Walker Circulation
Trades driven by
Walker Cell also
promote eastern
Pacific upwelling
and nutrient-rich
water
A. Evaporation from the warm tropical Pacific Ocean moistens the lower
atmosphere.
B. East to west surface trade winds transport the warm moisture laden air to
the western Pacific
C. Moist air rises and feeds rain in the western Pacific
D. Dried-out upper air moves eastward, and sinks in the eastern tropical
Pacific
http://www.gfdl.noaa.gov/research/climate/highlights/GFDL_V1N3_gallery.html
El Niño (warm ENSO, warm phase) events
• Trade winds weaken due to weakening pressure gradient across the
equatorial Pacific.
• The weakened trade winds allow warmer water from the western Pacific to
surge eastward, raising sea level in the eastern Pacific
• The rise in sea level in the eastern Pacific and a lack of upwelling lead more
warm surface water and a sinking of the thermocline (boundary between
warmer surface and colder deep water).
http://ww2010.atmos.uiuc.edu/(Gh)/guides/mtr/eln/def.rxml
• With warmer surface waters over the central and eastern Pacific,
convective clouds and thunderstorms shift eastward.
• This results in dry conditions in Indonesia and Australia with wetter
conditions in Peru and Ecuador.
http://ww2010.atmos.uiuc.edu/(Gh)/guides/mtr/eln/def.rxml
La Niña events are essentially the opposite of El Niño
events
- La Niña
- El Niño
http://eesc.columbia.edu/courses/ees/climate/labs/enso/index.html
Current La Niña event
El Niño events
Sea level and SST animation
ENSO conditions are measured by many methods, including satellites,
moored buoys, drifting buoys, and sea level analysis.
• sea surface temperatures (Nino 3, Nino 4, Nino 3.4)
• sea level pressure at Tahiti and Darwin, Australia (Southern Oscillation
Index, SOI)
• Multivariate ENSO index (MEI)
MEI included sea-level pressure, zonal and meridional components of the surface wind, sea surface
temperature, surface air temperature, and total cloudiness fraction of the sky.
http://www.cdc.noaa.gov/people/klaus.wolter/MEI/mei.html
Pacific Decadal
Oscillation
• A long-lived El Niño-like
pattern of Pacific climate
variability.
• Similar climatic pattern,
but different behavior in
time (greater
persistence)
• The climatic pattern is
most notable in the North
Pacific/North American
sector, with a weaker
center in the tropics
(opposite for ENSO).
http://jisao.washington.edu/pdo/
Typical wintertime Sea Surface Temperature (colors),
Sea Level Pressure (contours) and surface windstress (arrows)
anomaly patterns during warm and cool phases of PDO, ENSO
PDO index: the leading PC of monthly SST anomalies in
the North Pacific Ocean, poleward of 20N
http://jisao.washington.edu/pdo/
PDO and ENSO
• PDO and ENSO are not independent,
and PDO may actually be a manifestation
of slowly varying ENSO – the relationship
between the two is not yet completely
understood
Precipitation patterns related to PDO
Precipitation patterns related to ENSO
Hydroclimatology and ENSO
Gershunov, A. and T.P. Barnett. 1998. Interdecadal modulation of ENSO
teleconnections. Bulletin of the American Meterological Society, 79, 27152725. PDF
Rajagopalan, B., E. Cook, U. Lall, and B.K. Ray. 2000. Spatiotemporal
variability of ENSO and SST teleconnections to summer drought over the
United States during the 20th century. Journal of Climate 13, 4244-4255.
PDF
Climate and Water, 01/30/08
Gershunov and Barnett:
Possible discussion questions:
-This paper indicates that during high (low) NPO phases we should expect to see a
stronger climate response to El Nino (La Nina) . So what does this mean for
hydroclimatology in the Southwest? Would this mean more rain than "normal" El Nino
years and even less rain than "normal" La Nina years?
-The authors use SLP data and HPF data to show their results. Are there other forms
of data that may have been more useful, or are these two forms good enough for
their claim?
• I know we are in a weak La Nina right now, but this paper made me wonder what
the current NPO status is. I did some looking around the NOAA website, but couldn’t
find it. It looks like the southwest is impacted in a few of the combinations, so it would
be interesting to see what influence the NPO is having right now.
• In a world of secular warming trends (both ocean and surface) do any of the old
"rules" or associations apply? The pattern of SSTs and SLPs over the oceans will
obviously still exert strong atmospheric forcing, but will the storm track and/or
extremes change?
• A paper on the phasing of El Nino anomalies w/ respect to the annual cycle being
modulated by multidecadal variations in either the Atlantic or the Pacific?
Rajagopalan et al.
• As we accelerate climate change and sea temperatures rise (consequently
SSTs), can we also that the effects of ENSO on regional summer drought also
be amplified?
• Also, the article mentions other low-frequency climate indicators that lead to
drought. Can these known climate patterns also be used as a predictor of how
the climate will react to global warming?
• I am curious as to how the study of ENSO-PDSI (or any other drought index)
has evolved since 2000.
• It’s interesting that there are summer drought teleconnections with El Nino in
the SW. I’d thought that ENSO largely influenced frontal, winter precipitation
in the SW. Is this just a carry-over/memory effect of the lack of winter
precipitation, or is it possible that ENSO may actually moderate the
monsoon?
• “In terms of linear analyses,the key story appears to be the role of ENSO in
all three epochs as a modulator of drought, with the North Atlantic playing a
role in the eastern United States in the third epoch.” My question is, then,
what is modulating ENSO differently in the different epochs? Has there been
much progress on this since 2000? I know the “Perfect Ocean for Drought”
suggests CO2 forcing as one modulator, but what are others?
D.P. Brown and A. C. Comrie. 2004. A winter precipitation
‘dipole’ in the western United States associated with
multidecadal ENSO variability. GEOPHYSICAL RESEARCH
LETTERS, VOL. 31, L09203, doi:10.1029/2003GL018726
Winter (DJF) precipitation anomalies during
SON El Nino or La Nina years, for three
phases of PDO (warm, cool, warm)
http://www.climas.arizona.edu/forecasts/archive/nov2007/nov2007figs/9_prec_outlook.html
ftp://ftp.wcc.nrcs.usda.gov/support/water/westwide/snowpack/wy2008/sn
ow0801.gif
http://waterdata.usgs.gov/nwis/rt
Next week:
Climate forcing of moisture variability
Monday (back to hydroclimatology)
Guest speaker, Dr. Katie Hirschboeck.
Hirschboeck, K.K., 1988. Flood hydroclimatology, in Baker, V.R., Kochel,
R.C. and Patton, P.C., eds., Flood Geomorphology, John Wiley & Sons, 27 49.
Wednesday
McCabe, G. J., M. A. Palecki, and J. L. Betancourt. (2004). "Pacific and
Atlantic Ocean influences on multidecadal drought frequency in the United
States." Proceedings of the National Academy of Sciences 101(12): 41364141. PDF
Seager, R., Y. Kushnir, C. Herweijer, N. Naik and J. Velez, 2005: Modeling of
tropical forcing of persistent droughts and pluvials over western North America:
1856-2000. Journal of Climate, 18(19): 4065-4088. PDF