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Mechanisms and Climatological Structure of the Easterly Jet over Africa and the Atlantic. NASA/GSFC/GMAO: Man-Li C. Wu Siegfried D. Schubert, Max Suarez Randy Koster, Winston Chao GSFC/GLA/GEST: Oreste Reale University at Albany: Chris Thorncroft

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mechanisms and climatological structure of the easterly jet over africa and the atlantic

Mechanisms and Climatological Structure of the Easterly Jet over Africa and the Atlantic

NASA/GSFC/GMAO: Man-Li C. Wu

Siegfried D. Schubert, Max Suarez

Randy Koster, Winston Chao

GSFC/GLA/GEST: Oreste Reale

University at Albany: Chris Thorncroft

October 5, 2010 : NCU, Chungli, Taiwan

slide2

Outline

I Introduction

II Basic Structure and Variability of the Jet

in MERRA and Other Reanalyses

III Mechanisms

Wave and Jet Interaction

Mid-Latitude Influence

ENSO

MJO

III Validation/Case study of Hurricane Helene

IV Conclusions

schematic diagram
Schematic diagram

Our goal is to yield an integrative understanding of Atlantic storm and hurricane development. The schematic diagram here shows the interrelationship among the phenomena of interest to our studies.

slide4

AEJ Climatology (1989-2001)

u600

Reanalysis minus MERRA

MERRA

ERA-

Interim

ERA-40

NCEP-R2

JRA-25

Over all, MERRA, ERA-40 (ERA-Interim) are closer to each other

than the others

slide5

Resolution

MERRA: 0.5x0.625 degree

ERA-40: T159L60 (~125km or 1.1 degree)

NCEP-R2: T62L28 (~300Km or 2.5degree)

JRA-25 (T106L40 (~190km or 1.5degree)

ERA-Interim T255L60 (~80Km or 0.75 degree)

slide6

Climatology of AEJ (shaded), and K(y), and d[u]dy (contour) – (1980-2001)

K(y)=d(f(y)-d[u]/dy)/dy

d[u]/dy

MERRA

ERA-40

NCEP-R2

JRA-25

Note collocation of the region that favors barotropic instability with main development region of Atlantic cyclogenesis

slide7

Cross Section of AEJ (shaded), and K(y) and du/dy (contour) at 20W

K(y)=d(f(y)-d[u]/dy)/dy

d[u]/dy

MERRA

ERA-40

NCEP-R2

JRA-25

slide9

La Nina minus El Nino

v700 hPa variance based on MERRA 6-hrly data for JAS 1980-2001

02_10day

v-variance

Spatial distribution suggests there might be two different sources of disturbances.

slide10

La Nina minus El Nino

Variance based on MERRA 6-hrly data for JAS 1980-2001

v700-variance

02_10day

v-variance

06_09day

2.5_6day

slide11

La Nina minus El Nino

700 hPa Variance for JAS 1980-2001

2.5_6day

06_09day

MERRA

ERA-40

v-variance

NCEP-R2

JRA-25

slide12

Climatology of v700 Variance (1980-2001)

2.5_6 day

Percentage (%)

MERRA

ERA-40

NCEP-R2

JRA-25

slide13

La Nina minus El Nino

Climatology of Precipitation Variance (1980-2001)

Percentage (%)

2.5_6 day

MERRA

ERA-40

NCEP-R2

JRA-25

ii mechanisms

II Mechanisms

Wave and Jet Interaction

slide16

Time evolution from -3day to 1day based on MERRA data for JAS

u700 (shaded), streamfunction (contour)

55W and 22N

25W and 12N

day

-3

-2

-1

0lag

1

2

Jet serves as a wave guide/steering flow for the easterly waves

slide17

Time evolution from -3day to 2day based on MERRA data for JAS

u700 (shaded), -u’v’d[u]/dy (contour)

25W and 12N

day

55W and 22N

-3

-2

-1

0lag

1

2

Barotropic energy conversion dominant over the southern side of the Jet and also over the eastern side of the MDR

slide18

Time evolution from -3day to 1day based on MERRA data for JAS

u700 (shaded), -w’t’ (contour)

25W and 12N

day

55W and 22N

-3

-2

-1

0lag

1

2

Baroclinic energy conversion dominants over northern Africa

slide19

Composites: [u] (shaded) and Energy Conversion (contour)

5W and 26N

Land

25W and 12N

Ocean

-u’v’d[u]/dy

-w’t’

-v’t’d[t]/dy

ii mechanisms1

II Mechanisms

Mid-Latitude Influence

(6-9 days)

revisit variance distribution and time scales possible mid latitude influence
Revisit variance distribution and time scales

-possible mid-latitude influence

slide22

La Nina minus El Nino

Variance based on MERRA 6-hrly data for JAS 1980-2001

2.5_09day

v-variance

06_09day

2.5_6day

slide23

La Nina minus El Nino

Marginal Hilbert Spectra: v700 based onMERRA 6-hrly data for JAS in Neutral Years

20W and 26N

6-8day

~24day

v-variance

Spectral Density

Frequency: cycle/JAS

slide25

Lag Correlation Coefficient at 700 hPa

Location of index v700 is marked on panel (0lag)

2.5_6day

20W and 12N

06_09day

20W and 26N

day

-3

-2

-1

0

1

2

3

slide26

Lag Correlation Coefficient at 700 hPa; All data in 06_09

Index: v700 at 20W and 26N

Streamline of u and v, v (shaded)

day

-6

-2

-5

-1

0

-4

-3

1

slide27

Lag Correlation Coefficient at 700 hPa; All data in 06_09

Index: v700 at 20W and 26N

Streamline of u and v, precitation(shaded)

day

-6

-2

-1

-5

0

-4

-3

1

slide28

V-variance (2.5_6day) in JAS in Neutral Years between 1980-2001 based on MERRA

V-variance (2.5_6day) in JAS in Neutral Years between 1980-2001 based on MERRA

Wave CoherenceActivity

MERRA

ERA-40

slide29

Hilbert Spectra based on MERRA v700 data at 45W and 22N for JAS in Neutral Years between 1980-2001

60day

2.5-6day

6-9day

slide30

Composites of u, v, and z using v700 (2.5_6day) at (45W and 22N) as index for JAS in Neutral Years. Each panel is 1day apart from top down and left right. z(shaded), streamline (contour) of u and v at 700 hPa (2.5_9day)

day

day

-5

1

2

-4

-3

3

-2

4

-1

5

6

0-lag

slide32

1980-2001

Neutral Years: 1980, 81, 83, 84, 85, 89, 95, 96, 00, 01

El Nino Years: 1982, 86, 87, 90, 91, 92, 93, 94, 97

La Nina Years: 1988, 89, 99

slide33

Seasonal Genesis Parameters (SGP, Gray, 1975) based on MERRA (1980-2001)

El Nino

La Nina

SGP

Wind Shear

(u200 – u850)

Within 10m/s

Weighted RH (%)

(700 to 500 hPa)

SST > 26C

slide34

JAS Climatology (1980_2001) Jet Difference at 600 hPa

MERRA

ERA-40

NCEP-R2

JRA-25

La Nina minus

El Nino

During La Nina Years, Jet Moves Northward in

all reanalyses

El Nino

minus

Neutral

La Nina

minus

Neutral

slide35

JAS Climatology (1980_2001): V-variance (2.5-6day) Difference at 600 hPa

MERRA

ERA-40

NCEP-R2

JRA-25

La Nina minus

El Nino

El Nino minus

Neutral

La Nina minus

Neutral

v600 variance (2.5-6day) increased during the La Nina in all Reanalyses

slide37

Seasonal Genesis Parameters (SGP, Gray, 1975) based on MERRA

Neutral Years

SGP

Wind Shear

(u200 – u850)

Within 10m/s

Weighted RH (%)

(700 to 500 hPa)

SST > 26C

slide38

Time Evolution from -10day to 6day MERRA JAS

Using v700 (2.5_6day) at 15W and 14N as index to composite vlp200 (20-90days)

1980-2001

Neutral Years

day

-10

-8

-6

-4

-2

0

2

4

6

slide39

JAS Climatology based on MERRA data

VLP200 (20_90day) for Neutral Years

Negative Phase over Africa/Atlantic and Adjacent Regions

Positive Phase over Africa/Atlantic and Adjacent Regions

v700 var (2.5-6day) Dif (Negative Phase minus Positive Phase)

1980-2001

Neutral Years

slide40

Mechanism for Neutral Years

MJO Impact: Negative Phase minus Positive Phase

MERRA

ERA-40

NCEP-R2

JRA-25

Phase

dif

Var

dif

When negative phase of the MJO is over Africa and adjacent regions, v600 variance (2.5_6) is increased over Western Africa and Eastern Atlantic

slide41

Mechanism for Neutral Years

MJO Impact on Precipitation: Negative Phase minus Positive Phase

Neg-Phase

Pos-Phase

Dif

VLP200

Precipitation

Precipitation variance (2.5-6day) increased with

negative phase of the MJO

slide42

Conclusions (1 of 2)

  • AEJ:
  • All four reanalyses produce an AEJ, but they differ in details
  • MERRA is a little poleward and has less vertical tilt compared
  • to the others and is also stronger than the others. The NCEP AEJ
  • is shifted to the south. The JRA-25 AEJ is the weakest. MERRA
  • and ERA-40 (ERA-Interim) are closer to each other than the others
  • Gradients associated with energy conversions exhibit even
  • more differences, though again MERRA and ERA-40 (and
  • ERA-Interim) are closer
  • Variance:
  • Spatial distribution of wave activity has some frequency
  • dependence that is robust across the reanalyses: most regions
  • characterized by 2.5-6 days, but one region in particular is
  • dominated by 6-9 day time scales with links to middle latitudes
slide43

Conclusions (2 of 2)

  • Mechanisms:
  • All reanalyses reveal similar impacts from ENSO. The Jet is more poleward
  • and variance is larger over the MDR when comparing La Nina to El Nino years.
  • All reanalyses also show that the MJO enhances wave activity over the eastern
  • side of the MDR during the negative phase (ascending branch) – this is seem most
  • clearly when limiting the composites to ENSO neutral years
  • Wave-Jet interaction
  • •The Jet serves as a wave guide or provides the steering flow for the wave
  • disturbances.
  • •Barotropic energy conversion dominants over the eastern side of MDR, and
  • baroclinic energy conversion dominants on the northern side of the Jet over the
  • western Africa
  • •Some of the waves appear to have a middle latitude influence contributing to
  • the local maximum of 6-9 day variability off the coast of Northern Africa, as
  • well as play a role in enhancing 2.5-6 day variability over the central Atlantic
  • Over all, MERRA products are useful new resource for carrying out studies to understand AEJ/AEWs and their downstream effects.
hurricane helene 2006

Validation/A case study

to show MERRA’s ability to produce Hurricanes

Hurricane Helene, 2006

slide45

Radiosonde at Cape Verde Island (23.5W, 14.9N)

Meridional Wind (m/s)

Zonal Wind (m/s)

Obs

MERRA

ERA-

Interim

NCEP-R2

JRA-25

Aug 18

Aug 18

Sep 13

Sep 13

Aug 21

Debby

Sep 12/13

Helene

slide46

Time-Longitude Cross Section (11N-16N) at 600 hPa

Debby

v

Helene

rotational

vorticity

RH

wind

shear

speed

slide49

MERRA data at 12Z and Hurricane Helene, 2006, Starting on Sept 11

AEJ Axis (black), Curvature Vorticity (shaded), and Cyclone (black contour)

slide50

MERRA data at 12Z and Hurricane Helene, 2006, Starting on Sept 11

AEJ Axis (black), Vorticity Gradient (shaded), and Cyclone (black contour)

slide51

MERRA data at 12Z and Hurricane Helene, 2006, Starting on Sept 11

AEJ Axis (black), Wind Shear (shaded), and Cyclone (black contour)

slide52

MERRA data at 12Z and Hurricane Helene, 2006, Starting on Sept 11

AEJ Axis (black), Wind Speed (shaded), and Cyclone (black contour)

slide53

EOF

v700 (2.5_09day) -- based on MERRA

slide55

La Nina minus El Nino

Variance based on MERRA 6-hrly data for JAS 1980-2001

v700-variance

2.5_09day

v-variance

06_09day

2.5_6day

slide56

Lag Correlation Coefficient between v and u & v at 700 hPa,

v(shaded), streamline of u and v at 45W and 22N

2.5_6day

06_09day

day

-3

-2

-1

0

1

2

3

slide57

Lag Correlation Coefficient at 700 hPa

Index at 20W and 26N; All dada in 06_09 day

day

v (shaded)

u (shaded)

-3

-2

-1

0

1

2

3

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