NOAA Research Home » Outstanding Papers
NOAA Research 2007 Outstanding Scientific Paper Awards
“Weakening of tropical Pacific atmospheric circulation due to anthropogenic forcing”
ABSTRACT
Since the mid-nineteenth century the Earth's surface has warmed, and
models indicate that human activities have caused part of the warming by
altering the radiative balance of the atmosphere. Simple theories suggest
that global warming will reduce the strength of the mean tropical atmospheric
circulation. An important aspect of this tropical circulation is a large-scale
zonal (east–west) overturning of air across the equatorial Pacific
Ocean—driven by convection to the west and subsidence to the east—known
as the Walker circulation. Here we explore changes in tropical Pacific circulation
since the mid-nineteenth century using observations and a suite of global
climate model experiments. Observed Indo-Pacific sea level pressure reveals
a weakening of the Walker circulation. The size of this trend is consistent
with theoretical predictions, is accurately reproduced by climate model simulations
and, within the climate models, is largely due to anthropogenic forcing.
The climate model indicates that the weakened surface winds have altered
the thermal structure and circulation of the tropical Pacific Ocean. These
results support model projections of further weakening of tropical atmospheric
circulation during the twenty-first century. FULL
TEXT 
Summary of the linear trends in SLP gradient across the Indo-Pacific (ΔSLP) from observations and the various GCM historical radiative forcing experiments. Circles indicate the trend value from each observational data set: K, Kaplan (1854–1992); H, Hadley Centre (1871–1998); and B, a blend of Hadley and Kaplan, extended into 2005 using the NCEP gridded ship data (1854–2005). Model trends are computed over the period 1861–2000. Confidence intervals are computed from a 2,000-year control experiment, at the two-sided P = 0.05 level. (larger image)
Spatial pattern of observed and modelled sea level pressure linear trends. Linear trend of sea level pressure (SLP) from: a, Kaplan SLP reconstruction (1861–1992), and ensemble-mean of GCM experiments (1861–1992) as follows; b, all-forcing (five-member mean), c, natural forcing (three-member mean) and d, anthropogenic forcing (three-member mean). The trend averaged over the domain 15° S–15° N, 0°–360° is removed from each panel. Dashed rectangles indicate the regions used to define the large-scale Indo-Pacific SLP gradient index (ΔSLP). (larger image)
Observed and modelled equatorial Pacific zonal-mean zonal wind-stress anomaly, <τx>, and equatorial thermocline depth anomaly, Ztc. Upper panels: model/observed ,tx. and reconstruction using linear relation to ΔSLP; dashed line shows (1854–2005) trend in ,tx. reconstructed using blended Kaplan/Hadley/NCEP ΔSLP. Lower panels: Ztc in the western (black line, 2° S–2° N, 140° E–180° E) and eastern (blue line, 2° S–2° N, 130° W–90°W) equatorial Pacific. Left panels: ensemble-mean all-forcing CM2.1 GCM experiment, showing five-year running mean. Right panels: five-year running mean (thick lines) and annual-mean (thin lines) observational estimates. Observed stress is from European Centre for Medium Range Weather Forecasting Reanalysis 40, observed Ztc is from GFDL ocean data assimilation. Ztc is the location of the maximum vertical temperature gradient. Note different scales in each panel. (larger image)
NOAA Research Home » Outstanding Papers
CLIMATE · OCEANS,
GREAT LAKES, and COASTS · WEATHER
and AIR QUALITY
ABOUT US · RESEARCH
PROGRAMS · EDUCATION · HOME
10/23/07