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

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

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

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

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

6. Experimental design CTL SST 勾配プロファイル 45° SST gradient Winter Summer : SST front was smoothed ． winter summer Non-Front (NF) Integration for 120 months after spin-up. : SST front was shifted from 30° to 55° by 5° saving the gradient. 30° 55° 30° 55° SST [°C] [K/lat] : JJA mean SST of South Indian Ocean (OI-SST) Sensitivity experiments Non-front (NF) SST front locates at 45°. ⇒ Relation between SST front latitude and annular mode variability is investigated. NF

7. [m/s] [latitude] [SST frontal latitude] Front NF → Influence of SST front is overshadowed by atmospheric internal dynamics. (Robinson, 2006) Subpolar SST front ・ Primary westerly axis is in midlatitude away from SST front near the axis in NF. The surface westerly axis shifts together with the SST front and locates poleward flank of the front ． Subtropical or midlatitude SST front Ogawa et al. 2012 (GRL) [U ９２５ ] Climatological mean near surface westerly Axial latitudes

8. Climatological mean near surface westerly [m/s] [latitude] [U ９２５ ] [SST frontal latitude] Probability for the occurrence of [U925] maximum is sensitive to the frontal shift, while the is near the latitude of the distinct peak in NF experiment, with little sensitibity to SST front latitude. High latitude peak low latitude peak : Dual peaks ☞ The position of PFJ axis tends to go back and forth between the dual peak latitudes of the probability. NF axis Front ☞ Implication for annular variability in APE. Probability for the occurrence of [U925] maximum

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

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

11. Storm track fluctuation Storm track ⇔ U925 axis : consistent Standard deviation of V’ 925hPa ’ ： subweekly fluctuation PC < -1 Max lat. NF c limatology [SST front latitude] PC >1 Axial latitudes

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

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

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

15. Model annular mode represented “regime-like” characteristics. Summary Observed SAM shows similar characteristics. ☞ The difference in SST front latitude between the South Indian and Pacific Oceans may cause the inter-basin difference in the signature of SAM in winter. We investigated the dependence of annular mode characteristics on the latitude of SST front by a set of aqua-planet experiments. 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. Positive phase: Latitude of eddy-driven PFJ co-varies with the SST front latitude. -Strong thermodynamic effect by SST front. Storm track shows consistent variability.

17. SST FRONT EQUATORPOLE Global angular momentum budget (Nakamura et al. 2004) Storm track Introduction ・ Observed storm track and polar front Jet ( PFJ ) are dynamically coexisted with the SST front in climatological mean state. Nakamura et al. (2004) Formed along with SST front ・ Storm track ・ STJ: SubTropical Jet Formed by Angular momentum conservation of the parcel Formed and maintained by the eddy momentum flux from STJ ・ PFJ Friction Gives the angular momentum of solid earth to the atmosphere ・ Equatorial surface friction Raise atmospheric parcels ・ Equatorial deep convection Friction drives oceanic current ・ SST front Maintained together with the ocean current by the friction at the surface part of PFJ..

18. [m/s] [latitude] [U ９２５ ] [SST frontal latitude] [days] Mean residence time of the [U 925 ] axis (day) [SST frontal latitude] is sensitive to the frontal shift, while the is near the latitude of the distinct peak in NF run, with little sensitibity to SST front latitude. Residence time of [U 925 ] maximum High latitude peak low latitude peak : Dual peaks Substantially long residence time in both peaks suggests the low-frequency variability as represented by meridional migration between the peaks. NF axis Front

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

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

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

22. Shading: [U925] anomaly associated with AM : Climatological axial latitude of [U 925 ] [SST front latitude] Meridional fluctuation Strengthening SST front

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

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

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

26. NH-winter

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

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

29. NH-winter Atl. Pac. CompositeRegression

30. SH-winter V’ std. 925

31. NH-winter V’ std. 925

33. Z850 anomaly regressed on PC PC1 PC2

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

35. 55° 45° 55° 55° & 45° SST SST gradient Probability for the occurrence of [U925] maximum

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

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

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