Distribusi Percampuran Turbulen di Perairan Selat Alor (Distribution of Turbulence Mixing in Alor Strait)

Abstract

Selat Alor merupakan kanal terdalam setelah Selat Ombai di kepulauan Alor. Kontribusinya sebagai salah satu celah keluar Arus Lintas Indonesia (Arlindo) belum banyak dikaji hingga saat ini. Selat Alor memisahkan Laut Flores dan Laut Sawu, dan memiliki sill yang tinggi di dalamnya, diduga turbulensi akibat interaksi antara aliran selat dengan topografi dasar dapat memicu percampuran dan memodifikasi properti massa air yang melaluinya. Tujuan dari penelitian ini adalah untuk mengkuantifikasi transformasi massa air yang melalui Selat Alor dan mengkaji kemungkinan percampuran di dalam selat berdasarkan estimasi sesaat properti percampuran, yakni percampuran turbulen menggunakan metode skala Thorpe. Penurunan CTD dilakukan di 15 stasiun di perairan Selat Alor. Diperoleh hasil bahwa kontur kedalaman yang menghubungkan Laut Flores dengan Laut Sawu adalah ~300 m pada kanal utama. Salinitas maksimum massa air Subtropis Pasifik Utara (NPSW) dar i Laut Flores di Selat Alor banyak mengalami reduksi akibat intensifnya percampuran yang diduga dipicu oleh topografi dasar dan aliran selat yang menghasilkan turbulensi. Lapisan salinitas maksimum Massa Air Subtropis Samudera Hindia Utara (NISW) pada σ_θ = 23,5-24,5 terdeteksi di bagian selatan selat (Laut sawu). Jejak massa air NISW menurun dan banyak tereduksi mendekati pintu selatan selat. Intrusi Massa Air Lapisan Menengah Samudera Hindia Utara (NIIW) juga dijumpai di lapisan bawah Laut Sawu, konsisten dengan profil arus pada lapisan bawah. Rata-rata nilai difusivitas vertikal eddy (K_ρ)  di Selat Alor bagian utara memiliki orde of 10^-3 m² s^-1, dan di bagian selatan memiliki orde bervariasi, 10^-6-10^-4 m² s^-1. Penyempitan celah Selat Alor diduga merupakan pemicu turbulensi tinggi aliran yang berkontribusi pada tingginya nilai difusivitas vertikal.

Kata kunci: Arlindo, percampuran turbulen, difusivitas vertikal, Selat Alor

Alor Strait is the deepest channel in Alor islands after Ombai Strait. Contribution of the strait as one of the secondary exit passages of Indonesian Throughflow (ITF) has not been studied yet. The strait separates Flores Sea and Sawu Sea, and is featured by the existence of high sill within the strait, suggested that turbulence due to interaction between strait flow and bottom topography could drive mixing and then modify the water mass properties. The purpose of this study is to investigate transformation of ITF water mass and turbulent mixing process with Thorpe scale method. A hydrographic survey has been carried out in July 2011, in which 15 CTD casts were lowered in the strait. The results show that Alor sill depth is about 300 ms in the main gate. Maximum salinity of NPSW from Flores Sea within Alor Strait is significantly reduced due to strong mixing, perhaps driven by bottom topography and strait flow which creates turbulence. NISW (Northern Indian Subtropical Water) with maximum salinity layer at σ_θ = 23,5-24,5 is dominant in the southern part of Alor Strait (i.e. Sawu Sea). The existence of NIIW (North Indian Intermediate Water) is also found in the deeper layer of Sawu Sea. The average value of vertical eddy diffussivity (K_ρ) estimate in the thermocline layer and deep layer in northern part and central part of strait channel is within the order of 10^-3 m² s^-1. Lower order of K_ρ in the thermocline layer and deep layer were found in southern part of the Strait (Sawu Sea), ranging from 10^-6 to 10^-4 m² s^-1. These indicate that the existence of sills in the northern part and central part of Alor Strait could drive mixing significantly. Narrowing passage of Alor Strait probably contribute to the high value of vertical eddy diffusivity due to highly turbulence flow.

Keywords: Indonesian Throughflow (ITF), turbulent mixing, vertical diffussivity, Alor Strait