhanced ascent in their warm conveyor belt. • Moisture is key to the dynamics • The associated forcing of the large scale flow is likely to be mechanical, not thermal. • This “warm path” is not represented in coarse climate models GS weakens air-sea heat flux Nearly thermally adjusted air & water Summary • The Gulf Stream impacts on the warm sectors of cyclones through a weakening of air-sea interactions (alignment of air and sea isotherms), resulting in enhanced ascent in their conveyor belt. • Moisture is key to the dynamics • The associated forcing of the large scale flow is likely to be mechanical, not thermal. • This “warm path” is not represented in coarse climate models Interested in applying these ideas to your simulations (climate or weather)…? please tell me! GS weakens air-sea heat flux Nearly thermally adjusted air & water Extras From Deser & Blackmon (1993) Observed variability of wintertime surface climate SST EOF 1 (K) “Composites” (1939-68 minus 1900-1929) Surface winds and pressure (-3mb) VAR = 12% VAR=45% SST >1K Time (years) Working hypothesis: two different physics • “Cold path” (=GScold sector): akin to tropical airsea interactions. Synoptic systems drive surface heat fluxes, generating CAPE and shallow convection. • “Warm path”(=GSwarm sector): mid-latitudes proper. Weak air-sea heat fluxes; deep, slanted and moist adiabatic ascent. GS enhances air-sea heat flux GS weakens air-sea heat flux Nearly thermally adjusted air & water Extra-tropical cyclones: warm & cold sectors 13 December 2010 at 2231UTC (GOES, Infrared) Warm front WCB cold sector Cold front warm sector Weak SST gradient because of ocean eddy mixing Dynamic topography (cm) Time mean SST (contoured at 1K interval) and surface dynamic topography (color) Warm advection by the Gulf Stream NB: SST is ERA-interim (DJF 2002-2012), dyn. topo is Maximenko (yr mean, 1992-2002) Key dynamics at fronts • 2D geometry with geostrophic balance across the front • Transverse circulation: (i) maintains the thermal wind W Z M θ C (ii) is elongated along lines of constant “absolute momentum” M = Ug – fo y Eliassen (1962) Sections in the transverse plane Pressure vertical velocity (Pa/s) Relative humidity ... Cold side Warm side NB: Gulf Stream only Cold side Warm side New perspectives HR = ¼ deg LR = 1 deg • The cold and warm paths paradigm can help explain the dependence of atmospheric sensitivity to SST on spatial resolution (low res AGCMs only see the cold path) Smirnov et al. (J. Clim. 2015) UPSCALE project (Met Office UM model): frequency of fronts with weak stability in wintertime 25 km 135 km 60 km 25 km – 135km Outstanding issue Poleward heat flux Damping of temperature anomaly by turbulent air-sea heat fluxes SST contours Steering of temperature contours by synoptic waves Here the heat budget is closed by the synoptic waves ( u ) v t x y themselves. There is no “residual heating” to drive slower forms of motion. 2Is there in Nature? See v Parfitt’s talk / atyXX Climate as opposed to weather focus (i.e., impact of SST on storm-track) Poleward heat flux Damping of temperature anomaly by turbulent air-sea heat fluxes SST contours Steering of temperature contours by synoptic waves ( t u x ) v y v ( t ) / y 2 Climate as opposed to weather focus (i.e., impact of SST on storm-track) Poleward heat flux Damping of temperature anomaly by turbulent air-sea heat fluxes SST contours Steering of temperature contours by synoptic waves Eastward ( t u x ) v y v / y 2 Simulations with the Met Office Unified Model North Atl. domain (res 12km) Global domain (res 40km) Realistic SSTs W(5km) at t=18h in m/s +10m winds • Nested grid over a North Atlantic domain • One event: “bomb” storm passing over the Gulf Stream on Jan 14 2004 • Two experiments: Smoothed SSTs Simulations with the Met Office Unified Model North Atl. domain (res 12km) Global domain (res 40km) Realistic SSTs W(5km) at t=24h in m/s +10m winds • Nested grid over a North Atlantic domain • One event: “bomb” storm passing over the Gulf Stream on Jan 14 2004 • Two experiments: Smoothed SSTs Simulations with the Met Office Unified Model North Atl. domain (res 12km) Global domain (res 40km) Realistic SSTs W(5km) at t=36h in m/s +10m winds • Nested grid over a North Atlantic domain • One event: “bomb” storm passing over the Gulf Stream on Jan 14 2004 • Two experiments: Smoothed SSTs Simulations with the Met Office Unified Model North Atl. domain (res 12km) Global domain (res 40km) Realistic SSTs W(5km) at t=48h in m/s +10m winds • Nested grid over a North Atlantic domain • One event: “bomb” storm passing over the Gulf Stream on Jan 14 2004 • Two experiments: Smoothed SSTs