Storm track response to Ocean Fronts in a global high-resolution climate model R. Justin Small, Frank Bryan and Bob Tomas NCAR Young-Oh Kwon WHOI + 2 anonymous reviewers Department of Energy

Aims Investigate influence of ocean fronts on atmospheric storm track in winter – Surface storm track – Free-troposphere storm track What are the key storm track statistics that are affected? What affects baroclinicity? Using a global atmospheric climate model – 1. North Atlantic – 2. Southern Ocean – 3. North Pacific

Experiments Community Atmosphere Model version 4 – Developed at NCAR, Department of Energy, US labs – Hydrostatic, sigma-coordinate global model – ½ deg. grid spacing, 27 levels (<6 in lowest 1000m). Twin experiments, atmosphere-only. – 1. Control has realistic SST in region (e.g. N. Atlantic) – 2. Smooth Global SST experiment – SST is a climatology based on satellite/in situ data (1/4deg., Reynolds et al 2007). Each run for 60 years to gain some statistical significance.

Methods and Data We use a high pass filter – V’=V- – where is 5-day mean at surface, seasonal mean or monthly mean in free troposphere Compute climatological mean of quantities such as, Apply smoothing to SST fields as boundary condition for AGCM passes of filter 1 Comparisons are made with ERA-INTERIM data (ERA-I) and OAFLUX, QSCAT

North Atlantic case, Boreal winter (DJF). (a) (b) (c) SST FOR CONTROL SMOOTH SST EXPERIMENT SST difference SST DIFF  C /100km (d) SST gradient difference

Frequency distribution of strong SST gradients High-res coupled model Low-res coupled model Reynolds OI SST 1/4deg. Heavy smoothing of Reynolds OI SST 1/4deg. Light smoothing of Reynolds OI SST 1/4deg. Histograms of occurrence of binned SST gradients within 1deg. C/100km contour in North Atlantic including Gulf Stream. Uses data from DJF climatology. Units deg.C per 100km.

What storm track quantities are significantly affected by ocean front and what quantities are not?

10 -6 s -1 (a)(b) (c) control SMTH s -1 (d) Standard deviation of near –surface transient eddy vorticity variability. Filtered to retain only timescales less than 5 days. Note that differences (control-smooth, bottom right panel) of std. dev (  ’) overly SST anomalies, and reach up to 30% of smooth value s -1 Std.dev(  ’) Control Std.dev(  ’) Smooth Diff in Std.dev(  ’) +SST anomaly 30% OAFLUX obs- Joyce and Kwon 2009 Relative vorticity variability

SEA LEVEL PRESSURE VARIABILITTY. SLP sub- 5day variability and differences Add significance SMOOTH CONTROL DIFFERENCE hPa

Surface geostrophic vorticity sub-5day variability and differences GEOSTROPHIC VORTICITY variability. DERIVED JUST FROM SLP. SMOOTH CONTROL DIFFERENCE s %

Atlantic DJF: Meridional Heat Flux V’T’ ControlV’T’ ERA-I ms -1 K 30% In the right panel only differences significant at 95% are shown, and contours show SST differences of +/- 2  C from Fig. 1c. The number shown is the approx. ratio of the amplitude of the difference to the amplitude of the maximum in the smooth case, expressed as a percentage. V’T’ ERA-I ms -1 K V’T’ Diffn. Control-Smooth ms -1 C ms -1 K V’T’ ms -1 K V’T’ Control 25% V’T’ ERA-I ms -1 K Control-Smooth Transient eddy meridional heat flux 500hPa Transient eddy meridional heat flux 850hPa

m 2 s -2 Atlantic DJF: Meridional wind variance V’V’V’V’ Control m 2 s -2 10% V’V’ ERA-I m 2 s -2 Low-light – CAM wind variance (& heat-flux) is too high compared to ERA-I and MERRA. Therefore adding the ocean front worsens the comparison. Control-Smooth V’V’ Control V’V’ ERA-I m 2 s -2 15% V’V’ ERA-I m 2 s -2 V’V’ Transient eddy meridional wind variance 850hpa 7% std. dev Control-Smooth Transient eddy meridional wind variance 500hpa

BAROCLINICITY –WHAT COUNTERS THE EFFECT OF EDDIES IN REMOVING TEMPERATURE GRADIENT? –Latent heat release over western boundary currents helps maintain baroclinicity (Hoskins Valdes 1990,JAS) –Sensible heating maintains baroclinicity and anchors storm track (Nakamura et al 2008 GRL, Nonaka et al 2009, Sampe and Nakamura 2010 JCLIM, Ogawa et al 2012, Hotta and Nakamura 2011)

Baroclinicity Eady (1949)- growth rate of most unstable mode Baroclinicity Differences reduce to 7% at 500hPa (a)(b)(c) (e)(f)(d) 50% 30% 950hPa 850hPa SMTH ATL

Thermodynamic potential temperature budget at 950hPa. Units degC./day. DJF climatology mean (from 10 years) HOR. ADVECTION VER. ADVECTION -d/dy V’T’-d/dz W’T’ Condensational Heating sensible Heating BOUNDARY LAYER HEAT BUDGET –

Heat budget at 850hPa HOR. ADVECTION VER. ADVECTION -d/dy V’T’-d/dz W’T’ Condensational Heatingsensible Heating Thermodynamic potential temperature budget at 850hPa. Units degC./day. DJF climatology mean (from 10 years) FREE TROPOSPHERE HEAT BUDGET –

Vertically integrated total eddy heat flux divergence (color), for a) the SMTH case, b), ATL case and c) their difference. The corresponding climatological SST is shown as contours in a, b) and SST difference in c). SEE KWON AND JOYCE PRESENTATION (a)(b) (c) ATLSMTH TRANSIENT EDDY HEAT FLUX DIVERGENCE – CONTROL BY OCEAN FRONT

A few results from Southern Ocean focusing on South Indian Ocean.

ms -1 K 25%/83% SOUTHERN OCEAN CASE, JJA. Relationship of transient eddy heat flux to SST gradient. (a) V’T’ 850 DIFF (f) 33% BAROCLINICITY DIFF (b)  (SMOOTH)  C /100km (c) SST DIFFERENCE SST GRAD DIFF

(a) (b) SOUTHERN OCEAN CASE. Effective eddy diffusivity- eddy heat flux divided by mean temperature gradient m 2 s -1 SMTH CASE CONTROL CASE

Mean circulation response and interannual variability

20% reduction of zonal wind Fig. 1. Circulation response in the North Atlantic in DJF. a, c, d) show diffeernce between the ATL and SMTH runs for a) The sea level pressure, c) the 950hPa zonal wind and d) 500hPa geopotential height. Here stipling denotes 95% significance according to the t-test. b) shows the climatological mean zonal wind at 950hPa in the SMTH case. hPa gpmms -1 gpm (a)(b) (c) (d) ms -1 SEA LEVL PRESSURE DIFF U950 MEAN U950DIFF Z500 DIFF

30% Fig. 2. a) The climatological mean 250hpa zonal wind (U250) in the SMTH case for DJF over the North Atlantic. B) the difference in standard deviation of U250 between ATL and SMTH run. Stipling in b) denotes 95% significance according to the f-test. (a) (b) ms -1 U250 MEAN DIFF IN U250 INTERANNUAL STANDARD DEVIATION

Conclusions Ocean fronts induce large (~30%) changes in heat flux, moisture flux (~40%), and precip. in winter – Reaching well above the boundary layer – to > 500hPa –vorticity variance at surface (~30%) Smaller influence on wind (~10%) and sea level pressure (few% locally) variance Baroclinicity and eddy heat flux Maintained by sensible heating in boundary layer Condensational heating above that In Southern ocean v’T’~ dT/dy Results may be (very) model-dependent

Heat budget at 950hPa – diff unsmoothed minus smoothed

Heat budget at 850hPa – diff unsmoothed minus smoothed HOR. ADVECTION VER. ADVECTION -d/dy V’T’-d/dz W’T’ Condensational Heating sensible Heating

Sens. Ht. Fig. 14. Surface heating and tropospheric temperature differences between ATL and SMTH run. a, d, g) show surface sensible heating, surface latent heating, and precipitation respectively. b), e, and h) show temperature tendency at 950hPa (from sensible heat), and 850hPa and 500hPa (from condensational heating.) c), f), and I) show the corresponding air potential temperature. In right panels the corresponding SST anomalies of +2  C(-2  C) are shown as thick (thin) solid lines. (a)(b) T850 diff. (e) (b)(c) (d) (f) (g) (h)(i) Total Prec. dT/dt 950hPa dT/dt 850hPa T 950hPa T 850hPa dT/dt 500hPaT 500hPa Lat. Ht. Wm -2 mmday -1 Kday -1

Methods ERA, 5 day ERA, 30 day ERA, 90 day ERA-INTERIM “heat flux” V’T’ for DJF for different frequency bands ms -1 K

Frequency response 25deg.

Discussion Results get slightly shaky from here on…

To+2  To-2  T’=4  (a) Figure 17. Schematic showing scenarios for increases to v’T’ due to an ocean front. The solid lines are hypothetical isotherms deliniating a kink in a baroclinic zone (developing into a cold and warm front.) In a), b) there is no notable change to the displacement of the isotherm (no change to v’) Strong baroclinicity To+  To-  T’=2  (b) Weak baroclinicity Noting that v’T’ ~ baroclinicity (  T) leads to possible: Mechanism 1. Mixing length.

t=t0 t=t1t=t2 t=t0 t=t1 t=t2 (c) (d) Figure 17. Schematic showing possible scenarios for increases to v’T’ due to an ocean front. The solid lines are hypothetical isotherms deliniating a kink in a baroclinic zone (developing into a cold and warm front.) In c), d) there is a notable change to the displacement of the isotherm (change to v’). In c, d) a stronger baroclinicity leads to larger growth rate and displacements leads to larger changes in v’T’, particularly later in wave development. Weak baroclinicity Strong baroclinicity Eady growth rate would suggest… Mechanism 2. growth rate ~  T

A note on dynamical fields

Sea Level Pressure sub-5day variability and differences Add significance hPa ATL SMTH

Surface geostrophic V sub-5day differences ms -1 11% Surface geostrophic vorticity sub-5day differences s -1 30% ALL GEOSTROPHIC

Comparison with Aquaplanet Nakumura, Sampe et al. 2008, Sampe et al Maximum SST gradient changed by factor ~6 in zonal mean. SST anomalies 5  C or more (all one-sign) in zonal mean. Maximum SST gradient changed by factor ~3 locally (smaller in zonal mean). SST anomalies up to 5  C locally, more typically 2  C or less and have both signs.  C per 100km  C /100km SMOOTHCONTROL

Conclusions Ocean fronts induce large (~30%) changes in heat flux, moisture flux (~40%), vorticity variance, and precipitation in winter – Reaching well above the boundary layer – to > 500hPa Smaller influence on wind (~10%) and sea level pressure (few% locally) variance Comparison with reanalysis: – Model Heat flux agrees well with ERA-I and MERRA in Atlantic, is too high in Southern Ocean – Model wind variance is too high in both regions Results may be (very) model-dependent

Way ahead Moving to CAM-5, high-resolution – Improved convection schemes etc. – ¼ deg, 30 levels – maybe 1/8deg, more vertical levels Coupled simulations – ocean model 1/10 th deg. Parallel Ocean Program (POP) – 40+ years so far Spatial filtering on-line in model coupler (for SST, fluxes etc.) Show animation (if audience still awake)

Do ocean fronts influence storm tracks? Strong influence – Latent heat release over western boundary currents helps maintain baroclinicity (Hoskins Valdes 1990) – Ocean fronts essential to eddy variability associated with polar front jet (Nakamura et al 2008) Moderate Influence Ocean dynamics shifts location of storm track (Wilson et al. 2009, Brayshaw et al. 2011) No influence – Self-maintenance, eddies and mean jet, no role of ocean (Robinson 2006)

CAM model Courtesy Joe Tribbia, NCAR

From Minobe et al Low level convergence proportional to Laplacian of sea level pressure and to Laplacian of SST. CCSM. From Bryan et al FIG. 4. Laplacian of sea level pressure (color, 1029 Pa m22) and horizontal convergence of lowest model level wind (contours, interval s21, negative values dashed) for the winter season (Nov-Feb) in the Gulf Stream region: high-res CCSM4. Gulf Stream and atmospheric convection

Vertically integrated total eddy heat flux divergence Meridional component only i.e. d/dy v’T’ etc

Atlantic DJF: 850hPa V’T’ ControlV’T’ ERA-I V’V’ Control V’V’ ERA-I ms -1 K m 2 s -2 ms -1 K m 2 s -2 30% 15% In the right panel only differences significant at 95% are shown, and contours show SST differences of +/- 2  C from Fig. 1c. The number shown is the approx. ratio of the amplitude of the difference to the amplitude of the maximum in the smooth case, expressed as a percentage. V’T’ ERA-I ms -1 K V’V’ ERA-I m 2 s -2 V’T’ Diffn. V’V’ Transient eddy “heat flux” Transient eddy meridional wind variance 7% std. dev Control-Smooth

ms -1 C m 2 s -2 Atlantic DJF: 500hPa ms -1 K V’T’ ms -1 K V’T’ Control V’V’V’V’ Control m 2 s -2 25% 10% V’V’ ERA-I m 2 s -2 V’T’ ERA-I ms -1 K Transient eddy meridional wind variance Transient eddy “heat flux” Low-light – CAM wind variance (& heat-flux) is too high compared to ERA-I and MERRA. Therefore adding the ocean front worsens the comparison. Control-Smooth

Indian Ocean JJA: 850hPa V’T’ Control ms -1 K 30% V’T’ ERA-I ms -1 K V’T’ Diffn. 14% V’V’ Control V’V’ Diff’n m 2 s -2 V’V’ ERA-I m 2 s -2 In the right panel only differences significant at 95% are shown, and contours show SST differences of +/- 2  C from Fig. 1c. The number shown is the approx. ratio of the amplitude of the difference to the amplitude of the maximum in the smooth case, expressed as a percentage. Low-light – CAM wind variance (& heat-flux) is too high compared to ERA-I and MERRA. Therefore adding the ocean front worsens the comparison. ms -1 K Transient eddy “heat flux” Transient eddy meridional wind variance

Indian Ocean JJA: 500hPa In the right panel only differences significant at 95% are shown, and contours show SST differences of +/- 2  C from Fig. 1c. The number shown is the approx. ratio of the amplitude of the difference to the amplitude of the maximum in the smooth case, expressed as a percentage. V’T’ ControlV’T’ ERA-I ms -1 K 25% V’T’ ERA-I ms -1 K V’V’ Control V’V’ Diff’nV’V’ ERA-I 15% m 2 s -2 Transient eddy meridional wind variance Transient eddy “heat flux”