NCEP Climate Forecast Systems T62 vs. T126: Annual Cycle, ENSO and its Decadal Changes

Yan Xue

Climate Prediction Center

Acknowledgements:

Suru Saha, Wanqui Wang, Kyong-Hwan Seo, Boyin Huang

Climate Forecast System (CFS) – T62

1. Atmospheric component• Global Forecast System 2003

• T62 in horizontal; 64 layers in vertical

2. Oceanic component

• GFDL MOM3

• 1/3 in tropics; 1 in extratropics; 40 layers

• Quasi-global domain (74S to 64N)

3. Coupled model

• Once-a-day coupling

• No flux correction

• Sea ice extent taken as observed climatology

Wang et al. 2005

• Climatology well simulated• NINO3.4 amplitude larger than observation• Phase-locking to end of year• Early onset and late decay• ENSO too regular

Kirtman 2005

• Model’s low frequency modes differ from observation• Initialization shock

T62 vs. T126

• Climate Forecast System T62 went operational in 2004.

• Keeping the oceanic component unchanged, the horizontal resolution of the atmospheric component was increased from T62 to T126.

• Two sets of simulations with T126, each 100 year long, were studied here.

• Four sets of simulations with T62, each 32 year long, were studied here.

W S

S SW W

W S

Penland and Saha, 2005

T62

OBS

T126

• Characteristics of observed ENSO change with time: pre- and post-1976

• Mechanisms for decadal modulations of ENSO:– Background state changes (Fedorov and Philander 2000)– Interaction between annual cycle and ENSO– Atmospheric noise forcings – Interactions between the tropics and subtropics– Nonlinear dynamics

• What are the mechanisms controlling ENSO characteristics in T62 and T126?

• What are the biases of the mean and annual cycle in T62 and T126?

• How do the biases in models influence their ENSO characteristics?

• Why do the ENSO characteristics in T126 change from decade to decade?

Motivation

• ERSST in 1950-2005

• FSU wind stress in 1978-2005

• CMAP precipitation in 1979-2003

• Depth of 20oC isotherm from NCEP’s Global Ocean Data Assimilation System in 1979-2005

• Monthly and daily fields from T62

• Monthly and daily fields from T126

Data

Methodology• Spectrum analysis

• Linear regression

• Composite analysis

• Interannual Coupling Strength and Thermocline Coupling Strength

• Intraseasonal variance in 20-90 days

• Amplitude of SST anom. in T62 and T126 (1.2oC) is similar to obs except the maximum center is detached from the coast.

• Spatial structure of SST anom. is similar to obs except its meridional width is a little too narrow.

• Amplitude of prec. anom. in T62 and T126 (2.5 mm/day) is similar to obs except there are two maximum centers located to north and south of the equator.

• Spatial structure of prec. anom. is similar to obs except convection south of equator extended too far eastward, subsidence over Indonesian continent is too weak and subsidence to north of ITCZ too strong.

• Amplitude of westerly anom. in T62 (1.2 dyn/cm2) and T126 (0.9 dyn/cm2) is smaller than that of obs (1.5 dyn/cm2), but easterly anom. to north of westerly center is too strong.

• Spatial structure of zonal wind stress anom. is similar to that of obs except its meridional width is too narrow.

• Amplitude of positive D20 anom. in T62 (20 meter) and T126 (20 meter) is a little larger than that of obs (15 meter).

• Positive center in the eastern Pacific is too equatorially confined, and the negative center in the north-western Pacific is located too close to the equator, and extended too far eastward.

• SST anom. in T62 propagates eastward, while SST anom. in T126 is largely stationary, close to obs..

• Duration of SST anom. in T62 and T126 are too long compared to obs..

• Onset of ENSO is too early in T62.

Lag

Lag

• Prec. anom. in T62 propagates from the western Pacific to eastern Pacific, close to obs., while prec. anom. in T126 has no propagation.

• Prec. anom. in the far western Pacific in T62 and T126 is too weak.

• Prec. anom. in T62 is too large in the Indian Ocean.

Lag

• Zonal wind stress anom. in T62 propagates from the western Pacific to central Pacific, similar to obs., while T126 anom. is largely stationary.

• Easterly anom. in the far western Pacific in T126 is too weak.

• Anom. in the Indian Ocean in T62 is too strong.

Lag

• D20 anom. in T62 propagates from the western Pacific to eastern Pacific, similar to obs., while T126 anom. propagates little.

• T62 has too strong D20 precursor.

• T62 has too strong amplitude in the Indian Ocean.

early onset

2 year peaks

late onset

12 warm events

4 warm events

early onset

early onset

normal onset (12):

Dec-1 – Apr+0

late onset (3):

Jul+0 – Sep+0

early onset (4):

Sep-1 – Nov-1

late onset

onset: > 0.5oC Yr 0 normal onset (4):

Mar+0 – May+0

late onset (1): Aug

12 warm eventsearly onset

early onset

onset: > 0.5oC Yr 0

normal onset (12): Jan+0 (3), Apr +0 (1), May+0 (1), Jun+0 (3), Jul+0 (2), Aug+0 (2)

early onset (2): Oct-1

Air-See Feedback Loop

NINO4 TAUXNINO3.4 SST

NINO3 SST

D20

Interannual Coupling Strength (ICS)

Thermocline Coupling Strength (TCS)

warm SST large SST gradient

• Interannual Coupling Strength (ICS) is strongest during Feb-Mar and Aug-Sep, and weakest during May-Jul and Nov-Dec.

• ICS of T62 is weaker than that of observation.

T62

OBS

T126 CMIP2T126 CMIP1

• Both T62 and T126 are more stable than that of observation.

• T62 is more unstable than T126 during Nov-Feb.

TCS

weak upwelling

• Thermocline Coupling Strength (TCS) is strongest during Nov-Jan and weakest during Mar-Apr when upwelling is weakest.

• TCS of T62 is much stronger than that of observation, particularly in Feb-Apr and Jul-Sep.

• TCS of T62 and T126 are both stronger than that of observation, particularly in Feb-Apr and Jul-Sep.

• T62 is more unstable than T126 in May-Sep.

T126 CMIP2

T126 CMIP1

T62

TCS

T62

OBS

T126 CMIP2T126 CMIP1

• Both T62 and T126 are more stable than that of observation.

• T62 is more unstable than T126 during Nov-Feb.

T62T126 CMIP1 51-100 yr

T126 CMIP1 1-50 yr

T62 CMIP1

T126 CMIP1 51-100

T62 CMIP1

T126 CMIP2

W1

S1

S1 S2W1 W2

CMIP1 S1 – W1

CMIP2 S1 – W1

CMIP2 W2 – S1

CMIP2 S2 – W2

Summary • T62 and T126 have similar mean biases

– Negative SST biases (-1oC) in north-western Pacific, positive biases (2oC) in south-eastern Pacific

– Double ITCZ and too little prec. in the equatorial western Pacific

• T62 and T126 have different biases in annual cycle on the equator

– SST in the eastern Pacific peaks in May in T62 and in June in T126

– T126 has eastward propagating westerly and rain band in early spring in the far western Pacific

– Both T62 and T126 have too deep thermocline in June due to excessive westerly in the far western Pacific in spring

• T62 and T126 have similar phase-locking of warm events to end of year but T62 has onsets in Dec-1 – Apr+0 and T126 has onsets in Jan+0 – Aug+0.

Summary

• Interannual Coupling Strength of T62 and T126 are both weaker than that of observation, but they are compensated by their stronger Thermocline Coupling Strength.

• Interannual Coupling Strength of T126 is weaker than that of T62 in Nov-Feb, probably due to its confined rain band to the far western Pacific and delayed eastern Pacific warming.

• T126 is more stable than T62, and its onset of ENSO is more irregular than that of T62, suggesting that the system is near neutral and atmospheric stochastic forcings could play significant roles on its ENSO evolution.

• Decadal modulations of ENSO are related to background state changes – cool in the western Pacific, warm in the eastern Pacific, westerly anom. near the dateline, deep thermocline in the eastern Pacific and shallow thermocline in the western Pacific favor strong ENSO variability.

Future Work

• Explore the impacts of atmospheric noise forcings on ENSO.

• Explain why T62 has a much earlier onset than T126 does.

• Explore the roles of the Indian Ocean.

• Explore the roles of the subtropical Rossby waves on ENSO.

• Explore mechanisms for decadal modulations of ENSO.