Estimation of mixed layer depth from argo float data in Makassar strait during el niño southern oscilation

Makassar Strait is a pathway of water mass passing trough from the Pacific Ocean to the Indian Ocean. Makassar Strait is also one of the waters in Indonesia which passed by Indonesian Throughflow (ITF). Movement of water masses due to the ITF, wind system, and complex bathymetry conditions will cause the movement of vertical water masses (mixing). This makes the upper layers homogeneous, the objective of this study is to assess the variation of MLD using Argo Float data adjusted for ENSO (El Niño Southern Oscilation) events, under El Niño, La Niña, and normal conditions. MLD determination using temperature gradient criterion with value ≤ 0,1 oC/m. The data processing using Ocean Data View 5.0.0 software shows that the depth of homogeneous layer in Makassar Strait waters during El Niño years is various, between 37,73 – 73,62 m with temperature 25,7 - 28,99 oC. During La Niña, MLD found varying around 30,59 – 97,89 m with temperature profile slightly colder (26,7 – 29,8 oC). And during normal year, MLD located around 28,55 m to 112,98 m, shallowest and deepest from all data set, respectively. Average value of MLD in is vaguely normal deeper than La Niña and El Niño condition, as it should be shallower, showed that temperature gradient criterion with value ≤ 0,1 oC/m is not quite good to determine the MLD layer in Makassar Strait.


Introduction
Indonesia Waters have natural conditions with diverse topography, because it is the connecting system between two oceans, Pacific Ocean and Indian Ocean, then the properties and condition of Indonesia Waters are affected by these two oceans, especially the Pacific Ocean [1]. The Makassar Strait has an important role as a shipping lane in the central zone of Indonesia, because the Indonesian Through Flow (ITF) that connects the Sulawesi Sea in the north and the Java Sea in the south [2].
Vertical temperature is divided into three layers, namely a mixed layer, thermocline layer and deep layer. The depth of the mixed layer is characterized by a temperature value in depth have same value as the surface [3]. The homogeneous layer is also called the mixed layer depth (MLD), because has interactions between the ocean and the atmosphere. According to [4], mixed layer found on the surface is characterized by a layer that is almost homogeneous with variations in temperature, salinity and inconspicuous density (very small changes) in relation to temperature, salinity and depth [1] and also explained that due to the movement of water masses and seasonal wind changes, this homogeneous layer can vary in depth between 0 -100 meters in West Season and 0 -50 meters in East Season.
In a previous study [5] the method for determining the upper limit of the thermocline using temperature gradient criteria was smaller than 0,1 o C / m. According to [6] in [5] the definition of the

Data Processing Method
This study, analysis of MLD variation using Ocean Data View (ODV) software 5.0.0. Argo Float data has NetCDF format (network Common Data Form), data that has been downloaded, then imported into ODV software for processing. The file that is imported is the ARGO data format for its profile, to find the MLD value of the data that has been imported and then extracted into MS. Excel, in this study used temperature gradient criteria 0,1 o C / m to find the depth of the mixing layer written in equation 1, then ARGO data was plotted using ODV software.
To calculate the vertical temperature gradient according to the equation used by [7] : Correlation calculations are used to see the relation between mixed layer depth or layer depth mixing with the Ocean Nino Index (ONI). The correlation that used is a correlation analysis between two variables, there are variable x which is represented by the depth of the mixing layer, and variable y is represented by the Ocean Nino Index (ONI). The equation used is the Pearson linear correlation equation as follows [8].
where: r = Pearson correlation coefficient xi = variable that is correlated in the form of layer depth mixing x ̅ = variable x yi = variable correlated in the form of ONI y ̅ = average variable y

Data Analysis Method
Data analysis method used is descriptive statistical analysis, it is a statistics methods that used to analyze data by describing or describing data that has been collected as it is without intending to make conclusions that apply to the general [9]. The presentment of data analysis was done by grouping the data based on the conditions of ENSO (El Nino, La Nina and normal) in this study the depth of the mixing layer can be displayed in tabulation table and vertical profiles of temperature. Mixed Layer Depth structure was calculated by data distribution through average calculations, besides the results of the analysis are also displayed in the form of temperature vertical distribution.

Results and Discussion
This study analyzing the depth of mixing layers in the Makassar Strait using phenomenon (ENSO) adjusted Argo Float data, namely El Niño, La Niña, and normal. The parameters examined in this study are parameters of temperature, salinity and depth.
The results of ARGO data processing in the form of vertical temperature profiles provide an overview of changes in temperature to depth, starting from a depth of 0 m to 500 m. The temperature on the surface is relatively higher compared to the temperature values below the surface layer, which ranges from 28-30 o C, this happened because the location of the Makassar Strait waters is in the tropics that are close to the equator so it has an effect from warm pool of tropical in Pacific Ocean. According to [10] tropical warm water columns are characterized by surface temperatures greater than 28 o C.
This study, changes in the depth of the mixing layer with a gradient temperature of 0,1 o C / m are presented in Figure 1 based on the image formed by the pattern of water mass coating consisting of 3 parts namely mixed layer depth, thermocline layer and deep layer. This study, the thickness of the homogeneous layer (Mixed Layer Depth) that occurred in the ENSO phenomenon of La Niňa, El Niňo years, and normal years was observed.

El Niňo Conditions
The results of the temperature distribution in the form of a vertical profile in the condition of el nino presented in Figure 3 based on the image can be seen that there is a change in temperature to the depth of the surface of the gate. The average mixed layer depth layer ranges from 53.18 m, with an average temperature range of 27.9 oC with an average salinity of 34.19 psu.
In   The MLD layer is seen as a straight line from the surface and the curved part is the thermocline layer. While the transverse temperature results are clearly seen in the color change between the MLD layer, the thermoklin layer and the deep layer presented in Figure 4. Changes in the temperature in the MLD layer are characterized by changes in gradient ≤ 0.1 o C.    The MLD layer is seen as a straight line from the surface and the curved part is the thermocline layer. While the transverse temperature results are clearly seen in the color change between the MLD layer, the thermocline layer and the inner layer presented in Figure 6.

Discussion
According to [11] the depth of the homogeneous layer is effective for defining the depth of the upper limit of the thermocline layer, so that if the homogeneous layer of a deep water can be ensured that the thermocline layer will be deep as well. One of the factors that influence mixing is wind. According to [12] weak winds will cause stirring which does not reach deep depths so that the homogeneous layer above is thin. A thin homogeneous layer will be followed by a shallow depth of the mixing layer. The turn of the monsoon that changes regularly is marked by the blowing of monsoons alternately causing a direct impact on changes in the physical properties of seawater.
Model result showed that thermocline layer during La Niňa condition in Makassar Strait is found deeper than during El Niňo condition. In this study the Mixed Layer Depth and Ocean Nino Index relationship in the Makassar Strait has a Pearson correlation value of -0.27 negative linear correlation is low, this indicates that the second relationship is inversely proportional to where the deeper the depth layer depth, the smaller the ocean nino index value, p This shows that the effect of ENSO NINO 3.4 on the mixed layer depth is not as strong as shown in figure 9.

Conclusion
The depth of the mixed layer which is inversely proportional to this indicates that the NINO 3.4 ENSO is not very strong. Where the average depth of MLD was found to be deeper in Normal conditions which was 66.82 m. The average depth of La Niña is 54,63 m. Whereas, in flood El niňo conditions around 53,18 m.