Ege denizindeki etezyen rüzgar kaynaklı yükselici su hareketlerinin atmosfer-okyanus birleşik model sistemi ile simülasyonu
Analysis of coastal upwelling forced with etesian wind over Aegean sea using coupled atmosphere-ocean modeling system
Yazar: CEYHUN ÖZCAN
Danışman: DOÇ. DR. BARIŞ ÖNOL
İstanbul Teknik Üniversitesi / Fen Bilimleri Enstitüsü / Meteoroloji Mühendisliği Ana Bilim Dalı / Atmosfer Bilimleri Bilim Dalı
Derece: Yüksek Lisans, 2019
Konu: Meteoroloji = Meteorology

Özet:
Dünyada küresel ve bölgesel ölçekteki iklimlerin belirlenmesinde birçok etmen bulunmaktadır. Bu faktörlerin en önemlilerinden bazıları ise atmosfer-okyanus etkileşimi sonucu meydana gelmektedir. Atmosfer ve okyanus sürekli kuvvetli bir etkileşim içerisindedirler. Bu etkileşim çevre bölgelerdeki hava şartlarına etki etmekte ve okyanus içerisinde fiziksel ve kimyasal değişimlere sebep olmaktadır. Atmosfer ve okyanus birçok farklı parametre yardımı ile etkileşmekte olup bu parametrelerin en önemlilerinden bir tanesi de rüzgardır. Rüzgarın okyanus ve denizler üzerine etkisi genellikle yükselici su hareketi mekanizması ile ilişkilendirilmektedir. Genellikle kıyı bölgelerinde meydana gelen bu hadise, rüzgarın kıyıya paralel şiddetli bir şekilde esmesi ile oluşmaktadır. Kıyı boyunca esen rüzgar deniz yüzeyindeki su kütlelerinin açık denize doğru taşınmasına neden olmaktadır. Bu su kütlesi taşınımını dengelemek için alt tabakalardaki su kütlelerinin yükselmesine gerekmektedir. Bu mekanizma, deniz yüzeyinin daha soğuk, besleyici ve çoğunlukla daha yoğun olmasına sebep olmaktadır. Yükselici su hareketini tespit etmek için deniz yüzeyindeki bu parametrelerin değişimi gözlemlenebilir. Dünyada bu etkileşimlerin yoğun olduğu birçok bölge bulunmaktadır. Ege Denizi ve kıyıları bu bölgelerden birisidir. Bu çalışmada Ege Denizi'nde bölgesel kuzeyli Etezyen rüzgarlar nedeniyle oluşan yükselici su hareketlerinin, tekil ve birleşik atmosfer-okyanus modelleri ile simüle edilerek analiz edilmesi amaçlanmıştır. Bu çalışmada, 18-24 Temmuz 2004 tarihleri arasında gerçekleşen yükselici su hareketi vakası analiz edilmek için seçilmiştir. Seçilen bu tarihler arasında, Ege Denizi'nin doğusundaki ortalama rüzgar hızı yaklaşık 16.5 knot ölçülmüştür. Bu süreç sonucunda deniz yüzey sıcaklığında yaklaşık olarak 1.5 oC'lik düşüş gözlenmiştir. Seçilen bu vakanın simülasyonunda, RegCM 4.6 ve WRF 3.8 atmosfer modelleri ile ROMS okyanus modeli kullanılmıştır. Birleşik atmosfer-okyanus model sisteminin oluşturulması için ise RegESM'den yararlanılmıştır. Bu simülasyonlar ile yükselici su hareketi mekanizmasının anlamlandırılması ve meydana getirdiği sonuçların gözlenmesi için bir çok analiz yapılmıştır. Yapılan analizlerde deniz yüzeyi sıcaklığı, tuzluluk, karışım tabakası kalınlığı ve gizli ısı akısı gibi birçok parametredeki değişimler üzerine çalışılmıştır. Bunlara ek olarak Ekman'ın (1905) çalışmalarını ve Bakun'un (1973) hesaplarını temel alarak, rüzgarın yükselici su hareketi üzerine etkisini sayısal olarak ifade edilmesi için Yükselici su hareketi indeksi (YSHİ) oluşturulmuş ve bu indeks üzerinden incelemeler yapılmıştır. Çalışmada orjinal durum için simülasyonun yanı sıra, rüzgar hız değerlerinde %25 ve %50 oranlarında artış/azalışlar uygulanarak duyarlılık testleri yapılmıştır. Bu simülasyonlar, rüzgar şiddetindeki değişimlerin denizin nasıl ve ne derece tepki vereceğini incelemede faydalı olmuştur. Böylelikle, Etezyen rüzgar rejiminde iklim değişikliğinden kaynaklanacak bir değişim sonucunda oluşacak sonuçlar öngörülmüş olmaktadır. Bu simülasyon sonuçlarında, Ege Denizi'nin doğusundaki ortalama deniz yüzeyi sıcaklığı rüzgar hızının %25 oranında artması koşulunda 0.4 oC, rüzgar hızının %50 oranında artmasında ise 1.2 oC azalmıştır. Rüzgar hızının azalması koşullarında da yaklaşık aynı değişim oranları gözlenmiştir. Bu çalışmada izlenilen yöntemler diğer atmosfer-okyanus etkileşim süreçlerinin incelenmesi için de uygulanabilirdir.

Abstract:
Global and regional climates in the world are determined by many factors. Some of the most important of these factors occur as a result of atmospheric-ocean interaction. The atmosphere and the ocean are in constant interaction. This interaction affects weather conditions in the surrounding areas and causes physical and chemical changes in the ocean. The atmosphere and the ocean interact with the help of many different parameters, and one of the most significant of these parameters is the wind. The effect of wind on oceans and seas is often associated with the mechanism of upwelling. This phenomenon, which usually occurs in coastal areas, is caused by the strong winds blowing parallel to the coast. Water mass on the sea surface are transported towards the open sea by the wind blowing along the shore. In order to balance this water mass transport, the water mass in the sublayer need to rise. As a result of this mechanism, the sea surface is colder, nutritious and mostly denser. Variation of these parameters on the sea surface can be observed to detect upwelling. There are many regions in the world where these interactions are intense. The Aegean Sea and its coasts are one of these regions because of regional Etesian wind generally. In summer, the Ethesian wind is caused by the pressure gradient due to the high pressure center on Eastern Europe and the low pressure centers on the Middle East. This northern wind regime is active in most of the summer months. This wind blowing parallel to the eastern shores of Aegean Sea causes upwelling due to Ekman transport.  In this study, it is aimed to analyse the upwelling caused by Etesian winds in Aegean Sea by simulating with standalone and couple atmospheric-ocean models.
 In this study, a case of upwelling between 18-24 July 2004 in Aegen Sea is selected for analysis. Between these dates, the average wind speed in the eastern Aegean Sea was measured at approximately 16.5 knots. As a result of this process, a decrease of approximately 1.5 oC was observed in sea surface temperature. In the simulation of this selected case, RegCM 4.6 and WRF 3.8 atmosphere models and ROMS ocean model were used. The 0.75 degree resolution ERA-Interim data set produced by ECMWF was used to define the initial and boundary conditions in the atmosphere models, and the 7 km resolution reanalysis data created by MyOCEAN was used to determine the initial and boundary conditions of the ocean model. RegESM was used to create a combined atmosphere-ocean model system. The models run with the RegESM model system communicate information to each other with parameters such as sea surface temperature, momentum flux. This information is shared between the ocean and the atmosphere model at 1-hour intervals. In addition, RegESM allows processing and visualizing the outputs during the run of the models. Both models interact at 6 minute intervals to in-situ visualization. The atmosphere models were first run with 12 km horizontal resolution to cover the whole Mediterranean Sea. Then, a second model domain was created for the Aegean Sea with a resolution of 3 km by nesting within first model domain. CCMP, NOAA-BLENDED and ERA-Interim observation and reanalysis data sets were used to test the wind values of single atmosphere simulations. In these tests, the ERA-Interim generated by ECMWF and the OI-SST generated by NOAA sea surface temperature observation data sets were used to examine the relationship with the wind. When single atmosphere simulations and sea surface tempereatue and wind observation data are compared, it is calculated that the models represent the Aegean wind well and there is a strong and inverse correlation between the meridional component of the wind and sea surface temperature. In the next step, the ROMS ocean model was run as couple within the high-resolution RegCM model domain. The ROMS model has a horizontal resolution of 3 km and a 32 sigma level. With these simulations, a lot of analysis has been carried out to examine the mechanism of the upwelling and to observe the results. The analyses were conducted to examine the changes in parameters such as sea surface temperature, salinity, mixing layer thickness and latent heat flux. In addition, based on the studies of Ekman (1905) and Bakun's (1973) calculations, Upwelling Index (UI) was created to quantify the effect of the wind on the rising water movement and analyses were made on this index. This index is defined as the volume of water transported per unit time and unit distance from a section taken along the coast. To better see the effect of the wind on upwelling, sensitivity tests were performed by applying 25% and 50% increases / decreases in wind speed values in addition to simulation for the original situation. These simulations have been useful in examining how and to what extent the sea will react to changes in wind speed. Thus, possible consequences due to climate change in the Etesian wind regime were observed.
According to the results of simulation, It is determined that the amount of Ekman transport to the west and Upwelling Index value increase with increasing wind value. When the current values on the sea surface are analysed, it is seen that the current direction is generally western and in the scenarios where the wind value increases, the current intensity raises especially in the coastal regions. These results are consistent with Ekman transport. Upward flow of water from the lower levels is necessary to balance this water body carried horizontally. In order to observe the changes in this vertical, a vertical cross-section was taken along the shores of İzmir Karaburun within the ocean output. When the vertical flow rate is analysed, high upward values are observed in the regions close to the shore. In the scenarios where the wind is increased, it is seen that these vertical flow velocity values are further intensified. The flow rate in the reference simulation was 14.58 10-5 m/s on average, while the wind value was 21.57 10-5 m/s in the increased scenario and 8.64 10-5 m/s  in the reduced wind scenario. When the temperature distribution in the Aegean Sea is examined, it is seen that the temperatures decrease in the east of the Aegean Sea. The increase in wind value decreases the sea surface temperature more. The average sea surface temperature in the east of the Aegean Sea was found to be 22 oC in the reference simulation, 21.2 oC in the wind-enhanced scenario and 22.8 oC in the wind-reduced scenario. When the temperature profile of Karaburun vertical section is examined, it is seen that the same temperature curves slope towards the shore and thermocline layer reaches the surface. The movement of the cold water to the surface is determined. Furthermore, when the density profile in the vertical is examined, it is seen that the dense water body in the substrate is carried towards the surface. Another examination is performed for salinity. One of the most important factors affecting salinity in the Aegean Sea is the Black Sea water entering through the Dardanelles. The proportion of salt in the water coming from the Black Sea is quite low compared to the waters in the Aegean and Mediterranean. Simulation results showed that the northern wind changed the salinity of the Aegean Sea mostly to the north, because the wind affected the flow from the Çanakkale Strait to a great extent. With the decrease of the intensity of the wind, surface salinity decreases in the north of the Aegean Sea. However, when we look at the areas of upwelling, the increase in the wind force affects the salinity decreasing direction, but this rate is quite low compared to the change in the north of the Aegean Sea. When analyzed at the height of the mixing layer at sea, it is seen that it has a strong relationship with wind speed. Increasing the thickness of the mixing layer has a stratification reducing effect and facilitates the operation of the rising water movement mechanism and therefore plays an important role.
As a result of this study, wind-drived upwelling mechanism in Aegean Sea between 18-24 July 2004 could be examined in detail. The sensitivity of this upwelling system to the wind strength was analyzed with different simulations. During this event, the changes in the ocean and atmosphere were examined horizontally and vertically. Through this study, the effects of upwelling have been visualized and understood clearly. With this method, comprehensive studies on other upwelling events and other atmospheric-ocean processes can be studied. Ocean physics, chemistry and biology, fisheries and marine accidents can be studied.