عنوان مقاله [English]
Investigating the trend and fluctuations of the sea surface temperature (SST) data are critically important for understanding the interactions between the oceans, the atmosphere and the land in various spatial and temporal timescales. Such analysis of the SST time-series is also essential for the detection and modeling of climate change. The increase in the global SST is one of the primary physical impacts of climate change. Some recent investigations have shown that the fluctuations in SST data over the Persian Gulf and the western parts of the Indian Ocean regulates Iran's precipitation particularly over the southern districts. The present study was, therefore, motivated to analyze the trends in the SST data over the northwestern parts of the Indian Ocean waters containing 30 nodes of by (longitude and latitude) grids for the period 1950-2009. These grids were spread over various parts of the Persian Gulf and Arabian Sea, the water bodied between to North and between to East. The monthly SST data was gratefully extracted from the database of the physical sciences division of the National Oceanic and Atmospheric Administration (NOAA). According to their geographical positions, these 30 grids were classified into three groups, namely, the Persian Gulf, coastal areas (i.e., the grids off the coasts by ) and the Arabian Sea regions. Since the SST time-series generally have a normal distribution, a linear regression analysis was applied to detect the trend in the constructed time series for the classified regions in annual and seasonal timescales. The analysis was conducted by each grid individually as well as by averaging the SST data over each of the three mentioned zones. The seasonal time series were constructed by averaging monthly data so that winter, spring, summer and autumn consisted of the months Jan-March, April-June, July-Sep and Oct-Dec, respectively. The 60 years of the study period were also divided into three consecutive 20 years to assess the consistency in trend-line slope over time. The parametric statistical tests were used to investigate whether the detected trends are significant
The study revealed that during the 60 years of the study period, the SST of these 30 grids has inclusively increased by about 0.61°C. It is in general agreement with Deser et al. (2010) that reported the magnitude of the global SST trend during the 1900-2008 period as approximately 0.4–1.0°C per century in the tropics and subtropics and 1.2–1.6°C per century at higher latitudes. The amount of such increase for the Persian Gulf, coastal areas and the Arabian Sea was 0.5°C, 0.65°C and 0.63°C, respectively. Comparing with other seasons, for the large regions of the three classified zones the increasing trend was the greatest and the least for autumn and winter, respectively. While the spring and summer’s SST were increased by about 0.5°C during the last six decades, the corresponding increases for winter and autumn were found to be 0.60°C and 0.85°C, respectively. The autumnal upward trend was significantly greater than other seasons for the Persian Gulf and the coastal regions. However, the upward trend is statistically identical during autumn and winter over the Arabian Sea areas. With the exception of the spring, the slopes of the trend-lines were different between the Persian Gulf, coastal areas and the Arabian Sea during the other seasons.
When the considered 60 year period was divided into three consecutive 20 year periods, the trend exhibited a variety of differences between these new shorter data sets. While the spring and wintertime SSTs did not exhibit any significant trend during either 1950-1969 or 1970-1989 periods, the upward trend was significant for the period 1990-2009. In contrast to winter and spring, most of the considered SST time series (excluding the Persian Gulf data) were significantly warmed up during 1950-1969. No significant trend was observed for the period 1970-1989 on a seasonal scale. In spite of the fact that the Persian Gulf SSTs did not exhibit any significant positive trend during the summers or autumns for either 1950-1969 or 1970-1989 periods, the trend abruptly increased during the 1990-2009 period for these two seasons.