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Potential importance of midlatitude oceanic frontal latitude

Potential importance of midlatitude oceanic frontal latitude on the atmospheric annular mode variability as revealed from aqua-planet experiments. by Fumiaki Ogawa 1 , H . Nakamura 1 , K . Nishii 1 , T . Miyasaka 1 and A. Kuwano-Yoshida 2 1. RCAST , University of Tokyo, Japan

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Potential importance of midlatitude oceanic frontal latitude

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  1. Potential importance of midlatitude oceanic frontal latitude on the atmospheric annular mode variability as revealed from aqua-planet experiments by Fumiaki Ogawa1, H. Nakamura1 , K. Nishii1, T. Miyasaka1 and A. Kuwano-Yoshida2 1. RCAST, University of Tokyo, Japan 2. ESC, JAMSTEC, Yokohama, Japan

  2. Introduction JJA mean SST and SST front data: OI-SST (1982~2011) Winter : SST gradient exceeds 1 K/lat. • Latitudal of SST front differs from basin to basin: -Atlantic and Indian Oceans: 40~45° - Pacific Ocean: 55° Blue line: Latitudinal circle

  3. Mean atmospheric zonal asymmetry and SST front Black dots: SST fronts JJA mean U925 Winter ERA-Interim (1979-2011) •Latitudinal coincidence between SST front and U925 axis. [m/s] •SST front acts to intensify the storm track and associated eddy-driven polar front jet (PFJ). Nakamura and Shimpo (2004) •Zonal asymmetry inmidlatitude SST is the prominent factor for lower tropospheric zonal asymmetry. Inatsu and Hoskins (2004)

  4. Low-frequency annular variability and SST front Southern annular mode (SAM) U925 anomaly regressed on PC1 (Thompson and Wallace, 2000) Black dots : SST front Monthly U925 in JJA during 1979-2011 Winter EOF analysis SAM • Large amplitude over the Indian and Pacific oceans. •The nodal latitude coincides with SST front over these basins. SST front latitudes may affect the signature of SAM. [m/s]

  5. Objective and Strategy Objective •To study the dependence of annular mode characteristics on the latitude of SST front. Strategy •Aqua planet AGCM experiments • Maximizing the SST frontal effect •Focus: Winter

  6. Experimental design •Model: AGCM for Earth Simulator (AFES) •T79(150km grid), 56 levels. •Aqua planet with zonally uniform SST. •Perpetual winter [°C] SST Summer Winter winter summer CTL : JJA mean SST of South Indian Ocean (OI-SST) [K/lat] SST front locates at 45°. 30° 30° 55° 55° Integration for 120 months after spin-up. NF Sensitivity experiments : SST front was shifted from 30° to 55° by 5° saving the gradient. SST勾配プロファイル SST gradient 45° 45° : SST front was smoothed． Non-front (NF) Non-Front (NF) ⇒ Relation between SST front latitude and annular mode variability is investigated.

  7. Climatological mean near surface westerly Ogawa et al. 2012 (GRL) [U９２５] Axial latitudes Front [latitude] NF [m/s] Subtropical or midlatitude SST front [SST frontal latitude] The surface westerly axis shifts together with the SST front and locates poleward flank of the front． Subpolar SST front ・Primary westerly axis is in midlatitude away from SST front near the axis in NF. → Influence of SST front is overshadowed by atmospheric internal dynamics. (Robinson, 2006)

  8. Climatological mean near surface westerly Probability for the occurrence of [U925] maximum [U９２５] Front [latitude] NF axis [m/s] Probability for the occurrence of [U925] maximum • is sensitive to the frontal shift, while the is near the latitude of the distinct peakin NF experiment, with little sensitibity to SST front latitude. [SST frontal latitude] : Dual peaks ☞The position of PFJ axis tends to go back and forth between the dual peak latitudes of the probability. High latitude peak ☞ Implication for annular variability in APE. low latitude peak

  9. The model annular mode Extraction of annular mode:EOFanalysis on the 8-day low-pass filtered daily [U925]. ⇒ As the observed variability, we considered the model annular mode as : The meridional shift of U axis. ⇒EOF 1 represents the mode in most of the experiments. ( EOF2 represents the mode only when SST front is at 35°).

  10. Characteristics of the model annular mode Composite of [U925] Max lat. PC >1 SST Front [m/s] PC >1 [m/s] PC < -1 [SST front latitude] [ latitude ] Max lat. NF climatology Latitude of U925 axis [velocity(m/s)] Positive phase : Poleward of the SST front, shifting together with the SST front. [SST front latitude] : Near the mean axis in NF regardless of the SST front latitude. Negative phase ⇒ Dominance ofatmospheric internal dynamics (Robinson, 2006)

  11. “Regime-like” behavior of the annular mode [U925] composite [%] Probability for the occurrence of [U925] maximum Peak latitudes of probability POS [m/s] ・Dual peaks: Implication for regimes in [U]. (SST front at 55°) POS: poleward domain. Location of [U] maxima: [ latitude ] NEG: equatorward domain. SST front ⇔ composited results Climatological mean peak of probability in NF. POS NEG Annular mode mayrepresenttemporal variability inatmospheric sensitivity to SST front and leads to the “regime shift”. NEG [SST front lat.] [latitude] [latitude] Shifting together with SST front NEG POS Weak sensitivity to SST front

  12. Implication for the observed SAM in winter U925 anomaly regressed on PC1 U925 composited state NEG POS Black dots : SST front (PC1 < -1std.) (PC1 > 1std.) Indian sector (50~110°) SST front 45° Pacific sector (190~250°) SST front 55° •POS: U peakshifts by ~10°correspondingto the difference in SST front latitiude. •NEG:the shift of U peak is little despite the difference in SST front latitude. [m/s]

  13. Implication for the observed SAM in winter U925 composited state Aqua planet experiment NEG POS (PC1 < -1std.) (PC1 > 1std.) Front Indian sector (50~110°) 45° SST front 45° Pacific sector (190~250°) 55° SST front 55° •POS: U peakshifts by ~10°correspondingto the difference in SST front latitiude. •NEG:the shift of U peak is little despite the difference in SST front latitude. ⇔ Our APEs with corresponding latitudes of SST front

  14. Summary We investigated the dependence of annular mode characteristics on the latitude of SST front by a set of aqua-planet experiments. •Model annular mode represented “regime-like” characteristics. •Positive phase: Latitude of eddy-driven PFJ co-varieswith the SST front latitude. -Strong thermodynamic effect by SST front. •Negative phase: PFJ is located at a certain latitude regardless of SST front lat. - Similarity to non-front situation implies the dominance of atmospheric internal dynamics over SST front forcing. •Storm track shows consistent variability. •Observed SAM shows similar characteristics. ☞ The difference in SST front latitude between the South Indian and Pacific Oceans may cause the inter-basin difference inthe signature of SAM in winter.

  15. Ice edge ? No. JJA SST OI-sst 82-11

  16. Annular mode signature in SH winter. Regression Composite SH-winter -48 -44.25 Atl. -50.25 -45.00 Ind. -63.75 -45.75 Pac.

  17. How about in low-frequency annular variability? U925 composited state Black dots: SST front POS NEG PC1 > 1std. PC1 < -1std. •Strong zonal asymmetry •Westerly axes tend to locate poleward of SST front. POS •Almost zonally symmetric. •CorrespondencebetweenU axes and SST front is much weaker. NEG

  18. Shading: [U925] anomaly associated with AM Meridional fluctuation Strengthening SST front SST front [SST front latitude] [SST front latitude] :Climatological axial latitude of [U925]

  19. [%] Probability for the occurrence of [U925] maximum (SST front at 55°) EOF1 EOF2

  20. Characteristic of the phases near the tropopause Observed characteristics of the annular mode: Regime shift between the double jet ofSTJ and PFJ. the single jetof STJ Eichelberger and Hartmann 2007 Composite of [U250] for each phase PC < ー1 PC >1 PFJ STJ STJ APE results: Consistent with the previous work in all the experiments. POS: Position of PFJ is sensitive to the shift of SST front. NEG: Wind profile show little difference regardless of the SST frontal latitude.

  21. SST front forcing •Surface turbulent fluxes ---latent heat, sensible heat •The gradient of upward sensible heat flux across the SST frontis more important than latent heat flux for the recovery of thesurfacetemperature gradient relaxed by transient eddy heat flux. (Hotta and Nakamura 2011, Nonaka et al. 2009) ∂[SENS.-HF] /∂yPOS-NEG (positive for strengthening) Meridional gradient of sensible heat flux is stronger in POS than in NEGalong the SST front. ⇒ In POS, •Surface baroclinicity is more effectively recovered along the SST front ⇒Storm track and PFJ tend to be anchored

  22. NH-winter

  23. NH-winter (DJF) Atl. Pac.

  24. NH-winter Composite NEG ( PC < -1 ) POS ( PC > 1 )

  25. NH-winter Regression Composite Atl. Pac.

  26. SH-winter V’ std. 925

  27. NH-winter V’ std. 925

  28. Z850 anomaly regressed on PC PC1 PC2

  29. lon 30-120° lon 120-210° Codron (2007) U850 1958-2001 PC1 17.1% U925 1979-2011 PC2 16.0%

  30. Probability for the occurrence of [U925] maximum SST 45° 55° 45° 55° SST gradient 55° 45° 55° & 45° 55° & 45°

  31. Probability for the occurrence of [U925] maximum SST 62° 45° SST gradient

  32. SST U925.clim ctrl 55° 55° ctrl NF NF 55°strong 55°strong SST-gradient U300.clim 55° ctrl ctrl NF 55° 55°strong NF 55°strong

  33. Implication for the observed SAM in winter Aqua planet experiment Observed SAM ( winter ) NEG [U925] POS -50.25 -45.00 -54.5 -38.0 Ind. sector (50~110°) Front SST front 45° Pac. sector (190~250°) -63.75 -45.75 -63.5 -39.5 55° SST front POS: U axis shifts corresponding to the shift of SST front. NEG: U axis is located at very similar latitude. Our APEs with corresponding latitudes of SST front

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