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The associated monthly mean absolute vorticity gradient can be seen in Figure 1.

Figure 1. Monthly mean absolute vorticity gradient at 300 mb for the period 1980-1993 (from NMC
observations). Contour interval is 7.5 x 10^-5 s^-1. Values greater than 30 x 10^-5 s^-1 are shaded
in blue. Negative values are shaded in red and indicated with dashed contours. The thick black line
represents the zero wind line.

The two center figures are Hovmuller plots (time vs. latitude) of the annual cycle of absolute vorticity
gradient at 300 mb, zonally averaged between 135^o E-180^o (top) and 180^o-135^o W (bottom). Contour interval
is 7.5 x 10^-5 s^-1. Negative values are red; darker blue shading indicates larger values.

For example, Figure 2 shows the influence function for each month, when the target area chosen
is 30^o in diameter and is centered over the western U.S. (40^o N latitude, 115^o W longitude).
Also included in the figure is a histogram of the pattern correlation between the influence
functions of consecutive months. Thus, for example, J-F indicates the pattern correlation
between the January influence function and the February influence function. A value of 1 in
this plot means that the influence function is unchanged (apart possibly from a constant
multiplicative factor) between the two months.

In general, as time passes from January to June the sensitivity of western U.S. height
anomalies to forcing in the west Pacific increases and the sensitivity to forcing in the
tropical east Pacific decreases. Distinct month-to-month change is particularly noticeable
from March through July. The geographical scale of the pattern over the west Pacific steadily
decreases from one month to the next, resulting in lower pattern correlation between spring
months. Although the overall sensitivity declines from March to May, as one might perhaps
expect from the weakening of the Pacific jet, it increases in June, when the extrema have
larger amplitude than at any time in winter. (Actually, examination of the influence functions
computed using semi-monthly means shows that the period of greatest sensitivity is from mid-May
to mid-June.)

Since the total change in the response is due to the projection of the actual forcing on the
influence function, it is clear that the geographical scale of the influence function can be as
important as its amplitude. For example, consider a region of forcing south of the
30^o N latitude circle, all of one sign, between 120^o E and 180^o. In March,
this will produce a strong response over the western U.S., since the influence function in this
region is all of one sign. However, in June, this region of forcing will be too large:
forcing at 135^o E will be offset by forcing at 160^o E. Thus, although the
western U.S. response to June steady forcing can be large, the details of the forcing pattern
may be more important than during the winter months. Even if the region of forcing is not too
large, the reduced month-to-month pattern consistency of the influence function in the spring
months implies that applying the same steady forcing throughout spring could nevertheless
produce a non-steady response over North America. For example, Figure 2 suggests that steady forcing
over Southeast Asia will produce an increase in western U.S. height in April, a decrease in
May, and an increase again in June. A similar effect would occur during fall.

Figure 2. The influence function for the target area centered at 40^o N latitude, 115^o W longitude. The center
of the target area is indicated by a large filled circle. The shading interval is 6 x 10¹¹ ms^-2; values between
-6 x 10¹¹ ms^-2 and 6 x 10¹¹ ms^-2 are unshaded.

Newman, M. and P. D. Sardeshmukh, 1996: The impact of the annual cycle upon the North Pacific/North American response to low frequency forcing. Submitted to J. Atmos. Sci.

Extreme springtime weather events over North America

Traditionally in climate dynamics, much attention has been focused on understanding low
frequency variability during the Northern Hemisphere winter. This is perhaps in part because
this is the time of peak synoptic scale activity. However, low frequency variability has
substantial amplitude throughout the year. Furthermore, the societal impact of extreme weather
events is not limited to the winter. Heat waves can have a substantial affect upon both health
and energy consumption, and the agricultural impact of drought is well known.

As can be seen in the previous section, height anomalies over the western U.S. may have
enhanced sensitivity to west Pacific forcing during May and June. To what extent is this
observed? To help answer this question, Figure 3 shows some results of an
analysis of observed divergence and streamfunction fields at 200 mb, for the period May 15-June
15 of the years 1980-1993. On the left is global divergence linearly regressed against
streamfunction over south-central Canada, for lags of -12, -6, and 0 days prior to maximum
streamfunction amplitude. On the right is global streamfunction regressed against divergence
over the Philippines, for lags of -6, 0, and +6 days prior to maximum divergence amplitude.
(Similar results can be obtained if OLR is used instead of divergence.) These results indicate
that the theoretical region of sensitivity to forcing is in fact important in the late spring.
At the time of year when the southeast Asian monsoon is intensifying, anomalies in west Pacific
convection are followed about a week later by large streamfunction anomalies over North America.

Figure 3. Linear regression results, at various lags. Left: global 200 mb divergence patterns from
regression against 200 mb streamfunction over south-central Canada (averaged between 52.6^oN-58.1^oN
latitude and 95.6^oW-101.2^oW longitude). Right: global 200 mb streamfunction patterns from regression
against 200 mb divergence over the Philippines (averaged between 8.3^oN-13.8^oN latitude
and 118.1^oE-123.8^oE longitude).
Positive contours are blue; negative are red. Contour interval for the divergence plots (left) is
.3 X 10^-6 s^-1; for the streamfunction plots (right) 10⁶ m²/s. Green shading indicates the 99% confidence
level (correlation magnitude greater than .3).

1988 U.S. drought

Of particular interest is the drought that occurred over much of the contiguous United States
during the spring and summer of 1988. Overall damages to the society and environment made the
1988 drought one of the nation's greatest disasters of the twentieth century. In April-June
averages of upper-level geopotential height, the drought was associated with what appears to be
a wavetrain that emanates from the tropics east of the dateline and arcs across North America
(see Figure 4). For this reason, it has been suggested that the drought was caused by a northward
shift in the ITCZ over the east Pacific. This in turn may have been caused by the onset of a La Nina.

However, this shift in the ITCZ didn't occur until June, whereas the drought started in early
April. Furthermore, an analysis of the height data for this period shows that the wavetrain is
mostly an artifact of the averaging period used. (This can be seen in Figure 4, which shows the
monthly and seasonal mean 200 mb height anomalies for this period.)

Figure 4. April-May-June mean and April, May, and June monthly mean 200 mb geopotential height anomalies.

In fact, the April-May-June mean field is quite similar to one obtained by merely dividing the
June mean by three.

Further analysis of 10-day lowpass data shows that the June mean was in turn dominated by two
episodes of rapidly intensifying anticyclones. The development of both these anticyclones
appears to follow the regression results, with an increase in anomalous convection (and
associated upper-level divergence) occurring about a week prior to the height maximum over
north-central North America.

To demonstrate the extent to which the observed forcing over the west Pacific contributed to
the growth of the anticyclones, we show the results of a barotropic model run (Figure 5),
also started about a week prior to the onset of the early June anticyclone and using as forcing
the observed 200 mb divergence over only the southeast Asia/west Pacific region. This simple
model can nevertheless reproduce the timing and amplitude of the anomaly quite well, although
the position is in error by 30^o to the southwest.

Figure 5. (Left) Observed 10-day Lanczos filtered streamfunction anomalies at 200 mb. (Right) Modelled
streamfunction anomalies at 200 mb. Contour interval is 4 x 10⁶ m²/s.

Thus, it appears that the April-May-June mean height anomaly was primarily due to two
wavetrains which were forced by anomalous convection over the west Pacific (perhaps related to
the southeast Asian monsoon) during June. In this scenario, the shift of the ITCZ was perhaps
not important, although anomalous SST could still play a role. It should again be stressed
that, although the June anticyclones certainly produced the June heat wave and therefore played
an important role in enhancing the severity of the drought, they did not initiate the drought.
Also, the drought extended well into August, at which time height anomalies were again weak,
suggesting that the role of surface feedbacks could also become important. The causes for the
very dry spring that preceeded the June heat waves remain to be determined.

Chen, P. and M. Newman, 1996: Rossby-wave propagation and the rapid development of upper-level anomalous anticyclones during the 1988 U.S. drought. Submitted to J. Climate.

Chen, P. and M. Newman, 1995: Rapid development of upper-level anticyclones during the 1988 U.S. drought. Proceedings of the Twentieth Annual Climate Diagnostics Workshop, Seattle, Washington.

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