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= |10 5 η| 3/2 (H/50) 3 (V pot /70) 3 (1+0.1 V shear ) -2 η = absolute vorticity at 850hPa (s -1 ) H = relative humidity at 700hPa (%) V pot = potential intensity (m/s) V shear = magnitude of the vertical wind shear between 200 and 850hPa (m/s). K.A. Emanuel and D. Nolan, BAMS 85, (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, (1995). M. Bister and K.A. Emanuel, Meteor. Atm. Phys. 52, (1998)

Genesis Potential Climatology Febr. Sept.

Maximum Genesis Potential Index Climatology

Climatology - Basins Number of Tropical Cyclones Genesis Potential

Genesis Potential Anomalies & ENSO ASO (August - October) El Niño La Niña

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

Genesis Potential Index and Observations Difference: El Niño and La Niña – ASO II Genesis Potential Index Observed Track Density

Genesis Potential Anomalies & ENSO JFM (January-March) El Niño La Niña

Genesis Potential Index and Observations Difference: El Niño and La Niña - JFM Genesis Potential Index Observed Genesis Density

Genesis Potential Index and Observations Difference: El Niño and La Niña - JFM Genesis Potential Index Observed Track Density

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 variability Recalculated genesis potential index using climatology for 3 of the 4 variables and varying only the 4 th variable. Example: Vorticity, humidity, potential intensity: climatological values Vertical wind shear: observed values. Other combinations also tested (2 variables climatology, 2 observed values & 1 variable climatology, 3 observed values).

Genesis Potential – ENSO Difference ASO

Genesis Potential ENSO Variability ASO One 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.