مطالعه و بررسی پدیدۀ فراجوشی در سواحل شرقی خزر میانی با استفاده از شبیه‌سازی عددی

در این مطالعه پدیدۀ فراجوشی در سواحل شرقی خزر میانی دریای خزر با استفاده از مدل سه‌بعدی coherens برای سال 2004 بررسی شده است. در این شبیه‌سازی، شبکه‌بندی حوزه در راستای افق 0/046×0/046 درجه و در راستای قائم دارای 30 لایه سیگما در نظر گرفته‌شده است. همچنین تمامی واداشت‌های جوی (سرعت باد، فشار هوا، دمای هوا، آهنگ بارش، پوشش ابری و رطوبت نسبی) و واداشت رودخانه‌ای به‌عنوان شرایط مرزی و میانگین دما و شوری آب در ماه ژانویه به‌عنوان شرایط اولیه به مدل اعمال شده است. از آنجا که الگوی باد غالب، در تابستان برای دریای خزر به‌خصوص در سواحل شرقی خزر میانی، شمالی و شمال شرقی است، این مسئله باعث ایجاد پدیدۀ فراجوشی در سواحل شرقی خزر میانی در طول تابستان شده است. در این زمان یک گردش واچرخندی نیز در حوضۀ میانی خزر میانی مشهود است که در آگوست به همراه دو ناحیۀ فراجوشی کوچک‌تر دیده می‌شودکه آنکه نزدیک سواحل غربی است قوی‌تر است. بر اساس نتایج شبیه‌سازی‌شده در این مطالعه، جهت جریانات سطحی در سواحل شرقی خزر میانی از سمت ساحل به سوی مناطق مرکزی است که این جریان‌ها موجب حرکت آب از سواحل شرقی به سوی سواحل غربی و در نهایت انتقال آب‌های سرد زیرسطحی به طرف لایه‌های سطحی شده و میدان دمایی در نزدیک سواحل شرقی را تا چند درجه سردتر از سواحل غربی کرده است. همچنین با بررسی سرعت قائم در سواحل شرقی خزر میانی، بیشترین مقادیر سرعت قائم، در حدود عمق‌های بین 2 تا 5 متری از سطح آب مشاهده شده و سرعت جریانات در لایه‌های بالاتر و نزدیک به سطح آب دارای مقادیر کمتری شده است. این سرعت‌های قائم هم از نظر مقدار و هم وسعت در ماه آگوست ضعیف‌تر از ماه جولای بوده‌اند؛ به‌طوری‌که اندازۀ سرعت جریان‌های قائم در ماه‌های جولای و آگوست به ترتیب تقریباً 12 و 7 متر در ماه است که نشان‌دهندۀ ضعیف‌ترشدن این پدیده در ماه آگوست است.

The study of upwelling in the eastern coast of the Caspian Sea using numerical simulation

Upwelling areas in the ocean are important places for fishing as nutrients can be transported from deeper area to the near surface region. In this study, the upwelling in the eastern coastal area of the Caspian Sea for 2004 was studied using COHERENS, a threedimensional ocean model. In order to simulate the circulation in the Caspian Sea, the gridded fields were chosen as 0.046 × 0.046 degrees along the horizontal directions, which gives a grid size of about 5 km, and 30sigmalayers along the vertical (the layers’ numbers are represented as K so that the bottom layer begins with 1 and the layers go up towards sea level). Bathymetry and coastline locations are based on GEBCO data, that has been interpolated and slightly smoothed. The model was initialized for winter (January) using monthly mean temperature and salinity climatology obtained from Kara et al (2010). The model was forced by climatologic six hourly atmospheric forcing, air temperature and air pressure (0.5˚ ×0.5˚) derived from Reanalyses (ERA Interim) ECMWF data, wind velocity derived from modification of ECMWF (0.5˚ ×0.5˚) and precipitation rate, cloud cover and relative humidity (2.5˚×2.5˚) derived from NCEP/NCAR reanalysis data. Four rivers used in the model and the monthly mean values of the flows for the Volga (has three locations for discharge into the Caspian Sea in the model), Ural, Kura and Sepidrood (Sefidrood) are used. The salinity of river water was considered to be 0‰. Monthly mean discharge value for three major rivers (Volga, Ural, Kura) was obtained from the GRDC (The Global Runoff Data Centre) and for Sepidrood (Sefidrood) was obtained from the Water Research Institute.Results show that in the middle and southern parts of the Caspian Sea, the easterly and north easterly winds lead to upwelling near the east coast of the Caspian Sea. In the summer the eastern coast of the middle part of the Caspian Sea experiences an upwelling that is considered to be the most important thermal and dynamical phenomenon. The upwelling area is a region of 20 km wide and extends 10s of km along the coast also the timescale of such phenomena was about a few weeks. Anticyclonic circulation during this period in the middle basin of the Caspian Sea was also another feature during the upwelling period and was found to be stronger in August during which two strong upwelling areas are also observed in this basin one is particularly strong near the west coast. A southward current from the margin of upwelling area was also another important characteristic of upwelling off the eastern coasts of the Caspian Sea. From June to August the advection of cold upwelling waters occur from eastern area of the Caspian sea as also have been noted by Tuzhilkin and Kosarev (2005).The results show that the temperature of the east coast was lower than the west coast by 2 to 3 degrees Celsius when the upwelling occurred. Also, this phenomenon occurred down to a depth of less than 40 m, which is nearly consistent with Tuzhilkin and Kosarev's study (2005). Due to the upwelling, the depth of the thermocline near the coast raised by about 20 m. The vertical velocities in the upwelling area were also found to be about 12 and 7 m per month respectively for July and August. In August the horizontal extension of the upwelled area was also found less that for July. Also, another result of the simulation shows the existence of the vertical velocity in western part of the middle of Caspian Sea that, one can hypothesize the existence of topographicallyassociated upwelling phenomenon in the area because of the presence of especial topography that has disordered shape and steep slope.