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?
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
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 SSTbased PDO index, 1900-1993.
Contour interval is 0.2 C.
Contour map of correlation
coefficients between gridded
North American DecemberFebruary (DJF) precipitation
and the SST-based PDO index,
based upon data for the period
1900-93.
http://www.atmos.washington.edu/~mantua/REPORTS/PDO/PDO_cs.htm
PDO: 1998-07avg = -0.0065 (N-M); 0.1846 (ann)
1988-07avg = 0.2225 (N-M); 0.1977 (ann)
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 (JulyAugust-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
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 midlatitudes.
• 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/textbook/circulation/upper_tropospheric_flow.html
Jet Stream Cross
Sections
http://www.usatoday.com/weather/tg/wjstream/wjstream.htm - from M. Gleuck
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
Climate forcing of moisture variability
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
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?
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?
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?
Basin Overviews
Select 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 resources:
Hydro-Climatic Data Network (HCDN): Streamflow Data Set, 1874 - 1988
By J.R. Slack, Alan M. Lumb, and Jurate Maciunas Landwehr
USGS Water-Resources Investigations Report 93-4076
http://pubs.usgs.gov/wri/wri934076/1st_page.html
USGS Surface-Water Data for the Nation: http://waterdata.usgs.gov/nwis/sw
Climate data resources:
Western Regional Climate Center, Historical Climate Information:
http://www.wrcc.dri.edu/CLIMATEDATA.html
PRISM (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)