Sains Malaysiana 39(4)(2010): 533–542

Trends in Peninsular Malaysia Rainfall Data During the Southwest  Monsoon and Northeast Monsoon Seasons: 1975–2004

(Trend Hujan di Semenanjung Malaysia Ketika Monsun Barat Daya dan Monsun Timur Laut: 1975–2004)

Jamaludin Suhaila*

Mathematics Department, Faculty of Science

Universiti Teknologi Malaysia, 81310, Skudai, Johor, Malaysia

Sayang Mohd Deni

Center of Statistical Studies

Faculty of Information Technology and Quantitative Science

Universiti Teknologi MARA, 40450 Shah Alam, Selangor, Malaysia

Wan Zawiah Wan Zin & Abdul Aziz Jemain

School of Mathematical Sciences, Faculty of Science and Technology

Universiti Kebangsaan Malaysia, 43600, Bangi, Selangor, Malaysia

Diserahkan: 15 Disember 2008 / Diterima: 9 November 2009

ABSTRACT

This study investigated the spatial pattern and trends of the daily rainfall data in Peninsular Malaysia based on seasonal rainfall indices. Five rainfall indices which describe the main characteristics of rainfall, the total amount of rainfall, frequency of wet days, rainfall intensity, extreme frequency, and extreme intensity, were employed in this study. The statistics of rainfall indices were calculated in terms of their means for four regions in Peninsular Malaysia for the period 1975 to 2004. The findings indicate that the southwest monsoon had the greatest impact on the western part of the Peninsula, particularly in characterizing the rainfall pattern of the northwest region. During this season, the northwest region could be considered as the wettest region since all rainfall indices tested are higher than in other regions of the Peninsula. Otherwise, the northwest region is denoted as the driest part of the Peninsula during the northeast monsoon period. The northwest region is less influenced by the northeast monsoon because of the existence of the Titiwangsa Range, which blocks the region from receiving heavy rainfall. On the other hand, it is found that the lowlands areas such as the eastern part of the Peninsula are strongly characterized by the northeast monsoonal flow. Based on the results of the Mann-Kendall test, as the trend of the total amount of rainfall and the frequency of wet days during the southwest monsoon decrease at most of the stations, the rainfall intensity increases. In contrast, increasing trends in both the total amount of rainfall and the frequency of wet days were observed at several stations during the northeast monsoon, which give rise to the increasing trend of rainfall intensity. The results for both seasons indicate that there are significantly decreasing trends in the frequency of wet days during the extreme events for most of the stations on the peninsula. However, a smaller number of significant trends was found for extreme intensity.

Keywords: Extreme indices; frequency of wet days; Mann-Kendall test; rainfall intensity, total amount of rainfall

ABSTRAK

Kajian ini bertujuan menyelidiki pola taburan dan tren hujan di Semenanjung Malaysia berdasarkan indeks hujan mengikut musim. Lima indeks hujan yang menerangkan perlakuan hujan, amaun hujan, frekuensi hari basah, kelebatan hujan, frekuensi hari basah ekstrim dan frekuensi kelebatan ekstrim digunakan dalam kajian ini. Statistik indeks hujan dihitung berdasarkan nilai purata untuk empat rantau di Semenanjung Malaysia bagi tahun 1975 hingga 2004. Hasil kajian mendapati bahawa monsun barat daya mempunyai pengaruh yang besar dalam mempengaruhi pola taburan hujan di bahagian barat Semenanjung terutamanya rantau Barat Laut. Ketika monsun barat daya, rantau Barat Laut merupakan rantau yang paling basah kerana kesemua indeks kajian di rantau ini adalah lebih tinggi dari rantau lain di Semenanjung Malaysia. Sebaliknya, rantau Barat Laut merupakan rantau yang paling kering di Semenanjung Malaysia ketika monsun timur laut. Rantau Barat Laut kurang dipengaruhi oleh angin monsun timur laut disebabkan kehadiran Banjaran Titiwangsa yang menghalang rantau tersebut dari menerima hujan lebat. Di samping itu, keputusan turut mendapati bahawa kawasan kaki bukit di Timur Semenanjung lebih dipengaruhi oleh monsun timur laut. Berdasarkan keputusan ujian Mann-Kendall, tren amaun hujan dan hari basah di kebanyakan stesen didapati menurun ketika monsun barat daya seiring dengan peningkatan kelebatan hujan. Berlainan pula ketika monsun timur laut, di mana tren menaik diperolehi bagi indeks amaun hujan dan frekuensi hari basah di beberapa stesen, yang turut menyebabkan peningkatan tren kelebatan hujan. Keputusan kedua-dua monsun juga menunjukkan penurunan tren secara signifikan berlaku bagi indeks frekuensi hari basah ekstrim di kebanyakan stesen di Semenanjung. Walau bagaimanapun, hanya bilangan kecil tren signifikan diperolehi bagi kelebatan melampau.

Kata kunci: Amaun hujan; frekuensi hari basah; indeks ekstrim; kelebatan hujan; ujian Mann-Kendall

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*Pengarang untuk surat-menyurat; email: suhaila@mel.fs.utm.my