Me: Jared Persinger
Atmospheric Science
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Large Scale Drought 1998-2004
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1998-2002: Persistent La Nina
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2002-2004: weak El Nino conditions prevailed
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( Cool tropical troposphere, pole-ward shifted jet streams, eddy-driven descent in mid-lats )
( Western North America remains in drought, while rest of global climate adjusts accordingly )
This study compares the most previous drought with previous
five persistent North American droughts on record from mid
19th century to present day.
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Only recently has climate modeling become advanced enough to study the causes of North American
drought. It is now recognized that the American west experiences yearly reduced rain-fall when the
tropical pacific ocean is in an anomalously cold, La Nina-like state.
Many GCMs have been able to simulate short droughts as a response to these imposed SST
anomalies.
Many studies have been done, and all results agree that tropical pacific SST anomalies are
important for drought generation in the mid latitudes, but disagree to varying degrees to the roles
of the Pacific, Indian, and Atlantic SST anomalies.
Most studies also show zonal and sometimes hemispheric symmetry of precipitation.
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1.) Does the recent drought fit into the same global pattern of oceanic forcing and circulation and
precipitation response as prior persistent droughts.
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2.) To what extent was the turn of the century drought driven by tropical Pacific SSTs?
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3.)
What are the details of atmospheric circulation, moisture transport, subsidence, and surface
evaporation anomalies responsible for the most recent drought?
Precipitation Data: 1979-2005: Satellite-gauge precipitation data ( GPCP )
Before that: ( GHCN ) Gauge data in historical climatology network based
on old station data.
Circulation Data: Temperature, winds, etc.
CLIMATE MODELS:
1. POGA-ML: Tropical Pacific SSTs specified.
2. GOGA: SSTs specified globally. SST data from two sources of ship and satellite
observations.
Model-data comparisons: Basically model simulations are compared with previous data to see
how accurate the information is.
Problems: Computation of the moisture budget is
difficult because of errors introduced by re-gridding.
It will not be easy to represent the atmosphere over
complex topography, which can induce error in the
vertical motions that cause precipitation.
Annual Mean Precip.
In American West.
1982-83 El
Nino
90’s El Nino
86-87 Nino
1997-98 El
Nino
GHCN and GPCP and GOGA
model forced with global
SSTs.
Same estimates, but
using the POGA-ML
model.
1988-89 La Nina
All SST patterns show an El Nino signal with a warm
tropical Pacific Ocean, but there are noticeable
differences in the Atlantic Ocean. Interestingly enough,
the all observational and GOGA regressions have a
cooler than normal Atlantic Ocean associated with wet in
the West, while the POGA-ML produces a warm Atlantic,
consistent with a response to the El Nino state.
GOGA model with global-observed SST Precipitation
POGA-ML model precipitation
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Observed and modeled precipitation in the West are strongly
positively correlated with tropical Pacific SSTs and weakly with
Atlantic SSTs. Although not shown, the tropical Atlantic SSTs
include a component positively correlated to tropical pacific SSTs.
Basically suggests that tropical Atlantic SST anomalies influence
North American precipitation, but make clear it is a secondary
influence relative to tropical Pacific forcing.
The La Nina ends in 2002, and 4
years after the drought starts,
the eastern equatorial Pacific
warmed and remained warm
until 2005. Despite the change,
drought continued.
Relative to the base period of
1979-2005, both winter/summer
periods were dry from 19982002, according to satellite rain
gauge data.
In October through
March seasons, there is a
near zonal band of
reduced precipitation,
which indicated tropical
Pacific forcing.
It’s normal for southern
South America, Brazil
region, to be dry during
La Ninas, but during this
time period it actually was
wet.
Shortcomings:
The models make
the precipitation far
more zonal than
observed.
October and March from 2002-2004
both models show a typical El Nino
like pattern of precipitation
anomalies. Aka they make North
America wet and fail to reproduce
the drought continuation.
Increased precipitation
in sub-tropics.
Decrease in mid latitudes.
1998-2002, reduction of precipitation
at equator.
Above: GOGA and POGA-ML model
agree. They capture reduced precipitation
in summer years in northern latitudes but
fail to simulate drought in southern mid
latitudes in both seasons.
Bottom: POGA-ML model simulates
More precipitation between 30
and 40 degrees north, further
less north you go show more
typical el nino conditions.
increase in northern midlatitude precipitation
in the winter half years, but the GOGA does
not.
Both model and observed changes in the zonal wind for
two seasons from 1998-2002.
Both show a cooler tropical
troposphere, weak subtropical jets in
each hemisphere. Subtropical jet
strength is due to sub tropical
tropospheric temperature change and
not due to change in hadley cell
strength.
This figure: Shows anomalies in
zonal mean subsidence ( colors ), and
meridional transport of zonal
momentum by transient eddies.
Observed quantities show similar
relationship as the models.
The models show a clear relationship
between subsidence and momentum
fluxes along the lines expected in
both seasons and hemispheres.
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Getting to the dynamical point.
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All latitude anomalies of precipitation minus evaporation can be easily accounted for by a change in
moisture transport by the mean meridional circulation. The moisture transport by the mean meridional
circulation can be almost completely accounted for by a change in the MMC and is not really impacted
by anomalies in the specific humidity. The MMC changes can be explained by transient eddy induced
flow.
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The turn of the century North American drought fit into a period of protracted La Nina events with
characteristic changes in temperatures, winds, eddy momentum fluxes, and mean meridional
circulation. An eddy driven MMC is able to explain changes in precipitation minus evaporation. The
dynamic setup reversed in 2002 when the la Niña ended, a change simulated by the models, so the last
two years of the drought are distinct from early years and don’t fit in the usual global dynamical
configuration.
This figure shows the observed
GPCP precipitation over North
America during the winter and
summer half years of 1998-2002
with the GOGA and POGA-ML
models.
Winter: Shows reduced precipitation
over most of country. NE Canada remains
normal. SE USA is the driest. Model are
realistic, but too weak.
Summer: SE USA and Florida mostly
effected. Models failed to simulate wet
conditions over central Canada and dry
condition in Gulf Coast.
NOTE: Because the models are very broadly similar,
this moisture budget may contain useful information about
the mechanisms that created the drought.
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Terms in the moisture budget of the ( left ) POGA-ML model and ( right ) GOGA model for winter half
years of 1998-2002 drought.
Evaporation largely reduced, because of reduced
precipitation.
Reduced transient eddy moisture flux
convergence SE USA and N Mexico, related to
weaker eddy activity in subtropics.
Reduced moisture convergence by stationary
flow produced a tendency to reduce precipitation
over West Coast and ocean areas south and east
of the USA.
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The precipitation reduction is over the continent and is
strongly supported by a reduction of surface evaporation.
This is indicative of soil moisture feedback in extending
drought from the winter half year into the summer half year.
In the summer, changes in stationary flow convergence
tends to reduce precipitation across the drought region,
which is consistent with the prior droughts. The pattern of
anomalous stationary flow moisture convergence in both
seasons is accounted for by a change in circulation.
These figures shows 500 mb vertical pressure
anomalies and 700 mb horizontal flow
anomalies for both models for summer and
winter halves of the la Niña years.
Here we see that the regions of
reduced precipitation are regions of
anomalous subsidence. In the zonal
mean, eddy induced subsidence
forces the reduction in precipitation.
Signal to Noise Ratio:
Ratio
of GOGA ensemble mean precipitation
anomaly for a.) winter and b.) summer
half years of 1998-2002 period to
standard deviation of spread of
precipitation anomalies of ensemble
members from ensemble mean.
A ratio of -1 ( minus 2 ) means the
SST forced precipitation anomaly is
twice the size as the model internal
variability and most of the ensemble
members have negative precip.
Anomalies.
WINTER: Most of North America has
ratio above 1.
Summer: Ratio less
than that of winter.
Indicates smaller SST
forced signal, but the
west still is mostly dry.
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Recent Drought: Warm Indian Ocean
(Uncharacteristic for La Nina ) and cool
tropical Pacific.
Previous Droughts:
 1856-65 Civil War Drought
 1870’s and 1890’s Dust Bowl Drought
 1950’s
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No North American Drought has ever occurred that was not coincident with a sustained La
Nina
Pacific Ocean SST part of the drought is consistent drought to drought.
Each drought has dry conditions across the western and central United States.
The most recent drought is similar to all prior droughts in terms of the SST field that forced
it.
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1998-2002 period
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La Nina-cool tropical Pacific SSTs.
Cool Pacific cools troposphere and weakens pole-ward
movement of subtropical jet stream.
Pole-ward flow in the subtropics and descent in midlatitudes
suppressed precipitation.
Comparisons of simulations with global SST forcing and
others conducted with only tropical SST forcing indicate the
primacy of tropical pacific forcing, with Atlantic Ocean
possible playing a secondary role.
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Tropical Pacific SSTs became warm as weak el Niño sets in.
Stronger subtropical jets, induced equator-ward flow, and
ascent in the mid-latitudes.
These two years of drought were divorced from the usual
global dynamical context in North America.