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Tropical Climate Variations and Their Impacts on the Northwest Indian Ocean – Northeast Africa – Southwest Asia Region. Capt Damon Vorhees, USAF Damon.Vorhees@vandenberg.af.mil Tom Murphree murphree@nps.edu Lt Col Karl Pfeiffer, USAF kdpfeiff@nps.edu Department of Meteorology

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Tropical Climate Variations and Their Impacts on the Northwest Indian Ocean – Northeast Africa – Southwest Asia Region


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Tropical Climate Variations and Their Impacts on the Northwest Indian Ocean – Northeast Africa –Southwest Asia Region

Capt Damon Vorhees, USAF

Damon.Vorhees@vandenberg.af.mil

Tom Murphree

murphree@nps.edu

Lt Col Karl Pfeiffer, USAF

kdpfeiff@nps.edu

Department of Meteorology

Naval Postgraduate School

Monterey, CA 93943-5114

1

Vorhees et al., Tropical Met Conf Apr06,

POC: Damon.Vorhees@vandenberg.af.mil

questions and hypotheses
Questions and Hypotheses
  • Develop foundation for climate forecasting in Northwest Indian Ocean – Northeast Africa – Southwest Asia (SWA) region --- especially in SWA.
  • Main questions:
    • How do El Nino (EN), La Nina (LN), Indian Ocean Zonal Mode (IOZM), Madden-Julian Oscillation (MJO), North Atlantic Oscillation (NAO) alter climate in this region during autumn-winter?
    • What are the mechanisms by which these climate variations alter climate in this region?
  • Main hypotheses:
    • Tropical climate variations set up tropical Rossby-Kelvin response that alters subtropical long-wave and moisture transport patterns.
    • Precipitation anomalies occur through anomalous moisture advection over the tropical northwest Indian Ocean and tropical and subtropical north Africa.

2

Vorhees et al., Tropical Met Conf Apr06,

POC: Damon.Vorhees@vandenberg.af.mil

prior research
Prior Research
  • Mariotti et al. (2002, 2005):
    • Autumn (winter) SWA precip above (below) normal during EN (LN). Anomalous onshore moisture flux from Arabian Sea into SWA during periods of above (below) normal precip in SWA. But no mechanisms offered.
  • No prior studies on effects of IOZM on SWA. Few studies of NAO impacts.
  • Barlow et al. (2005):
    • Convective (subsidence) component of MJO in eastern Indian Ocean (IO) leads to below (above) normal precipitation in SWA.
    • Upper-level Rossby-Kelvin response to MJO extends over SWA.
    • Only speculated on how MJO affects low level circulation and moisture transports into SWA.

3

Vorhees et al., Tropical Met Conf Apr06,

POC: Damon.Vorhees@vandenberg.af.mil

data and methods
Data and Methods
  • Monthly and daily global NCEP/NCAR reanalysis fields for Oct-Dec and Jan–Mar.
  • Indices used:
    • Multivariate ENSO Index (MEI) to identify EN / LN / neutral periods during 1960-2005.
    • Dipole Mode Index (DMI) to identify positive and negative IOZM periods during 1981-1996
    • Real-time Multivariate MJO (RMM) Index to identify convection and subsidence components of MJO in the eastern IO during 1975 to 2005.
    • CPC NAO Index to identify positive and negative NAO periods during 1960-2005
  • Analyzed circulation and precipitation anomalies associated with each climate variation. Only some results shown in this talk.

4

Vorhees et al., Tropical Met Conf Apr06,

POC: Damon.Vorhees@vandenberg.af.mil

autumn el nino 200 hpa height and wind composite anomalies
Autumn El Nino 200-hPaHeight and Wind Composite Anomalies

Northern component

of Rossby-Kelvin

response at 200 hPa

(m s-1)

Decreased SSTs and Convection

(m)

  • Broad, elongated troughing over southern Asia is the Rossby- Kelvin response to below normal convection over the Maritime Continent
  • Note UL convergence over MC, northwesterlies over N IO

5

Vorhees et al., Tropical Met Conf Apr06,

POC: Damon.Vorhees@vandenberg.af.mil

autumn el nino 850 hpa wind specific humidity anomalies

Anticyclone over southern Asia is the Rossby-Kelvin response to below normal convection over the Maritime Continent. Note LL divergence over MC, SE-erlies over N IO

  • During autumn of EN periods, there is an anomalous onshore southerly flow over SWA and NE Africa
  • This represents a weakening of the offshore monsoon
  • More days with onshore flow into SWA and NE Africa
Autumn El Nino 850-hPaWind & Specific Humidity Anomalies

Northern component

of Rossby-Kelvin

response

at 850 hPa

(m s-1)

wet

(g kg-1)

dry

6

Vorhees et al., Tropical Met Conf Apr06,

POC: Damon.Vorhees@vandenberg.af.mil

autumn el nino 850 hpa moisture advection composite anomalies
Autumn El Nino 850-hPa Moisture Advection Composite Anomalies

(m s-1)

wet

dry

(g kg-1 s-1 X 108)

  • Southerly wind anomaly advects moist air from the Arabian Sea and NW IO into SWA and the Horn of Africa
  • Northerly wind anomalies over the Caucuses and the Caspian Sea bring drier air south

7

Vorhees et al., Tropical Met Conf Apr06,

POC: Damon.Vorhees@vandenberg.af.mil

autumn el nino precip rate surface temp composite anomalies
Autumn El Nino Precip Rate &Surface Temp Composite Anomalies

(°C)

(mm day-1)

  • Net result of EN anomalies is general trend during autumn for SWA to be wetter than normal
  • Despite northerly wind anomaly over NW Iran and Iraq, there is an increase in precip as northerly winds run into high terrain in this region
  • Temperature anomalies better reflection of the wind anomalies

8

Vorhees et al., Tropical Met Conf Apr06,

POC: Damon.Vorhees@vandenberg.af.mil

slide9

Autumn Pos IOZM 200-hPaHeight and Wind Composite Anomalies

  • Strong Rossby-Kelvin response to decrease in SST/convection over the eastern IO
  • Similar patterns to autumn EN case, but anomalies have strengthened
  • Note Rossby-Kelvin response over eastern Africa to increase in SST and convection over the western IO

(m s-1)

Decreased SSTs and convection

(m)

9

Vorhees et al., Tropical Met Conf Apr06,

POC: Damon.Vorhees@vandenberg.af.mil

slide10

Autumn Pos IOZM 850-hPa Wind and Specific Humidity Anomalies

(m s-1)

dry

wet

(g kg-1)

  • Strong R-K response leads to southerly anomalies in low levels that transport moist air into SWA and NE Africa from MC and tropical IO

10

Vorhees et al., Tropical Met Conf Apr06,

POC: Damon.Vorhees@vandenberg.af.mil

autumn positive phase iozm precip rate surface temp anomalies
Autumn Positive Phase IOZM Precip Rate & Surface Temp Anomalies

(°C)

(mm day-1)

  • During strong positive IOZM periods in the autumn, SWA and NE Africa are generally much wetter and much warmer than usual

11

Vorhees et al., Tropical Met Conf Apr06,

POC: Damon.Vorhees@vandenberg.af.mil

slide12

Increase in convection in the eastern IO leads to a very prominent Rossby-Kelvin response over the Maritime Continent, IO, and SWA

  • Opposite pattern is observed when subsidence component of the MJO is in the eastern IO

MJO Convective Component in E. IO 200-hPa Height & Wind Composite Anomalies

(m s-1)

Convective Component of MJO

(m)

12

Vorhees et al., Tropical Met Conf Apr06,

POC: Damon.Vorhees@vandenberg.af.mil

mjo convective component in e io 850 hpa wind specific humidity anomalies
MJO Convective Component in E. IO 850-hPa Wind & Specific Humidity Anomalies

(m s-1)

Convective Component of MJO

dry

wet

(g kg-1)

  • Note offshore anomalies similar to analogous situation for LN in fall
  • Strong LL convergence into area of increased convection is apparent

13

Vorhees et al., Tropical Met Conf Apr06,

POC: Damon.Vorhees@vandenberg.af.mil

mjo convective component in e io precip rate surface temp anomalies
MJO Convective Component in E. IO Precip Rate & Surface Temp Anomalies

(mm day-1)

(°C)

  • Due to circulation anomalies shown in prior slide, net results is tendency for when the convective phase of the MJO is over the E. IO, SWA tends to be cooler and drier than normal.
  • Result of continental air infiltrating from Asia

14

Vorhees et al., Tropical Met Conf Apr06,

POC: Damon.Vorhees@vandenberg.af.mil

slide15

Mechanisms for Above NormalPrecipitation in SWA

Above Normal

Precip

L

H

Below Normal

Convection

  • Low level anomaly pattern during periods of above normal precip. Pattern occurs

during specific phases of EN/LN, IOZM, MJO, and/or NAO. Example: much of 2004-05

winter.

  • Opposite patterns for below normal precipitation in SWA.
  • Mechanisms involve teleconnections from tropics and midlatitudes to SWA.
  • ENLN, IOZM, MJO, and NAO are predictable at leads of one week to six months  SWA

precip may be predictable on these time scales.

  • Next step: develop forecasting system based on these climate variations.

15

Vorhees et al., Tropical Met Conf Apr06,

POC: Damon.Vorhees@vandenberg.af.mil

summary
Summary
  • Identified major climate variations that affect NE Africa - SWA region, and mechanisms by which they alter fall-winter circulation and precipitation in this region.
  • Implications for long-term forecasting in this region:
    • Expect impacts from tropics.
    • Develop quantitative tools based on indices of these climate variations.
    • Account for interactions of two or more climate variations.

16

Vorhees et al., Tropical Met Conf Apr06,

POC: Damon.Vorhees@vandenberg.af.mil

autumn el nino 850 hpa gph wind composite anomalies
Autumn El Nino 850-hPa GPH-Wind Composite Anomalies

Northern component

of Rossby-Kelvin

response at 850 hPa

  • Anticyclone over southern Asia is the Rossby-Kelvin response to below normal convection over the Maritime Continent. Note LL divergence over MC, SE-erlies over N IO
  • During autumn of EN periods, there is an anomalous onshore southerly flow over SWA and NE Africa
  • This represents a weakening of the offshore monsoon
  • More days with onshore flow into SWA and NE Africa

21

Vorhees et al., Tropical Met Conf Apr06,

POC: Damon.Vorhees@vandenberg.af.mil

autumn la nina 850hpa wind and specific humidity anomalies
Autumn La Nina 850hPa Wind and Specific Humidity Anomalies

Higher SSTs

Increased Convection

  • R-K response leads to NE-erly anomalies that transport dry air into SWA and eastern Africa from Asian continent
  • During LN periods, there is an offshore anomaly in the low-level winds over SWA.
  • This represents a strengthening of the offshore monsoon
  • Drier, continental air from the interior of Asia brought in SWA

22

Vorhees et al., Tropical Met Conf Apr06,

POC: Damon.Vorhees@vandenberg.af.mil

autumn la nina precipitation temp composite anomalies
Autumn La Nina Precipitation & Temp Composite Anomalies
  • Net result of LN anomalies is general trend during autumn for SWA and NE Africa to be drier than normal
  • Temperatures now generally cooler throughout SWA and NE Africa
  • Southerly wind anomalies lead to slightly warmer conditions over the Caucuses and the Caspian Sea

23

Vorhees et al., Tropical Met Conf Apr06,

POC: Damon.Vorhees@vandenberg.af.mil

winter enln anomalies
Winter ENLN Anomalies
  • Anomalies tend to be opposite to those observed during the autumn
    • EN—drier than normal
    • LN—wetter than normal
  • Precip/temp patterns more complicated than in autumn
  • Observed reversal of trends could be due to the following factors
    • Center of activity for ENLN shifts further south and east during the boreal winter
    • Westerlies strengthen and dip further south over SWA during the winter

24

Vorhees et al., Tropical Met Conf Apr06,

POC: Damon.Vorhees@vandenberg.af.mil

mjo subsidence component in e io precipitation and temp anomalies
MJO Subsidence Component in E. IO Precipitation and Temp Anomalies
  • Due to circulation anomalies shown in prior slide, net results is tendency for when the subsidence phase of the MJO is in the eastern IO, SWA tends to be warmer and wetter than average
  • Result of weakened offshore winds
  • More days with onshore flow from the Arabian Sea

25

Vorhees et al., Tropical Met Conf Apr06,

POC: Damon.Vorhees@vandenberg.af.mil

slide22

Mechanisms for Above NormalPrecip in SWA

Above Normal

Precip

L

L

Below Normal

Convection

H

H

Gulf of Mexico – Midwest U.S. analogy

  • Low level anomaly pattern during periods of above normal precip. Pattern occurs

during specific phases of EN/LN, IOZM, MJO, and/or NAO. Example: much of 2004-05

winter

  • Mechanisms involve teleconnections from tropics and midlatitudes to SWA.
  • ENLN, IOZM, MJO, and NAO are predictable at leads of one week to six months  SWA

precip may be predictable on these time scales.

  • Next step: develop forecasting system based on these climate variations.

27

Vorhees et al., Tropical Met Conf Apr06,

POC: Damon.Vorhees@vandenberg.af.mil