Influence of intraseasonal surface wind variability on ENSO in CFS predictions

Wanqiu Wang, Mingyue Chen, Arun Kumar, Yan XueClimate Prediction Center, NCEP/NOAA

NOAA's 34th Climate Diagnostics and Prediction WorkshopOctober 26-30, 2009 in Monterey, CA

1. Background

2. Objective

3. The NCEP CFS forecasts

4. Influence of intraseasonal variability (ISV) on ENSO prediction

Outline

1. ISV in the W. Pacific affects subsequent SSTs in E. Pacific.

2. A sequence of MJO events may result in the development of an ENSO event.

3. Strong ISV can lead to the termination of ENSO.

Impacts of ISV on ENSO

Background

1. Observed ISV that occurs before the start of the ENSO forecast may add to prediction skill.

2. Observed ISV that occurs during the forecast period could become a limiting factor for forecast skill.

3. Within an ensemble spanning a range of initial conditions, Individual forecasts may vary with the regime of initial ISV .

How does ISV affect ENSO prediction?

Background

Forecast with no clear separation among members

Background

IC: May 2006

Nino3.4: 190E-240E; 5S-5N

Nino3.4 SST forecasts from the CFS

Nino3.4 SST forecasts from the CFSForecast with separation among members

IC: May 2005

IC: Aug 2008

IC: Nov 2006

IC: Jun 2007

1. To diagnose to what extent the spread and the separation among the forecast members arise from initial intraseasonal surface wind anomalies.

2. To examine the impact of the initial ISV on the ENSO prediction skill and the dependency on the intraseasonal activities during the forecast period.

Objectives

1. Produced daily for 9 target months

2. 2005 –2007: 2 runs/day

2008 – Now: 4 runs/day

The NCEP CFS forecasts

U(t0) observed initial surface wind

T(t0,) forecast Nino3.4 SST

t0 = 1Jan2005 to 31Dec2008= 1 to 270 days

Influence of ISV on ENSO predictionIntraseasonal anomalies with respect to initial time

Applying a 10-70-day filter to t0 (NOT ) givesUI(t0)

TI(t0,)

Intraseasonal anomalies with respect to initial time

Raw anomalies

Intraseasonal anomalies with respect to initial time

Raw anomalies

CFS Nino3.4 SST anomalies

T(t0,)

TI(t0,)

t0 = 11 Jun to Jul 10, 2007 = 1 to 270 days

< 60 days: Local wind influence

Correlation: Initial intraseasonal U10m and forecast Nino3.4 SST

Where is the ISV that has strong impacts on ENSO prediction?

> 60 days: Remote impacts from W. Pac.

Lead

tim

e

(days)

Correlation: W. Pac (120E-160E) U10m and forecast Nino3.4 SST

How long does it take for the initial ISV to affect forecast ENSO?

Impacts of W. Pac. winds on Nino3.4 SST has a maximum around 80-day lags and the impacts persist throughout the entire target period.

Impacts of initial W. Pac. winds are carried through via subsurface Kelvin wave propagation.

Correlation: initial W. Pac. U10m and forecast SSH

How are the impacts of ISV carried through in the forecasts?

Impacts of W. Pac. winds on SSTs at different longitudes.

No effects on surface advection. We would otherwise see surface warming propagates from WPac to EPac.

Correlation: Initial W. Pac. U10m and forecast SST

CFS Nino34 SST (K)

Substantial change in forecasts from Dec 1-5 to Dec 6-10

10S-10N U850 (m/s)

Impacts of the MJO event in Dec 2006Influence of ISV on ENSO prediction

Dec 1

Dec10

Forecasts after Dec 5

Forecasts before Dec 5

Impacts of the MJO cycles in Nov-Jan 2007/2008

Observed U10m anomalies CFS Nino3.4 SST

Initia

l d

ate

Target date

1 Nov 2007 25 Oct 2008

t0 = 1 Nov 2007 to 31 Jan 2008 = 1 to 270 days

CFS Nino3.4 SST anomalies

U(t0) TI(t0,)

t0 = 1 Nov 2007 to 31 Jan 2008 = 1 to 270 days

1 Nov 2007 25 Oct 2008

U10m (120E-160E average)

Initia

l d

ate

Target date

Intraseasonal anomalies with respect to initial time

Summary

• There exists a substantial spread among the ENSO forecasts started from different days, and at times, the ensemble members show a clear separation.

• The separation among the forecast members are found to be related to differences in subseasonal variability of surface winds in the western Pacific.

• It takes about 80 days for the impacts of the initial winds in the western Pacific to be carried through to the eastern Pacific by subsurface Kelvin waves.

• ENSO forecasts during Dec-Jan 2006/2007 and Nov-Jan 2007/2008 strongly depend on the phase of the MJO during these periods.