Genesis Potential Index and ENSO

Suzana J. Camargo

Collaborators:

Kerry A. Emanuel Program in Atmospheres, Ocean and Climate

Massachusetts Institute of Technology, Cambridge, MA Adam H. Sobel Department of Applied Physics and Applied Mathematics

Department of Earth and Environmental Sciences

Columbia University, New York, NY

Motivation

Genesis potential index: dependent on large scale fields developed using statistical fitting to observed genesis of tropical cyclones globally.

Examine how the Genesis potential index describes ENSO – Tropical Cyclone (TC) variability globally.

Comparison with various tropical cyclone indices in different basins.

Possible use in forecasting seasonal TC variability using large scale fields from AGCMs.

Analysis of which variables are responsible for the ENSO response in the Genesis Potential index.

Genesis Potential Index

Refinement of Gray’s tropical cyclone genesis index using Reanalysis data (Emanuel & Nolan 2004).

GP= |105 η|3/2 (/50)3 (Vpot/70)3 (1+0.1 Vshear)-2

η = absolute vorticity at 850hPa (s-1) = relative humidity at 700hPa (%)

Vpot = potential intensity (m/s)

Vshear = magnitude of the vertical wind shear between 200 and 850hPa (m/s).

K.A. Emanuel and D. Nolan, BAMS 85, 667-668 (2004).

Potential Intensity

Variables that enter the definition of the potential

intensity (taking into account dissipative heating):

SST – sea surface temperature SLP – sea level pressure CAPE – convective available potential energy Atmospheric temperature (various pressure levels)

Mixing ratio (various pressure levels)

K.A. Emanuel, JAS 52, 3969-3976 (1995).M. Bister and K.A. Emanuel, Meteor. Atm. Phys. 52, 233-240 (1998)

Genesis Potential Climatology

Febr.

Sept.

Maximum Genesis Potential Index Climatology

Climatology - Basins

Number ofTropical Cyclones

Genesis Potential

Genesis Potential Anomalies & ENSOASO (August - October)

El Niño

La Niña

Genesis Potential Index and ObservationsDifference: El Niño and La Niña - ASO

GenesisPotential

Index

ObservedGenesisDensity

Genesis Potential Index and ObservationsDifference: El Niño and La Niña – ASO II

GenesisPotential

Index

ObservedTrack

Density

Genesis Potential Anomalies & ENSOJFM (January-March)

El Niño

La Niña

Genesis Potential Index and ObservationsDifference: El Niño and La Niña - JFM

GenesisPotentialIndex

ObservedGenesisDensity

Genesis Potential Index and ObservationsDifference: El Niño and La Niña - JFM

GenesisPotentialIndex

ObservedTrackDensity

Interannual Variability - Atlantic

Correlations: Genesis Potential & Number of Tropical Cyclones

Eastern North Pacific

Atlantic

Correlations: Genesis Potential & Number of Tropical Cyclones

Genesis Potential Index Difference: El Niño and La Niña - ASO

Various Correlations:Genesis Potential - South Pacific

Number of TC days

Correlations

Tropical Cyclone variability indices variables with positive correlation with Genesis Potential Index in various basins:

Number of tropical cyclones Number of named tropical cyclones Number of hurricanes Number of major hurricanes Number of Tropical Cyclone days (track density) ACE (Accumulated Cyclone Energy)

Consistent highest correlations (for all variables): South Pacific Atlantic Eastern Pacific Western Pacific

pre-typhoon season (FMA-MJJ) Eastern Part – year around Western Part – JFM-AMJ, OND-NDJ, Year

Other basins: specific TC variables for definite seasons, mainly pre or post the peak of the tropical cyclone season.

Example: Australian basin: OND - peak JFM.

Interannual Variability

ENSO connection

Variables responsible for ENSO variabilityRecalculated genesis potential index

using climatology for 3 of the 4 variables and varying only the 4th variable.

Example: Vorticity, humidity, potential intensity: climatological

values Vertical wind shear: observed values.

Other combinations also tested (2 variables climatology, 2observed values & 1 variable climatology, 3 observed values).

Genesis Potential – ENSO Difference ASO

Genesis Potential ENSO Variability ASOOne variable observations & 3 variables climatology

HUMIDITY VORTICITY

Potential Intensity Vertical Wind Shear

Conclusions

Genesis Potential index pattern reproduces well known ENSO effects on TC activity.

Genesis potential index and number of tropical cyclones per basin is correlated in basins with large ENSO influence (South Pacific, Atlantic).

Genesis potential index is correlated with various tropical cyclone activity indices (number of hurricanes, ACE, number of major hurricanes, track density values – number of TC days).

Conclusions II

Most important variables responsible for genesis potential shifts can be identified in different regions: Atlantic: wind shear (mainly) and SST (PI). Western North Pacific: combination of humidity,

vorticity and wind shear Eastern North Pacific: wind shear and SST (PI).

Possible application – forecasting TC activity using Genesis Potential index in AGCMs.