Climate forcing of moisture variability

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Climate forcing of moisture variability

• Combine effects from ocean/atmospheric
  variability on hydroclimatic variability across
  North America
• Droughts due to the Pacific, Atlantic Ocean, and
  /or global SSTs, temperature trends?
• The role of the equatorial Pacific (SSTs) in
  Great Plains and SW US annual (and seasonal to
  some extent) precipitation
• Role of midlatitude SSTs?

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• 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).

Typical wintertime Sea Surface Temperature
(colors),  Sea Level Pressure (contours) and
surface windstress (arrows) anomaly patterns
during warm and cool phases of PDO, ENSO
http//jisao.washington.edu/pdo/
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Relationship between PDO and NH Winter Climate
Characteristic warm-phase PDO October-March air
temperature anomalies, in degrees C. This field
is based on linear regressions between the
gridded surface air temperature data and the
SST-based PDO index, 1900-1993. Contour interval
is 0.2 C. Contour map of correlation
coefficients between gridded North American
December-February (DJF) precipitation and the
SST-based PDO index, based upon data for the
period 1900-93.
PDO 1998-07avg -0.0065 (N-M) 0.1846 (ann)
1988-07avg 0.2225 (N-M) 0.1977 (ann)
http//www.atmos.washington.edu/mantua/REPORTS/PD
O/PDO_cs.htm
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Atlantic Multidecadal Oscillation
AMO index the ten-year running mean of detrended
Atlantic SST anomalies north of the equator.
Correlation of the AMO index with gridded SSTA
over the world ocean (all seasons). The thick
contour is zero and thin contours denote the 95
signicance level.
Correlation of the AMO index with climate
division rainfall with the Mississippi basin
highlighted by light gray fill. The larger
highlighted circles indicate correlations above
the 90 signicance level. The color bar applies
to correlations in both panels.
Enfield et al. 2001 In all but the summer season
(July-August-September) the patterns are
different from Fig. 1c and have far fewer
significant correlations. The summer season
pattern is similar and has many significant
correlations. We therefore believe that
multidecadal variations in summer rainfall are
mainly responsible for the observed relationship.
annual AMO has been positive since 1997
http//www.aoml.noaa.gov/phod/docs/enfield/enfield
_etal2001.pdf
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Mid-latitude Fronts, and Jet Streams

• The latitudinal temperature gradient across the
  Earth's surface, (and thus, the latitudinal
  pressure gradient), achieves a maximum in the
  mid-latitudes.
• This coincides with the polar front, the
  boundary between polar and tropical types of air.
  
• The polar front jet stream, is a high speed
  corridor of air responsible for creating and
  moving large pressure systems through the
  midlatitudes.
• Additionally, a subtropical jet stream forms at
  approximately 30 degrees due to convergence of
  air aloft at that latitude (higher up, lower wind
  speeds due to a lower pressure gradient).

http//www.uwsp.edu/geo/faculty/ritter/geog101/tex
tbook/circulation/upper_tropospheric_flow.html
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Jet Stream Cross Sections
http//www.usatoday.com/weather/tg/wjstream/wjstre
am.htm - from M. Gleuck
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• Polar Jet Stream, Circumpolar Vortex, and Rossby
  Waves
• Over the poles, very cold, subsiding air
  creating high pressure at the surface. 
• The surface subsidence creates a large area of
  low pressure aloft called the circumpolar vortex.
  
• At the equatorward boundary of the vortex lies
  the polar jet stream.
• The polar vortex expands and contracts through
  the year as a result of the shifting of surface
  energy receipt.
• The equatorward edge, along the jet stream,
  takes on wave-like forms called Rossby Waves (or
  planetary waves), with cold air invading into the
  middle latitudes and warm air pushing poleward,
  and a mixing of air masses.

http//en.wikipedia.org/wiki/Rossby_wave
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Climate forcing of moisture variabilityMcCabe,
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) 4136-4141. PDFSeager, 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
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• McCabe et al. questions to consider
• While this study shows nicely how much of the
  multi-decadal variability in drought might be
  explained by PDO, AMO and secular trends in
  temperature, how much of the overall variability
  in drought is captured by the multi-decadal band?
• What does it mean for water resources across the
  country that we seem to be entering a positive
  (warm) phase of the AMO, and potentially a cool
  phase of the PDO (especially in the SW)?
• How relevant is McCabe's assessment of the PDO
  and AMO today? Are we still in a positive AMO
  trend and if so, are McCabe's assumptions about a
  prolonged drought viable?
• More generally, what does it really mean to a
  water manager that there is a correlation between
  PDO, AMO and 20-year drought frequency? This
  seems like it could be very useful information in
  a general sense, but what does it mean on the
  timescales that these managers are doing their
  forecasting or allocating?
• Although it is not addressed directly in this
  paper, it would also be interesting to talk about
  how the different phases of AMO and PDO interact
  with the shorter-term variability in drought
  forced by ENSO, and how they may be related to
  one another (if at all).
• What about the seasonality in precipitation
  regimes seasonal signal in AMO, PDO?

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• More McCabe et al. questions
• Is the 100-year record used in this study long
  enough to robustly establish the relationships
  pointed out by the authors (e.g. we only really
  have a record that includes one and a half
  cycles of the AMO)?
• Is a 20-year running mean a reasonable (or the
  best) way to remove the annual and interannual
  signal in drought variability?
• Does the Monte-Carlo approach to determining the
  significance of the correlations between the
  running averages make sense? Is this a reasonable
  way to establish significance when the degrees of
  freedom are so low?
• McCabe used a Monte Carlo approach involving
  shuffling of the raw climate data to smooth the
  data. How reliable is this type of approach and
  has it been used by any other study?

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• Seager et al. questions
• Seager et al. found "a zonally and
  hemispherically symmetric component to the
  persistent droughts and pluvials". So when it is
  dry in the plains, it is also dry in the
  midlatitudes of each hemisphere, indicating a
  tropical origin. How does a mechanism for this
  work?
• What constitutes zonal versus meridional
  circulation and flow in the paper?

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Basin OverviewsSelect a river basin of interest
in western US (or North America). Investigate the
climate of region (range and seasonality,
variability, circulation influences), gage
records available, use the longest to show
hydrograph (variations on flow over the year) and
variability (year to year), discuss source of
water supply (e.g., summer/winter systems, snow
vs rain), flow characteristics, critical
activities and/or  water supplies issues
(endangered species, dams, hydro) in the basin. 
Develop a short (15 minutes) power point
presentation, and write a short summary for the
blog.Hydrologic data resourcesHydro-Climatic
Data Network (HCDN) Streamflow Data Set, 1874 -
1988By J.R. Slack, Alan M. Lumb, and Jurate
Maciunas LandwehrUSGS Water-Resources
Investigations Report 93-4076 http//pubs.usgs.gov
/wri/wri934076/1st_page.htmlUSGS Surface-Water
Data for the Nation http//waterdata.usgs.gov/nwi
s/swClimate data resourcesWestern Regional
Climate Center, Historical Climate Information
http//www.wrcc.dri.edu/CLIMATEDATA.htmlPRISM
(precipitation and temperature data in a
continuous, digital grid of monthly and  annual
estimates) http//prism.oregonstate.edu/ WestMap
http//www.cefa.dri.edu/Westmap/ (climate by
county, div)
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