Atmospheric response to North Pacific SST The role of model resolution and synoptic SST variability Guidi Zhou, Mojib Latif, Wonsun Park*, Richard Greatbatch GEOMAR Helmholtz Centre for Ocean Research Kiel, Germany *presenting Frontal-scale Air-sea Interaction Workshop Boulder, CO, USA August 6, 2013

 Background and motivation  Observed atmospheric response to re- emergent SST  AGCM response  Sensitivity to background SST and horizontal resolution Outline

 Extra-tropical SST experiences annual reemergence [Alexander et al. 1999…]  The atmosphere responds to extra-tropical SST anomalies [Czaja & Frankignoul 1999, 2002…]  Response is small compare to Tropical response and internal atm. variability [Palmer & Sun 1985; Kushnir et al. 2002…]  But potentially persistent due to the large thermal inertia of the oceanic mixed layer [Kushnir et al. 2002…]  Transient eddies are crucial [Peng & Whitaker 1999…]  Oceanic fronts are important for anchoring the storm-track [Minobe et al. 2008; Taguchi et al. 2009…] Background and motivation

Observed atmospheric link to reemergent SST anomalies Regressed SST on NP SST PC1* (ENSO removed)  ERSST 80-year winter- mean data  Remove ENSO from NP SST using linear regression (PC1*)  Lagged regression of SST on PC1* (PC1* leads)  SST reemergence found in NP

Observed atmospheric link to reemergent SST anomalies Regressed SLP on NP SST PC1* (ENSO removed)  ERSLP 80-year winter-mean data  Remove ENSO from NP SST PC using linear regression (PC1*)  Lagged regression of SLP on PC1* (PC1* leads)  Significant links between SLP and PC1* of previous winters  Deepened Aleutian Low

Observed atmospheric link to reemergent SST anomalies Regressed SST on NP SST PC1* (ENSO removed)Regressed SLP on NP SST PC1* (ENSO removed)

AGCM response to NP SSTa  Model: ECHAM5  Resolution: T213 (~0.56°) L31  Background SST: NOAA-OI-Daily SST (time-varying , to preserve the fronts)  SST anomaly forcing: +/- lag-0 regression pattern in NP  4-month (NDJF) integrations  Initialized Nov.1, +/- 10 winters each  Mean (DJF) differences between +/- SSTa experiments studied Experimental setup

AGCM response to NP SSTa SST anomaly (doubled) PDO-like anomaly pattern (K)

The relative position of the SST anomaly with respect to the full SST Contour: winter-mean climatological SST (°C) Shading: PDO-like SSTa (°C) Positive PDO-like SSTa pattern would reduce the front

AGCM response to NP SSTa response characteristics Deepened Aleutian Low (hPa)

AGCM response to NP SSTa response characteristics (W/m 2 ) Upward heat fluxes reinforce the SSTa in some regions

AGCM response to NP SSTa response characteristics Geopotential height (m) equiv. barotropic U-velocity (m/s)Vertical velocity (Pa/s) accelerated jet streamdeep convection Zonally averaged over NP

AGCM response to NP SSTa eddy forcing div. eddy vorticity flux (1/s 2 ) conv. eddy vorticity flux divergence (1/s)div. eddy zonal momentum flux (m/s 2 ) upper div., surface conv.conv. eddy momentum flux Consider 40°N (over cold SSTa)

AGCM response to NP SSTa Characteristics Diagnostics (over cold SSTa) T213 Response summary  Equivalent barotropic low  Upward heat flux reinforcing the SSTa in some regions  Accelerated jet stream  Deep convection  Deep diabatic heating  Enhanced storm-track  Convergence of eddy vorticity flux is balanced by divergence in circulation, resulting in ascent at mid-levels and low-level convergence. The vortex stretching associated produces the surface low.  Convergence of eddy momentum flux reinforces the jet.  Consistent with Kushnir et al

High vs. low resolutions T31- response Geopotential height (m) T213- response

Influence of synoptic SST variability Snapshot, Jan 01, Fine spatial/temporal structures on T213 Synoptic SST variability only in NP ctrl, NP SSTa experiments Climatological state, Jan 01 Smoothed spatial/temporal structures on T213 ctrl, NP SSTa experiments Sea surface temperature (K)

Observed daily varying SST vs. climatological SST as background T213-daily varying North Pacific background SST T213-climatological background SST no significant response significant eddy-mediated response

 North Pacific atmosphere response to underlying SST anomalies in observations  This can be simulated only with higher resolution of AGCM, e.g. T213  Temporal resolution is also important in this aspect  The response is eddy-mediated Summary In order to obtain a realistic response to midlatitude SST anomalies one needs to resolve the synoptic SST variability. This requires employing sufficiently high horizontal resolution. Guidi Zhou

Zonal mean SST gradient Zonal (120°E-160°W) mean