Observed Changes in Atmosphere and Ocean   

 

1. Introduction

In seeking to understand the cause of the dramatic shift to a wetter precipitation regime at Devils Lake in the early 1990s, Prescient Weather has examined a variety of atmospheric reanalysis datasets, together with the history of well-known large-scale climate oscillations.  The goals of this investigation were to develop a physical understanding of the atmospheric circulation changes that have generated increased precipitation in North Dakota, and to identify, if possible, the role of large-scale climate phenomena in contributing to the regional changes.

 

2. Analysis of Atmospheric Flow Changes

The broad spatial scale of the precipitation increases across central North America in the most recent two decades indicates that the atmospheric circulation has undergone large-scale, though perhaps subtle, changes and that the new average pattern is more favorably disposed to wet conditions in many locations.  While it is beyond the scope of this study to determine exhaustively the nature of the circulation changes across the entire continent, it is possible to identify a significant summertime pattern shift that has occurred in the northern Great Plains.  Figure 1 shows the difference in the May-July average relative vorticity at 500 hPa between the 1948-1990 period and the 1991-2011 period; this analysis uses the NCEP-NCAR Global Reanalysis 1 (NCGR1) dataset (Kalnay et al. 1996).  A zone of markedly increased average vorticity is observed near the U.S.-Canada border, extending from western Montana to the western Great Lakes.  Larger relative vorticity reflects the presence of lower pressure and more storminess, which generate increased precipitation.

The recent shift to a regime of higher average vorticity is illustrated in Figure 2, which shows the sequence of May-July average relative vorticity values within the green box outlined in Figure 1.  The shift in the early 1990s stands out sharply, as 12 of 21 years since 1991 show May-July vorticity higher than the highest value in the 1948-1990 period.

Figure 1. Difference in average 500 hPa relative vorticity in May, June, and July, between the period 1948-1990 and the period 1991-2011.  The green rectangle shows the area in which the vorticity values were averaged to obtain Figure 2; the north-south black dotted line through the Great Plains shows the baseline for the cross-section in Figure 4.

 

Description: rvort_5_7.PNG

Figure 2.    Average 500 hPa relative vorticity in May, June, and July within the region outlined in green in Figure 1.

 

In addition to larger average relative vorticity in recent years, the frequency of positive vorticity events has also increased in the vicinity of Devils Lake.  A positive vorticity event is sometimes referred to as a “short wave” and its passage aloft is often accompanied by a distinct precipitation event.  Figure 3 shows the difference in average frequency of short waves exceeding a relatively low threshold vorticity of 2 x 10-5 s-1, confirming a concentration of increased frequency near the U.S.-Canada border of North Dakota.

Figure 3.    Difference in average frequency of occurrence of 500 hPa relative vorticity values of at least 2x10-5   s-1 in May, June, and July, between 1948-1990 and 1991-2011.

 

The vertical structure of the average vorticity change is illustrated in a north-south cross-section along 100 °W (Figure 4); the baseline of the cross-section is marked by the black dotted line in Figure 1.  The 100 °W meridian passes just west of Devils Lake, and the latitude of Devils Lake is approximately 48 °N.  Figure 4 reveals that the post-1991 increase in vorticity is vertically coherent and is centered almost directly above the Devils Lake region in this north-south vertical plane.

Figure 4.    Top panel: cross-section of average May-July relative vorticity along 100 °W, for 1948-1990 (black lines) and 1991-2011 (red lines).  Contours for negative values are dashed.  Bottom panel: cross-section of the difference in average May-July relative vorticity along 100 °W, between 1948-1990 and 1991-2011.  Devils Lake lies just to the east of the cross-section at approximately 48 °N.

 

In addition to generating increased precipitation, the shift to more cyclonic flow aloft in summer has been accompanied by reduced temperature in the lower troposphere; Figure 5 shows the post-1990 change in May-July potential temperature along the 100 °W meridian.  The average temperature has dropped in response to increased cloudiness and precipitation, and also because northerly flow has become more common in these months.  Monthly average streamlines of 10-m wind reveal that flow from the south tended to be stronger prior to 1991, and in May the mean wind direction has changed from southeasterly to northeasterly, so that cool northern air has invaded the Devils Lake region with greater frequency.  Figure 6 shows the frequency of occurrence of air mass source regions in July, based on 15-day back-trajectory calculations using the six-hourly 10-m winds in the NCGR1 dataset.  Air masses originating in Canada and the Arctic have become more frequent, at the expense of air masses originating south of the U.S.-Canada border.  Similar monthly analyses reveal that the frequency of Arctic air masses has increased in every summer month (May through August); this is consistent with the observation of weaker southerly flow and lower average temperatures.

Figure 5.    Mean potential temperature (top panel) and difference between 1991-2011 and 1948-1990 (bottom panel) for May through July along the 100 °W meridian.  Devils Lake is located close to this cross-section at approximately 48 °N.

http://devilslake.prescientweather.com/airmassfreq_jul.PNG

Figure 6.    Frequency of occurrence of air mass source regions for Devils Lake in July.

 

In summary, atmospheric reanalyses show that upper-level troughs have become considerably more frequent in the vicinity of Devils Lake in summer, leading to an increased frequency of Arctic and Canadian air masses, and lower mean temperatures.  The shift to more frequently cyclonic circulation has generated increased precipitation via the large-scale lifting that accompanies such disturbances.  It appears very likely that this change in the mean circulation pattern is the principal direct cause of the change to a wetter regime in summer at Devils Lake.  It is also of interest to note that similar changes have taken place in October (Figure 7), which has seen the highest percentage increase in precipitation of any single month at Devils Lake.  In October, the increase in average relative vorticity is centered farther west, along the Montana-Canada border, but also extends over all of North Dakota.

Figure 7.    Difference in average 500 hPa relative vorticity in October, between the period 1948-1990 and the period 1991-2011.

 

3. Role of Large-Scale Climate Oscillations

Numerous studies in the past 20 years have discussed the importance of large-scale modes of decadal to interdecadal climate variability in affecting regional climate, including rainfall distributions, across the globe (e.g. Parker et al. 2007, Meinke et al. 2005).  Among the phenomena that show decadal variability are the El Niño - Southern Oscillation (ENSO, Rodgers et al. 2004), the Pacific Decadal Oscillation (PDO, Mantua and Hare 2002), the Atlantic Multidecadal Oscillation (AMO, Enfield et al. 2001), and the North Atlantic Oscillation (NAO, Hurrell 1995).  Some of these climate modes are described principally by variations in sea surface temperature (e.g. PDO, AMO), some by atmospheric variations (e.g. NAO), and some are coupled oscillations that can be measured by either oceanic or atmospheric changes (e.g. ENSO).  The annual and decadal averages of the indices that represent the phase of these oscillations are shown in Figure 8.

Description: climateindices.png

Figure 8.    Annual (shaded) and decadal (black lines) averages of ENSO, PDO, AMO, and NAO.

 

An early hypothesis concerning the recent changes at Devils Lake was that phase shifts in one or more of the decadally-varying climate indices would be identified as causally related to the precipitation shift in North Dakota.  However, the annually- and decadally-averaged climate indices do not appear to exhibit any marked and lasting changes on or around 1991, and data mining analysis has not revealed any clear connection between the annual climate indices and the Devils Lake precipitation.  Nevertheless, closer examination of the months in which precipitation has increased most significantly (May through July, and October) reveals significant climate index changes that coincided with the 1991 onset of wetter conditions.

First, ENSO in the months of May through July has shifted to a more positive state since 1991.  Figure 9 shows the sequence of May-July average ENSO index values and the running decadal average.  Beginning in 1991, five consecutive years occurred in which the May-July ENSO index was positive, and the decadal average has remained higher in recent years than at most times in the record prior to 1991.

Description: ensochanges.PNG

Figure 9.    Yearly values of average ENSO index values in May, June, and July (blue columns), and centered decadal averages of the same (red line).  The ENSO index is the Bivariate ENSO Time Series index, which is an equally-weighted combination of the Niño3.4 sea surface temperature anomaly and the Southern Oscillation Index anomaly (Smith and Sardeshmukh 2000).

 

The apparently close connection between ENSO phase and Devils Lake precipitation is illustrated by Table 1, which shows the mean ENSO index by month before and after the 1991 shift, along with the change in mean precipitation by month.  The greatest increases in ENSO index have occurred in the summer months, which have also witnessed the largest increase in average precipitation.  The simultaneity - both by season and by year - of the ENSO changes and the precipitation changes at Devils Lake are suggestive of a causal relationship.  This hypothesis is consistent with previously published studies showing a connection between ENSO phase and precipitation anomalies in the Northern Plains (e.g. Bunkers et al. 1996, Khandekar 2004).


 

Table 1.      Mean monthly ENSO index values and Devils Lake basin-average precipitation prior to and after the 1991 regime shift.  The ENSO index is the Bivariate ENSO Timeseries Index.

Month

Mean ENSO Index
1895-1990

Mean ENSO Index
1991-2011

Difference

Mean
Precipitation
1895-1990 (in)

Mean
Precipitation
1991-2011 (in)

Difference (in)

Jan

-0.01

-0.05

-0.04

0.53

0.45

-0.08

Feb

-0.01

-0.10

-0.09

0.41

0.44

+0.03

Mar

-0.03

+0.10

+0.13

0.74

0.75

+0.01

Apr

-0.05

+0.17

+0.22

1.10

1.10

0.00

May

-0.03

+0.36

+0.39

2.07

2.81

+0.74

Jun

-0.03

+0.30

+0.33

3.22

3.79

+0.57

Jul

-0.01

+0.19

+0.20

2.52

3.34

+0.82

Aug

0.00

+0.18

+0.18

2.23

2.53

+0.30

Sep

0.00

-0.02

-0.02

1.77

1.88

+0.11

Oct

-0.01

+0.08

+0.09

1.02

1.50

+0.48

Nov

+0.02

-0.05

-0.07

0.62

0.82

+0.20

Dec

0.00

-0.07

-0.07

0.49

0.64

+0.15

 

To investigate possible mechanisms by which the ENSO phase could influence Devils Lake precipitation, the NCGR1 was used to examine the effects of the May- July ENSO phase on mid-tropospheric heights across the tropics and northern hemisphere.  Figures 10 and 11 show the average 500 hPa height anomalies associated with the strongest El Niño and La Niña episodes since 1948 in May through July.  In the tropics, the figures show the expected response in which El Niño years are associated with higher than normal heights, corresponding to above-normal temperatures in the lower troposphere; in contrast, La Niña years tend to bring cooler conditions and lower heights.  In the northern North Pacific Ocean, both El Niño and La Niña induce generally low heights, but important differences are evident.  During El Niño episodes, the strongest negative height anomaly is located west of the International Date Line and close to Kamchatka, whereas La Niña episodes bring below-normal heights to the Bering Sea, the Gulf of Alaska, and far western Canada.  Moreover, El Niño brings low heights and troughing to the region north of Hawaii and to the southwestern United States, whereas La Niña tends to cause ridging in the central Pacific.

Description: hgtanom_elnino_500hPa_5_7.png

Figure 10.  Average 500 hPa height anomaly in May, June, and July, for the 10 years since 1948 having the most positive average ENSO index value in those months. 

Description: hgtanom_lanina_500hPa_5_7.png

Figure 11.  Average 500 hPa height anomaly in May, June, and July, for the 10 years since 1948 having the most negative average ENSO index value in those months. 

 

The ENSO-related upper-level height changes shown in Figures 10 and 11 bring about significant changes in the location and orientation of the jet stream flow over the northern and eastern Pacific Ocean, as well as over western North America.  During La Niña conditions, the strong trough in the Gulf of Alaska favors a more southerly component to the upper flow over western North America, which is not conducive to the production of short wave troughs in the northern Great Plains.  However, during El Niño conditions the tendency for ridging near the Pacific coast of Canada does favor cyclonic curvature of the flow on the east side of the ridge over the Great Plains.  This conceptual model of the ENSO-related flow variation is consistent with Castro et al. (2001), who argue that the Pacific SSTs significantly influence the North American southwest monsoon.

A second major climate shift that appears to bear some relation to the Devils Lake problem is the recent emergence of high-amplitude summertime phase peaks in the Quasi-Biennial Oscillation (QBO).  The QBO is a quasi-periodic oscillation of the equatorial zonal wind in the stratosphere; the zonal wind alternates between easterly and westerly phases with a period of 2-3 years.  The phase of the QBO is known to influence mid-latitude seasonal climate as well as monsoon precipitation and Atlantic tropical cyclone frequency (Baldwin et al. 2001); it is therefore plausible that changes in QBO behavior could be related to Devils Lake precipitation changes.  A comparison between May-July (MJJ) precipitation and MJJ average QBO index reveals that 8 of the 10 wettest (and 15 of the 20 wettest) MJJ periods since 1948 have occurred when the QBO was more negative (easterly) than usual.  On average, then, the negative QBO phase seems to favor increased summer precipitation at Devils Lake.

The correlation between QBO phase and Devils Lake precipitation exists on an annual basis both prior to and after the 1991 increase in precipitation.  However, the QBO behavior also shifted markedly in 1991, with high-amplitude index values becoming much more common in May to July (Figure 12).  The change consisted of both an overall increase in QBO amplitude, and also a shift in seasonality so that QBO peaks now tend to occur preferentially in the summer half of the year.  Prior to 1991 there was a nearly equal partition of QBO peaks between summer and winter, but since 1991 nearly all of the QBO peaks have occurred between April and September.

http://devilslake.prescientweather.com/dlpcp_qbo1.PNG

Figure 12.  Comparison of the observed average QBO index in May through July (blue diamonds, right scale) and the May through July precipitation in the Devils Lake basin (red bars, left scale).

 

The combination of higher amplitude QBO oscillations and the shift to summer phase peaks means that 8 of the 10 most negative QBO episodes in MJJ have occurred since 1991.  Given that the correlation between QBO phase and Devils Lake precipitation also existed prior to 1991, it is unlikely that the post-1990 QBO and precipitation changes are unrelated.  However, the physical mechanisms by which QBO phase might influence precipitation in the Northern Plains remains unclear and is a worthy topic for future investigation.

Third, and finally, the NAO has shifted to a more negative state since 1991 in October, as shown in Figure 13.  Negative values of the October NAO began in 1991 and have continued in most years since.  It is possible that the recent dominance of the negative NAO phase in autumn is attributable to the reduction in summer and autumn Arctic sea ice extent, because the reduced meridional temperature gradient acts to weaken the jet stream and promote high-latitude blocking.

Description: naochanges.PNG

Figure 13.  NAO index values in October (blue columns), and centered decadal averages of the same (red line).  Source: Climate Analysis Section, Climate and Global Dynamics Division, NCAR Earth Systems Laboratory.

 

4. Conclusions

The summertime atmospheric circulation over the northern Great Plains has undergone notable change since 1991, with a more cyclonic pattern favoring increased precipitation in recent years.  The elevated frequency of upper-level troughs appears to be the primary and immediate cause of the shift to a wetter regime at Devils Lake.  Simultaneous and rather dramatic shifts in the summer behavior of ENSO and the QBO suggest that these large-scale pattern changes are plausible culprits for the Devils Lake change.  A phase shift in the NAO index also appears to be related to increased precipitation in October.

In the case of ENSO, the increased frequency of El Niño conditions in recent summers appears to have favored the production of shortwave troughs over the U.S. northern Plains by altering the dominant ridge-trough structure over the North Pacific.  El Niño conditions in summer favor a dominant trough position in the western North Pacific rather than in the Gulf of Alaska, with the former arrangement being more favorable for storminess in central North America.  Further investigation of the physical mechanisms connected with summer ENSO variations would be worthwhile, and there also remains a significant challenge in identifying the nature of the circulation changes associated with the observed NAO and QBO changes.

 

References

Baldwin, M.P., L. J. Gray, T. J. Dunkerton, et al., 2001: The Quasi-Biennial Oscillation.  Rev. Geophys., 39, 179-229.

Bunkers, M. J., J. R. Miller Jr., and A. T. DeGaetano, 1996: An Examination of El Niño - La Niña - Related Precipitation and Temperature Anomalies across the Northern Plains.  J. Clim., 9, 147-160.

Castro, C. L., T. B. McKee, and R. A. Pielke Sr., 2001: The Relationship of the North American Monsoon to Tropical and North Pacific Sea Surface Temperatures as Revealed by Observational Analyses.  J. Clim., 14, 4449-4473.

Enfield, D. B., A. M. Mestas-Nuñez, and P. J. Trimble, 2001: The Atlantic Multidecadal Oscillation and Its Relation to Rainfall and River Flows in the Continental U.S.  Geophys. Res. Lett., 28, 2077-2080.

Hurrell, J. W., 1995: Decadal Trends in the North Atlantic Oscillation: Regional Temperatures and Precipitation. Science, 269, 676-679.

Kalnay, E., and co-authors, 1996: The NCEP/NCAR 40-Year Reanalysis Project.  Bull. Amer. Meteor. Soc., 77, 437-471.

Khandekar, Madhav L., 2004: Canadian Prairie Drought: A Climatological Assessment.  Alberta Environment, Pub. No: T/787

Mantua, N. J., and S. R. Hare, 2002: The Pacific Decadal Oscillation.  J. Oceanography, 58, 35-44.

Meinke, H., P. DeVoil, G. L. Hammer, S. Power, R. Allan, R. C. Stone, C. Folland, and A. Potgieter, 2005:  Rainfall Variability at Decadal and Longer Time Scales: Signal or Noise?  J. Climate, 18, 89-96.

Parker, D., C. Folland, A. Scaife, J. Knight, A. Colman, P. Baines, and B. Dong, 2008: Decadal to Multidecadal Variability and the Climate Change Background.  J. Geophys. Res., 112, D18115, doi: 10.1029/2007JD008411.

Rodgers, K. B., P. Friederichs, and M. Latif, 2004: Tropical Pacific Decadal Variability and Its Relation to Decadal Modulation of ENSO.  J. Climate, 17, 3761-3774.

Smith, C. A., and P. Sardeshmukh, 2000: The Effect of ENSO on the Intraseasonal Variance of Surface Temperature in Winter.  Int. J. of Climatology, 20, 1543-1557.