the basic ingredients of the north atlantic storm track david brayshaw, brian hoskins and mike...

The basic ingredients of the North Atlantic storm track

David Brayshaw, Brian Hoskins and Mike Blackburnd.j.brayshaw@reading.ac.uk

• Brayshaw et al. (2008) The storm track response to idealized SST perturbations in an aquaplanet GCM, J. Atm. Sci, 65, 2842-2860.

• Brayshaw et al. (2009) The basic ingredients of the North Atlantic storm track. Part I: land-sea contrast and orography, J. Atm. Sci, 66, 2539-2558.

• 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 C20ensemble

NCEP CMIP3 -NCEP

From Ulbrich et al (2008).

CMIP3 C20ensemble

NCEP CMIP3 -NCEP

Errors in storm track:Location, strength, orientation

From Ulbrich et al (2008).

CMIP3 C20ensemble

NCEP CMIP3 -NCEP

CMIP3:C21 – C20

Errors in storm track:Location, strength, orientation

From Ulbrich et al (2008).

CMIP3 C20ensemble

NCEP CMIP3 -NCEP

CMIP3:C21 – C20

Errors in storm track:Location, strength, orientation

Changes in storm track:Location, strength, orientation

From Ulbrich et al (2008).

CMIP3 C20ensemble

NCEP CMIP3 -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 (HadAM3 @1.5o resolution)

Perpetual equinox

Orographic, land surface and SST profiles based on “real” boundary condition data

Accompanying full-aquaplanet simulations

Land properties

OrographySST 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 (m2)

30N

60N

90W 0E 90E

Baroclinicity

Eady growth rate (850 hPa)

30N

60N

90W 0E 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 = differencehatches (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 hPa Baroclinicity 850 hPa

Extratropical land-sea contrast:The “Atlantic” sector

30N

60N

90W 0E

Small rectangular continent Semi-realistic North American continent

Storm track localised over North Atlantic

ocean basin

Storm track 850 hPaStorm track 850 hPa

Colours =absolute values, contours = differencehatches (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 NWWarm 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 trackEnhanced 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 Blackburnd.j.brayshaw@reading.ac.uk

• Brayshaw et al. (2008) The storm track response to idealized SST perturbations in an aquaplanet GCM, J. Atm. Sci, 65, 2842-2860.

• Brayshaw et al. (2009) The basic ingredients of the North Atlantic storm track. Part I: land-sea contrast and orography, J. Atm. Sci, 66, 2539-2558.

• 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.