October’s Climate in Brief
Temperature October 2009
Climate Update for Tonga
October 2009 Climate Summary
Dec2009 to Feb2010 Rainfall Outlook
•
Niuatoputapu and Vava’u is currently in Drought
. Rainfall has been below normal for the last six months.
• Drought Warning is current for Ha’apai.
• Well below normal rainfall was recorded across the country during October..
October was very much dry throughout Tonga. Weather was dominated by the existence of ridges of high pressure systems
from the south to southwest bringing cool southerly winds and fine weather to the country. Weak frontal systems during the
first few weeks brought the little rain that was received during October. Vava’u recorded the lowest temperature for the month
at 11.7
o
C and Ha’apai recorded the lowest rainfall at 7.7mm. October rainfall totals throughout Tonga continue to reflect El
Nino type conditions.
El Niño conditions exist in the equatorial Pacific. Many dynamical climate models project the continuation of El Niño through
2009 and into 2010
Rainfall outlook for the next 3 months: Above normal rainfall is favoured for Niuafo’ou, normal rainfall is favoured for central
Tonga and the chances for below normal rainfall is the highest for southern Tonga.
Rainfall in Tonga is reduced during an El Nino Event and the frequency of occurrence of Tropical cyclones increases accord-
ing to historical records.
Issued 10
th
November 2009
Location
Mean
Temperature
(
o
C)
Departure
from
Normal
Comments
Niuatoputapu NA
NA
NA
Vava’u 22.0
-2.8
Below
normal
Ha’apai 23.8
-0.3
Below
normal
Nuku’alofa 22.0
-1.1
Below
normal
Fua’amotu
21.9
-0.6
Below normal
Niuafo’ou 25.7
-0.5
Below
normal
Lowest Mini-
mum Temp
19.4
11.7
Record
low
18.2
14.5
13.1
Highest
Maximum
Temp
31.5
29
30.9
27.3
27.0
Mean air temperatures was below normal across the country during October. The mean temperature for the month
was 23.1
o
C which was 1.5
o
C cooler than normal but 0.1
o
C warmer than September 2009. Daytime maximum tem-
peratures were cooler than normal across the country except Niuafo’ou where it was 0.8
o
C warmer than normal.
Overnight minimum temperatures were cooler than normal in the north and Fua’amotu except for Ha’apai and Nu-
ku’alofa where it was 0.3
o
C warmer than normal.
Table 1: Temperature October 2009
Forecast Validation for the last 3 months, August - October 2009.
Rainfall October 2009
Page 2
Climate Update for Tonga
The rainfall outlook for the August
to October 2009 period was for
above normal for northern Tonga
and below normal to normal condi-
tions for central and southern
Tonga with a low skill level. The
observed rainfall for the last 3
months was above normal in south-
ern Tonga and below normal else-
where. Generally, the forecast for
Aug—Oct period was moderately
consistence with what was predicted
for central and southern Tonga but
inconsistent for the north.
Table 3. Aug– Oct 2009 Rainfall
Ha’apai recorded extremely low rainfall in October. Niuafo’ou and Vava’u recorded well below normal rainfall. The
highest daily rainfall was 22.0mm recorded in Nuku’alofa on the 6th. Niuatoputapu and now Vava’u is currently in
drought. Rainfall has been below normal since May, same for Ha’apai except that in June it received above normal
rainfall and July to October rainfall was below normal.
Drought warning is current for Ha’apai
.
Table 2. Rainfall October 2009
Location
Aug - Oct 2009
Forecast
Range
Aug – Oct
2009
Observed
Data
(mm)
Comments
33%tile
Rainfall (mm)
67%tile
Rainfall (mm)
Niuafo’ou
330.3 475.0 269.5 Below
Normal
Niuatoputapu
N/A
Vava’u
317.3 470.9 181.2 Below
Normal
Ha’apai
236.0 353.3 167.6 Below
Normal
Nuku’alofa
262.0 392.0 489.9
Above
Normal
Fua’amotu 276.0 394.3 475.6 Above
Normal
Station
(data period)
October
Total
(mm)
Forecast Probability
Comments
33%tile
Rainfall
(mm)
67%tile
Rainfall
(mm)
Median
Rainfall
(mm)
Niuafo’ou
1971-2009
20.6
134.3 183.3 154.
Well below normal
3rd lowest in its record
Niuatoputapu
1947-2009
N/A
Vava’u
1947-2009
29.7
100.3 172.0
137.8
Well Below Normal
7th lowest in its record
Ha’apai
1947-2009
7.7
60.0 122.0 92.0
Well below Normal
4th lowest in its record
Nuku’alofa
1945-2009
51.7
53.3 130.7 99.0
Below normal
Fua’amotu
1980-2009
48.3
42.2 110.0 62.0
Below normal
September
Total
(mm)
151.3
76.5
91.1
102.5
357.2
322.7
August
Total
(mm)
97.6
45.7
60.7
57.4
80.4
104.6
Seasonal Predictions for Tonga
(December09 - February 2010)
Drought Status
(Summary analysis)
Niuatoputapu and Vava’u is currently in drought, rain-
fall has been below normal for the last six months. At
least 379mm of rainfall is required next month to end
this drought. Drought Warning is current for Ha’apai.
El Niño Southern Oscillation (ENSO) status.
Pacific Ocean temperatures remain at El Niño event
levels as they have in the last three months however
there has been some cooling in the last fortnight.
As would be expected during an El Niño event, cloudi-
ness near the Dateline has been greater than normal
over the last four weeks. However, compared to past
El Niños, the current trend in cloudiness is weak. Pa-
cific sea surface temperatures are predicted to remain
above El Niño thresholds until the end of the year ac-
cording to most international climate models.
The confidence level in the outlook is related to how con-
sistently the Pacific Ocean affects the rainfall in Tonga.
Therefore the outlook confidence level for the December
to February 2010 period according to climatological his-
torical data shows this effect to be moderate to good .
Current Conditions
Page 3
December—February 2010 Rainfall Outlook
Rainfall Outlook
SCOPIC model outlook for December09 to February
2010 period is for rainfall to be above normal for Niua-
fo’ou , normal for Niuatoputapu, Vava’u and Ha’apai
and below normal for Tongatapu. The skill level of the
forecast is moderate to good.
:
Note the rainfall values are for the three month period (December09— February2010).
Location
Below normal
probability (%)
33%tile
Rainfall (mm)
Normal
Probability %
66%tile
Rainfall (mm)
Above normal
Probability (%)
Niuafo’ou
7
676.0
12
924.0
81
Niuatoputapu
36
647.0
37
857.0
27
Vava’u
37
606.0
52
883.0
10
Ha’apai
12
431.0
54
678.7
34
Nuku’alofa
64
429.3
30
714.3
6
Fua’amotu
53
400.5
23
768.7
24
Table 4.
Seasonal Rainfall Outlook for December 2009—February
Rainfall Recorded over the last 13 month October 2008 to October 2009
Page 4
15.0
17.0
19.0
21.0
23.0
25.0
27.0
29.0
31.0
0
50
100
150
200
250
300
350
Oct
Nov
Dec
Jan
Feb
Mar
Apr
May
June
July
Aug
Sept
Oct
T
e
mp
er
a
t
ur
e (
o
C)
M
ont
hl
y
R
a
i
n
f
a
l
l
T
o
t
a
l
s
(
m
m
)
Fua'amotu Airport
Normal Rainfall (1961-1990)
Monthly Total Rainfall
Monthly avg Max Temp
Monthly avg Min Temp
17
19
21
23
25
27
29
31
33
0.0
50.0
100.0
150.0
200.0
250.0
300.0
Oct
Nov
Dec
Jan
Feb
Mar
Apr
May
Jun
Jul
Aug
Sep
Oct
T
e
mp
e
r
at
ur
e (
o
C)
Mo
n
t
h
l
y
R
a
i
n
f
a
l
l
T
o
tal
s
(
mm)
Salote Pilolevu Airport
Average Rainfall (1961-1990)
Monthly Total Rainfall
Monthly avg Max Tem
Monthly avg Min Tem
15.0
17.0
19.0
21.0
23.0
25.0
27.0
29.0
31.0
0.0
50.0
100.0
150.0
200.0
250.0
300.0
350.0
400.0
Oct
Nov
Dec
Jan
Feb
Mar
Apr
May
Jun
July
Aug
Sep
Oct
T
e
m
p
er
at
u
r
e
(
o
C)
Mo
n
t
h
l
y
R
a
in
f
a
ll T
o
t
a
ls
(mm)
Lupepau'u Airport
Average Rainfall (1961-1990)
Monthly Total Rainfall
Monthly avg Max Temp
Monthly avg MinTemp
This summary is prepared as soon as possible following the end of the month, once climate data is received from
recording stations around Tonga so the values may change on receipt of later information and the ENSO informa-
tion is received from various Meteorological Agencies. Delays in data collection, communication and processing
occasionally arise. While every effort is made to verify observational data, the Tonga Meteorological Service does
not guarantee the accuracy and reliability of the analysis and rainfall predictions presented, and accepts no liabil-
ity for any losses incurred through the use of this summary and its contents. This information should be used as for
guidance only. All requests for data and for further information about this forecast should be directed to the Direc-
tor of the Tonga Meteorological Service, at P.O. Box 845, Nuku’alofa. Or email at
fmt_met@met.gov.to
Note:
The Tonga Meteorological Service currently uses the Seasonal Climate Outlook for Pacific Island Countries
(SCOPIC) Model for its seasonal rainfall prediction, validation and drought analysis. The system analyses current
sea surface temperature patterns across the Pacific Ocean and then matches the most similar patterns experienced
through the available historical period. The terms “Below normal” refers to rainfall in the lowest 33%, “Normal”
refers to rainfall between the 33% and 67%, “Above normal” refers to rainfall in the highest 67% . Table 4 shows
the percentage chance of receiving rainfall in each category from meteorological stations in Tonga. If conditions
are Climatology then it means that we are forecasting an equal chance of rainfall to be in any tercile.
Average day time and night time temperature is the average daily maximum and minimum temperature recorded
throughout Tonga.
The drought analysis summary is based on the ENSO phases on a 4 month aggregate drought index using the Stan-
dardised Precipitation Index (SPI) method. SCOPIC focuses mainly on meteorological drought as it uses only
rainfall for analysis. “Drought is a weather-related natural disaster that occurs when there is not enough water
for users' normal needs. As people use water in many different ways, there is no universal definition”. In this
analysis we refer to
drought
as a prolonged period of below-normal rainfall (6-month rainfall that is below the
40th percentile.
In this issue the use of Southern Tonga refers to Tongatapu and ‘Eua, Central Tonga refers to Ha’apai and Vava’u,
Northern Tonga is referred to Niuatoputapu and Niuafo’ou.
Disclaimer
Page 5
Significant Event
Southwest Pacific Wet Season and also the Cyclone Season
Wet Season which is also the cyclone season starts in November to the following April. During an El Nino event,
the rainfall in Tonga is reduced and the frequency of occurrence of cyclones increases from one to two cyclones to
affect Tonga in a season.
• Niuatoputapu and Vava’u is currently in drought. Rainfall has been below normal for the last six months.
• Drought warning is current for Ha’apai.