byFumiaki Ogawa1, H. Nakamura1 , K. Nishii1,

T. Miyasaka1 and A. Kuwano-Yoshida2

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

Introductiondata: 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.

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

Low-frequency annular variability and SST front

Southern annular mode (SAM)

Monthly U925 in JJAduring 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.

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

Experimental design

CTLSST 勾配プロファイル

45°SST gradient

Winter

•Model: AGCM for Earth Simulator (AFES)•T79(150km grid), 56 levels.•Aqua planet with zonally uniform SST.•Perpetual winter

: SST front was smoothed ． winter

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°

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

[m/s]

[latit

ude]

[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

Climatological mean near surface westerly

[m/s]

[latit

ude]

[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 peaklow 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

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.

SST Front

Max lat.

[SST front latitude]

[m/s]

PC < -1

Max lat.

NF climatology

[m/s]

[SST front latitude]

Characteristics of the model annular mode[ l

atitu

de ]

[velocity(m/s)]

PC >1 PC >1

Latitude of U925 axisPositive 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.

“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.]

[ lat

itude

]

POS

NEG

SST front

Peak latitudes of probability

Climatological meanpeak 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]

POSNEG

(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:

Black dots : SST front

U925 composited state

(PC1 > 1std.)POS NEG

SST front

(PC1 < -1std.) Indian sector(50~110°)

Pacific sector(190~250°)

SSTfront

[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°

U925 composited state

(PC1 > 1std.)POS NEG

SST front

(PC1 < -1std.) Indian sector(50~110°)

Pacific sector(190~250°)

SSTfront

•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 winterAqua planet experiment

45°Front

55°

45°

55°

⇔ Our APEs with corresponding latitudes of SST front

•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.

JJA SST OI-sst 82-11

Ice edge ? No.

Composite

Atl.

Ind.

Pac.

-48-44.25

-50.25-45.00

-63.75-45.75

Annular mode signature in SH winter.SH-winter Regression

U925 composited statePOS

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

NEGPC1 < -1std.

Shading: [U925] anomaly associated with AM

: 　 Climatological axial latitude of [U 925][SST front latitude] [SST front latitude]

Meridional fluctuation Strengthening

SST front

SST front

Probability for the occurrence of [U925] maximum(SST front at 55°)

[%]

EOF1

EOF2

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 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

⇒ 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

NH-winter

NH-winter (DJF)

Atl.

Pac.

NH-winter Composite

POS ( PC > 1 ) NEG ( PC < -1 )

NH-winter

Atl.

Pac.

CompositeRegression

SH-winter V’ std. 925

NH-winter V’ std. 925

Z850 anomaly regressed on PC

PC1 PC2

PC1 17.1%

PC2 16.0%

Codr

on (2

007)

U850

195

8-20

01U

925

197

9-20

11lon 30-120° lon 120-210°

55°

55°

45°

45°

45°

55°

55° & 45°

55° & 45°

SST

SST gradient

Probability for the occurrence of [U925] maximum

SST

SST gradient

45°62°

Probability for the occurrence of [U925] maximum

U925.clim

U300.clim

SST

SST-gradient

55°

55°strong

ctrl

NF

55°

55°strong

ctrlNF

ctrl

ctrl55°

55°strongNF

55°55°strongNF

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.NEG: U axis is located at very similar latitude.

POS NEG

SST front

Ind. sector(50~110°)

Pac.sector(190~250°)

SSTfront

Observed SAM ( winter )[U925]

45°Front

55°