The basic ingredients of the North Atlantic storm track David Brayshaw, Brian Hoskins and Mike Blackburn Brayshaw et al. (2008) The storm track response to idealized SST perturbations in an aquaplanet GCM, J. Atm. Sci, 65, Brayshaw et al. (2009) The basic ingredients of the North Atlantic storm track. Part I: land-sea contrast and orography, J. Atm. Sci, 66, Brayshaw et al. (2011) The basic ingredients of the North Atlantic storm track. Part II: sea surface temperatures, J. Atm. Sci, minor revisions. Sauliere et al. (2011) Further investigation of the impact of idealised continents and SST distributions on the Northern Hemisphere storm tracks, J. Atm. Sci, in prep.

Introduction NH storm tracks Strongest in winter Two primary regions Strong impact on European climate and weather Fig: NCEP storm tracks Fig from Ulbrich et al (2008). Winter storm track activity (2-6d BPF Geo 500)

From Ulbrich et al (2008). CMIP3 C20 ensemble NCEPCMIP3 - NCEP

From Ulbrich et al (2008). CMIP3 C20 ensemble NCEPCMIP3 - NCEP Errors in storm track: Location, strength, orientation

From Ulbrich et al (2008). CMIP3 C20 ensemble NCEPCMIP3 - NCEP CMIP3: C21 – C20 Errors in storm track: Location, strength, orientation

From Ulbrich et al (2008). CMIP3 C20 ensemble NCEPCMIP3 - NCEP CMIP3: C21 – C20 Errors in storm track: Location, strength, orientation Changes in storm track: Location, strength, orientation

From Ulbrich et al (2008). CMIP3 C20 ensemble NCEPCMIP3 - NCEP CMIP3: C21 – C20 Errors in storm track: Location, strength, orientation Changes in storm track: Location, strength, orientation What features determine the basic structure of the storm track? Land-sea contrast? Orography? SST gradients/ anomalies? Tropical circulation structures? In isolation and in combination

North Atlantic experimental design Full atmospheric GCM o resolution) Perpetual equinox Orographic, land surface and SST profiles based on “real” boundary condition data Accompanying full-aquaplanet simulations Land properties Orography SST profiles

Control simulation (no land) Statistically zonally symmetric storm track and jet Broad jet – “almost split” Storm track 850 hPa 2-6 day band pass filtered geopotential height variance (m 2 ) 30N 60N 90W0E 90E Baroclinicity Eady growth rate (850 hPa) 30N 60N 90W0E 90E

Extratropical land-sea contrast: A small rectangular continent Storm track 850 hPa Storm track weakened over land: reduced moisture availability increased surface drag Baroclinicity enhanced over land: stronger surface dT/dy increased surface drag Baroclinicity 850 hPa Colours =absolute values, contours = difference hatches (on storm track plots) = 90% signif

Extratropical land-sea contrast: A small rectangular continent Storm track weakened over land: reduced moisture availability increased surface drag Baroclinicity enhanced over land: stronger surface dT/dy increased surface drag Stronger storm track downstream Storm track 850 hPaBaroclinicity 850 hPa

Extratropical land-sea contrast: The “Atlantic” sector 30N 60N 90W0E Small rectangular continentSemi-realistic North American continent Storm track localised over North Atlantic ocean basin Storm track 850 hPa Colours =absolute values, contours = difference hatches (on storm track plots) = 90% signif

The Rocky Mountains Introduce orographic feature Temperature (shading) and streamfunction (contours) anomalies at 700 hPa Streamfunction at 1000 hPa

The Rocky Mountains Temperature (shading) and streamfunction (contours) anomalies at 700 hPa Streamfunction at 1000 hPa Northward deflection: isentropic ascent “over” the hill Southward deflection: isentropic descent “around the hill” or blocked return flow

The Rocky Mountains Cold dry air pool to NW Warm moist air to SE Enhanced baroclinicity along SW-NE axis of coastline Eady growth rate 850 hPa Temperature (shading) and streamfunction (contours) anomalies at 700 hPa Streamfunction at 1000 hPa

The Rocky Mountains Temperature (shading) and streamfunction (contours) anomalies at 700 hPa Streamfunction at 1000 hPa Enhanced storm growth along SW-NE axis of coastline Storm track 850 hPa

The Gulf Stream Storm track 850 hPa Gulf Stream in aquaplanet Tight SST gradient

The Gulf Stream Storm track 850 hPa Gulf Stream in aquaplanet Tight SST gradient Enhanced storm track

The Gulf Stream Storm track 850 hPa Gulf Stream in aquaplanet Tight SST gradient Enhanced storm track Gulf Stream in “semi-realistic”

The Gulf Stream Storm track 850 hPa Gulf Stream in aquaplanet Tight SST gradient Enhanced storm track Weaker storm track (BC reduced at coast) Gulf Stream in “semi-realistic”

Other features North Atlantic Drift (warm NE Atlantic):  weakens storm track  possible northward shift North Atlantic SSTs include (sub)tropics  Affects tropical circulation and subtropical jet  Relationship between subtropical jet and extratropical SST gradients important South American continent Southern Eurasia Pacific sector (with Jerome Sauliere, submitting to JAS.)

Extensions/links NERC Fellowship “Semi-realistic” framework Mediterranean storm track Relationship to poleward energy transport UK hydrological extremes project (NERC CWC) Other UK/European energy systems Holocene storm track changes (last 12,000 years) Brayshaw, Hoskins and Black; Phil Trans A (2010)

The basic ingredients of the North Atlantic storm track David Brayshaw, Brian Hoskins and Mike Blackburn Brayshaw et al. (2008) The storm track response to idealized SST perturbations in an aquaplanet GCM, J. Atm. Sci, 65, Brayshaw et al. (2009) The basic ingredients of the North Atlantic storm track. Part I: land-sea contrast and orography, J. Atm. Sci, 66, Brayshaw et al. (2011) The basic ingredients of the North Atlantic storm track. Part II: sea surface temperatures, J. Atm. Sci, minor revisions. Sauliere et al. (2011) Further investigation of the impact of idealised continents and SST distributions on the Northern Hemisphere storm tracks, J. Atm. Sci, in prep.