the relationship between post 1997/1998 westerly wind events (wwes) and recent lack of enso related...
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![Page 1: 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)](https://reader036.vdocuments.us/reader036/viewer/2022062407/56649d625503460f94a4534b/html5/thumbnails/1.jpg)
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
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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.
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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
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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)
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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
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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.)
![Page 7: 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)](https://reader036.vdocuments.us/reader036/viewer/2022062407/56649d625503460f94a4534b/html5/thumbnails/7.jpg)
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.)
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The increase in equatorial easterlies is seen mostly during WWE intervals
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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.
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2oN-2oS Average SSTA
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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
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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
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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 ?