শুধুই দেখি...

গ্লোবাল ওয়ার্মিং আমাদের বংগোপসাগরের সাইক্লোন একটিভিটির উপর কী প্রভাব ফেলিবে গ্লোবাল ওয়ার্মিং! সরল এবং জট্টিল একটা বিষয়। জলবায়ু-গাছপালা-প্রাণিজগতের উপরে এর প্রভাব নিয়া ব্যাপক বিতর্ক আছে। গ্রীণহাউজ গ্যাসের নিঃসরণের কারণে জলবায়ু পরিবর্তন এবং সাম্প্রতিক সাইক্লোন "নার্গিস" এর ব্যাপক ধ্বংসলীলা ও প্রাণহানি। তো এই গ্লোবাল ওয়ার্মিং আমাদের বংগোপসাগরের সাইক্লোন একটিভিটির উপর কী প্রভাব ফেলিবে__বিশেষ করে এই একবিংশ শতাব্দিতে তার উত্তরটা জানা জরুরী। এর একটা ইনটারেশটিং উত্তর পাওয়া গিয়াছে। ইনডিয়ার গো‌য়াতে অবস্থিত নেশনাল ইনস্টিটিউট ওফ ওশেনোগ্রাফির, সায়েন্টিসট এবং ডেপুটি ডিরেক্টার, ড রাজেশ কুমার তার স হকর্মীদের নিয়া এক গবেষণায় ঠিক এই প্রশ্নের উত্তর খুজতে গিয়ে তারা ইনটারেশটিং একটা এনসার খুজে পেয়েছেন: বে ওফ বেংগলে সাইক্লোনের সংখ্যা কমবে। কিন্তু যেই সাইক্লোনেরা হবে তাহারা হবে ভয়ংকর। প্রচন্ড। ধংসের হোলি চালাবে এরা। রিপোর্ট খানা এই রকম ছিলো "cyclones in the Bay of Bengal will be fewer, but those that do form will be of a greater intensity." There is a concern that the enhanced greenhouse effect may be affecting extreme weather events such as tropical cyclone frequency, duration and their intensity. The north Indian Ocean offers a reliable long-term record of tropical cyclone activity though it may not be representative of the rest of the tropical regions. There is substantial scientific evidence that the enhanced greenhouse effect is predominantly of anthropogenic origin. There have been many suggestions based on global circulation and theoretical modelling studies that increases may occur in tropical cyclone frequency. In order to look into the global warming scenario, Dr. Ramesh Kumar and his team analyzed a long term National Centre for Environmental Prediction/National Centre for Atmospheric Research reanalysis dataset of the tracks of depressions, storms and severe cyclonic storms in the north Indian Ocean for the past 100 years. They further divided the 100 years as two epochs, one from 1901 to 1950 as normal conditions (epoch I) and the period 1951 to 2006 under the influence of global warming conditions (epoch II) to see whether there are any trends in their frequency. The study has not yet been published in a peer-reviewed journal. They restricted their present study to Bay of Bengal only as the number of cyclones which form in the bay is several times higher than that which form in the Arabian Sea. They also looked at the total number of cyclones on a yearly basis rather than individual months as they were more interested in looking at the role of the global warming on their formation. The study clearly shows that the total number of depressions over the Bay of Bengal is showing an increasing trend from 1901 to 1950. The number, which was 3 per year, rose to about 11 in 1950. However, the epoch II clearly showed a decreasing trend — from 11 to about 3. The analysis was repeated with the number of storms and severe cyclonic storms to check for any significant differences in their formation and trends during the above two epochs. While there was only a marginal decrease in the number of the storms during epoch I, there was a substantial decrease from two storms per year to almost one per year during epoch II. Decreasing trend In the case of intense or severe cyclonic storms, a small decreasing trend was seen for epoch I. However, epoch II showed a small increasing trend. For cyclone genesis and sustenance, low level relative vorticity (at 1.5km; 850 hecta Pascals) and mid-tropospheric relative humidity at 500 hPa (hecta Pascals) should be high and the vertical wind shear (difference of the 850 hPa wind and 200 hPa wind) should be low. Dr. Kumar said, “Our study clearly showed that the mid tropospheric humidity showed a substantial decrease from 1951 to 2006. Thus, we feel that this decrease in the mid tropospheric humidity in epoch II may be a major factor for the decrease in the number of depressions and storms in the Bay of Bengal. “Whereas relative vorticity and vertical wind shear between the lower troposphere (i.e. at 850 hecta pascals) and upper troposphere (i.e. at 200 hecta pascals) show a small increase and decrease respectively, which are conducive for cyclogenesis which could be the reason for the slight increase in the number of severe cyclonic storms in the Bay of Bengal.” The important question now arises: what is the role of increased sea surface temperatures (SST) on cyclone formation, intensity in the 21st century? Dr. Kumar is of the opinion that even though many believe that rising SSTs may lead to more cyclones and more cyclones of greater intensity, if most of the parameters are not conducive then even in a warm oceanic environment cyclones cannot form. The number of depressions showed an increasing trend during 1901-50 In the case of severe storms epoch II (1951-2006) showed an increasing trend source: Click This Link সো খবর আছে