The Impact of Climate Modes on Summer Temperature and Precipitation of Darwin, Australia, 1870-2011

doi:10.4236/acs.2012.24051

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Atmospheric and Climate Sciences, 2012, 2, 562-567

http://dx.doi.org/10.4236/acs.2012.24051 Published Online October 2012 (http://www.SciRP.org/journal/acs)

The Impact of Climate Modes on Summer Temperature

and Precipitation of Darwin, Australia, 1870-2011

Cameron Hunter, Jacqueline Binyamin*

University of Winnipeg, Department of Geography, Winnipeg, Canada

Email: *[email protected]

Received August 23, 2012; revised September 25, 2012; accepted October 5, 2012

ABSTRACT

Monthly mean summer (DJF) temperature and precipitation from Global Historical Climate Network (GHCN-V3) for

the period of 1870-2011, are analyzed to assess the role of teleconnections on climate of Darwin, Australia. Indices of

El Niño-Southern Oscillation (ENSO), Antarctic Oscillation (AAO), Pacific Decadal Oscillation (PDO), North Atlantic

Oscillation (NAO), Arctic Oscillation (AO), and Pacific North American Oscillation (PNA) are extracted from monthly

means and compared with climatic data of Darwin. Most of these climate modes are shown to have a strong influence

on the monthly mean summer temperature and precipitation. ENSO is shown to have a positive relationship with the

amount of precipitation received and a negative relationship with the temperature. Where an El Niño event produces

warmer drier conditions and a La Niña event produces colder wetter conditions. The AAO is shown to cause cold and

dry condition s during the positive phase and warm and wet conditions during the neg ative phase. The PDO is shown to

cause El Niño like co ndition during the p ositive phase causing warmer, drier weather, and La Niña like conditio ns dur-

ing the negative phase causing cooler, wetter weather. Through the analysis it is also shown that the NAO, AO, and

PNA have little effect on the temperature and precipitation patterns of Darwin.

Keywords: Climate Modes; Teleconnections; Climate of Darwin; Climate Change; Climate Variability; ENSO; AAO;

PDO

1. Introduction

Darwin, Australia latitude 12.4˚ south, longitude 130.9˚

east is located on Australia’s northwestern coast. Month-

ly average temperature and total precipitation data for

Darwin International Airport, which is approximately 3

km from the Indian Ocean and is 30 m above sea level

are obtained from Global Historical Climate Network

(GHCN-V3) for the period of 1870-2011.

Darwin receives an average of 1693 mm of precipita-

tion yearly, with the distribution of precipitation being

very seasonal. Most of Darwin’s yearly total precipita-

tion is received in the summer months (DJF), while Dar-

win receives nearly no precipitation in the winter months

(JJA). Darwin’s average annual temperature is 27.6˚C

and due to its proximity to the ocean does not fluctuate

much throughout the year. The temperature ranges from

approximately 29˚C in summer to a low of 25.5˚C in

winter. The proximity to the ocean mediated Darwin’s

temperature, cooling the air during the summer months

and warming the air during the winter months. This mod-

erating effect causes Darwin’s small temperature range.

This paper investigates the effect of six teleconnec-

tions on summer temperature and precipitation of Darwin.

These teleconnections include El Niño-Southern Oscilla-

tion (ENSO), Antarctic Oscillation (AAO), Pacific De

cadal Oscillation (PDO), North Atlantic Oscillation

(NAO), Arctic Oscillation (AO), and Pacific North

American Oscillation (PNA). Our study is important be-

cause little work has been done on the effect that climate

modes have on Darwin’s temperature and precipitation.

The ENSO is categorized by sea surface temperature

and atmospheric pressure anomalies, and measured using

the Southern Oscillation Index (SOI) which is the pres-

sure difference between Tahiti, and Darwin. ENSO has

the strongest effect on the climate of the South Pacific,

causing below average rainfall amounts during El Niño

and above average rainfall amounts during La Niña for

Darwin [1,2]. Several studies have explored the precipi-

tation variability over Darwin and across the Australian

continent [3-5]. While others have explored ENSO’s

effect on the Australian continent [1,2,6-9].

The AAO is measured using the Antarctic Oscillation

Index (AOI) which is the difference between the surface

pressures at 40˚S and 65˚S [10]. The two phases of AAO,

the positive phase and the negative phase, are classified

by the strength of the subtropical high and the subpolar

*Corresponding a uthor.

C. HUNTER, J. BINYAMIN 563

low. The positive phase shows a strengthening of the

subtropical high and a deepening of the subpolar low,

creating a greater than normal pressure difference be-

tween the two [11]. While the negative phase shows a

weakening of th e subtropical high and subpolar low, cre-

ating a weak pressure gradient [10]. The AAO has a

strong effect on the location of the southern hemisphere

midlatitudejet stream and through this affects the tem-

perature and precipitation patterns throughout southern

hemisph ere [4,10].

The PDO is a long-lived El Niño-like pattern of Pa-

cific climate variability. Its events can persist for 20 - 30

years and have a strong effect on the climate of North

America. Fluctuating sea surface temperatures (SSTs) in

the central North Pacific and along the west coast of

North America distinguishes the positive and negative

phases of this climate mode. When the PDO is in its

warm or positive phase, SSTs in the western coast of

North America are relatively warm and central North

Pacific SSTs are cooler and vice versa in the cool or

negative phase causing a change in the temperature and

precipitation patterns in these areas [12,13 ].

The NAO is the result of pressure differences between

the Azores high and the Icelandic low [14,15]. NAO af-

fects the northern hemisphere winter climates of eastern

North America and Europe, and as well affects the posi-

tion of the Atlantic storm track.

The AO is identified by the pressure difference be-

tween the polar high and the atmospheric low south of

the pole [16]. The strong pressure gradient causes the

circumpolar vortex to strengthen keeping the cold polar

air north [17,18]. The strengthening of the circumpolar

vortex causes mild weather throughout Europe and cold

weather in Greenland and Newfoundland during the nor-

thern hemisphere winter. The weak polar vortex brings

cold weather to Europe and the United States and warm

weather to Greenland and Newfoundland during the nor-

thern hemisphere win t e r.

The PNA affects the climate of the northern hemi-

sphere. It deals with the 700 mb heights for the locations,

Hawaii, the Aleutian Islands, southeastern United States

and the Intermountain region of North America. The

positive phase of the PNA shows higher than average

heights over Hawaii and the Intermountain region of

North American, while below average heights are re-

corded over the Aleutian Islands and the southeastern

United States. The negative phase shows a reversal of the

heights of these areas [19]. These height anomalies affect

the position of the East Asian jet stream and in the posi-

tive phase produce above average temperature in the

western United States and Canada and below average

temperatures across the southern United States. PNA also

affects the precipitation patterns of North America with

the positive phase causing above average precipitantion

on the Gulf of Alaska and below average precipitation

for the Midwestern United States [19,20].

Section 2 describes temperature and precipitation resu-

lts; and Section 3; summary and conclusions.

2. Results and Discussion

2.1. Temperature

Most of the Australian continent has experienced a war-

ming trend in the past century, except for a region of

cooling in the northwest [5]. Risbey [5] links this region

of cooling to the increased amount of precipitation re-

ceived in this area in the past century, and a greater per-

mcentage of cloudy days blocking incoming solar radia-

tion. Figure 1 shows that the temperature of Darwin has

cooled approximately by 1.5˚C over the past 129 years.

While there is a large amount of year-to-year variation in

the annual average temperature, there is clearly a strong

cooling trend in the temperature. As Darwin is located

along the northwest coast of Australia and as it is shown

in Section 2.2., has seen an increase in the total precip ita-

tion received, this blocking of solar radiation by cloud

cover could be one of the causes of the cooling tempera-

ture trend. Crowley [21] suggested that this cooling could

be due to the northern hemisphere exporting cold water

into the southern hemisphere. A seesaw effect is ob-

nserved where warming in the northern hemisphere

causes cold water from deep North Atlantic to circulate

and surface in the southern hemisphere thereby cooling

despite high CO2 levels [21].

Figure 2 shows again the trend of decreasing tem-

peratures in Darwin, but it also reveals a period of colder

than normal temperature. The period of 1942 to 2011 is

shown to be on average 0.5˚C cooler than would be ex-

pected by the trend. This period of cooling may be at-

tributed to increased amounts of aerosols in the earth’s

atmosphere specifically sulfate aerosols created by vol-

canic activity, as well as anthropogenic activity. These

Figure 1. Average annual temperature (˚C) for Darwin,

1882-2011. Data are incomplete for some years and there-

fore are omitted.

C. HUNTER, J. BINYAMIN

564

Figure 2. 30-year temperature (˚C) averages for Darwin, for

the period 1882-2011.

aerosols reflect incoming solar radiation and reduce the

amount of radiation reaching the surface, as well as act-

ing as cloud condensation nuclei and increasing the cloud

cover. Through th ese processes, sulfate aerosols d ecrease

the temperature of the surface and lower atmosphere.

We believe that sulfate aerosols are one of the possible

causes of this anomalous cold period from 1942-2011.

The ENSO, AAO and the PDO show the strongest

correlation with Darwin’s temperature. ENSO shows a

strong negative correlation with the summer temperature

at Darwin. The correlation coefficient of –0.36 means

strong negative values of the SOI will produce above

average temperatures. Large negative SOI values repre-

sent strong El Niño events, these events seem to produce

warm temperature anomalies, while positive SOI values

represent La Niña event and produce cold anomalies at

Darwin. Figure 3 shows this relationship between El

Niño/La Niña events and the summer temperature in

Darwin. The same relationship between temperature and

SOI has also been recognized previously by Power et al.

[6] and Suppiah [2]. This temperature trend could be due

to the pressure systems above Darwin in summer during

El Niño and La Niña events. During El Niño events a

high-pressure system persists above Darwin , with air sin-

king and diverging at the surface. Therefore there is no

uplift over Darwin and thus no clouds to reflect incoming

solar radiation. During La Niña events the opposite oc-

curs and a low-pressure system over Darwin causes con-

verging air and uplift. Greater amounts of cloud over can

be expected to reflect solar radiation increasing the local

albedo and so less radiation is received at the surface and

lower temperatures are experienced.

AAO also s eems to h ave a s trong eff ect on the av erag e

summer temperature of Darwin with a correlation coeffi-

cient of –0.42. Figure 4 shows strong positive AAO

events produces colder temperatures while negative

events produces warmer temperatures. The shift of the

southern hemisphere midlatitude jet stream by the change

in the pressure gradient between the polar and the sub-

tropical pressure centers produces bands of varying tem-

perature and rainfall differences in the southern hemi-

sphere, with a latitudinal band of cool temperatures fal-

ling on Australia during the positive phase and a latitu-

dinal band of warmer temperatures during the negative

phase [4].

PDO also shows a strong correlation with the average

summer temperature of Darwin shown in Figure 5. The

strong positive correlation coefficient of 0.37 shows

PDO’s strong effect on the summer temperatures of Dar-

win. The positive correlation means positive PDO even ts

produce warm temperature anomalies while negative

PDO events produce cold temperature anomalies. As we ll

as affecting the atmosphere and oceans in the northern

Pacific, the PDO also produces ENSO like conditions in

the South Pacific. The PDO produces El Niño like condi-

tions during the positive phase of the PDO, and La Niña

like conditions during the negative phase of PDO [12].

The analogous ENSO conditions produced by the PDO

explain the strong effect the PDO has on Darwin’s tem-

perature.

Figure 3. Correlation between average summer (DJF)

Southern Oscillation index (SOI) and average summer te m-

perature (˚C) for Darwin, 1882-2010. SOI data are from:

http://www.cru.uea.ac.uk/cru/data/soi/soi.dat.

Figure 4. Correlation between average summer (DJF) Ant-

arctic Oscillation (AAO) index and average summer tem-

perature (˚C) for Darwin, 1980-2010. AAO Index data are

from:

http://www.cpc.ncep.noaa.gov/products/precip/CWlink/dail

y_ao_index/aao/monthly.aao.index.b79.current.ascii.table.

C. HUNTER, J. BINYAMIN 565

Figure 5. Correlation between summer (DJF) Pacific De-

cadal Oscillation (PDO) index and the average summer

temperature (˚C) for Darwin, 1902-2010. PDO Index data

are from: http://jisao.washington. edu/pdo/PDO.latest.

The NAO, AO, and PNA are shown to have little ef-

fect on the temperature of Darwin. As these three tele-

connections affect the atmospheric and oceanic circula-

tion in the northern hemisphere, their effect on Darwin if

they have any, is too small to be discovered. If there is

any effect on the climate of Darwin, it is likely masked

by other interactions due to the large distance separating

Darwin from the centers of action of these oscillation

systems.

2.2. Precipitation

Darwin has experienced an increase in the yearly total

precipitation received in the last 142 years (Figure 6).

On average Darwin received 650 mm more precipitation

in a year now than in 1870. The trend in precipitation at

Darwin follows the trend of precipitation across all of

Australia, with the coastal wet regions receiving more

precipitation and the interior dry regi ons receiving le ss [5].

As with temperature the ENSO, AAO, and PDO all are

shown to have strong effects on the yearly summer pre-

cipitation totals. Again AO, NAO and PNA have little

effect on Darwin’s precipitation. The correlation coeffi-

cient between summer averages of SOI and total summer

precipitation for Darwin is 0.21 (Figure 7). The strong

positive correlation means that La Niña events, (positive

SOI values) causes greater than normal precipitation and

El Niño events (negative SOI values) produces less pre-

cipitation than normal. These precipitation anomalies are

due to the persistent high and low pressures above Dar-

win during La Niña and El Niño. During El Niño the

anomalous high-pressure over Darwin causes divergence

and sinking air, as there are few processes of up lift in

this area, therefore, less precipitation is received. La Niña

events have the opposite effect, anomalous low-pressure

over Darwin produces lots of uplift and thus lots of pre-

cipitation in the area [4,6,11]. Many strong El Niño/La

Niña events have caused large precipitation anomalies in

Darwin. Both the summers o f 1982-1983, and 1997-1998

were strong El Niño events with summer average SOI

values of –3.06 and –2.16 respectively; these years re-

ceived 576 mm, and 532 mm less than normal precipita-

tion. The strong La Niña event in the summers of 1998-

1999 had an av erag e SOI v alue of 1.16 , whic h re - cord ed

148 mm of precipitation more than normal (Figure 6).

The AAO has the strongest effect on the summer pre-

cipitation totals for Darwin. Figure 8 clearly shows the

positive relationship between the AAO index and the

total summer’s precipitation values with a correlation

coefficient of 0 .4. The positive cor relation between AAO

index and summer p recipitation means the positiv e phase

of AAO will cause greater than normal precipitation val-

ues in Darwin, and the negative ph ase of AAO will cau se

less than normal precipitation values. The movement of

the southern hemisphere midlatitude jet stream south in

the positive phase produ ces a dry band b etween 30˚S and

50˚S and a wetter band between 20˚C and 30˚S [4]. Dar-

win (latitude 12.4˚S) is located nearest to the later band

and obviously receives some residual effect of the wet

band. While the negative ph ase of AAO cause a dry band

between 20˚S and 30˚S and this must cause less precipi-

tation in Darwin.

The PDO also shows a strong influence on the pre-

cipitation of Darwin. As stated above the PDO creates El

Niño/La Niña conditions in the southern Pacific, as it

affects the Northern Pacific [12]. The correlation coeffi-

cient of –0.21 shows that dry El Niño type conditions are

produced by the positiv e phase of PDO and wet La Niña

like conditions are produced during by the negative

phase of PDO (Figure 9). The figure also shows the

negative relationship between PDO and summer precipi-

tation values.

3. Summary and Conclusions

Darwin has undergone a cooling of 1.5˚C of its average

Figure 6. Total annual precipitation (mm) received in Dar-

win, 1870-2011.

C. HUNTER, J. BINYAMIN

566

Figure 7. Correlation between the Southern Oscillation

Index (SOI), and the total summer (DJF) precipitation for

Darwin, 1871-2010. SOI data are from:

http://www.cru.uea.ac.uk/cru/data/soi/soi.dat.

Figure 8. Correlation between Antarctic Oscillation (AAO)

Index and the total summer (DJF) precipitation for Darwin,

1980-2011. AAO Index data are from:

http://www.cpc.ncep.noaa.gov/products/precip/CWlink/dail

y_ao_index/aao/monthly.aao.index.b79.current.ascii.table.

Figure 9. Correlation between the Pacific Decadal Oscilla-

tion (PDO) Index and the total summer (DJF) precipitation

for Darwin, 1902-2011. PDO Index data are from:

http://jisao.washington.edu/pdo/PDO.latest.

annual temperature dur ing the period of 1882-2011 . This

cooling can be attributed to increased precipitation in the

northwestern regions of Australia causing these areas to

receive less solar radiation due to blocking of solar radia-

tion by cloud cover. Another explanation of this cooling

could be due to cold water from deep north Atlantic sur-

facing in the southern hemisphere and cooling the south-

ern oceans. The atypical cool period from 1942-2011

could possibly be due to increased sulfate aerosols in th e

atmosphere, which reflect solar radiation. Sulfate aero-

sols also act as cloud condensation nuclei, increasing the

cloud cover and reducing the amount of radiation reach-

ing the surface leading to cooling of the surface and the

lower atmosphere.

The ENSO, AAO and PDO have all been shown to

have a large effect on Darwin’s average summer tem-

perature. The ENSO is shown to produce warmer condi-

tions at Darwin during El Niño events and cooler condi

tions during La Niña events. The AAO produces cold

weather during the positive phase and warm weather

during the negative phase. Unlike AAO the PDO’ posit-

ive phase is shown to produce warm weather and the ne-

gative phase is shown to produce cooler weather at Dar-

win.

Precipitation in Darwin increased during the 1870-

2011 period by an amount of 650 mm. Also, ENSO, AAO

and PDO are all observed to have a large effect on the

summer precipitation in Darwin. El Niño events produce

much dryer summers and La Niña events produce wetter

summers. The low-pressure present during La Niña

events causes convergence and precipitation over Darwin,

while the high pressure present during El Niño events

causes divergence and clear skies. The AAO is shown to

cause drier summer weather during the negative phase

and wetter summer weather during the positive phase.

Finally the PDO causes Darwin to experience dry sum-

mer weather during the positive phase and wet summer

weather during the negative phase.

Climate modes are shown to have a large effect on the

Darwin’s summer temperature and precipitation and have

significantly changed its yearly average temperature and

total precipitation in the past 1 40 years. A greater change

in the Earth’s climate will likely have an effect on the

frequency and strength of the climate modes discussed

and will thus greatly alter Darwin’s climate as well as the

climate of other locations. Future research is needed to

understand the relationship between climate change and

variability and the climate modes in order to be able to

forecast the effect of climate change on other locations.

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