Introduction
Sea level rise is a major climate indicator that directly or indirectly affects a significant portion of the world’s population, especially those in coastal communities where one-third of the world’s population resides [1-3]. As global warming intensifies, the need for sea-level research has become more important in recent years [4-6]. Sea-level rise is one of the major anticipated consequences of ongoing climate change and the resultant changes in ocean heat content, along with the melting of ice [2,3]. Understanding physical and biological processes in the upper layer of the ocean requires knowledge of sea level, which is one of the most critical parameters in the current warming environment [2-8].
Satellite altimetry-derived sea-level estimates provide the opportunity for relatively long-term and continuous observation of sea-level changes for the oceanic region away from the coast all over the globe, which was previously limited to tidal stations. This has helped in developing a detailed understanding of ocean dynamics globally, particularly in remote areas. The observed sea-level variability can be a deterministic contribution (response to a direct forcing) or a nondeterministic contribution (response to regional small-scale turbulent processes) [3]. The link between sea-level variability and different factors, including circulation, atmospheric pressure changes, wind stress, and large-scale oscillations, is complex [9-14]. Numerical models are used to understand the contribution of various factors to overall variability. The Intergovernmental Panel for Climate Change (IPCC) reports project significant regional variability in sea level for the 21st century [1,2]. Considerable efforts have been made to understand long-term variability at the global and regional scales in recent decades [2-12,15,16]. Tide gauge measurements and altimetry have shown that sea level has risen during the last century and continues to rise at a faster rate in the present century. The projected sea level for different IPCC scenarios also predicts a future increase in the rate of sea-level rise.
The region of the Persian Gulf (Figure 1), along with the Strait of Hormuz and the Gulf of Oman, is one of the busiest shipping routes in the world and is also important for the oil industry. The Persian Gulf is geographically located between 23◦ N and 31◦ N and between 47◦ E and 56◦ E. On average, a minimum of one ship passes through the Strait of Hormuz every 6 minutes [17]. The width of the Persian Gulf varies from 56 km at the strait to 338 km inside the Gulf, with an area of approximately 2.39 × 105 km2 and a length of approximately 1000 km from the strait to the northern end. The average depth of the region is 36 m, and the average volume is 8.63 × 103 km3. The Persian Gulf is connected to the Indian Ocean through the narrow Strait of Hormuz. Sills are absent at the entrance of the Strait of Hormuz, and the depth at the strait rapidly increases from 100 to 2000 m within just 200 km towards the Indian Ocean side.
The meteorological and oceanographic conditions of the Persian Gulf have many similarities with those of the adjacent Mediterranean Sea and Red Sea, except in the depth of the basin. The depth of the Persian Gulf is less than 100 m, while it reaches more than 2000 m in both the Mediterranean Sea and the Red Sea. The northwestern and western Persian Gulf are shallower than the rest of the region. A trough extends from the Strait of Hormuz to the Northern Persian Gulf, which collects denser bottom water and guides outflow to the Arabian Sea.
The Persian Gulf is characterized by intense evaporation due to the surrounding arid climate and its shallow nature.
Figure 1. Study area. Northern, and southern regions of the Persian Gulf are marked.
Climatological Features in Mean Sea Level:
Annual, Seasonal, and Monthly 3.1.1.
Annual The annual climatology of sea level, prepared from monthly mean sea-level data covering nearly three decades from 1993 to 2020, is shown in Figure 2. The sea level in the Persian Gulf is generally higher on the southern and western sides. The presence of mesoscale eddies is visible from multiple patches in the annual climatology. The figure shows the presence of a cyclonic eddy along the eastern coast near the Strait of Hormuz, showing the possible propagation of cyclonic and anti-cyclonic eddies from the Arabian Sea to the Gulf along the coast as coastal high and low centers of sea level. Figure 1. Study area. Northern, and southern regions of the Persian Gulf are marked. The circulation in the Persian Gulf is mainly associated with tides, winds, and density-driven currents. The contribution of the tide is significant at smaller scales such as a horizontal length of less than 10 km and a period of one day. The residual circulation is mainly driven by the wind system and the density gradient [21]. Wind-dominated circulation is prominent in the northern side, while density-dominated circulation prevails in the central and southern regions. The exchange of high salinity and low salinity water occurs at the Strait of Hormuz. The denser surface water formed by intense evaporation sinks and outflow to the Gulf of Oman as a high-salinity undercurrent. The Persian Gulf is characterized by reverse estuary flow similar to that of the Mediterranean Sea. Additionally, a prominent secondary coastal current is visible in the opposite direction (southward) in the direct measurements as well as satellite images along the Iranian coast [17,21]. Sea-level variability in the Persian Gulf is mainly driven by the meteorological conditions in the region and the water exchange along the Strait of Hormuz [6,22]. Analysis based on meteorological parameters has shown the dominance of annual along with semi- annual variations in the sea level [6]. Higher sea levels are observed during summer and lower sea levels are observed during winter [23]. Sultan [6] reported that the contribution of pressure to the sea-level variability is approximately 75%, while that of the steric effect is 25%. Previous studies based on tide gauges in the Persian Gulf reported different values for the long-term trend in sea level mainly due to the difference in the selected period for the study [6,24,25]. The analysis based on an 11-year record from two tide gauges showed an average sea-level trend of 2.1 mm/year [6]. An elaborated study based on nine tide gauges and a longer time scale showed a trend of 1.7 mm/year [24]. A further study based on altimetry data for 1990 to 1999 reported a faster rate of sea-level rise (2.8 mm/year) at the Northern Persian Gulf [25]. A subsequent study focusing on the entire Gulf by Hosseinibalam [26] showed that the rate of the rise in sea level is 2.34 mm/year. Alothman [27] reported that the mean rate of sea-level rise is 2.4 mm/year based on 1979 to 2007 data. Globally, the recent records from the satellite altimetry display a faster sea-level rise of 3.3 ± 0.5 mm/year for the 1993–2017 period.
Results 3.1 Climatological Features in Mean Sea Level: Annual, Seasonal, and Monthly
3.1.1.Annual
The annual climatology of sea level, prepared from monthly mean sea-level data covering nearly three decades from 1993 to 2020, is shown in Figure 2. The sea level in the Persian Gulf is generally higher on the southern and western sides. The presence of mesoscale eddies is visible from the existence of multiple patches in the annual climatology. The figure shows the presence of a cyclonic eddy along the eastern coast near the Strait of Hormuz, showing the possible propagation of cyclonic and anti-cyclonic eddies from the Arabian Sea to the Gulf along the coast as coastal high and low centers of sea level. Figure 1. Study area. Northern, and southern regions of the Persian Gulf are marked. The circulation in the Persian Gulf is mainly associated with tides, winds, and density-driven currents. The contribution of the tide is significant at smaller scales such as a horizontal length of less than 10 km and a period of one day. The residual circulation is mainly driven by the wind system and the density gradient [21]. Wind-dominated circulation is prominent in the northern side, while density-dominated circulation prevails in the central and southern regions. The exchange of high and low salinity water occurs at the Strait of Hormuz. The denser surface water formed by intense evaporation sinks and outflows to the Gulf of Oman as a high-salinity undercurrent. The Persian Gulf is characterized by reverse estuary flow similar to that of the Mediterranean Sea. Additionally, a prominent secondary coastal current is visible in the opposite direction (southward) in the direct measurements and satellite images along the Iranian coast [17,21]. Sea-level variability in the Persian Gulf is mainly driven by the meteorological conditions in the region and the water exchange along the Strait of Hormuz [6,22]. Analysis based on meteorological parameters has shown the dominance of annual along with semi-annual variations in the sea level [6]. Higher sea levels are observed during summer and lower sea levels are observed during winter [23]. Sultan [6] reported that the contribution of pressure to the sea-level variability is approximately 75%, while that of the steric effect is 25%. Previous studies based on tide gauges in the Persian Gulf reported different values for the long-term trend in sea level mainly due to the difference in the selected period for the study [6,24,25]. The analysis based on an 11-year record from two tide gauges showed an average sea-level trend of 2.1 mm/year [6]. An elaborated study based on nine tide gauges and a longer time scale showed a trend of 1.7 mm/year [24]. A further study based on altimetry data for 1990 to 1999 reported a faster rate of sea-level rise (2.8 mm/year) at the Northern Persian Gulf [25]. A subsequent study focusing on the entire Gulf by Hosseinibalam [26] showed that the rate of the rise in sea level is 2.34 mm/year. Alothman [27] reported that the mean rate of sea-level rise is 2.4 mm/year based on 1979 to 2007 data. Globally, the recent records from the satellite altimetry display a faster sea-level rise of 3.3 ± 0.5 mm/year for the 1993–2017.
Figure 2. Annual climatological mean sea level for the Persian Gulf. Sea level given in meters
3.1.2. Seasonal
Sea level in the Persian Gulf is higher during fall compared to the rest of the year (Figure 4). A significant spatial and temporal variability is observed in the Persian Gulf sea level from season to season. During winter, the northern Gulf has a relatively lower sea level, while the rest of the region has a higher sea level. This pattern continues in spring also, with the northern lower sea level extended to a wider area towards the southwestern coast. The pattern is reversed during summer with higher sea level towards the northern and western sides. An extension of lower sea level from the Gulf of Oman to the Persian Gulf through the Strait of Hormuz is also visible. During fall, the northern higher sea level almost spread into the entire Gulf. The maximum and minimum sea levels are observed during the fall and spring seasons, respectively. The results show that the fluctuations in sea level outside the Persian Gulf gradually propagate into the Gulf through the Strait of Hormuz as a “tongue” of a high or low level and moves towards the Northern Persian Gulf. 3.1.3. Monthly Maps of the monthly mean sea level are shown in Figure 5. The northern low in sea level began by December in the northern end and spreads to the south gradually during January to April, and weakens thereafter. The pattern reverses by July, where the northern Gulf experiences a higher sea level and intensifies to a wider area in the following months. The maximum sea level in a seasonal cycle is observed during November and the minimum during April. Figure 5a shows that the extension of a higher sea level from the Gulf of Oman (or the Arabian Sea) to the Persian Gulf is limited to the southeastern side during January. The condition is similar in almost all months (Figure 5b–l), where the signatures of sea level variability are propagated to the Persian Gulf through the Strait of Hormuz but largely limited to the southern and eastern Gulf. A significant difference is observed in sea-level variability between the Persian Gulf and the Gulf of Oman due to the presence of the Strait of Hormuz as a barrier.
Figure 3. Annual climatological mean sea level for the adjacent marginal seas and gulfs. Sea level given in meters.
3.2 Comparision of seasonal cycles
Sea level in the Persian Gulf is higher during fall compared to the rest of the year (Figure 4). A significant spatial and temporal variability is observed in the Persian Gulf sea level from season to season. During winter, the northern Gulf has a relatively lower sea level, while the rest of the region has a higher sea level. This pattern continues in spring also, with the northern lower sea level extended to a wider area towards the southwestern coast. The pattern is reversed during summer with higher sea level towards the northern and western sides. An extension of lower sea level from the Gulf of Oman to the Persian Gulf through the Strait of Hormuz is also visible. During fall, the northern higher sea level almost spread into the entire Gulf. The maximum and minimum sea levels are observed during the fall and spring seasons. The results show that the fluctuations in sea level outside the Persian Gulf gradually propagate into the Gulf through the Strait of Hormuz as a “tongue” of a high or low level and moves towards the Northern Persian Gulf.
Figure 5. Monthly climatological mean sea level for the Persian Gulf (sea level given in meters).
