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El Nino and La Nina Effects on Tropical Cyclones. LT Bruce W. Ford June 9, 2000 Master’s Thesis Presentation. The Mechanisms. Advisor: Tom Murphree Second Reader: Pat Harr. Motivation. Clarify role of EN and LN events on western Pacific TCs Explore mechanisms behind TC effects

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el nino and la nina effects on tropical cyclones

El Nino and La Nina Effects on Tropical Cyclones

LT Bruce W. Ford

June 9, 2000

Master’s Thesis Presentation

The Mechanisms

Advisor: Tom Murphree

Second Reader: Pat Harr

1

motivation
Motivation
  • Clarify role of EN and LN events on western Pacific TCs
  • Explore mechanisms behind TC effects
  • Explore impacts on Atlantic TCs
  • Test to see if our results apply to a single event (1999)

2

slide3

Hypotheses

  • Equatorial Pacific heating anomalies lead to alterations in western Pacific circulations, and thereby to altered TC numbers, formation sites, intensities, and tracks.
  • EN and LN events aid in creating anomalous wave trains which extend into the Atlantic, thereby altering Atlantic TC activity.

3

slide4

Study Methodology

Past Studies

This Study

  • Considers numbers, formation sites, intensities, and tracks
  • Analyses large-scale dynamics
  • Examines both EN and LN events
  • Covers 1949-1999
  • Explores teleconnections between basins
  • Focused on overall numbers of TCs
  • Mostly statistical
  • Focused on EN events
  • Conducted too early to include events of the 90’s
  • Did not consider links to other basins

4

slide5

Data

  • NCEP/NCAR Reanalysis Data
    • Monthly fields, Jan 1948-present
    • Covers global heating and circulation fields, subject to reanalysis errors
    • 2.5o latitude x 2.5o longitude grid (144x73 points)
  • JTWC Best Track Data
    • 1536 storm files in dataset, 1945-1999
    • Covers western North Pacific TCs, but limited east of dateline
  • NHC Best Track Data
    • 571 storm files in dataset, 1945-1999
    • Covers North Atlantic TCs

5

slide6

Methods

  • Event identification: Multivariate ENSO index (MEI)
    • Incorporates SLP, U and V surface winds, SST, Tair, OLR
  • Composites used:
    • 9 strongest EN and LN events
    • All events since 1949 EN and LN events
  • Anomalies calculated using 1968-1996 mean

6

slide7

Number of Western Pacific TC Reports

Number of Best Track Reports

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

peak

peak

El Niño Events

La Niña Events

Climo

7

slide8

Average Number of Western Pacific TCs

Number of TCs

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

peak

peak

El Niño Events

La Niña Events

Climo

8

slide9

Average Number of Strong Western Pacific TCs

Strong -  80 knots (41 ms-1) maximum intensity

Number of TCs

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

peak

peak

El Niño Events

La Niña Events

Climo

9

slide10

El Niño Events

La Niña Events

Average Number of Weak Western Pacific TCs

Weak - < 80 knots (41 ms-1) maximum intensity

Number of TCs

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

peak

peak

Climo

10

slide11

August TC Formation Sites

El Niño

La Niña

El Niño formations

200 hPa geopotential height anomaly

Vertical shear, U200-U850

La Niña formations

11

slide12

30N

H

Eq

H

30S

180E

30N

L

Eq

L

30S

180E

Circulation Response to Equatorial Heating, Z200

Rossby-Kelvin Wave Response

El Niño

La Niña

Tropospheric cooling

Tropospheric warming

12

slide13

Tracks

Schematic Western Pacific Vertical Shear, August

La Niña

U200

Climatology

El Niño

20oN 150oE

U850

-4 0 4 ms-1

U200

15oN 165oE

U850

-4 0 4 ms-1

13

slide14

Tracks

September TC Formation Sites

El Niño

La Niña

El Niño formations

200 hPa geopotential height anomaly

Vertical shear

La Niña formations

14

slide15

October TC Formation Sites

El Niño

La Niña

El Niño formations

200 hPa geopotential height anomaly

Vertical shear

La Niña formations

15

slide16

November TC Formation Sites

El Niño

La Niña

El Niño formations

200 hPa geopotential height anomaly

Vertical shear

La Niña formations

16

slide17

September TC Tracks

El Niño

La Niña

El Niño formations

200 hPa geopotential height anomaly

Vertical shear

La Niña formations

17

slide18

October TC Tracks

El Niño

La Niña

El Niño formations

200 hPa geopotential height anomaly

Vertical shear

La Niña formations

18

slide19

November TC Tracks

El Niño

La Niña

El Niño formations

200 hPa geopotential height anomaly

Vertical shear

La Niña formations

19

slide20

Westernmost Longitudes

September

October

65% greater chance of LN landfall

November

285% greater chance of LN landfall

El Niño TCs

La Niña TCs

No EN landfalls

20

slide21

SST Anomalies, August – November

Climo

El Niño

La Niña

21

slide22

OLR Anomalies, August – November

Climo

climo

El Niño

La Niña

22

slide23

Z200 Anomalies, August – November

Climo

climo

El Niño

La Niña

23

slide24

Shear Anomalies, August – November

Climo

climo

El Niño

La Niña

Atlantic

24

mechanisms pacific
Mechanisms - Pacific

 SST in equatorial Pacific

 Convective heating in equatorial Pacific

 Circulation in tropical and subtropical Pacific

responses)

(Tropical Rossby-Kelvin

and

extratropical Rossby wave

 Vertical shear in tropical and subtropical western Pacific

 TC activity: formation sites, numbers, intensities, tracks

Wave Train

27

slide29

HypothesizedMechanisms - Atlantic

 SST in global tropics

 Convective heating in global tropics

 Circulation in global tropics and subtropics

extratropical Rossby wave

responses)

and

(Tropical Rossby-Kelvin

 Vertical shear in tropical and subtropical North Atlantic

 TC activity: formation sites, numbers, intensities, tracks

29

slide30

El Niño Events

La Niña Events

1999

Numbers of Atlantic TC Reports

Number of Best Track Reports

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

peak

peak

Climo

30

slide31

Average Number of Atlantic TCs

Number of TCs

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

peak

peak

El Niño Events

La Niña Events

Climo

31

slide32

Average Number of Strong Atlantic TCs

Strong -  80 knots (41 ms-1) maximum intensity

Number of TCs

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

peak

peak

El Niño Events

La Niña Events

Climo

32

slide33

Average Number of Weak Atlantic TCs

Weak - < 80 knots (41 ms-1) maximum intensity

Number of TCs

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

peak

peak

El Niño Events

La Niña Events

Climo

33

slide34

Summary

  • Western North Pacific TC numbers, formation sites, intensities, and tracks are effected by equatorial Pacific circulation anomalies brought about by EN and LN events.
  • SST changes do not seem to be reason for more (less) intense storms in EN (LN).
  • Extratropical wave trains alter upper level flow which modifies Atlantic vertical shear and TC activity.
  • Atlantic alteration mechanisms are dynamically similar to those in the Pacific.

34

slide35

Future Research

  • Examine year following event peaks
  • Conduct in-depth study of Atlantic(formations, tracks, etc.)
  • Develop indices to monitor and forecast EN and LN impacts.
  • Apply these results to improve medium and long-range TC forecasting (Systematic Approach)
  • Conduct modeling studies of EN and LN wave trains and impacts on TCs

35

slide37

Number of Best Track Reports

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

peak

peak

Back

37

slide38

Number of TCs

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

peak

peak

Back

38

slide39

Number of TCs

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

peak

peak

Back

39

slide40

Number of TCs

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

peak

peak

Back

40

slide41

Intensity (knots)

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

peak

peak

41

slide47

Vertical Wind Shear Climatology

(1968-1996)

Shear Anomalies

47

slide48

200 hPa Geopotential Height

Climatology (1968-1996)

Z200 Anomalies

48

slide49

850 hPa Geopotential Height

Climatology (1968-1996)

49

slide50

Number of Best Track Reports

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

peak

peak

Back

50

slide51

Number of TCs

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

peak

peak

Back

51

slide52

Number of TCs

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

peak

peak

Back

52

slide53

Number of TCs

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

peak

peak

Back

53

slide54

Intensity (knots)

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

peak

peak

Figure 58. The long-term average maximum intensity of western North Pacific TCs per month for 1949-1998. Red and blue bars at bottom of figure indicate the duration of composite El Niño and La Niña events, with event peaks indicated in darker colors. The TC season in this region extends from April through December.

54

slide55

Figure 2. Modeled geopotential height response at 200 hPa to a positive tropical heating perturbation centered on the equator at 130E (indicated by oval contours). Positive (negative) height responses are indicated by solid (dashed) contours. Height contour interval = 5 m. The background flow is zonally symmetric with 25 ms-1 jets at 25oN and 32oS. The arching pattern of positive and negative height responses over the East Asian – North Pacific – North American – North Atlantic region reveals a quasi-stationary wave train initiated by the perturbation heating. A similar wave train occurs over the Australian – South Pacific – South Atlantic region.

55

slide56

Circulation Response to East Asian-WestPac Heating

Extratropical Rossby Wave Response

L

H

L

H

L

Pacific

Based on Nitta 1987

56

slide57

a

b

Figure 6. Multivariate ENSO index for the seven strongest historic El Niño (a) and La Niña (b) events since 1950. Figure from http://www.cdc.noaa.gov/~kew/MEI/mei.html. See Wolter and Timlin (1993) for more information on MEI.

57

slide58

Height (hPa)

i

Height (hPa)

j

Figure 32 (continued). Composite anomalies averaged from 5oN-15oN for August-November prior to El Niño or La Niña event peak: (i) cross section of El Niño zonal wind anomalies in ms -1; (j) cross section of La Niña zonal wind anomalies in ms-1. See Chapter 2 for information on evolution of events and selection of event years.

58

slide59

Height (hPa)

K

Height (hPa)

l

Figure 32 (continued). Composite anomalies averaged from 20oN-30oN for August-November prior to El Niño or La Niña event peak: (i) cross section of El Niño zonal wind anomalies in ms -1; (j) cross section of La Niña zonal wind anomalies in ms-1. See Chapter 2 for information on evolution of events and selection of event years.

59

slide60

Figure 33. Correlation of sea level pressure in the Philippine Sea (5-15N, 120-160E) with sea level pressure elsewhere. See Chapter 2 for information on correlations.

60

slide61

El Niño Events

La Niña Events

1999

Number of TCs

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

peak

peak

Figure 36. The average number of strong (>80 knot (41 ms-1) maximum intensity) Atlantic TCs per month for composite El Niño and La Niña events, based on the nine strongestevents during 1949-1998 and for 1999. Red and blue bars at bottom of figure indicate the duration of composite El Niño and La Niña events, with event peaks indicated in darker colors. The TC season in this region extends from April to December. This study focuses on the first season shown in this figure, the season that occurs during and immediately before the event peaks (see Chapter 2 for rationale). See Chapter 2 for information on evolution of events and selection of event years.

61

slide62

El Niño Events

La Niña Events

1999

Number of TCs

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

peak

peak

Figure 37. The average number of weak (<80 knot (41 ms-1) maximum intensity) Atlantic TCs per month for composite El Niño and La Niña events, based on the nine strongestevents during 1949-1998 and for 1999. Red and blue bars at bottom of figure indicate the duration of composite El Niño and La Niña events, with event peaks indicated in darker colors. The TC season in this region extends from April to December. This study focuses on the first season shown in this figure, the season that occurs during and immediately before the event peaks (see Chapter 2 for rationale). See Chapter 2 for information on evolution of events and selection of event years.

62

slide63

El Niño Events

La Niña Events

1999

Intensity (knots)

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

peak

peak

Figure 38. The average maximum intensity of Atlantic TCs per month for composite El Niño and La Niña events, based on the nine strongestevents during 1949-1998 and for 1999. Red and blue bars at bottom of figure indicate the duration of composite El Niño and La Niña events, with event peaks indicated in darker colors. The TC season in this region extends from April to December. This study focuses on the first season shown in this figure, the season that occurs during and immediately before the event peaks (see Chapter 2 for rationale). See Chapter 2 for information on evolution of events and selection of event years.

63

slide64

El Niño Events

La Niña Events

1999

Number of Best Track Reports

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

peak

peak

Figure 39. The average number of best track reports for western Pacific TCs per month for the nine strongest composite El Niño and La Niña events, based on the nine strongestevents during 1949-1998 and for 1999. Red and blue bars at bottom of figure indicate the duration of composite El Niño and La Niña events, with event peaks indicated in darker colors. The TC season in this region extends from April to December. This study focuses on the first season shown in this figure, the season that occurs during and immediately before the event peaks (see Chapter 2 for rationale). See Chapter 2 for information on evolution of events and selection of event years.

64

slide65

El Niño Events

La Niña Events

1999

Number of TCs

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

peak

peak

Figure 40. The average number of western Pacific TCs per month for composite El Niño and La Niña events, based on the nine strongestevents during 1949-1998 and for 1999. Red and blue bars at bottom of figure indicate the duration of composite El Niño and La Niña events, with event peaks indicated in darker colors. The TC season in this region extends from April to December. This study focuses on the first season shown in this figure, the season that occurs during and immediately before the event peaks (see Chapter 2 for rationale). See Chapter 2 for information on evolution of events and selection of event years.

65

slide66

El Niño Events

La Niña Events

1999

Number of TCs

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

peak

peak

Figure 41. The average number of strong (>80 knot (41 ms-1) maximum intensity) western Pacific TCs per month for composite El Niño and La Niña events, based on the nine strongestevents during 1949-1998 and for 1999. Red and blue bars at bottom of figure indicate the duration of composite El Niño and La Niña events, with event peaks indicated in darker colors. The TC season in this region extends from April to December. This study focuses on the first season shown in this figure, the season that occurs during and immediately before the event peaks (see Chapter 2 for rationale). See Chapter 2 for information on evolution of events and selection of event years.

66

slide67

El Niño Events

La Niña Events

1999

Number of TCs

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

peak

peak

Figure 42. The average number of weak (<80 knot (41 ms-1) maximum intensity) western Pacific TCs per month for composite El Niño and La Niña events, based on the nine strongestevents during 1949-1998 and for 1999. Red and blue bars at bottom of figure indicate the duration of composite El Niño and La Niña events, with event peaks indicated in darker colors. The TC season in this region extends from April to December. This study focuses on the first season shown in this figure, the season that occurs during and immediately before the event peaks (see Chapter 2 for rationale). See Chapter 2 for information on evolution of events and selection of event years.

67

slide68

El Niño Events

La Niña Events

1999

Intensity (knots)

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

peak

peak

Figure 43. The average maximum intensity of western Pacific TCs per month for composite El Niño and La Niña events, based on the nine strongestevents during 1949-1998 and for 1999. Red and blue bars at bottom of figure indicate the duration of composite El Niño and La Niña events, with event peaks indicated in darker colors. The TC season in this region extends from April to December. This study focuses on the first season shown in this figure, the season that occurs during and immediately before the event peaks (see Chapter 2 for rationale). See Chapter 2 for information on evolution of events and selection of event years.

68