The relationship between post 1997/1998 Westerly Wind Events (WWEs) and recent lack of ENSO

related cold-tongue warming

D.E. Harrison and

A.M. Chiodi (presenting)

Joint Institute for the study of the Atmosphere and the Ocean, University of Washington

andNOAA Pacific Marine Environmental Laboratory, Seattle, WA, USA

Motivation: observed differences in WWE-related cold tongue warming

Why is there little warming post 1997/98 ?

Results shown are for WWEs that occur with |Nino-3| index < 0.75 C.The left panel is reproduced following Vecchi and Harrison (2000.) Black boxes denote WWE regions.

Outline

• WWE description

• Compare pre and post 1997/98 WWE wind anomalies

• Equatorial Pacific modeling results

-Effects of single WWEs

-Implications for WWE-driven cold

tongue warming during an ENSO event

WWE Description• Associated with various atmospheric

phenomena (e.g. tropical cyclones, cold surges, MJO-type convective clusters)

• WWE zonal wind anomalies have large amplitudes (mean 7-10 m/s, peak > 15m/s) and large zonal scales (1400-2500km) but short timescales (6-20 days).

• Associated with development and maintenance and of ENSO related cold-tongue warming

• WWEs have been shown to be crucial to accurate model hindcasts of several warm ENSO events.

• Modeling studies have shown increases up to 0.5oC for single WWEs with stress anomalies along the equator

Above is a NOAA NGDC satellite photograph of a tropical cyclone (Super Typhoon Keith) that was likely associated with a 1997 WWE when it formed near the Marshall Islands (approx. 8o N, 170oE)

WWE composite 10 m wind anomalies

W-type

Data from 12-hourly ECMWF operational analysis.Base period for climatology is 1986-1996.Significant agreement with TAO buoys (west of 120oW.)

A significant increase in the easterlies is seen during post 97/98 WWEs

WWE composite 10 m wind anomalies

C-type

Data from 12-hourly ECMWF operational analysis.Base period for climatology is 1986-1996.Significant agreement with TAO buoys (west of 120oW.)

WWE composite 10 m wind anomalies

E-type

Data from 12-hourly ECMWF operational analysis.Base period for climatology is 1986-1996.Significant agreement with TAO buoys (west of 120oW.)

The increase in equatorial easterlies is seen mostly during WWE intervals

Effects of single WWEs on model SST

• Apply basin scale WWE zonal stress anomaly to the model

• Compare effects of pre and post 1997/98 WWEs

Model description (NOAA GFDL’s MOM4) • Anomaly added everywhere west of 120oW (based on comparison with TAO data)• Zonal stress anomaly added for 40 days centered in March• Initial conditions taken from a 20-year climatological run designed to reproduce the seasonal cycle of the upper equatorial Pacific Ocean• Here SSTA is determined from a run integrated without the WWE• Surface heat flux based on Philander and Siegel (1985) and Harrison (1989)•Grid: 27 vertical layers, 1/3o meridional x 1o zonal resolution along equator, realistic coastlines, no bottom topography•Psuedo-stress anomaly is computed using Cd = 1.3 x 10-3 and added to the COADS-based climatology (Harrison 1989) used in model spin-up.

2oN-2oS Average SSTA

ENSO-related WWE effects: What happens if a string of WWEs is applied?

• Multiple W, C and E zonal stress anomalies added to the climatology

• WWE timing based four recent El Nino events

• Compare effects of pre and post 1997/98 WWEs

Yearly Comparison of SSTA and average WWE strength

Period of year

WWE 1 Jun – 1 DecSSTA 1 Sep – 1 Feb ( year + 1)

The correlation improves when stress is averaged over a (larger) region that resolves the increased easterlies.

0.66 Correlation

0.74 Correlation

Conclusions• WWE-related cold tongue warming has been weak, on average, during

1999-2006.

• Averaged over 1999-2006, there has been a dynamically significant increase in the equatorial easterlies during WWE intervals.

• Model results show that these easterly anomalies are capable of canceling out the warming that would otherwise occur due to the WWEs.

• These results suggests that it is best to consider basin scale wind anomalies when assessing or predicting the effects of WWEs.

• What causes the easterly anomalies discussed here ?