MOISTURE TRANSPORT IN THE SOUTHERN

HEMISPHERE BY EXTRA-TROPICAL CYCLONES

VICTORIA SINCLAIR1, GLENN CARVER2, TIMO VIHMA3 AND HEIKKI JÄRVINEN1

1. Department of Physics, University of Helsinki, Finland

2. OpenIFS, Numerical Aspects, ECMWF, Reading, UK

3. Finnish Meteorological Instititute, Finland

Thanks to Helen Dacre, Matt Hawcroft and Kevin Hodges, University of Reading, UK

• Motivation: Why investigate moisture transport by extra-tropical

cyclones in the southern hemisphere?

• Questions

• Tools: ERA-Interim and OpenIFS

• Method: how to calculate the cyclone-related meridional moisture flux

• Results: cyclones transport 60 – 80% of moisture

strong cyclones are disproportionally important

• Conclusions

OVERVIEW

• The storm tracks are likely to change position and intensity in the future

• How will changes in the intensity and track of extra-tropical cyclones affect:

(1) the amount of meridional moisture transport?(2) precipitation patterns in Antarctica, Australia, South America etc. ?(3) The surface mass balance of Antartica?

WHYSTUDYMOISTURETRANSPORT BYCYCLONES?

• What percentage of the meridional moisture transport in the southern hemisphere is due to extra-tropical cyclones?

• How important are the strongest extra-tropical cyclones in transporting moisture?

QUESTIONS

• ERA – INTERIM reanalysis• 1979 – 2013 (35 years)• T255 (~0.75 degrees), 60 levels• Based on IFS version CY31r2

• OpenIFS (Open Integrated Forecast System)• Version of ECMWF’s IFS which is available to academica under licence• Based on IFS version CY38r1• Forecast-only capability of IFS (no data assimilation, wave or ocean model)• http://www.ecmwf.int/en/research/projects/openifs

TOOLS

• Mainly consider ERA-Interim (same method can be applied to OpenIFS)• Calculate the total vertically integrated meridional moisture flux (MMF)

between 1000 hPa and 300 hPa

• Compare this total MMF to the “cyclone-related” MMF

• How do we calculate the cyclone-related MMF?

METHOD

• Typically cyclone-related moisture flux is taken to be the moisture fluxdue to transient eddies (TE).

• Disadvantage: Cannot easily subset cyclones e.g. the strongestcyclones, those that travel more poleward etc.

• Need a new approach

CYCLONE RELATEDMOISTURE FLUX

• Use TRACK (Hodges, 1995; Hodges 1999) – an objective feature tracking algorithm

• Aim to identify synoptic-scale cyclones

• 850-hPa relative vorticity truncated to T42 resolution

• Filter tracks: last at least 2 days and travelat least 1000 km

• Track all cyclones in the southernhemisphere

• Focus on cyclones south of 30S

IDENTIFY CYCLONESseason Mean # cyclones

DJF 381MAM 431JJA 459SON 428

CREATEA CYCLONE MASK

Blue * represent the localised 850‐hPa relative vorticity maxima (e.g. cyclones)

• Apply a masking methodpreviously used by Hawcroftet. al. (GRL, 2012)

• For each timestep, find the location of all cyclonesidentified by TRACK

• Approximate eachcyclone by a circle

• Mask the areas whichare ”cyclones”

CREATEA CYCLONE MASK

Red: mask = 1White: mask = 0 

Cyclone mask

• Calculate the vertically integratedmeridional moisture flux due to cyclones

CREATEA CYCLONE MASK

Blue: moisture transport towardsthe polesOrange: moisture transport towardsthe equator

• What radius should the circle have?(1) Constant radius: try 8, 10, and 12 degrees(2) Rossby radius of deformation (Lr = NH / f)

BUT HOW BIG IS A CYCLONE?

RESULTS

TOTAL VS. CYCLONE MMF

Assuming a cycloneradius of 12 degrees

Zonal and temporal means (by season)Net meridional moisture transport 

total cyclones

RATIO OF POLEWARDTRANSPORT DUETO CYCLONES

Assuming a cycloneradius of 12 degrees

Between 50 – 70S cyclones transport at least 80% of the totalnet MMF in allseasons

SENSITIVITYTO CYCLONERADIUS DEFINITION

Lr = Rossby Radius of deformation

Cyclones equatorward of 30oS areassumed to have the same radius as ifthey were at 30oS

MAXIMUM RELATIVEVORTICITY DISTRIBUTION

JJA50%  25%  10%

How importantare the strongestcyclones in the total meridionalmoisturetransport?

IMPORTANCE OF THE STRONGEST CYCLONESBased on cyclones with R = Rossby radius of deformation

At 60oS,  the strongest 10 % of cyclones transport 23% of the moisture, the strongest 25% transport 53 % 

(Results are similar for R = 12 degrees: strongest 10%: 27%, strongest 25%: 59%)

OPENIFS VS ERA-INTERIM

3 months (March, April, May 2009)

Similar number of cyclone tracks in ERA-I (448) and OpenIFS (434)

ERA has more weak storms

OIFS has more “quite strong” storms

• Longer simulation with OpenIFS• Sensitiviy experiments with OpenIFS• What controls the amount of meridional moisture transport by cyclones in

the southern hemisphere?• Sea surface temperatures• Sea ice distribution

FUTURE PLANS

• Cyclones are the dominantprocess for transporting moisturepolewards

• 60 – 80 % of the total vertically integratedpoleward moisture flux in the southernhemisphere mid-latitudes is due to cyclones

• Strong cyclones play a disproportionately large role

• The strongest 10% of cyclones in terms of relative vorticity (in JJA) transport 23% of the moisture; the strongest 25% transport 53%

CONCLUSIONS

THANKYOUVICTORIA.SINCLAIR@HELSINKI.FI

• Use TRACK (Hodges, 1995; Hodges 1999) – an objective feature tracking algorithm

• Aim to identify synoptic-scale cyclones• 850-hPa relative vorticity truncated to T42

resolution• Filter tracks: last 2 days and travel at least

1000 km• Track all cyclones in the southern

hemisphere

1. IDENTIFY CYCLONESNumber of cyclones in JJA

season Mean # cyclonesDJF 381MAM 431JJA 459SON 428

STORM DENSITY

Spiral storm track is evident in winter (JJA)

Agrees well with previous studies

CYCLONE MERIDIONALMOISTURE FLUX

• Using ERA-40 data Tietäväinen and Vihma (2008) found that transienteddies are significant in transporting mpoisture poleward

PREVIOUS RESULTS

Transient eddies

total

Tietäväinen and Vihma (2008) 

RELATIVE VORTICITYDISTRIBUTIONS

DJF JJA

DOES RELATIVE VORTICITYCORRELATE WITH MMF?Cyclones near the equator: weakvorticity and transport moistureequatorward

Net MMF doesnot dependstrongly on the size of the cyclone Only a subset of cyclones included! 

Every 25th cyclone from JJA

Strongercyclones don’talways lead to more MMF transport 

MMF AND DISTANCETRAVELLED

Cyclones thattravel the farthestpoleward havethe largest MMF and largevorticitymaximum