The Great Plains Low-Level Jet In Global and Regional Reanalyses: Climatology and Interannual Variability

Scott J. Weaver Department of Atmospheric and Oceanic Science

And

Sumant Nigam

Department of Atmospheric and Oceanic Science, and Earth System Science Interdisciplinary Center

University of Maryland, College Park, MD 20742-2425

(Submitted to Journal of Climate on February 15, 2006)

Corresponding Author: Scott J. Weaver 4337 Computer and Space Sciences Bldg. University of Maryland, College Park, MD 20742-2425 Email: sweaver@atmos.umd.edu

1. Introduction

Means (1952) was the first to use the term low-level jet (LLJ) defined as a zone of strong

southerly flow occurring below 850 mb. In the United States LLJ’s are found along the east and

west coasts and in the Great Plains. The Great Plains low-level jet (GPLLJ) has the most

frequent occurrence of all three geographic locations. Additionally, LLJ impacts are more robust

in the central United States than in any other region. This is due partly to the vast amounts of

moisture that is transported to the central United States via the LLJ as well as the dynamical

influences that the LLJ has on the regional divergent circulation.

There are several plausible theories for LLJ formation as well as many studies describing the

effects and impacts of the LLJ on weather and climate. The GPLLJ was shown to form due to

supergeostrophic wind speeds as a consequence of the spatially uneven and diurnally varying

heating of the terrain slope to the west, the so called thermal wind effect (Wexler 1961). Diurnal

LLJ variability was also related to boundary layer frictional effects and subsequent rotation of

the ageostrophic wind vector (Blackadar 1957, Hoxit 1975). Uccellini and Johnson (1979)

analyzed the symbiotic relationship of the GPLLJ with upper level jet streaks.

One of the most important discoveries was the influence of the LLJ on Great Plains nocturnal

thunderstorm formation and maintenance. Naturally this led to other linkages such as the

connection between the LLJ and Mesoscale Convective Complexes (MCC’s) (Maddox 1980,

Maddox 1983, Wetzel et al 1983). As higher resolution sampling and modeling of the

atmosphere began to take hold, LLJ studies became somewhat more intricate. Wind profilers

were used to document the details of the LLJ on an hourly or sub hourly timescale (Mitchell et al

1995, Arritt et al 1997, Zhang et al 2005). Mesoscale models were also used for studying the

LLJ (Zhong et al 1996, Pan et al 2004, Zhang et al 2005). Aspects of the diurnal cycle were

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elucidated (Arritt 1997, Bonner 1968, Mitchell 1995, Parish 1988). This enabled investigators to

understand the LLJ in much more detail and begin to look at possible connections to other

aspects of weather and climate. The results of these high temporal and spatial resolution studies

lead to increased understanding of the role of topography and surface heat fluxes in forcing the

GPLLJ. Some studies have even focused on the influence of the LLJ on the transport of

chemical constituents (Zhang et al 2001).

Other perspectives related to the GPLLJ were also forged. Climatological studies began to

show larger scale long-term linkages. It has been show that the LLJ is correlated to hydroclimate

in the Great Plains via moisture fluxes. (Helfand and Schubert 1995, Higgins et al 1997,

Schubert et al 1998) Additionally, many authors show a direct link of the GPLLJ to floods or

droughts in the United States (Mo et al 1995, Bell and Janowiak 1995, Trenberth and Guillemot

1996, Mo et al 1997). Studies using the NCEP global reanalysis and comparison to wind profiler

datasets show that the NCEP reanalysis has been able to represent most of the gross properties of

the LLJ and its relationship to precipitation in the Central United States (Anderson and Arritt

2001). However, due to the coarse resolution of the original reanalysis it is very likely that some

aspects of the LLJ (height of wind max, speed direction etc.) have evaded detection or have been

misrepresented.

While the high resolution and long-term climatological studies have been important for

comprehension of the nature of the LLJ on various timescales there are shortcomings associated

with each of these viewpoints. High-resolution observational and modeling studies are essential

to our understanding of the physical processes and short term (hours-days) impacts of the LLJ.

However they cannot be used to diagnose the lower frequency (monthly) processes associated

with the GPLLJ and subsequent LLJ impacts due to the truncated output record. Conversely, the

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long term reanalysis products used for low frequency studies suffer from low spatial and

temporal resolution, at least in the context of LLJ evolution; The jet can mature and decay within

6 hours. Such limitations can be over come with the advent of the North American Regional

Reanalysis (NARR). NARR is a consistent, 24-year, high-resolution dataset spanning the North

American domain and is viewed as a major improvement over the original global reanalysis. Not

only is the spatial and temporal resolution much finer than the NCEP reanalysis, NARR,

additionally makes use of the regional ETA model and the attendant recent advances that were

made to the ETA.

The present study’s unique attributes are:

• Expansive data record (24 years) used to generate a GPLLJ climatology.

• GPLLJ traced by climatological low-level flow instead of event frequency.

• Creation of a GPLLJ index to study interannual variability.

• GPLLJ connections to local and remote circulation features and Great Plains

hydroclimate.

• EOF analysis on GPLLJ signature to diagnose recurrent variability patterns and the

associated precipitation characteristics and circulation features.

The present study is motivated by the surge in scientific interest in understanding regional

hydroclimate – an important goal of the U.S. water cycle science plan (Hornberger 2001). The

GPLLJ figures prominently in this scheme given the recent findings that gulf moisture transports

(via GPLLJ) dominate local evaporation in the generation of precipitation variability. The

GPLLJ fluctuations have, moreover been shown to be associated with larger scale circulation

anomalies, some originating in the Atlantic and Pacific basins (Ruiz-Barradas and Nigam 2005).

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The GPLLJ is a key player in bringing moisture from source (Gulf of Mexico) to sink (Great

Plains precipitation).

Although the LLJ is primarily a mesoscale, warm-season (April – September), nighttime

phenomena, its persistent occurrence over the Great Plains during the summer months should

allow the LLJ to reveal itself in the summertime monthly climatology of the low-level flow.

While most studies employ the Bonner criterion of a wind max of at least 12 m s -1 and a

decrease of at least 6 m s -1 to the 3 km level, due to the monthly averaged flow being used here

that definition is not applicable. The aim is to assess the structure of the monthly averaged low-

level flow and interannual variability of the GPLLJ. Some pertinent questions along this front

are:

• What are the characteristics of the diurnal cycle of monthly meridional water vapor

fluxes?

• How is the GPLLJ related to monthly synoptic flow features?

• What are the possible large-scale forcing mechanisms of such patterns?

• Are there any links to Atlantic or Pacific SST anomalies?

• How realistic is NARR in representing GPLLJ precipitation linkages?

• What are the recurrent patterns of GPLLJ variations and there circulation and

precipitation linkages?

Monthly warm season climatology of the low-level flow is constructed using NARR.

Features of the jet climatology are used as the basis for a GPLLJ index definition with which to

study interannual variability of the GPLLJ and large-scale circulation linkages. A vertical jet

like profile is seen in the monthly averages and as such we refer to this as the GPLLJ although

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we concede that the traditional definition of the GPLLJ is typically defined as a nocturnal

maximum in wind speed with attendant vertical shear. Monthly, diurnally averaged GPLLJ

index variations show the presence of interannual variability and some relationship to previously

noted wet and dry periods. Regressions of the monthly averaged nocturnal GPLLJ index on

various circulation fields and SST’s in the Atlantic and Pacific basins show patterns similar to

previous studies dealing with Great Plains precipitation anomalies supporting the view that the

GPLLJ is modulated by remote regions and is a part of the mechanism for precipitation

anomalies in the Great Plains. Section 2 will describe the data used in this study. In section 3

we will discuss the low-level flow structure. Sections 4 and 5 will present the GPLLJ index and

regressions respectively, while section 6 will discuss EOF analysis of the GPLLJ. Section 7 will

provide a brief summary of findings and suggestions for future work

2. Data Description

The North American Regional Reanalysis (NARR) is a 25 year (1979-2003), consistent,

high-resolution dataset that covers the North American domain. Some comparisons with other

reanalysis datasets will necessitate the use of a slightly shorter 23-year NARR time period. The

NARR dataset is similar to the original NCEP global reanalysis (GR), however has several

improvements over the GR. Most of these lend themselves nicely to the study of long-term

mesoscale variability and the impacts on hydroclimate. The most striking difference between the

NARR and GR is the spatio-temporal resolution upgrade. The NARR has a 3-hour time step and

one-third degree spatial resolution. Additionally, NARR assimilates direct observations of

precipitation whereas the original NCEP GR uses forecasts of latent heating to diagnose the

accumulated precipitation. The NARR also takes advantage of the regional ETA model

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including many of the upgrades that were made to this model and the data assimilation scheme

that ETA employs. The ETA upgrades are associated with GCIP funded initiatives in

hydrological research, land atmosphere coupling, and improvements in the NOAH land surface

model. Some additional improvements include: Direct assimilation of radiances, the use of

additional sources of data (table 2 Mesinger et al 2003), and improved data processing.

Additional information can be found at (http://www.emc.ncep.noaa.gov/mmb/rreanl/)

The ERA-40 is a global reanalysis project with output spanning the period September 1957-

August 2002. It is comprised of conventional observations and satellite data streams. Analyses

were produced at 0Z, 6Z, 12Z, and 18Z for the entire period. The atmospheric model used in

generation of the analyses is comprised of 60 vertical levels and 125 km grid spacing for surface

and other gridded fields. For more information please go to:

(http://www.ecmwf.int/products/data/archive/descriptions/e4/)

Modest use of the NCEP/NCAR reanalysis is employed in this study. The reader is referred to

Kalnay et al (1996) for a complete description.

3. Climatology of Low Level Flow Characteristics in NARR

a. Meridional Water Vapor Flux

The signature of the GPLLJ can be seen at many vertical levels within the lower troposphere.

Additionally the jet is a major component that governs the strength of meridional moisture

fluxes. We begin our description of the GPLLJ by depicting climatological characteristics of the

warm season (April-September) monthly averaged 1000-700mb integrated meridional water

vapor flux (VFLX). Commencing the LLJ analysis via VFLX is advantageous as the entire

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vertical range of LLJ occurrence and the impact of the LLJ on VFLX can be compactly viewed

through inspection of one variable.

Figure 1 depicts the evolution of VFLX throughout the warm season as seen in NARR.

Enhanced warm season meridional moisture fluxes can be seen in various locations. Modest

inflow from the Gulf of Mexico occurs in mid spring and increases throughout early summer

reaching a peak of 180 kg m –1 s -1 over extreme northeastern Mexico in July. There is a

pronounced coupling of the Caribbean Sea and the Gulf of Mexico, which influence the flux of

moisture onto the North American Continent in June and July. Additionally, the moisture fluxes

associated with the Bermuda high over the western North Atlantic is clearly evident especially in

the summer months of June, July, and August. Some other features include the California LLJ

and the moisture flux associated with the Monsoonal flow over the desert southwest in late

summer.

Focusing on the Central United States not only does the VFLX increase in magnitude

throughout late spring and summer but the geographical extent moves further northward

encompassing the entirety of the southern and central Great Plains region. July exhibits the

sharpest tongue of moisture flux into the Central U.S. Progressing toward the end of summer we

see a further northward march of the core of max VFLX culminating over the Central Plains in

September with a significant drop off in magnitude. The rectangular box in the June and July

panels outline a subjectively chosen spatially coherent region that encompasses the range of

significant VFLX influence.

Figure 2 shows the monthly diurnal amplitude and phase of the VFLX in NARR for May,

July, and September obtained through harmonic analysis. The boxed area outlines the region

with the maximum amplitude as in figure 1. The length of the arrow indicates VFLX magnitude,

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while the arrow’s direction illustrates the time of VFLX maximum. For example, an arrow

pointing directly northward shows a VFLX maximum at 6 AM CST. As in figure 1 it is obvious

that the VFLX increases in magnitude with the progression from late spring to summer and

decays toward early fall. The diurnal cycle follows a nocturnal rise and decay just as many

previous studies of the GPLLJ have noted. Blackadar (1957) showed that the early morning

maximum in low-level flow was due to the periodicity of the inertial oscillation of the

ageostrophic wind vector. Inspection of the Great Plains region shows that in every month there

is a pronounced early morning maximum in VFLX. In the Caribbean Sea the preference seems

to be a late night max in VFLX. The east coast region of the U.S. also shows an early morning

preference for maximum VFLX albeit with a smaller magnitude. A common feature between the

Great Plains and the east coast is that of a mountain range situated to the west (Rockies,

Appalachians). LLJs have been shown to form in an area of sloping topography.

b. Mean Meridional Wind Profiles

While examining the characteristics of integrated meridional moisture flux shows the role of

the GPLLJ on promoting positive northward moisture flux from the Gulf of Mexico it lacks in

the depiction of vertical characteristics and the latitudinal variability. Figure 3 shows the warm

season (April-September) monthly averaged meridional wind at 25°N and 30°N. Longitude-

height profiles of meridional wind for May, July, and September were produced from NARR

between 92-104°W and 1000-700 mb. At First glance it is immediately apparent that the LLJ

influence is robust in the monthly averages of low-level flow even though it is primarily a

nocturnal phenomena. At 25°N the influence of the topography is evident as the core of

maximum meridional wind is strongly associated with the mountain barrier. A much weaker

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secondary core of maximum meridional wind occurs between 100-102°W located over the peak

topography. Significant average vertical wind shear is present above the core in all months

except September. As the warm season progresses the average low level max in meridional flow

increases reaching a peak of 8 m s -1 in July and drops off rapidly during September. In all

months the core of maximum meridional wind is located near 925 mb at 25°N.

The difference in low-level flow between the GPLLJ onset (May) and decay (September) is

noteworthy and can be the effect produced by the enhanced lee side cyclogenesis and dynamic

forcing of the low-level flow to the east of the Sierra Madre Oriental Range during springtime 1.

The tightly packed core indicating a preferred location of LLJ occurrence could be due to the

duel thermal effects created by the close proximity of the steep topography to the west and the

land sea interface to the east. Land sea boundaries have also been noted to have an impact on the

LLJ as these areas can generate shallow baroclinicity (Zhang et al 2005). Of notable interest is

the eastward stretching of the core at 25°N during late spring. Perhaps this is a reflection of the

thermodynamic influence of the underlying sea surface at these longitudes. That being the

greater diurnal land sea horizontal temperature gradient during May. Another factor could be

the reduced nocturnal maximum caused by the decreased frictional influence of the Gulf of

Mexico in the time mean flow.

The right column of figure 3 also shows the meridional wind over the same longitude domain

although at 30°N. There are marked differences in the height (pressure), strength, and width of

the core of maximum meridional wind. The topographic influence is perhaps lessened due to the

reduced incline of the westward slope and reduction in preferred location of thermal wind

1 The structure of low-level flow for April is similar to May albeit slightly weaker. However it is still much stronger than September suggesting that the dynamic forcing of the GPLLJ in spring is more robust than in early fall.

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influences. Additionally, there is no land sea interface directly to the east as 30°N just skirts the

Gulf coast. The lack of an eastern temperature gradient influence is shown by a manifestation of

a weaker west to east meridional wind gradient than at 25°N suggesting less spatial constraint of

the location of the GPLLJ as influenced by surface characteristics, which promote horizontal

temperature gradients. In general there is a similar evolution of the meridional wind field at

30°N with the exception of the presence of a slight double maximum in the meridional wind

during July. This could be a northern signature of the core of maximum wind found at 25°N,

which was found a bit further to the east, suggesting the possibility of a double jet structure.

Two very interesting differences between the meridional wind at 25°N and 30°N is that at

30°N the core has shifted further to the west and is now seen at higher level around 875 mb. The

eastward down sloping of the monthly wind max profile is a signature of the influence of the

gently sloping terrain in the lee of the topographic barrier. The increase in height, westward

location of the core, and zonal stretching of the monthly meridional wind profile at this latitude

demonstrate the significant influence that topography has on LLJ structure and preferred location

when compared with GPLLJ characteristics at 25°N. The core of max wind at 35 °N (not

shown) is much more elongated and less intense than the core at 25°N and 30°N with meridional

wind maximum generally extending from 925-850 mb.

c. Diurnal cycle of Meridional wind

Up to this point we have focused on GPLLJ characteristics as seen in the monthly NARR

output. The traditional definition of the GPLLJ is one that includes the presence of a diurnal

cycle. It has been shown in figure 2 that VFLX is a maximum in the Great Plains at some point

during the overnight hours. Figure 4 shows meridional wind profiles in a similar fashion to those

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in figure 3. Here we are looking at the July average meridional wind at the time of maximum (7

AM CST) and minimum (4PM CST) magnitude. Immediately apparent is that there is a

considerable difference in the strength of meridional wind between early morning and late

afternoon. Some previous studies have noted a doubling of the daytime wind speed at night

(Stensrud 1996). NARR does not produce such a strong diurnal oscillation in the time-averaged

flow at 25°N. Average meridional wind speeds here range from 6 m s–1 at the time of local

minimum to 10 ms–1 during local maximum, which is considerable but not quite double.

However at 30°N the range is from 4 ms–1 to 10 ms–1 which is more than double. This suggests

that at lower latitudes the diurnal oscillation in low level flow is not as strong as at northerly

latitudes.

4. Low-Level Jet Index

a. Index Definition

Recent interest in interannual variability of Great Plains hydroclimate requires the study of

not only the sources (oceans) and sinks (precipitation) of moisture but of the mechanisms

through which moisture is transported from one location to another. The detailed high-resolution

view obtained from an inspection of the structure of the Great Plains LLJ and meridional

moisture fluxes is merely a first step in a comprehensive analysis of the GPLLJ. Since the

GPLLJ has been shown to be an influential aspect of Great Plains hydroclimate via moisture

fluxes and precipitation over the central U.S. has been demonstrated to have significant

interannual variability then a natural question to ask is: What are the characteristics of GPLLJ

variability?

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To facilitate the study of GPLLJ variability we begin by choosing the box of 100°W-95°W

and 25°N-35°N based on inspection of the salient features of the previously presented

climatology. These being that a majority of the max values associated with moisture fluxes and

meridional wind occur in this region. This is the same box outlined in figures 1 and 2.

As a check on the robustness of choosing this area for a GPLLJ index and for facilitating

further refinement of the index by selecting an adequate pressure level for index definition,

figure 5 shows monthly vertical profiles of the area averaged meridional wind within the box in

NARR (April-Aug) and NCEP and ERA-40 reanalyses (July only) 2. The lower vertical

resolution (75 mb) NCEP and ERA-40 reanalyses capture the maximum in wind quite well but

are challenged in depicting the vertical shear. This is most likely due to the coarse vertical

resolution at play. In all months the maximum in low-level meridional flow occurs at either 900

or 925 mb in NARR and thus it could be argued that either of these levels would be sufficient for

a GPLLJ index definition. Based on the common view of LLJs that there be significant shear

above the jet core we choose 900 mb as there is a stronger presence of vertical shear above 900

mb than above 925 mb in all months in NARR. In fact there is no decrease in wind speed above

925 mb in April and May, and an insignificant amount in June. Thus we define the Great Plains

low-level jet (GPLLJ) index as the area averaged meridional wind at 900 mb over the latitude

longitude range of 25-35° N and 100-95° W.

2 An examination of the effect of defining different latitude-longitude ranges was investigated. Two additional indices were defined using the same longitude range as the aforementioned box (100-95° W) however with different latitude ranges of 22-30° N and 30-37° N respectively. These indices were then cross-correlated to check if there was a noticeable difference of the LLJ index’s dependence on these latitude variations. It was found in NARR that the 22-30°N index correlated at .941 to the 25-35° N index, while the 30-37° N index correlated at .935 with the 25-35° N index.

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b. Index Variations

Figure 6 shows the time series associated with the GPLLJ index from NARR and ERA-40.

The first two panels show the GPLLJ index for the entire ERA-40 time period. The bottom

panel represents the GPLLJ index in NARR during the overlapping (1979-2001) time period

with ERA-40 3. Each data triangle represents the monthly (April – September) GPLLJ index in

accordance with our previous definition. The GPLLJ index has substantial intraseasonal

variability and switches sign in every year. Index variations in both NARR and ERA-40 are

strong and mostly consistent between the two reanalysis datasets with SD of 1.3 in ERA-40 and

1.4 in NARR. The correlation during the overlapping period is a robust .971 suggesting similar

skill in tracking the anomalies defined by the GPLLJ index.

Interannual variability can be seen by the solid continuous bold lines, which are generated by

a 1-2-1 smoothing of the warm season (April-September) mean index anomalies. Another

interesting feature is the characterization of the drought of 1988 and the flood of 1993 in both

NARR and ERA-40. In 1988 all months with the exception of September show anomalously

low meridional wind speeds (reduced VFLX). In 1993 the June-September meridional wind

speeds were anomalously high with the strongest wind occurring in July (enhanced VFLX). As

previously mentioned, several investigators found a direct link of the anomalous GPLLJ with

droughts and floods in the central United States. This might indicate at least subjectively that the

GPLLJ index is adequately representing the phenomena with respect to drought and flood years.

3 Please note that 925 mb was used in lieu of 900 mb in calculating the ERA-40 GPLLJ index as 900 mb is unavailable in ERA-40.

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5. GPLLJ Linkages

a. Index regressions

Indices are attractive in that they can be used for compactly characterizing a phenomenon

over a spatially coherent region and can aid in determining linkages to other important fields. A

drawback associated with index regressions is that the information gleaned from these analyses

are not suitable for determining causality in light of competing influences. However, they are

invaluable in commencing an objective assessment of linkages to the phenomenon of interest. It

is understood that much of the LLJ fluctuation occurs on a diurnal timescale, however large scale

circulation features are essential to providing the background time mean flow on which the

forcing of the diurnally varying LLJ operates and as such are vital to determining the strength

and position of the GPLLJ on stationary timescales.

Figure 7 displays the contemporaneous July GPLLJ index regressions on NARR

precipitation (top), ERA-40 200 mb geopotential height (middle) and ERA-40 sea level pressure

(bottom). July is chosen as this is the month with maximum GPLLJ phase and amplitude 4. The

GPLLJ is associated with Great Plains precipitation anomalies approaching a max of 1.6 mm day

–1. Inverse anomalies are positioned in the southeast United States and in the Gulf of Mexico

with a magnitude of about half the value in the Great Plains. This pattern shows that on the

monthly timescale the GPLLJ is positively correlated with Great Plains precipitation. With the

exception of a considerable anomalous structure off the west coast of Central America and

weaker anomalies on the edges of the NARR domain in the Atlantic and Pacific oceans, the

positive spatial pattern showing the linkage of the GPLLJ to Great Plains precipitation is similar

4 The index for these regressions is calculated using the July monthly averaged 12Z value of meridional wind from NARR as this is the time of maximum LLJ amplitude. The NARR index is used for regressions on NARR and ERA40 fields. The correlation between NARR and ERA40 GPLLJ index is a robust .971 and as such the cross regression should not have a significant impact on regression results.

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to Barradas and Nigam (2005) figure 6 which shows regressions of a Great Plains precipitation

index on precipitation. The correlation between the GPLLJ index and Ruiz-Barradas and

Nigam’s Great Plains precipitation index is .66.

The middle panel of figure 7 shows the 12Z GPLLJ index (NARR) regressions on the ERA-

40 200 mb geopotential height. The structure of the 200 mb height in association with the

GPLLJ during July shows a wave train with a distinct quadrapole pattern emanating from the

west North Pacific eastward into western North America and culminating over the southeast

United States. This pattern suggests the possibility of GPLLJ variability being influenced by

summertime teleconnection patterns.

The sea level pressure field associated with the GPLLJ shows a weakened pacific pattern and

an enhanced and more coherent Atlantic pattern in comparison to the top panel. The Atlantic

pattern resembles the North Atlantic Oscillation (NAO) signature. The NAO can modulate the

strength and spatial extent of the summertime Bermuda high and as such could impact low-level

moisture fluxes east of the Rocky mountains. Recent findings suggest that the GPLLJ is

modulated by enhanced upper level (200 mb) zonal flow over and upstream of the Rockies and

attendant downstream cyclonic circulation on the monthly timescale (Byerle and Paegle 2003).

The anomalous height gradients and attendant wave train activity in the upper atmosphere over

the Pacific coupled with the low level SLP anomalies shown in the Atlantic could suggest a dual

influence of upper level Pacific anomalies in resonance with lower level Atlantic anomalies on

GPLLJ variability and impacts on Great Plains hydroclimate.

Figure 8 shows GPLLJ index regressions on surface to 200 mb integrated diabatic heating

(top panel) from ERA40 and SST’s (lower panel) from the Hadley Center. There are

considerable diabatic heating anomalies in the tropical western and eastern Pacific in association

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with the large-scale pattern of GPLLJ index regressions. Could the strong diabatic heating

anomalies in the tropical western and eastern Pacific force the standing wave pattern that is

associated with enhanced GPLLJ phase and amplitude? Using a barotropic model Lau and Peng

(1992) found that a northward shift of the eastern Pacific ITCZ during northern hemisphere

summer as prescribed by diabatic heating anomalies north of 9°N and concurrent equatorial west

pacific heating anomalies could force wave train activity emanating from the extratropical

Pacific into the United States. The exact structure of the modeled wave train with that of the

middle panel of figure 7 differs, however the association of the locations of the two heating

anomalies is interesting.

The SST pattern in the bottom panel of figure 8 associated with the GPLLJ is reminiscent of

features found in both the Pacific Decadal Oscillation (PDO) and ENSO. Barlow et al 2001

show the first three Pacific SST modes for JJA gleaned from rotated principal component

analysis. The first and second modes are ENSO and PDO respectively. The ENSO mode is

characterized by enhanced eastern and central Pacific equatorial SST anomalies and negative

anomalies in the western North Pacific primarily west of the international date line. The Pacific

decadal pattern is recognized by a horseshoe pattern of positive SST anomalies stretching from

the western coast of North America southwest toward the date line. Negative anomalies in the

mid latitudes hover between 140°W and 180°W and center on about 40°N.

The SST regression associated with the GPLLJ seems to possess features that are common to

both patterns with negative North Pacific anomalies centered on 180°W and stretching westward

toward Japan (ENSO like) and a somewhat horseshoe shape emanating form the western coast of

the U.S. southwestward toward the equator (PDO like). The weak positive SST anomaly tied to

the coast of South America is not seen in the PDO pattern. In fact the PDO pattern is

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characterized by negative anomalies at this location. So it seems there could be a competing

and/or offsetting influence of these two modes. In the Atlantic basin negative SST anomalies are

found from 30-50° N stretching from North America to the European continent. This suggests

that there is also possible Atlantic basin atmospheric and/or oceanic influences on the GPLLJ,

however in light of the overlapping modal possibilities any further pronouncement of

mechanistic forcing is prohibited.

6. EOF Analysis

a. Spatial patterns and PC time series

The possible superposition of competing influences motivates the use EOF analysis to draw

out the most recurrent patterns of variability and hydroclimate impacts. The analysis was

conducted on NARR 900 mb meridional wind in the domain 105-85°W and 20-50° N. An added

benefit of the analysis technique is that due to the orthogonality each mode is non-influential on

all other modes. The left column of figure 9 displays the June-August climatological 900mb

meridional wind (>4 m s–1 shaded) and the first three EOF’s (contours). The right column

shows the principal components associated with each EOF. These three LLJ modes have

percentage of explained variance of 35.7%, 24.1%, and 14.1% respectively.

Mode 1 (Central Plains) is characterized by substantial strengthening and spatial expansion

of the jet core. Although there is a northward shift of the core away from the primary moisture

source (Gulf of Mexico) the meridional expansion of the LLJ keeps the jet tied to the Gulf. The

enhanced strength of the jet core would presumably augment the dynamical forcing at the LLJ

exit region via stronger low-level convergence. An inspection of the principal component time

series shows substantial intraseasonal oscillations with the PC switching signs in 21 of the 24

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summers analyzed. The strong anomalous central U.S. drought and flood summers of 1988 and

1993 are represented by strong negative (positive) values in June 1988 (June, July, and August of

1993).

LLJ mode 2 (Northern Plains) shows a more northward shifted LLJ than that seen in mode 1

with negative values occurring over the northern Gulf of Mexico. This mode shows the LLJ core

between 40°N and 45°N effectively isolating the LLJ from any moisture source. As can be seen

from inspection of the PC time series the positive phase of this mode was operative in 1988 with

June showing the strongest amplitude. Not surprisingly June 1988 also exhibited the strongest

negative value of PC 1 out of all 24 years. There is less intraseasonal variability in PC 2 when

compared with PC 1 as evidenced by sign changes in 16 of the 24 summers analyzed.

GPLLJ mode 3 (Southern plains) shows collocated strengthening of the climatological LLJ

along with a dipole structure between the southern plains and the northeastern Gulf of Mexico.

Strengthening (weakening) of the climatological LLJ is associated with floods (droughts) in the

central U.S. due to enhanced (suppressed) moisture flux convergence. While the positive phase

would have a slightly reduced northward moisture flux due to the climatologically opposing 900

mb meridional wind in the eastern two-thirds of the Gulf of Mexico, the enhanced low-level

convergence would contribute substantially to the moisture flux convergence at the jet exit

region in the central plains. Additionally, there would still be a positive contribution to the

northward flux of moisture in the western third of the Gulf of Mexico. Analyzing the PC time

series from the perspective of the 1988 (1993) drought (flood) years shows that this PC exhibited

negative values during all summer months during 1988. The negative phase would have

decreased LLJ strength in the southern plains with an eastward shift of moisture flux

19

convergence. In 1993 July and August show positive values of the principal component

suggesting enhanced meridional flow over the area associated with the climatological LLJ.

b. PC index regressions

Figure 10 shows the principal component regressions on NARR precipitation (left column)

and ERA40 SLP (right Column). The top panels associated with EOF 1 (Central Plains) show

very similar regression patterns to those in figure 7 albeit with slight differences. This principal

component shows the strongest positive association with GP precipitation and correlates with the

GPLLJ index at a robust .86. Additionally there seems to be a strong association with the north

Atlantic SLP field perhaps associated with an NAO induced southwestward shift of the Bermuda

high.

The middle panels (PC 2) show a more broad precipitation pattern with the eastern half of the

U.S. showing less precipitation when the positive phase is active. Not surprisingly the

correlation of this PC with the GPLLJ index is -.15. The SLP field shows signs of a possible

pacific wave train influence with southward central U.S. low–level flow as diagnosed from

geostrophic considerations. This flow opposes the climatological low-level jet and as such

would decrease positive northward moisture flux.

The bottom panels representing the regressions of PC 3, correlated at .46 with the GPLLJ

index, show inhibited precipitation over the south central U.S. and northern Gulf of Mexico with

a slight positive contribution to Great Plains precipitation. While the smaller scale positive SLP

pressure anomaly located over the Texas-Mexico Gulf coast inhibits local precipitation it could

contribute positively to increased LLJ strength via enhance boundary layer stability in which

diurnally varying LLJ’s thrive (Stensrud 1996).

20

7. Discussion

This study has exploited the long term high spatial and temporal resolution NARR dataset in

representing aspects of the low level flow over the Great Plains and in particular the signature of

the GPLLJ. It is found that the primarily nocturnal GPLLJ is a robust feature in the monthly

climatology of meridional wind especially in July. The 1000-700 mb meridional water vapor

flux has similar characteristics. There is significant variability of the GPLLJ with respect to

latitude. At 25°N the LLJ seems to be a more persistent feature regardless of time of day with

nighttime values being approximately 1.67 times the daytime value. Further north at 30°N the

LLJ exhibits a strengthened diurnal cycle with nighttime values 2.5 times the daytime minimum.

At both latitudes the GPLLJ is highly associated with the steepness of the topographic barrier to

the west. The center of maximum wind is located roughly 50-75 mb AGL, however this

amounts to the pressure level of the max wind being markedly different with latitude due to the

varying topography.

Evaluating the characteristics of meridional wind and moisture fluxes in the monthly

climatology facilitated choosing an area suitable for GPLLJ index definition. On average it was

found that most GPLLJ activity occurs in the box outlined by 100-95°W and 25-35°N. Area

averaged wind profiles showed 900 mb to be the best pressure level for defining a GPLLJ index.

This index was computed using the European Center’s ERA-40 reanalysis and the National

Centers for Environmental Prediction North American Regional Reanalysis (NARR). The

standard deviation of the index was 1.3 in ERA-40 and 1.4 in NARR. The indices were

correlated at .971 for the 23 year overlapping ERA-40 and NARR period. The GPLLJ index

shows intraseasonal as well as interannual variability. Interesting notable features were the low

21

(high) index values during 1988 (1993). 1988 was a major central U.S. drought year, while 1993

had some of the worst flooding over the Midwest during the twentieth century.

July Nocturnal GPLLJ index regressions on precipitation show a strong link of the GPLLJ

with Great Plains precipitation in NARR. Correlation of the summertime (JJA) GPLLJ index

and Ruiz-Barradas and Nigam’s Great Plains precipitation index is .66. Regressions of the

Nocturnal July GPLLJ index on 200 mb geopotential height and sea level pressure show the

presence of a standing wave pattern with PNA and NAO like structures in the Pacific and

Atlantic basins. Diabatic heating associated with the GPLLJ show significant east and west

tropical Pacific anomalies with the largest amplitude and spatial extent found in the western

Pacific. This may be the forcing for the standing upper level wave pattern seen in the 200 mb

regression analysis. The SST pattern associated with the July GPLLJ shows a structure similar

to a blend of ENSO and PDO SST patterns noted in previous studies. North Atlantic SST

anomalies are also noted and could suggest some downstream influence of atmospheric or

oceanic anomalies on the GPLLJ.

EOF analysis of 900 mb meridional wind in NARR shows that the first 3 (35.7%, 24.1%,

14.1%) modes explain approximately 74% of the total variance with modes 1 (central plains)

and 2 (northern plains) exhibiting substantial spatial shifting and intensification of the LLJ core.

Mode 3 (southern plains) shows collocated strengthening of the climatological LLJ and a dipolar

structure. Principal component time series analysis shows traction with the extreme hydrologic

events of the drought and flood summers of 1988 and 1993 respectively. In the summer of 1988

all 3 modes were active, while in 1993 modes 1 (central plains) and 3 (southern plains)

displayed the most activity.

22

Principal component regressions on NARR precipitation show the strongest positive

(negative) association with mode 1 (central plains) (mode 2(northern plains)) over the Great

Plains, while mode 3 (southern plains) is characterized by large negative precipitation anomalies

over the southern U.S. and a small positive node in the Great Plains. PC regressions on ERA40

SLP show a strong link with the Atlantic basin via the NAO for mode 1 (central plains) and a

Pacific wave train for mode 2 (northern plains). Mode 3 (southern plains) shows a localized

SLP anomaly and is reasoned to enhance the diurnally varying LLJ via augmented static stability

in which diurnally varying LLJs flourish.

While some aspects of GPLLJ climatology and interannual variability have come to light

more analysis needs be conducted with regard to improving the understanding of the role of the

GPLLJ variability mechanisms on Great Plains hydroclimate. This study focused on the monthly

timescale as a first step to assessing GPLLJ fluctuation characteristics. Future work will focus

on:

• Higher frequency mechanisms associated with GPLLJ variability in weekly averaged

NARR output.

• Adjacent ocean basin forcing.

• Deeper investigation of diabatic heating anomalies on GPLLJ variability via forced

wave train patterns.

23

Acknowledgments Alfredo Ruiz-Barradas, Chi-Fan Shih (NCAR), NCAR Scientific Computing Division

24

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Figure Captions Figure 1. Seasonal evolution of the NARR 1000-700 mb meridional water vapor flux. Units are kg m –1 s –1 Contour interval is 25. Values in excess of 75 are shaded. Figure 2. Seasonal evolution of the diurnal amplitude and phase of meridional water vapor flux. Units are kg m –1 s –1. Direction of the arrow determines time of maximum amplitude. Arrow length denotes magnitude. Boxed region denotes area of maximum amplitude. Figure 3. Longitude-height cross section of the seasonal evolution of meridional wind at 25 N (left column) and 30 N (right column) in NARR. Contours > 5 m s–1 are shaded. Topography is blocked out.

Figure 4. Longitude-height cross section of July diurnal evolution of meridional wind at 25 N (left column) and 30 N (right column) for 4PM CDT and 7 AM CDT. Figure 5. Area averaged (100-95 W, 25-35 N) meridional wind profile for the warm season months of May-September in NARR. For July the ERA-40 and NCEP reanalyses are included for comparison. The vertical resolution in both ERA-40 and NCEP is 75 mb. Units are m s–1 . Figure 6. GPLLJ index anomalies during the warm season (April-September) in (top 2 panels) ERA-40 and in (bottom panel) NARR. Monthly values are shown using a triangle while the smoothed index obtained from a 1-2-1 averaging of the seasonal mean anomalies is displayed using solid lines. Horizontal dashed lines mark the plus minus 1 standard deviation range in each panel. Figure 7. July regressions of the nocturnal GPLLJ index on (top panel) ERA-40 200 mb geopotential height and (bottom panel) ERA-40 sea level pressure. Height contours are 3 m while sea level pressure contours are .2 mb. In both panels the positive values are shaded. Figure 8. July regressions of the nocturnal GPLLJ index on (top panel) NARR precipitation and (bottom panel) ERA-40 400 mb diabatic heating. NARR precipitation contours are .2 mm day –1 while diabatic heating contours are 10 k day –1. In both panels the positive values are shaded. Figure 9. 900 mb meridional July wind climatology (shaded) and the first three EOF modes (contours) for JJA in the left column. JJA principal component time series associated with each mode in the right column. Figure 10. Principal component regressions on NARR precipitation in mm day –1 (left column) and ERA40 SLP in mb (right column). The contour interval is .2 for both columns.

28

Figure 1. Seasonal evolution of the NARR 1000-700 mb meridional water vapor flux. Units are kg m –1 s –1 Contour interval is 25. Values in excess of 75 are shaded.

29

Figure 2. Seasonal evolution of the diurnal amplitude and phase of meridional water vapor flux. Units are kg m –1 s –1. Direction of the arrow determines time of maximum amplitude. Arrow length denotes magnitude. Boxed region denotes area of maximum amplitude.

30

Figure 3. Longitude-height cross section of the seasonal evolution of meridional wind at 25 N (left column) and 30 N (right column) in NARR. Contours > 5 m s–1 are shaded. Topography is blocked out.

31

Figure 4. Longitude-height cross section of July diurnal evolution of meridional wind at 25 N (left column) and 30 N (right column) for 4PM CDT and 7 AM CDT.

32

Figure 5. Area averaged (100-95 W, 25-35 N) meridional wind profile for the warm season months of May-September in NARR. For July the ERA-40 and NCEP reanalyses are included for comparison. The vertical resolution in both ERA-40 and NCEP is 75 mb. Units are m s–1 .

33

Figure 6. GPLLJ index anomalies during the warm season (April-September) in (top 2 panels) ERA-40 and in (bottom panel) NARR. Monthly values are shown using a triangle while the smoothed index obtained from a 1-2-1 averaging of the seasonal mean anomalies is displayed using solid lines. Horizontal dashed lines mark the plus minus 1 standard deviation range in each panel.

34

Figure 7. July regressions of the nocturnal GPLLJ index on (top panel) ERA-40 200 mb geopotential height and (bottom panel) ERA-40 sea level pressure. Height contours are 3 m while sea level pressure contours are .2 mb. In both panels the positive values are shaded.

35

Figure 8. July regressions of the nocturnal GPLLJ index on (top panel) NARR precipitation and (bottom panel) ERA-40 400 mb diabatic heating. NARR precipitation contours are .2 mm day –1 while diabatic heating contours are 10 k day –1. In both panels the positive values are shaded.

36

Figure 9. 900 mb meridional July wind climatology (shaded) and the first three EOF modes (contours) for JJA in the left column. JJA principal component time series associated with each mode in the right column.

37

Figure 10. Principal component regressions on NARR precipitation in mm day –1 (left

column) and ERA40 SLP in mb (right column). The contour interval is .2 for both columns.

38