A STUDY OF ATMOSPHERIC CIRCULATION OVER THE SOUTH ISLAND OF NEW ZEALAND (1961-1980)
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53
A STUDY OF ATMOSPHERIC CIRCULATION OVER THE
SOUTH ISLAND OF NEW ZEALAND (1961-1980)
A. P. Sturman
Cooperative Institute f o r Research in Environmental Sciences
Campus Box 449
University o f Colorado
Boulder, Colorado 80309"
A. C . Trewinnard and P. A . Gorman
Department o f Geography
University o f Canterbury
Christchurch 1, N e w Zealand
ABSTRACT
A subjective synoptic classification scheme is used to examine atmospheric
circulation over the South Island. Daily surface isobaric charts have been
categorised f o r twenty years (1961-1980) and monthly circulation indices
derived. B o t h data sets are used t o describe general characteristics o f
atmospheric circulation over the South Island, particularly the annual
regime and circulation variability over the twenty years. Anticyclonic circu-
lation and gradient winds from one o f the westerly directions clearly
dominate, with anticyclonic southwesterly being the most frequent type In
all seasons. Evidence o f biennial, annual and semi-annual variations i s
given by the derived circulation indices. Distinct fluctuations of atmospheric
circulation are identified over the twenty years, complementing results o f
previous research.
INTRODUCTION region, climatic variability appears to be the
normal s t a t e o f affairs, receiving m e d i a
As for much of this century, great variations attention during periods o f extreme weather
of climate have occurred over the globe in the (e.g. drought, severe storms, floods and heavy
two decades 1961-1980. Lockwood (1979) snowfall) largely because of the serious impact
suggests that from 1950 t o 1975 the annual on agricultural activities. Recent research
mean surface temperature i n t h e Northern suggests that there has been a general warming
Hemisphere decreased a t a rate o f between over New Zealand since about 1950 (Salinger
0.1 a n d 0.2°C p e r decade, w h i l e i n t h e and Gunn, 1975; Salinger, 1976, 1979, 1982;
Southern Hemisphere cooling occurred until Trenberth, 1977), although there i s some
the mid-1960's, followed by a slight warming. debate over this (Hessell, 1980). Study o f
Lack o f suitable data makes analysis o f precipitation variations suggests that relatively
Southern Hemisphere trends tentative, how- strong periodicities occur keeping approxi-
ever. M o r e recently, Jones e t a l . (1982) mately i n phase with the quasi-eleven year
produced evidence that Northern Hemisphere solar cycle (Tomlinson, 1980a, 198011; Vines
cooling ceased i n t h e 1970's, subsequently and Tomlinson, 1980). Trenberth (1976)
followed by warming. Over the New Zealand examined fluctuations i n atmospheric circula-
* O n leave f r o m University o f Canterbury, Christchurch, N e w Zealand.54 A Study of Atmospheric Circulation
tion over the Australasian region using indices to examine the influence of synoptic circulation
based o n pressure data. H e concluded that, on regional climatic characteristics o f t h t
up to 1975, pressures had become higher south South Island, N e w Zealand. T h e primary
of 40°S, with less westerly flow between 30 objective o f the present study is t o provide
and 45'S, more westerly south o f 45'S, and an initial analysis o f atmospheric circulation
more northeasterly or less southwesterly across patterns and derived indices from which further
New Zealand. However, his study made no studies can develop. This paper provides a
direct examination o f vorticity fields. description of an atmospheric circulation data
base which is already being used i n further
The research mentioned above suggests that research (e.g. Sturman a n d Soons, 1984).
variability occurs i n several aspects o f the Comments are also made here on the general
climate o f N e w Zealand. However, f e w characteristics of atmospheric circulation over
detailed studies exist of the nature of synoptic the South Island, i n particular t h e annual
scale circulation and i t s interrelations w i t h regime and the nature of circulation variability
such climate elements as precipitation, tem- between 1961 and 1980.
perature and cloud cover. D e Lisle (1956)
produced one such study, while Kerr (1944), ATMOSPHERIC C I R C U L AT I O N TYPES
Watts (1947) and de Lisle (1957) concentrated
solely o n describing atmospheric circulation Sea level atmospheric circulation over the
features i n the New Zealand region. Salinger South Island, N e w Zealand (Fig. 1 ) was
(1980a, 1980b) examined relationships between classified daily (1200 local time) for the period
Trenborth's (1976) indices and temperature and 1January 1961 t o 31 December 1980 using
drecipitation over the whole of New Zealana. a subjective technique similar to that of Lamb
(1950, 1965, 1972). Isobaric charts w e r e
This paper is the first o f several planned obtained from the New Zealand Meteorological
Os
700
TA M A N NORTH
ISLAND
SEA S.
Aoo
1
Itar- 1
1 1
1
1
SOUTH 1
ISLAND1
TASMANIA
0 2 0 0
CDLAN
00D OVER 1 1
km 5cfs
Fig. 1 : Location o f t h e study area. T h e dotted lines indicate t h e area used f o r the synoptic classification.A Study o f Atmospheric Circulation 55
Service. It was presumed that map construction role o f sub-tropical anticyclones which are
technique would show no major changes over frequently located over the Tasman Sea t o
the period. Although i t i s impossible t o the northwest. Anticyclonic circulation is most
maintain homogeneity o f analysis over the frequent i n summer (52.8%) suggesting a
period, each month of charts was not analysed southward movement o f sub-tropical a n t i -
in chronological order, b u t i n a n irregular cyclones i n this season. However, i t is least
order divided between t w o analysts. Conse- frequent in spring when westerly flow increases.
quently, the construction and analysis of charts Maunder (1971) indicated that there are no
should have had little impact on the nature semi-permanent anticyclones in the region, but
of the results, particularly long term trends. that a n eastward migration o f anticyclones
takes place, roughly a t weekly intervals. H e
The percentage frequencies o f t h e 2 7 also suggested that these anticyclones reach
possible circulation types for the whole study their furthest south in February and north in
period a r e listed i n Ta b l e 1 . Anticyclonic October, similar to the results produced here.
circulation (48.6%) and airflow from any o f Cyclonic circulation, however, has a maximum
the westerly directions (56.2%) clearly domin- frequency in winter and a minimum in summer
ate South Island atmospheric circulation over as could b e expected from a n annual lati-
the whole period. This supports Maunder's tudinal shift in atmospheric circulation zones
(1971) general conclusions. The most frequent between pole and equator. I t therefore seems
circulation type is anticyclonic southwesterly that there is no simple northward and south-
(12.4%) followed b y non-directional a n t i - ward movement o f pressure features during
cyclonic (9.4%), emphasising t h e dominant the year. These apparent paradoxes i n the
TA B L E 1 : P E R C E N TA G E F R E Q U E N C I E S O F D A I L Y AT M O S P H E R I C C I R C U L AT I O N T Y P E S BY
SEASON A N D F O R T H E W H O L E P E R I O D 1 J A N U A R Y 1961 T O 31 D E C E M B E R 1980.
Spring (Sept.-Nov.)
Non- Total
NE SE SW NV NW directional
Anticyclonic 2.7 3.9 1.0 1.0 3.6 12.5 4.7 6.6 7.5 43.5
Cyclonic 1.5 2.8 0.8 0.7 1.5 7.4 6.0 11.0 3.7 35.4
Unspecified 1.3 1.3 0.4 0.3 1.5 6.3 3.8 4.6 1.6 21.1
Total 5.5 8.0 2.2 2.0 6.6 26.2 14.5 22.2 12.8 100.0
Summer (Dec.-Feb.)
Non- Total
N NE E SE S S W W NW directional
Anticyclonic 4.3 6.1 2.4 1.4 3 . 5 11.9 4 . 0 7.7 11.5 52.8
Cyclonic 1.2 2.0 0.4 0.7 1 . 4 7 . 5 5 . 6 7.0 3.2 29.0
Unspecified 1.2 0.9 0.1 0.4 1 . 2 4 . 8 3 . 8 3.6 2.2 18.2
Total 6.7 9.0 2.9 2.5 6 . 1 2 4 . 2 13.4 18.3 16.9 100.0
Autumn (March-May)
Non- Total
N NE E SE S S W NW directional
Anticyclonic 3.0 4.7 2.2 1.7 4 . 7 14.0 4.6 6.1 10.0 51.0
Cyclonic 1.7 2.4 0.8 0.7 1 . 6 7 . 2 4.3 7.9 3.8 29.9
Unspecified 1.5 1.7 0.3 0.2 1 . 0 5 . 6 3.5 3.2 2.1 19.1
Total 6.2 8.8 3.3 2.6 7 . 3 26.8 12.4 16.7 15.9 100.0
Winter (June-Aug.)
Non- Total
NE SE SW NV NW directional
Anticyclonic 2.6 5.8 2.8 2.3 4.4 11.3 3.3 6.1 8.8 47.4
Cyclonic 2.6 3.6 1.1 1.2 3.2 7.4 5.2 7.1 4.2 35.6
Unspecified 1.3 1.6 0.9 0.7 1.0 4.5 2.0 3.4 1.6 17.0
Total 6.5 11.0 4.8 4.2 8.6 23.2 10.5 16.6 14.6 100.0
Whole period
Non- T o t a l
NE SE SW W N W directional
Anticyclonic 3.1 5.1 2.1 1.6 4.1 12.4 4.1 6 . 6 9.4 4 8 . 6
Cyclonic 1.8 2.7 0.8 0.8 1.9 7.4 51 8 1 3.7 3 2 . 5
Unspecified 1.3 1.4 0.4 0.4 1.2 5.3 3.3 3 . 7 1.9 1 8 . 9
Total 6.2 9.2 3.3 2.9 7.2 25.1 12.7 18.4 15.0 1 0 0 . 056 A S t u d y o f Atmospheric Circulation
temporal variability of atmospheric circulation at 500 mb over New Zealand. Although there
of the South Island w i l l be examined later is this distinct change in jet structure i n the
in more detail using circulation indices. middle troposphere, t h e mean tropospheric
wave pattern varies only slightly during the
On examining the directional classes, i t is year. A n upper level ridge occurs over New
interesting t o note fewer occurrences o f true Zealand i n the summer which moves slightly
westerlies compared t o northwesterlies a n d to the west i n winter (van Loon, 1972b).
southwesterlies. However, this relates t o the
passage o f frontal troughs over t h e South C I R C U L AT I O N INDICES
Island. These are preceded b y northwesterly Four monthly circulation indices w e r e
flow and followed directly b y southwesterly. derived f r o m t h e daily classification i n a
True westerly gradient flow occurs f o r rela- similar manner to Murray and Lewis (1966).
tively short periods when anticyclones occur However, minor changes were made t o allow
to the north with depressions immediately to for local conditions. For example, we divided
the south. A s these t w o features normally their progression index into separate easterly
move a t different speeds from west t o east and westerly indices because o f the major
this situation i s n o t frequent. Comparing impact these two airflow directions have on
easterly and westerly directional classes, flow local weather of the South Island. This is due
from one o f the easterly directions reaches a particularly to the role o f the Southern Alps
maximum frequency in winter (with 20%) and as a significant barrier t o air masses. Their
a minimum i n spring. General westerly flow general meridional index was not used as i t
shows a reverse trend with westerly and north- was considered t h a t a southerly index ( i n
westerly directions particularly important i n which northerly a n d southerly components
spring. However, southwesterlies a r e m o r e were differentiated) would be more useful i n
frequent i n autumn, particularly anticyclonic examining relationships between atmospheric
southwesterlies. With regard to easterly circula- circulation and climatic characteristics over the
tion types (NE, E , SE), i n all seasons anti- South Island. This is particularly important,
cyclonic easterlies dominate w i t h cyclonic as weather conditions are so strongly influenced
easterlies increasing significantly i n frequency by airflow direction. T h e southerly index
only i n winter. T h e higher frequencies o f differs, then, from the westerly and easterly
northeasterlies a n d northerlies compared t o indices in that monthly values become negative
easterlies a n d southeasterlies c o n f i r m t h e when northerlies dominate and positive when
importance o f anticyclones t o the east and southerlies dominate.
southeast o f New Zealand (Trenberth, 1976).
This area is said to experience relatively high TA B L E 2 : D A I L Y I N D E X VA L U E S U S E D T O
frequencies o f blocking compared to the rest C A L C U L AT E M O N T H L Y C I R C U L A T I O N
of the Southern Hemisphere (Streten, 1980; INDICES.
Trenberth, 1976). Streten (1980) indicates that Where a n i n d e x value i s given f o r a directional
anticyclonic blocking i n t h e N e w Zealand classification (e.g. westerly), i t i s t h e same f o r a l l
classifications having that direction (e.g. unspecified
region is frequent all year with a maximum westerly, anticyclonic westerly and cyclonic westerly).
in winter. Trenberth, however, suggests that CYCLONICITY Non-directional anticyclonic — 2
"anticyclones are situated mainly east of New INDEX Directional anticyclonic — 1
Zealand in summer and west of New Zealand A l l unspecified 0
in winter, but can occur i n either location." Directional cyclonic 1
Non-directional cyclonic 2
(Trenberth, 1976, p . 74). However, he does
WESTERLEY Westerly 2
say that "Since the 1940's there has been a INDEX Northwesterly or southwesterly 1
trend f o r more anticyclones to occur east o f A l l other classifications 0
New Zealand and loss i n the Australia and SOUTHERLY Northerly — 2
Tasman Sea area." (Trenberth, 1976, p. 74). INDEX Northwesterly or northeasterly —1
It is interesting t o note that, on the basis o f Westerly, easterly, anticyclonic,
data f o r 1972-78, i n winter the 500 m b jet cyclonic o r unspecified 0
splits n o r t h a n d south o f N e w Zealand Southwesterly or southeasterly I
Southerly 2
(Trenberth, 1979). This is a preferred situation EASTERLY Easterly 2
for the occurrence of blocking activity (Austin, INDEX Northeasterly or southeasterly I
1930). I n summer a weaker single jet occurs All other classifications 0A Study o f Atmospheric Circulation 5 7
Table 2 describes t h e derivation o f the indices o f cyclonicity, westerly, southerly and
circulation indices. Each of the four monthly easterly circulation were therefore obtained for
indices was calculated as the sum of the daily the period 1961 t o 1980. These data series
values as given i n t h e table. 240 monthly were then smoothed using 3 and 12 term filters,
the first to remove inter-monthly fluctuations
15
but retaining the annual cycle and the second
10
to remove intra-annual variations to illustrate
5
more clearly longer term variability.
0
-5 Figure 2 shows clearly the seasonal varia-
-10 tion of atmospheric circulation over the South
-15 Island u s i n g t h e f o u r indices described.
-20 Cyclonicity exhibits an annual variation with
-25 a marked minimum i n February when anti-
cyclones dominate. However, there is no clear
winter maximum but rather minor oscillations
40
about a constant level between about July and
35
November. T h e westerly i n d e x shows a n
30
interesting variation containing annual a n d
25
semi-annual cycles. The westerly maximum in
20
October relates t o the strengthening o f the
latitudinal temperature and pressure gradient
15
in spring as recognised b y Maunder (1971).
10
However, a sub-maximum also occurs during
5
early autumn suggesting a twice yearly o r
0
semi-annual variation of the pressure gradient
as noted b y Maunder (1971) and van Loon
(1972a). The winter minimum in the westerly
25
index is also quite clear. The southerly index
20
shows a similarly complex annual variation,
15 again with a possible semi-annual periodicity.
tO Southerlies indicated by positive values appear
5 to dominate in late autumn and early winter
0 (May and June) with distinct minima in March
-5 and August when northerlies (indicated b y
-10 negative values) clearly dominate. I n spring,
-I5 the southerly index stays about zero suggesting
a reduction i n meridional exchange with the
increase in westerly flow.
25
The existence o f biennial (two-yearly) and
20
semi-annual (twice-yearly) variations in atmos-
15
pheric circulation of the New Zealand region
1111111f has been suggested b y a number o f authors.
10
5
The biennial variation w a s discussed b y
0 Trenberth (1976), while Maunder (1971), van
-5 Loon (1972a) and van Loon and Rogers (1984)
-10
identified the semi-annual variation, particu-
J M A M J .1 A S O N S
15 larly to the east and south of the area. Spectral
analysis of the circulation indices derived here
Months confirmed the existence of a biennial variation
of the cyclonicity and southerly indices, while
Fig. 2 : Average monthly circulation indices derived the semi-annual variation occurred only in the
from the daily synoptic classification, 1961-1980. The cyclonicity index. This differs from the average
circle represents t h e mean, t h e bars represent t h e
standards deviation ( C = cyclonicity, W = westerly, monthly indices in Figure 2 which appear to
S = southerly, E e a s t e r l y ) . show stronger semi-annual variations i n the0-
C -10
58
-20
W 20
301
,01
10-
o-
-10-
30
E 20
10
I r - r 1 v
19611 6 2 6 3 6 4 65 6 6 6 7 6 8 6 9 7 0 71 7 2 7 3 7 4 7 5 7 6 7 7 7 9 7 9 1980
YF
Fig. 3 : T i m e series o f t h e f o u r circulation indices, 1961-1980, smoothed using a 3 t e r m moving average
(labels as i n Figure 2).
30
20
10
10-
0
- 10
3°-
2°-
10-
1961 6 2 6 3 6 4 65 6 6 6 7 68 6 9 7 0 71 7 2 7 3 7 4 7 5 7 e 7 7 78 7 9 1980
YEARS
Fig. 4 : Ti m e series o f t h e f o u r circulation indices, 1961-1980, smoothed using a 1 2 t e r m moving average
(labels as i n Figure 2).A Study of Atmospheric Circulation 59
westerly index and perhaps the southerly index. an unusual year being one o f "the warmest
Only t h e cyclonicity a n d westerly indices on record i n New Zealand" (Trenberth, 1976,
showed an apparent annual variation. How- p. 73) and experiencing persistent northeasterly
ever, the time series are probably not long winds. This shows up clearly i n Figure 4 as
enough t o provide a reliable assessment o f a marked decrease i n southerly and increase
periodic fluctuations. Finally, t h e easterly in easterly indices. In 1972, drought conditions
index shows only minor variations w i t h a continued in eastern areas, but also spread to
clear maximum i n winter followed b y a northern South Island (Burrows and Green-
marked reduction i n spring a s westerlies land, 1979), as southwesterly airflow became
increase. Although this maximum coincides more frequent (Trenberth, 1975). Figure 4
with a minimum i n the westerlies the two shows t h a t t h e m o s t marked change i n
indices are n o t so closely related a t other meridional circulation occurred between 1971
times. T h e easterly index shows much less and 1972 as the dominant airflow direction
variability. However, b y examining t h e changed from northerly t o southerly.
standard deviations (given by the vertical bars
in Figure 2 ) o f all four indices the year t o TA B L E 3 : M E A N , S L O P E , A N D S TA N D A R D
year variability of monthly circulation appears D E V I AT I O N O F T H E F O U R C I R C U L AT I O N
to be quite large, particularly with cyclonicity. INDICES, 1961-1980.
Mean Slope Standard
This inter-annual variability o f monthly deviation
circulation indices is illustrated i n Figure 3. Cycloni city —6.63 0.014 9.1
Th:se monthly time series are smoothed using Westerly 20.96 —0.008 6.7
a 3 term simple moving average, b u t still Southerly 0.67 0.001 8.0
Easterly 5.68 0.009 4.2
retain significant variability. The annual cycle
noted i n the monthly averages is apparent in These short term climatic variations have
parts of all four series but particularly in the
cyclonicity index. I n the westerly and easterly significant impact on local weather conditions
indices i t is particularly prominent during the and subsequently t h e economy o f N e w
period 1976 to 1980. Variation of the southerly Zealand, particularly as i t i s based largely
index appears much less regular. on agriculture. In order to examine the longer
term trends, the slope of each series is provided
The longer term variations are shown i n together with basic statistics in Table 3. I t is
Figure 4 i n which the monthly time series clear from the means that the South Island
have been filtered using a 1 2 term simple is dominated b y anticyclonic circulation (the
moving average. Clear fluctuations appear in cyclonicity index being negative) and persistent
all four indices, but they are seldom coincident. westerly airflow. A s i m i l a r frequency o f
The most marked variation occurs i n t h e southerly a n d northerly winds produces a
cyclonicity index between 1968 and 1969, two southerly mean close to zero, while as might
years which experienced h i g h l y contrasting be expected easterly airflow is o f much less
weather conditions. Burrows and Greenland importance than are westerlies. The slopes o f
(1979) indicate that eastern and northern parts the f o u r time series suggest quite marked
of the South Island experienced severe flooding changes i n circulation over the twenty-year
in 1968 followed b y drought conditions i n period. T h e maximum slope i s 0.014 i n
1969. I t is interesting that Trenberth's (1975, cyclonicity, representing a n increase i n the
1976) analysis, based largely on flow direction, monthly index o f 3.36 over the 240 months.
does n o t identify t h i s distinct circulation This can be thought of as an increase of 2 to
change. A similar b u t smaller change i n 3 days o f cyclonic circulation p e r month.
cyclonicity took place between 1971 and 1972, This increase in cyclonicity coincides with an
but i n this case i t occurred simultaneously increase in easterlies, a decrease in westerlies,
with a change from predominantly northerly and a v e r y small increase i n southerlies.
to southerly flow, as well as a decrease i n Student's-t tests were used t o assess t h e
easterlies. T h e resulting local weather was significance o f these circulation changes b y
similarly more complex, with severe flooding comparing means of the first and second halves
in western and northern South Island in 1971, of each of the index series in a similar manner
but also drought conditions i n t h e east. to Trenberth (1976). I n spite o f the greater
Trenberth (1975, 1976) indicated that 1971 was slope, the change in cyclonicity is not statis--60 A Study o f Atmospheric Circulation
tically significant. I t appears that cyclonieity Analysis o f t h e circulation indices data
varies so much month-to-month and year-to- provides some evidence o f biennial, annual
year, evidenced b y i t s greater standard and semi-annual variations, although not i n
deviation (see Figure 2 and Table 3), that any all four indices. These variations are consistent
long term trend is difficult to confirm statis- with results found by other researchers includ-
tically. T h e slope o f the southerly series i s ing Trenberth (1975, 1976, 1977), van Loon
also not significant. However, the change i n (1971, 1972a, 1972b, 1974) and van Loon and
westerly and easterly airflow is significant at Rogers (1984), and have a major influence
the 2.5% and 0.5% levels respectively using on the regional weather. This aspect is to be
a two-tailed test (1% and 0.1% using a one- followed u p i n further research.
tailed test). This clearly supports Trenberth's
(1976, 1977) major conclusion that easterlies, Longer term trends i n t h e indices data
particularly northeasterlies, h a v e increased suggest that distinct fluctuations in atmospheric
circulation o f this area have occurred over
frequency a t the expense o f westerlies. This
is attributed t o the increased occurrence o f the last t w o decades. T h e most significant
anticyclones to the east and southeast o f the change is an increase in easterly airflow and
South Island, as well as a poleward movement this appears t o occur a t the expense o f the
of the westerly wind belt. westerlies. Trenberth (1976) suggested t h a t
the westerly wind belt has moved poleward
DISCUSSION since t h e 1940's, during which t i m e anti-
cyclones have become more frequent t o the
This paper provides an initial analysis o f east and southeast o f New Zealand. A s a
synoptic scale atmospheric circulation over the result, Trenberth indicated that northeasterlies
South Island, New Zealand. I t has a dual have become more frequent.
purpose i n firstly describing an atmospheric
circulation data scouree based on twenty years CONCLUSION
of surface synoptic chart data. T h i s data
source is currently being used i n studies o f The classification scheme a n d derived
coastal erosion and deposition, alpine precipi- indices used here have been applied success-
tation (Sturman and Soons, 1984), and local fully b y researchers i n other parts o f the
wind systems. Secondly, t h e classification world (e.g. Perry 1969, 1970, 1975; Murray
scheme and derived indices used here provide and Benwell, 1970; Sowden and Parker, 1981;
a new approach to the description and analysis Storey, 1982). They have proved quite success-
of atmospheric circulation over t h e South ful i n identifying relationships between
Island o f New Zealand. atmospheric circulation and regional climatic
characteristics (such as precipitation, cloudi-
The results describe the major atmospheric ness, and temperature). The initial application
circulation characteristics o f the South Island of this approach to the study of the atmosphere
area. The dominance o f anticyclonic circula- over the South Island produced results that
tion and westerly airflow is clearly evident, are comparable to those o f previous studies.
with anticyclonic southwesterly being t h e However, t h e circulation types a n d indices
most frequent type i n a l l seasons. Seasonal derived here seem to provide greater flexibility
variability of classified types is consistent with in analysis and presentation o f results. This
previous research (Maunder, 1971), but much initial study will be followed by extension of
more detail is provided. The development o f the classification both backward and forward
circulation indices allows a clear description in time, as well as application to analysis of
of temporal variation of atmospheric circula- particular climatic characteristics o f t h e
tion over the area i n terms o f both vorticity South Island.
and airflow direction. Monthly average indices
for t h e twenty-year period have illustrated ACKNOWLEDGMENTS
intra-annual variability not previously noted
or presented in this form. The time series o f The authors acknowledge the continuing
circulation indices show important variations assistance of the New Zealand Meteorological
over the period, especially when smoothed. Service i n providing the necessary data, i n
In particular, a marked difference in circula- particular their librarians and M r. G. F. A .
tion between 1968 and 1969 is clearly indicated. Ward. Financial assistance o f the UniversityA Study of Atmospheric Circulation 61
of Canterbury is also much appreciated, as 1 9 7 0 : Changesi n d u r a t i o n a n d frequency
of synoptic types over the British Isles. Weather,
is t h e technical support provided b y t h e 25: 123-126.
Geography Department. We are also indebted 1975: Eastern N o r t h A t l a n t i c sea-surface
to R. G. Barry, for his helpful comments on temperature anomalies and concurrent tempera-
the manuscript, as well as N . Ware and R . ture and weather patterns over the British Isles.
Weather, 30: 258-261.
McDonald, University of Colorado, for typing Salinger, M . J . , 1976: N e w Zealand temperatures
and final drafting. since 1300 A.D. Nature, 260: 310-311.
1 9 7 9 : N e w Zealandc l i m a t e : t h e tempera-
ture record, historical data and some agricultural
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