The Turn of the Century North American Drought: Global Context, Dynamics, and Past AnalogsRichard Seager

Me: Jared PersingerAtmospheric Science

Introduction• Large Scale Drought 1998-2004

• 1998-2002: Persistent La Nina• ( Cool tropical troposphere, pole-ward shifted jet streams, eddy-driven descent in mid-lats )

• 2002-2004: weak El Nino conditions prevailed• ( 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.

It’s a new thing 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.

Three Big Questions

1.) Does the recent drought fit into the same global pattern of oceanic forcing and circulation and precipitation response as prior persistent droughts.

2.) To what extent was the turn of the century drought driven by tropical Pacific SSTs?

3.) What are the details of atmospheric circulation, moisture transport, subsidence, and surface evaporation anomalies responsible for the most recent drought?

Data/ModelsPrecipitation 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.

GRAPHS…..

1982-83 El Nino

90’s El Nino

1997-98 El Nino86-87

Nino

Annual Mean Precip. In American West.

GHCN and GPCP and GOGA model forced with global SSTs.

Same estimates, but using the POGA-ML model.

1988-89 La Nina

Correlations 1979-2005All 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.

• 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.

GOGA model with global-observed SST Precipitation

POGA-ML model precipitation

Global Context of this drought (Observed Precipitation Anomaly for 1998-2002 Period)

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 1998-2002, 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.

Same figure, diff. time period.

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.

YES! Zonal Mean Perspectives ( Observed and Modeled zonal mean precipitation anomalies for northern ( left ) summer and ( right ) winter half years of the 1998-2002 period and 2002-2004 period )

1998-2002, reduction of precipitation at equator.

Increased precipitation in sub-tropics.Decrease in mid latitudes.

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.

More precipitation between 30 and 40 degrees north, further less north you go show more typical el nino conditions.

Bottom: POGA-ML model simulates increase in northern midlatitude precipitation in the winter half years, but the GOGA does not.

Zonal mean temp/wind Colors: Observed and modeled zonal mean temperature Contours: zonal wind anomalies

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.

AgainThis figure: Shows anomalies in zonal mean subsidence ( colors ), and meridional transport of zonal momentum by transient eddies.

The models show a clear relationship between subsidence and momentum fluxes along the lines expected in both seasons and hemispheres.

Observed quantities show similar relationship as the models.

Skipping a few graphs

Getting to the dynamical point. 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.

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.

No title for this slideThis 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.

Moisture Budget 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.

Same as last slide, but Summer half. 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.

Circulation Anomalies

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.

Almost Done ( signal to noise ratio )

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.

Comparisons

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

Comparisons 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.

Let’s Conclude

1998-2002 period 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.

2002-2004 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.

My thoughts/conclusions