11/5/09

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Tropical Atlantic Variability the Role of the Ocean

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Sahel Rainfall Interhemispheric SST Contrast

NH Cold

NH Warm

Strong relationship to SST changes

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A West African Monsoon Index

The West African summer monsoon index (WASMI) is defined as an area-averaged seasonally (JJAS) at 850 hPa within the West African monsoon domain (5º-17.5ºN, 20ºW-40ºE) (Li and Zeng, 2002, 2003, 2005)

Correlation maps between the seasonal (June-September) rainfall and WASMI (1979-97). The shaded areas indicate significant at the 95% confidence level.

A West African Monsoon Index

The West African summer monsoon index (WASMI) is defined as an area-averaged seasonally (JJAS) at 850 hPa within the West African monsoon domain (5º-17.5ºN, 20ºW-40ºE) (Li and Zeng, 2002, 2003, 2005)

Sahel Rainfall Interhemispheric SST Contrast

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Results from perfect SST “AMIP” ensembles (L. Goddard, IRI)

… but current seasonal SST forecasts are without skill!

SST along the equator

  as observed (thick black)

  in coupled ocean–atmosphere general circulation models (GCMs)

⇒ Strong biases in GCMs including a failure of reproducing cold tongue

Errors in AGCMs during spring

Possibility of improving coupled simulations

Richter and Xie (2008)

Atlantic SST bias in coupled climate models

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Zonal averaged circulation in the Tropical Ocean. Is it changing?

Subtropical Cell (STC)

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summer ITCZ

spring ITCZ

ITCZ position and rainfall intensity affect densely

populated regions in West Africa

Sahel rainfall climatology

Guinea rainfall climatology

Atlantic ITCZ annual cycle

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Atlantic ITCZ Variability

First EOF (23%) of the June-August rainfall from GPCP 1979-2001 (contours in mm/day). June-August SST anomaly (colors, in °C & white contours, every 0.2°) and surface wind anomaly (vector, in m/sec) are determined by regression on the time series of the rainfall EOF.

First EOF (33%) of the March-April rainfall from GPCP 1979-2001 (contours in mm/day). March-April SST anomaly (colors, in °C & white contours, every 0.2°) and surface wind anomaly (vector, in m/sec) are determined by regression on the time series of the rainfall EOF.

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Zonal Mode - „Atlantic Nino“

Dominant mode during boreal summer

Increased precipitation (grey shaded, mm/day) during warm events

⇒ SST in the equatorial cold tongue important for regional climate prediction

Kushnir et al. (2006)

NTA

MEAN

Covariance

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Point 1

•  Tropical Atlantic Variability fundamentally involves the interaction between the ocean surface and the ITCZ�

•  Its patterns are highly seasonal�

•  In the spring the North-South SST gradient anomaly dominates causing rainfall fluctuation over Brazil�

•  Are ocean dynamics involved?

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  NCAR Climate Community Model (CCM3)

  couple to simple 75m ocean ML model

  Standard T42 configuration

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ML-coupled

SST anomaly

SST anomaly as a function of time and latitude

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Precipitation as a function of time and latitude

CCM3

AML

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Point 2

• In the spring the North-South SST gradient anomaly develops well in a Mixed-Layer Ocean Model.�

•  Ocean Dynamics are NOT crucial here. �

•  However, the signals are possibly to strong in this model and one can assume that ocean mixing will damp the SST anomalies�

•  How large is the upper ocean mixing?

Free falling microstructure profiler:

•  two vertical velocity shear sensors

•  high resolution temperature sensor

•  acceleration sensor

•  depth range: 0-500m

Quantify diapycnal mixing in the upwelling regions of the Tropical Atlantic Ocean

Marcus Dengler�(IFM-GEOMAR)

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Heat Budget

Direct estimates of the diapycnal heat flux at the base of the mixed layer: up to 90 W/m² during cold tongue onset

Microstructure measurements with a loosely tethered profiler (M. Dengler, IFM-GEOMAR)

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Tropical Atlantic Climate Experiment

Spring Summer First EOF of rainfall anomaly [mm/day] and regressed SST and surface wind

after Kushnir et al. (2002)

SST and circulation scheme

⇒ Potential for prediction of Rainfall over Africa and Brazil

after Schott et al. 2004

Motivation/Introduction

Equatorial Cold Tongue

Cold SSTs develops during boreal summer in the eastern equatorial Atlantic

Strong interannual variability and long term warming trend

ATL3 annual cycle: Max in April ~29°C Min in Aug. ~24°C

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Why are the rain fall predictions over Africa so bad?

Dominant pattern of precipitation error associated with dominant pattern of SST prediction error based on persistent SST anomalies (Goddard & Mason ,Climate Dynamics, 2002)

FLAME – Mean (MJJ) 15m Temperatures

Cold tongue in the eastern equatorial Atlantic with box for cold tongue index

Tropical Atlantic Climate Experiment

FLAME – Model

•  1/12° horizontal resolution •  45 vertical levels (10m near the surface, 250m below 2000m) •  simulation from 1990 to 2002

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Tropical Atlantic Climate Experiment

Cold Tongue (MJJ) – Temperature Anomaly

Temperature Transport Anomaly (MJJ)

Point 4

•  The zonal (Antlantic NINO) mode affects the West African Monsoon and thus rainfall in the boral fall season over Subsaharan Africa.

•  Significant societal impacts have been demonstrated, for example drought mitigation, tropical disease mitigation..

•  Ocean dynamics are at the heart of the cold SST development in the eastern tropical Atlantic.

•  Current assimilation models perform poorly

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TACE: Tropical Atlantic Climate Experiment TACE core period six years: 2006/7 - 2011/12

To advance understanding of coupled ocean-atmosphere processes and improve climate prediction for the Tropical Atlantic region

Specific goals are: a) To advance understanding of the key processes that control SST, interactions with the AMI (Atl. Marine ITCZ), and related climate predictability in the eastern tropical Atlantic. b) To contribute to the design of an enhanced sustained observing system for the tropical Atlantic region.

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1) Determine oceanic processes important in regulating SST in the tropical Atlantic and associated atmospheric responses

2) Improve SST forecasts on seasonal to interannual time scales in the tropical Atlantic

3) Provide parameterizations and model improvements to global and regional prediction centers

4) Investigate response of tropical Atlantic region to global warming, including teleconnection patterns

Tropical Atlantic Climate Experiment

Cruise Track M68/2

Mooring array at 23°W BMBF Verbundvorhaben Nordatlantik

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Tropical Atlantic Climate Experiment

3 Moorings at 23°W 0.75°N, 0°N, 0.75°S (including the PIRATA ADCP in the central mooring) and one at 21.5°W, 0°N

CTD-profiler at 23°W, 0°N 1000m - 3500m (John Toole, WHOI)

CTD/O2-profiler At 23°W, 5°N 120m – 1000m (new SFB)

Ocean Observing System

Interannual Variability

Seasonal dependence of interannual variability: Strongest during boreal summer (June/July)

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Interannual Variability

Boreal summer cold tongue:

Warm events in 2002, 2006, 2007

Cold event in 2005

EUC and cold tongue variability

warm (cold) event in JJA 2002 (2005), with relaxed (intensified) winds in the west during boreal spring (MAM)

EUC embedded in shallower (deeper) thermocline at 23°W during boreal summer 2002 (2005)

Hormann and Brandt (2009)

Interannual Variability

Zonal Velocity Temp. Gradient Temperature

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