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 . 0
56                                                                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        0
A 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 the
0-

          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 University
A 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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