Anomalous northern hemispheric summer mid-tropospheric circulation over West Africa

Isaac K. Tetteh & Fredrick H.M. Semazzi Climate Modeling Laboratory, NCSU

29th Conference on Hurricanes and Tropical Meteorology, Tucson, Arizona

10-14th MAY 2010

Presentation Outline

i. Introduction ii. Motivation & Objectivesiii. Data and Methodsiv Resultsv. Conclusionsvi. Future Worki. Acknowledgmentsii. References

Introduction

• Previous studies have contributed to our understanding of climate variability over West Africa/Africa (Janowiak 1988; Nicholson; 1986; Lamb 1972; Lough 1986)

• Some specifics Nicholson (1986) - used linear correlation analysis to delineate 6 spatially precipitation coherent patterns over Africa - emphasized the role of Hadley and Walker circulations in strongly

modulating the principal dipole over Africa Janowiak (1988) - used rotated EOF solutions to isolate the dominant mode of precip. variability of Africa eg JJAS : 3 modes delineated EOF mode 1: Continental Interior mode (Sahel mode), modulated by Atlantic SST anomaly (SSTA) patterns.

Introduction

EOF mode 2: located between equator and 10 deg. South

EOF mode 3: coastal West Africa Lamb (1972); Lough, 1986 : established the close link between

global SST and precipitation variability over West Africa

Nicholson and Grist (2003); Nicholson( 2008 ) : investigated the mean annual cycle of rainfall and general circulation features over West Africa and central Africa, where the roles, seasonal excursions and the orientations of TEJ and AEJ-N were clearly established.

e.g. vertical alignments of the AEJ-N and TEJ axes enhance a weak

rainbelt and widespread drought over West Africa.

Motivation

• Some limitations of previous work

was confined to using a single variable or very few variables (egs, precipitation, temperature, humidity, SST) to investigate West African climate variability egs. Janowiak (1988), Semazzi et al.(1988)

also, previous work failed to consider simultaneous link between SST, dominant precipitation modes, and circulation

. Advancement of previous work

By doing this will provide opportunity for comprehensive understanding the W. African monsoon system

Objectives

• The objectives of the study are:

isolate the dominant precipitation modes over West Africa during NH summer (June-July-August-September; JJAS) season.

obtain corresponding circulation anomalies in the middle level

represented by 500 hPa, and

relate them to surface circulation and SST anomalies on the basis of the dominant precipitation modes to infer cause and effect from a more or less 3-dimensional perspective.

Study Domain: West Africa

Gulf of Guinea coast

Soudano-Sahel

Sahel

Data and Methods

Methods and Analysis Techniques

• The climate/meteorological variables used (1948-2006)

i. NCEP/NCAR Reanalysis

- horizontal winds (1000 hPa and 500 hPa)

- omega

- geopotential height

ii. University of Delaware

- Terrestrial precipitation

III. NOAA’s Improved ERSST

Data and Methods

i. EOF Analysis (regular non-rotated) performed on JJAS precip. ii. Retention of statistically significant modes (North et al. 1982) iii. EOF precipitation time series (TS) were decomposed into very wet and

very dry at one sd from the meaniv. Anomalies from other climatic fields were computedv Precipitation TS composited with the anomaly fieldsvi. From horizontal wind and precip. anomaly composites, streamfunction and rotational winds, computed using the following equation: Vh= Vψ + VФ (Krishnamurti 1971) streamfunction velocity potential (rotational) (divergence)

Precip.field

anom.

500 hPaVert. vel.(Omega)anom.

500 hPa wind anom.

500 hPaGeopot.

Hgtanom.

SSTanom.

sfcHorizon.

Windsanom.

Pos(Neg)Precip.

Time Series

Composites

Streamfunction & rotational

Velocity potental &divergence

Streamfunction & rotational

Velocity potental &divergence

Results

• Four dominant precipitation modes were statistically significant

• Composites of the precipitation TS with the fields yielded a total of 8 events (4 dry and 4 wet)

• We present results excluding the omega, geopotential, and divergent circulation since these were presented in the 90th annual meeting in Atlanta (Tetteh and Semazzi 2010).

JJAS Precip. EOF analysis

36.0% var.

9.56% var.

5.45% var.

3.65% var.

- Sahel mode - Associated with multivariate. ENSO Index, Indian OceanSST index,Global SST EOF 3 (Interhemispheric SST asymmetry)

- GOGC mode-Associated with Trop. Atlantic Dipole mode

Mode 3 : west coastal Africa (dipole) and eastern part of W. Africa ( dipole) associated with trop South Atlantic mode

-Mode 4: western Sahel and eastern part of W. Africa (a dipole) associated with Global SST EOF 3

POS EVENTS NEG. EVENTS

JJAS PRECIP. MODES, SFC WINDS, & SST PATTERNS: Anomaly COMPOSITES

Key features (differences)

1. Contrasting SST patterns -Mediterranean Sea - Atlantic and Indian Oceans1. Damping or strengthening of the cold tongue complex (CTC)3. Strength of the southwesterlies over tropical Atlantic4. Strength of momentum flux convergence/diverrgence5. Strength of low-level troughs & ridges6. Strength of horizontal advection

Precip. fieldWet

Precip. field Dry

Mid-level (500 hPa) Anomalous Circulation

Key features (differences) in mid-level anomalous

circulation

1. Size of troughs and ridges over land and Atlantic Ocean

2. Centers of action of troughs and ridges1. Strengths and number of

the trough and ridges4. Strength of the diabatic of heating/cooling1. Strength of vertical

ascent/moist convection and subsidence

2. Intensity of precipitating moist convection

3. Changes in mid-level circulation

POS EVENTS NEG. EVENTS

Key features (differences) in low-level anomalous

circulation

1. Size of troughs and ridges over land and Atlantic Ocean

2. Centers of action of troughs and ridges1. Strengths of the trough and ridges4. Strength of the diabatic of heating/cooling1. Strength of vertical

ascent or moist convection/ maintenance of subsidence from mid-level

Low-level Anomalous circulation

Conclusions • The sizes, strengths, and centers of actions of mid- and low-level troughs

and ridges, together with SST patterns over Mediterranean Sea, Atlantic and Indian Oceans can be used to gauge the spatial extent of the precipitation field over West Africa in JJAS.

In particular,

i. Mode 1: Sahel mode Wet Sahel is associated with: (1) well-developed CTC (2) large-scale mid- and low-level trough traversing from east to west Africa,

bounded by the Sahel belt and projects unto or over tropical N. Atlantic Ocean (TNAO)

Dry Sahel is associated with: (1) a dampened CTC, and a large low-level ridge centered over mid-Sahel and

TNAO, and large trough located over tropical South Atlantic Ocean (TSAO) (2) a large-scale mid-level ridge similar but opposite to its counterpart in the wet phase

Conclusions

ii. Mode 2: GOGC mode

Wet GOGC is associated:

(1) with warm tropical equatorial SST anomalies (SSTAs)

(ii) large-scale trough over land, large-scale ridge over TSAO, and a very

small TNAO trough

Dry GOGC is associated:

(1) a well-developed CTC in TSAO and equatorial eastern Atlantic

(2) a well-developed ridge similar but opposite to the tough in the wet phase of Sahel.

Conclusions

• Mode 3: west coastal Africa (dipole) and eastern part of W. Africa ( dipole)

wet phase is associated with:

(1) general warming of the two oceans and the Medit. Sea

(2) large-scale low-level troughs on land and over TSAO

(3) four mid-level ridges and a dipole in TSAO

Dry phase is associated with:

(1) cold SSTAs in over the two oceans and the sea

(2) Small-scale low-level roughs on land and tropical Atlantic Ocean

(3) number of troughs/ridges tend to increase with decreasing sizes.

Conclusions

• Mode 4 : western Sahel and eastern part of W. Africa (a dipole)

wet phase is associated with:

(1) warmer Indian Ocean anomalies as with theAtlantic

(2) large-scale low-level troughs on land and over TSAO

(3) three mid-level ridges aligned in the southeast direction from TNAO

to land

Dry phase is associated with:

(1) general warming of the two oceans and the sea

(2) Small-scale low-level roughs on land and tropical Atlantic Ocean

(3) one large-scale and one small-scale ridges centered over land and TNAO, respectvely.

Future Work

• Modeling combined with observational studies• Climate Forecast

Acknowledgments

• NCEP/NCAR (reanalysis)• University of Delaware (precip) • NOAA’s (ERSST)• Ghana Government – my sponsor

References

• Janowiak, JE. 1988. An Investigation of Interannual Rainfall Variability in Africa. Journal of Climate 1:240-255

• Krishnamurti TN. 1971. Tropical east-west circulations during the northern summer. Journal of Atmospheric Sciences 28: 1342-1347. .

• Nicholson SE. 2008. On the factors modulating the intensity of the tropical rain belt over West Africa. International Journal of Climatology DOI: 10.1002/joc.1702.

• Nicholson SE, Grist JP. 2003. The Seasonal Evolution of the Atmospheric Circulation over West Africa and Equatorial Africa. Journal of Climate. 16 (7): 1013-1030.

• Nicholson SE. 1986. The spatial coherence of African rainfall anomalies: Interhemispheric teleconnections. Journal of Climate and Applied Meteorology 25: 1365-1381.

• .Semazzi FHM, Mehta V, Sud YC. 1988. An investigation of the relationship between sub-Sahran rainfall and global sea surface

temperatures. Atmosphere-Ocean, 26:118- 138 .

• Tetteh, I. K. & Semazzi, F.H.M (2010). Three-dimensional characterization of boreal spring-summer climate variability over West Africa. 90th American Meteorological Society (AMS) Meeting. 22nd Conference on Climate Variability and Change, Atlanta, GA, USA. 17–21 January 2010.