Subseasonal Variability of Hurricane Activity

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Subseasonal Variability of Hurricane Activity

Kathy Pegion

Center for Ocean-Land-Atmosphere Studies

Philip Pegion (CPC),

Tim DelSole (COLA), & Mihai Sirbu (COLA)

Tropical cyclones tend to cluster in time and space on subseasonal timescales.

Is this predictable?

Can it be forecasted with a dynamical model?

Counts of Atlantic TS and Hurricanes every 6 hrs

Courtesy of the Clivar MJOWG 3

Week 2 Outlook – Valid: Sep 23 – 29, 2008 Issued: 9/15

1. An increased chance for tropical cyclogenesis for the eastern Pacific, western Gulf Of Mexico and western Caribbean. The favorable phase of the MJO increases the threat for tropical development during the period. Confidence: Moderate

2. An increased chance for above-average rainfall for the eastern Pacific, southern Mexico and parts of Central America. The enhanced phase of the MJO is expected to result in wet conditions during the period. Confidence: Moderate

3. An increased chance for below-average rainfall for the equatorial Indian Ocean and western Indonesia. The suppressed phase of the MJO is expected to result in dry conditions during the period. Confidence: Moderate

4. An increased chance for above-average rainfall stretching from Southeast Asia into the western Pacific. The enhanced phase of the MJO is expected to result in wet conditions during the period. Confidence: Moderate

5. An increased chance for tropical cyclogenesis for the South China Sea and the far western Pacific. The favorable phase of the MJO increases the threat for tropical development during the period. Confidence: Moderate

SEE TEXT NOTATION: Conditions are expected to begin to become more favorable for tropical cyclogenesis across the deep tropical Atlantic Ocean late during the period but development is more likely during week 3.

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1. Can the relationship between MJO and tropical cyclone activity be captured using common hurricane indices (eg. SGP and GPI)?

2. How well does the CFS simulate the relationship?

3. Can forecasts of these indices by the CFS be used to predict where tropical cyclone formation is more/less likely on 1-4 week timescales?

Questions

Seasonal Genesis Parameter (Gray 1979)

SGP=vorticity*coriolis*shear*thermal energy*moist stability *RH

Genesis Potential Index (Emanuel and Nolan 2004) GPI=absolute vorticity*RH * Potential Intensity * shear

Hurricane Indices

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Data

NCEP Reanalysis-2 1979-2007

CFS03 (T62L64) 52-year simulation

Filtering

1. Five-day running mean applied to fields prior to calculating SGP or GPI

2. ENSO removed from fields prior to calculating SGP or GPI using linear regression with Nino 3.4

All tropical storm formations from the Hurricane Best Track Database 1979-2007

Climatology and Standard Deviation of SGP & GPI

NCEP Reanalysis-2

RMM1 9

RMM2

Phase 1

Phase 2 Phase 3

Phase 4 Phase 5 Phase 6

Phase 7

Phase 8

Maritime Continent

Indian Ocean Western Pacific

W. Hemisphere & Africa

MJO Index

SQRT(RMM1²+RMM2²) > 1

Real-time Multivariate MJO Index (Wheeler and Hendon 2004)

(obtained from Matt Wheeler, BMRC; 1975-2000)

1. Based on combined EOFs of U200, U850, and OLR

2. First two PC timeseries (RMM1 & RMM2) represent the MJO amplitude and phase

Composite OLR (W/m²) anomalies (Jun-Nov)

• Large-scale region of

enhanced/suppressed convection

• Eastward propagation

• Note enhanced/supressed

convection in Central America &

Atlantic

Observed CFS03

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Composite anomalies (Jun-Nov) Based on observed RMM

•SGP & GPI vary with phase of MJO

•SGP & GPI are very similar

R2 SGP R2 GPI

Obs OLR

Composite SGP & GPI anomalies (Jun-Nov) Reanalysis-2 (1979-2007)

Location of tropical storm formations

from Hurricane Best Track Database 1979-2007 obtained from Unisys

• Actual formations are generally

consistent with regions of +/- SGP/GPI anomalies

SGP GPI

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Quantifying the Relationship

H0: P(Hur Index | MJO Phase) = P(Hur Index)

Hurricane Index: GPI or SGP Terciles MJO Phase : 8 phases + null phase



M  P

HurIndex



^{log P}

P

P

Mutual Information

0 5 10 15 20 25 30 35

1 2 3 4 5 6 7 8 9

all year Jun-Nov

P(MJO phase)

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GPI SGP

33% (Jun-Nov) 33% (Jun-Nov)

67% (Jun-Nov) 67% (Jun-Nov)

P(Hur Index) Each Tercile = 33%

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P(GPI, MJO Phase)

GPI <= 33%ile

33 %ile >

GPI < 66%ile GPI >= 66%ile

Phase 1 Phase 2 Phase 3

Phase 4 Phase 5

Phase 6 Phase 7 Phase 8

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Mutual Information between Hurricane Index and MJO

GPI SGP

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NCEP/R2 CFS

SGP Climatology

(Jun-Nov)

SGP Standard Deviation

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Composite SGP (Jun-Nov)

CFS is generally able to capture the overall behavior of the

relationship between the MJO and SGP

NCEP/R2 CFS03/T62

NCEP/R2 CFS03/T62 ERA40

MJO Phase 21

No/Weak MJO

0 5 10 15 20 25 30 35 40 45

1 2 3 4 5 6 7 8 9

R2:all year R2:Jun-Nov CFS:Jun-Nov

P(MJO phase)

33% (Jun-Nov)

67% (Jun-Nov)

CFS SGP

33% (Jun-Nov)

67% (Jun-Nov)

P(Hur Index) Each Tercile = 33%

R2 SGP

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P(Hur Index) Each Tercile = 33%

CFS GPI

33% (Jun-Nov)

67% (Jun-Nov)

33% (Jun-Nov)

67% (Jun-Nov)

R2 GPI

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CFS SGP

Mutual Information between SGP and MJO

R2 SGP

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Mutual Information between GPI and MJO

R2 GPI CFS GPI

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Conclusions

1. SGP & GPI indices capture the relationship between MJO and tropical cyclone development.

2. The CFS is generally able to capture the relationship 3. Mutual information between SGP and MJO shows a

significant relationship in the eastern Pacific.

4. Mutual information between GPI and MJO shows a significant relationship throughout the domain.

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Week 2 Outlook – Valid: Sep 23 – 29, 2008

Please note: Confidence estimates are subjective in nature and are not based on an objective scheme. The estimates are given to provide additional information to the user.

Issued: 9/15

1. An increased chance for tropical cyclogenesis for the eastern Pacific, western Gulf Of Mexico and western Caribbean. The favorable phase of the MJO increases the threat for tropical development during the period. Confidence: Moderate

2. An increased chance for above-average rainfall for the eastern Pacific, southern Mexico and parts of Central America. The enhanced phase of the MJO is expected to result in wet conditions during the period. Confidence: Moderate

3. An increased chance for below-average rainfall for the equatorial Indian Ocean and western Indonesia. The suppressed phase of the MJO is expected to result in dry conditions during the period. Confidence: Moderate

4. An increased chance for above-average rainfall stretching from Southeast Asia into the western Pacific. The enhanced phase of the MJO is expected to result in wet conditions during the period. Confidence: Moderate

5. An increased chance for tropical cyclogenesis for the South China Sea and the far western Pacific. The favorable phase of the MJO increases the threat for tropical development during the period. Confidence: Moderate

SEE TEXT NOTATION: Conditions are expected to begin to become more favorable for tropical cyclogenesis across the deep tropical Atlantic Ocean late during the period but development is more likely during week 3.

Courtesy of CPC from the Global Tropical Benefits/Hazards Website (http://www.cpc.noaa.gov/products/precip/CWlink/ghazards/ghaz.shtml)

Caveats & Future Work

1. Indices: maybe the hurricane indices and/or the MJO index are not the right one, also test different

variables.

2. So far, we have only quantified one piece of the problem. Have not quantified P ( TC | Hur Index), P(TC | MJO).

What we really want to know is:

H0: P(TC | Hur Index & MJO) = P(TC)

3. What is the prediction skill of SGP and GPI and are

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Week-1 Week-2

Week-3 Week-4

Correlations between CFS forecasts and NCEP/R2 (Aug-Oct 2005-2007)

Full Fields of SGP, not anomalies, no MJO filter