Part II:Observed Multi-Time Scale Variability in the Tropical Atlantic

Part I: Biases in the NCEP CFS in the Tropical Atlantic

Diagnosing CGCM bias and analyzing multi-time scale variability in the tropical Atlantic sector using satellite observations

GEC-hri Project Meeting: COLA, Oct. 30, 2007

Part I: Biases in the NCEP CFS in the Tropical

Atlantic

Zeng-Zhen Hu

(COLA)

Bohua Huang(GMU & COLA)

Kathy Pegion(COLA)

Note: Since the GEOS5 is not running at COLA yet, here we have used CFS as an example. We plan to look into GEOS5 in

the same way as we have done for CFS.

Data & CFSCFS Model& Simulations

• AGCM: NCEP T62&L64

• OGCM: MOM3, 1/3o(10oS-10oN), 1o(higher latitudes); 74oS-64oN, L40

• Free run: Jan 1985-Dec 2036

Observations

• NCEP/NCAR reanalysis I (Kalnay et al. 1996); ERA40 (Uppala et al. 2005)

• Cloud coverage data by ISCCP on 2.5ox2.5o, Jul1983-Dec2004 (Rossow and Dueñas 2004); Corresponding radiation calculated by Zhang et al. (2004)

• The SST dataset is the extended reconstruction (ER-v2) on 2ox2o (Smith and Reynolds 2003)

CFS Well Simulates Leading ModesC

FS

ER

-v2

1st16%

STA

2nd15%

NTA

3rd10%

SSA

STA

3rd10%

NTA

2nd11%

SSA

1st12%

CFS Systematic Biases

MAM

JJA

SON

DJF

Low Clouds Short-Wave SST

There are coherent biases in the southeastern Atlantic Ocean:

(1) Too few low clouds

(2) Too much short-wave radiation reaching the sea surface

(3) Warm biases

JJA Low Clouds & Vertical T Gradient

T700-T850

Low Clouds

T850-T925

CFS: Region A OBS: Region B

A B

Low Clouds & Vertical T Gradient

CFS Region A OBS Region B

T850-T925

T700-T850

Low Clouds

Inversion layerNO inversion layer

Quasi-inversion layerNO quasi-inversion layer

Also low clouds may be

associated with different climate

modes:

CFS: Subtropical South Atlantic Mode (SSA)

OBS: Atlantic Niño/Southern

Tropical Atlantic Mode (STA) or

Zonal Mode

Regression: Low Clouds in region B &UV+SST

CFS OBS

Summary of Part I: Biases in the NCEP CFS in the tropical Atlantic

• CFS: Deficit low-cloud and excessive SW radiation can cause the warm SST bias in the southeastern Atlantic. In return, the warm bias does not favor inversion layer and also low-cloud formation.

• CFS can not correctly simulate the vertical inversion layer of the temperature and the formation of low cloud. Low-cloud formation: Observed: (T850-T925); CFS: (T700-T850)

Publications(partly supported by this project and a NOAA project)

• Hu, Z.-Z., B. Huang, and K. Pegion, 2007: Leading patterns of the tropical Atlantic variability in a coupled GCM. Climate Dynamics, DOI 10.1007/s00382-007-0318-x.

• Hu, Z.-Z., B. Huang, and K. Pegion, 2007: Low cloud errors over the southeastern Atlantic in the NCEP CFS and their association with lower-tropospheric stability and air-sea interaction. J. Geophys. Res. (atmosphere), (submitted).

Part II:Observed Multi-Time Scale

Variability in the Tropical Atlantic

Bohua Huang(GMU & COLA)

Zeng-Zhen Hu(COLA)

Variance of Daily Precipitation in Different Time Scales

EOF1&2 of Daily Precipitation at 25-80 Day Time Scales

The Oscillation of EOF1&EOF2 (Period about 60 days)

Connection of the Tropical Oceans at 25-80 Day time Scales

_ ++

Connection with SST Anomalies

EOF1

Summary of Part II: Observed Multi-Time Scale Variability in the Tropical Atlantic

• There are multi-time scale variability in the tropical Atlantic Ocean.

• The variability at 25-80 day time scales in the tropical Atlantic is a part of intra-seasonal variability of global tropical oceans. There is a clear connection between global tropics and tropical Atlantic: out-of-phase: the tropical western Pacific; in-phase: Indian Oceans.

• The variability in the atmosphere is weakly coupled with SST.

Clouds in ISCCP & CFSISCCP (Weare 1999;

Rossow&Dueñas

2004)

CFS (Xu&

Randall 1996)

BL (>900hPa)

Middle (350-

650hPa)

Low (650-

900hPa)

High

Middle (400-680

hPa)

High

Low (680-

surface)

QuickTime™ and aTIFF (LZW) decompressor

are needed to see this picture.

QuickTime™ and aTIFF (LZW) decompressor

are needed to see this picture.Unstable &

convective cloudsStable & stratus clouds

ISCCP CFS