Benjamin A. Schenkel (bschenkel@fsu.edu) and Robert E. Hart

AMS Tropical Conference 2012

Department of Earth, Ocean, and Atmospheric Science

The Florida State University

Assessing the Climate Footprint of Tropical Cyclones: Pertinent Players or

Irrelevant Pawns?

Research Sponsored by NASA Earth and Space Science Fellowshipand NSF Grant #ATM-0842618

Motivation

Assessing the TC Climate Footprint Benjamin A. Schenkel and Robert E. Hart The Florida State University 2/13

Introduction Methodology Large Scale Impacts of TCs

Motivation

Assessing the TC Climate Footprint Benjamin A. Schenkel and Robert E. Hart The Florida State University 2/13

Introduction Methodology Large Scale Impacts of TCs

Motivation

???

How do tropical cyclones (TCs) impact the large scale circulation?

Assessing the TC Climate Footprint Benjamin A. Schenkel and Robert E. Hart The Florida State University 3/13

Introduction Methodology Large Scale Impacts of TCs

HighEnergy

Polar Jet

SST Cold WakeCooler and Drier

Atmospheric Response to TC Passage

Assessing the TC Climate Footprint Benjamin A. Schenkel and Robert E. Hart The Florida State University 4/13

0

45°N

Sfc.

Tropopause

TC

Polar Jet

L H

Does the tropical environmental response extend beyond the SST cold wake?

Significant heat and moisture transports

Excitation of Rossby wave train

Introduction Methodology Large Scale Impacts of TCs

LowEnergy

Significant reduction in meridional energy gradient

Methodology: Quantifying the Large Scale Response to TC Passage

Assessing the TC Climate Footprint Benjamin A. Schenkel and Robert E. Hart The Florida State University 5/13

• Objective: To analyze the response of the tropical atmospheric

environment to TC passage

• Evaluation of mean environmental response will occur using four-

dimensional storm-relative composites of normalized anomalies

• Composites are constructed using the NCEP Climate Forecast System

Reanalysis (Saha et al. 2011) for category 3 to 5 Best-Track TCs in the

Western North Pacific from 1982-2009 (N=257 TCs)

• Space-time spectral analysis (Wheeler and Kiladis 1999) used to

determine which equatorial waves are responsible for anomalies at

tropical latitudes

Introduction Methodology Large Scale Impacts of TCs

Large Scale Pressure Anomalies in the Tropics Following TC Passage

Assessing the TC Climate Footprint Benjamin A. Schenkel and Robert E. Hart The Florida State University 6/13

Introduction Methodology Large Scale Impacts of TCs

Large Scale Pressure Anomalies in the Tropics Following TC Passage

Assessing the TC Climate Footprint Benjamin A. Schenkel and Robert E. Hart The Florida State University 6/13

Introduction Methodology Large Scale Impacts of TCs

• Large scale response similar to TC Tip with significant anomalies through day 11

• Anomalies are significant (0.2σ) over an area extending over one-quarter of the circumference of the globe

• Reduced need for the Hadley cell to do work

Large Scale Pressure Anomalies in the Tropics Following TC Passage

Assessing the TC Climate Footprint Benjamin A. Schenkel and Robert E. Hart The Florida State University 7/13

Introduction Methodology Large Scale Impacts of TCs

Large Scale Pressure Anomalies in the Tropics Following TC Passage

Assessing the TC Climate Footprint Benjamin A. Schenkel and Robert E. Hart The Florida State University 7/13

Introduction Methodology Large Scale Impacts of TCs

Vertical cross-sections will be presented next through the domain center

• Large scale response similar to TC Tip with significant anomalies through day 11

• Anomalies are significant (0.2σ) over an area extending over one-quarter of the circumference of the globe

• Reduced need for the Hadley cell to do work

Physical Processes Responsible for Cooling and Drying of Equator

Assessing the TC Climate Footprint Benjamin A. Schenkel and Robert E. Hart The Florida State University 8/13

Introduction Methodology Large Scale Impacts of TCs

• Weakening of Hadley cell following TC passageConvergence

Divergence

Subsidence

NorthSouth

Physical Processes Responsible for Cooling and Drying of Equator

Assessing the TC Climate Footprint Benjamin A. Schenkel and Robert E. Hart The Florida State University 9/13

Introduction Methodology Large Scale Impacts of TCs

• Weakening of zonal mean circulation following TC passage

• Cooling and drying in lower troposphere due to reduction in low level convergence of heat and moisture into ITCZ

• Anomalous diabatic cooling from reduction in deep convection

NorthSouthNext, we will examine the role of equatorial waves in generating these anomalies

Attributing Physical Processes to Equatorial Waves

Assessing the TC Climate Footprint Benjamin A. Schenkel and Robert E. Hart The Florida State University 10/13

• Normalized mean sea level pressure anomalies averaged for points within five degrees of the equator

• Negative sea level pressure anomaly associated with TC

• Positive sea level pressure anomaly at equator

Introduction Methodology Large Scale Impacts of TCs

EastWest

Attributing Physical Processes to Equatorial Waves: MJO

Assessing the TC Climate Footprint Benjamin A. Schenkel and Robert E. Hart The Florida State University 11/13

• Normalized mean sea level pressure anomalies averaged for points within five degrees of the equator

• Negative sea level pressure anomaly associated with TC

• Positive sea level pressure anomaly at equator

• Dashed black contours: convectively active MJO

Introduction Methodology Large Scale Impacts of TCs

EastWest

Attributing Physical Processes to Equatorial Waves: MJO

Assessing the TC Climate Footprint Benjamin A. Schenkel and Robert E. Hart The Florida State University 11/13

• Normalized mean sea level pressure anomalies averaged for points within five degrees of the equator

• Negative sea level pressure anomaly associated with TC

• Positive sea level pressure anomaly at equator

• Dashed black contours: convectively active MJO

• Solid black contours: convectively suppressed MJO

Introduction Methodology Large Scale Impacts of TCs

EastWest

Attributing Physical Processes to Equatorial Waves: MJO

Assessing the TC Climate Footprint Benjamin A. Schenkel and Robert E. Hart The Florida State University 11/13

• MJO amplitude is maximized at time of TC passage although its magnitude is not significant according to Wheeler and Hendon (2004)

• Magnitude and relative location of positive pressure anomalies is beyond what can be attributed solely to MJO suggesting that TCs may also have an impact

Introduction Methodology Large Scale Impacts of TCs

EastWest

Attributing Physical Processes to Equatorial Waves: Kelvin Waves

Assessing the TC Climate Footprint Benjamin A. Schenkel and Robert E. Hart The Florida State University 12/13

• Dashed orange contours: convectively active Kelvin wave

Introduction Methodology Large Scale Impacts of TCs

EastWest

Attributing Physical Processes to Equatorial Waves: Kelvin Waves

Assessing the TC Climate Footprint Benjamin A. Schenkel and Robert E. Hart The Florida State University 12/13

• Dashed orange contours: convectively active Kelvin wave

• Solid orange contours: convectively suppressed Kelvin wave

• Triggering of convectively suppressed Kelvin wave is stronger and further east than expected for MJO event

Introduction Methodology Large Scale Impacts of TCs

EastWest

Attributing Physical Processes to Equatorial Waves: Kelvin Waves

Assessing the TC Climate Footprint Benjamin A. Schenkel and Robert E. Hart The Florida State University 12/13

• Timing and magnitude of the anomalies may suggest that TCs modulate and possibly initiate Kelvin waves potentially due to the following processes:

1. Suppression of deep convection by the subsident secondary circulation outside the TC inner core

2. TCs “steal” heat and moisture rich air at low levels from their environment

Introduction Methodology Large Scale Impacts of TCs

EastWest

Conclusion: A Conceptual Model

Assessing the TC Climate Footprint Benjamin A. Schenkel and Robert E. Hart The Florida State University 13/13

0

45°N

Sfc.

Tropopause

Polar Jet

Introduction Methodology Large Scale Impacts of TCs

Convectively Suppressed MJO

Convectively Suppressed MJO

Convectively Active MJO

Conclusion: A Conceptual Model

0

45°N

Sfc.

Anomalous flux of heat and moisture converging into TC at expense of environment

Tropopause

Suppression of precipitation yields cold anomalies due to reduction in diabatic heat release

Polar Jet

Assessing the TC Climate Footprint Benjamin A. Schenkel and Robert E. Hart The Florida State University 13/13

Introduction Methodology Large Scale Impacts of TCs

Suppressed PrecipitationPositive pressure

anomalyConvectively Suppressed Kelvin Wave

Modulation of MJO or ENSO???

Significant positive pressure anomalies occur across the entire basin

TC

Reduction in future TC activity???

What is the aggregate impact of anomalous TC activity over an entire season?

Time Series of Sensible Heat Tendency Term Anomalies

Assessing the TC Climate Footprint Benjamin A. Schenkel and Robert E. Hart The Florida State University 30/13

Introduction Introduction Methodology Large Scale Impacts of TCs

Vertical Structure of Moisture Anomalies

Assessing the TC Climate Footprint Benjamin A. Schenkel and Robert E. Hart The Florida State University 20/13

• Moistening of upper troposphere due to TC

• Drying of boundary layer due to reduction in sea surface fluxes from cold SST wake from TC

• Anomalous drying of troposphere following TC passage in region in which TC did not directly pass through

Introduction Methodology Large Scale Impacts of TCs