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El Nino and La Nina Effects on North Atlantic Tropical Cyclones: Impacts and Mechanisms. Master’s Thesis Presentation by LT Paula Hildebrand March 15, 2001 Advisor: Prof. Tom Murphree Second Reader: Prof. Pat Harr. Motivations for This Study.

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el nino and la nina effects on north atlantic tropical cyclones impacts and mechanisms

El Nino and La Nina Effects on North Atlantic Tropical Cyclones: Impacts and Mechanisms

Master’s Thesis Presentation

by LT Paula Hildebrand

March 15, 2001

Advisor: Prof. Tom Murphree

Second Reader: Prof. Pat Harr

motivations for this study
Motivations for This Study
  • Clarify impacts of EN and LN events on North Atlantic TCs
  • Explore mechanisms for EN/LN impacts in the North Atlantic
  • Follow on to LT Bruce Ford’s study of EN and LN impacts on WPAC TCs
hypotheses
Hypotheses

EN/LN events occur in the tropical Pacific

Alterations of global tropical circulation

Changes in

Tropical Easterly Jet (TEJ)

Development of anomalous extratropical wave trains

Changes in North Atlantic shear and steering flow

Changes in North Atlantic TC activity:

- numbers and intensities

- formation sites

- tracks

background
Past Studies

Focused on overall number of TCs

Mainly statistical analyses

Used localized indices to select EN/LN years

Used individual years to identify EN/LN impacts

Analyzed conditions in and near North Atlantic

This Study

Considers numbers, intensities, formation sites, and tracks

Analyzes large-scale dynamics

Uses a Pacific-wide index during TC season to select EN/LN years

Uses 10 year composites to represent EN and LN events

Explores teleconnections from Pacific via TEJ and extratropical wave trains

Background
selection of en and ln years

MEI : a Pacific-wide index

Years Selected

Selection based on MEI average during

North Atlantic TC season (Jun-Nov)

Long Term Mean (LTM):

1970-1999

Selection of EN and LN Years
data and methods
Data and Methods

NHC best track and NCEP reanalysis fields

  • EN/LN differences in TC activity:
    • - formation numbers and sites
    • - TC tracks and landfall

EN/LN differences in basin scale factors:

- vertical shear, SST, OLR, UL & LL vorticity

EN/LN differences in global scale factors:

- TEJ, wave trains, UL westerlies, trades

tc numbers and intensities

LTM

EN

LN

Ave TC intensity (kts) per month

LTM

EN

LN

Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec

TC Numbers and Intensities

Ave # of TCs >25kts per month

Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec

regions with differences in tc formations
1 - Gulf of Mexico (GOMEX)

2 - Subtropical Atlantic (SATL)

3 - Western Caribbean (WCAR)

4 - Central Tropical Atlantic (TATL)

5 - West Africa (WAF)

1

2

3

4

5

Regions With Differences in TC Formations
tc formations vertical shear in jas
TC Formations & Vertical Shear in JAS

El Niño

La Niña

El Niño formations

Vertical shear, U200-U850

La Niña formations

tc formations sst anomaly in jas

El Niño formations

Sea Surface Temperature Anomalies

La Niña formations

TC Formations & SST Anomaly in JAS

El Niño

La Niña

tc formations olr anomaly in jas

El Niño formations

Outgoing Longwave Radiation Anomalies

La Niña formations

TC Formations & OLR Anomaly in JAS

El Niño

La Niña

formations shear ul height anomaly in jas

El Niño formations

200 hPa geopotential height anomaly

Vertical shear, U200-U850

La Niña formations

Formations, Shear, & UL Height Anomaly in JAS

El Niño

La Niña

slide14

Global Scale Factors in JAS

Anomalous 200 hPa Heights and Winds

L

EN

H

H

H

L

L

LN

tc formations vertical shear in son
TC Formations & Vertical Shear in SON

El Niño

La Niña

El Niño formations

Vertical shear, U200-U850

La Niña formations

tc formations sst anomaly in son

El Niño formations

Sea Surface Temperature Anomalies

La Niña formations

TC Formations & SST Anomaly in SON

El Niño

La Niña

tc formations olr anomaly in son

El Niño formations

Outgoing Longwave Radiation Anomalies

La Niña formations

TC Formations & OLR Anomaly in SON

El Niño

La Niña

formations shear ul height anomaly in son

El Niño formations

200 hPa geopotential height anomaly

Vertical shear, U200-U850

La Niña formations

Formations, Shear, & UL Height Anomaly in SON

El Niño

La Niña

global scale factors in son
Global Scale Factors in SON

Anomalous 200 hPa Heights and Winds

H

L

H

L

L

L

L

H

H

H

EN

L

H

L

H

H

H

H

L

L

L

LN

comparison of tc tracks during en ln
Comparison of TC Tracks During EN & LN
  • Actual tracks
  • Westernmost longitude
  • Formation in relation to ridge axis
slide21

TC Tracks & 200 hPa Heights in JAS

El Niño

La Niña

El Niño formations

200 hPa geopotential height anomaly

TC tracks

La Niña formations

slide22

TC Tracks & 200 hPa Heights in SON

El Niño

La Niña

El Niño formations

200 hPa geopotential height anomaly

TC tracks

La Niña formations

hurricane westernmost longitudes

SON

JAS

EN: 8

LN: 19

EN: 12

LN: 26

El Niño positions

La Niña positions

Hurricane Westernmost Longitudes

Would you want to be in the coastal regions during a LN?

tc formations 500 hpa heights in jas
TC Formations & 500 hPa Heights in JAS

El Niño

La Niña

500 hPa heights

TC formations

tc formations 500 hpa heights in son
TC Formations & 500 hPa Heights in SON

El Niño

La Niña

500 hPa heights

TC formations

anomalous steering flow in son
Anomalous Steering Flow in SON

El Niño

La Niña

500 hPa height anomalies

500 hPa wind anomalies

track and steering flow differences
Track and Steering Flow Differences
  • height anomalies due to extratropical wave trains
  • anomalous wind
  • anomalous steering

EN

500 hPa height anomalies

500 hPa wind anomalies

formations differences and explanations

LN

EN

Basin Factors that Explain

Formation Differences

Formations: Differences and Explanations
  • Differences:
    • More tropical formations during LN
    • More subtropical formations during EN
  • Explanations:
    • Vertical shear due to anomalous TEJ and extratropical wave trains
    • SST anomaly
    • OLR anomaly
tracks differences and explanations

LN

LN

LN

Tracks: Differences and Explanations
  • Differences:
    • More westward tracks during LN
    • More landfalls during LN
  • Explanation:
    • TC formation sites
    • Wind anomalies due to anomalous extratropical wave trains