عنوان مقاله [English]
Long-term synoptic maps related to sea levels show the presence of a semi-permanent, low-pressure system in the warm season in the Indian subcontinent, southern Afghanistan, Pakistan and Central Plateau of Iran, known as Low-Pressure Pakistan or Gang. The above mentioned low pressure refers to thermal low pressure created on Sindh basin in Pakistan due to the entry of solar radiation in May and June. Since the daily radiation is more than the nightly radiation, this thermal low pressure has become a permanent summer system (Boucher, 1975, 132; Kaviani and Alijani, 1999, 192). Pakistan thermal low pressure is an important and effective system in the climate of Pakistan, the northwest of India, the southwest of Afghanistan, and the southeast and east of Iran. Therefore, extensive studies have been conducted to examine the effect of this system on other parameters and climatic phenomena such as the studies by Iqbal and Jawaid (2008), Faisal and et al., (2013); Alijani et al., (2011); Khosravi et al., (2011). The study by Bollasina and Nigam (2011) which is a complete study on Pakistan low pressure, on the origins and changes of summer low pressure in Pakistan and northwest India, concluded that Pakistan-India low-pressure current influence two regional and remote forces. In regional scale, the effects of Hindu Kush Mountains were stronger than those of Earth surface and talent temperatures. The effects caused by remote factors, rooted in convection currents in Gulf of Bengal and East India in June and July, are also of importance. According to the importance of this system and its effect on the Iranian climate especially in the southeast, it is essential to study the tempo spatial variation. Therefore, the main questions outlined here are as follows: What are the changes of Pakistan low pressure during the statistical period? How are the spatial changes over time? How are the changes in terms of intensity over time?
2- Data and Methodology
ECMWF (ERA-Interim Version) with a spatial resolution of 1*1° was used in the geographical area (Latitude of 20-50° N and Longitude of 20-90° E) for the warm seasons (June to September). The statistical period 1980-2015 was taken into account. In order to extract the central pressure, the longitude of 62.5° to 72.5°E and latitude of 22.5°to 32.5°N were defined. First, the annual minimum pressure was extracted in the form of time series. Man-Kendal non-parametric trend tests were employed to study the variations.
3- Results and Discussion
36-year medium maps (1980-2015) for sea level pressure over the warm seasons show the deployment of the low-pressure system with two closed cells. As it is clear in the literature review, the system is a feature of warm seasons usually created as a result of extreme heats of underlying atmosphere along with the dynamical subduction originating from the West Indies in the south of Pakistan (Ramage, 1966). According to the time-series results, the lowest pressure was reported in 1984 with the central pressure of 996, and the maximum was reported in 2015 (999). A review of monthly Pakistan low pressure shows that the central pressure of this system has different fluctuations. It experiences a rising trend over all months. The increase was evaluated at three confidence levels (90%, 95%, and 99%). They are not significant, however. The greatest rising trend was observed in June and lowest in July. As a result, the system experiences a slightly increasing trend. Decade anomaly of central pressure was also studied in monthly and yearly scale. Table 1 shows the results. According to the results, the central pressure of the system is weakening and the last decade has the highest central pressure.
Table 1: Anomaly of Central Pressure of Pakistan Low Pressure (1981-2015), Based on Spatial Period (Latitude of 22.5*32.5°N and of Longitude 72.5*62.5° E)
Total Average (1981-2015)
Five years of latest
Finally, Hoff Muller Maps were used to describe the static continuity variations, that is, spatial changes of this low-pressure region during two periods (1980-1989 and 2005-2016). The results showed that Pakistan low pressure has no specific spatial extent or displacement of the East or the West direction. In other words, the system has equal extent over the initial and closing decades.
Pakistan low pressure is an important, effective system in South Asian regions. It is of great importance in the region. This article aimed to study the tempo variations of this system using Man-Kendal nonparametric test and the Sen's estimator of slope. The monthly and annual review show that this system has a rising central pressure trend. In other words, it is weakening. Note that the increase is slight and is not significant at 95%, 90%, and 99%. Anomaly map review shows verified it, too so that the greatest central pressure was observed in the recent decade. The system did not show any specific spatial variation.