This study mainly aims to examine the relationship between oceanographic properties and climate change in the conjunction of the Istanbul Strait (IS), specifically in the east of the Sea of Marmara (SoM) via evaluating observational temperature and salinity profiles at six stations with monthly atmospheric pressure and air temperature during the period of 1997-2010. The annual cycle of temperature and salinity of the upper and lower layers in the east of SoM have strong seasonal variation: The upper layer temperature rises during summer while decreases in winter. Conversely, the lower layer temperature is higher in winter compared to spring and summer. Additionally, salinity levels in the upper layer are higher in winter and lower in summer, whereas in the lower layer, the pattern is reversed, with lower salinity in winter and higher salinity in summer. In this context, linear trend analyzes are calculated by using monthly time series of temperature and salinity and compared with the trends of atmospheric conditions and the surrounding seas. The average temperature trend in the east of the SoM is increase of 0.067°C/y in the upper layer and increase of 0.028°C/y in the lower layer during this period. The salinity trend is increase of 0.105 psu/y in the upper layer and increase of 0.012 psu/y in the lower layer. The air temperature trend is increase of 0.086°C/y, and the air pressure trend is decrease of 0.034 mbar/y. These values closely align with those observed in the surrounding seas. The air temperature and upper layer temperatures are consistent with each other in terms of both long-term series and trend values.

In this study, temporal and spatial trends of sea surface temperature (SST), wind speed, air temperature, sea surface salinity (SSS), sea-level pressure (SLP), and SST-based upwelling index are examined and compared with data from previous studies. Analyses of these parameters are performed for five critical locations that best illustrate the characteristic meteorological and oceanographic structures of the Aegean Sea. Upwelling index calculations are conducted with respect to latitude in order to reveal variations and detect upwelling strength easily. Long-term variations of all parameters are investigated using long-term ECMWF ERA-Interim data covering the period from 1979 to 2018; moreover, the Copernicus Marine Environment Monitoring Service (Global SSS/SSD L4) is used to obtain SSS data that cover the period of 2000-2015. Linear trend and Mann-Kendall tests are applied to identify tendencies and long-term variability. Monthly-average SST and air temperature data exhibit rising trends over the past 40 years. The results also reveal a decreasing trend in wind speed and SLP over the Aegean Sea. It is hard to determine, however, whether the current warming trend is a natural reaction of Earth or not.

İzmit Bay, a 50 km long inlet at the eastern end of the Marmara Sea, is crucial for the region’s economy, culture, and marine ecosystem. The bay’s water circulation regulates nutrient distribution, stratification, sedimentation, oxygen levels, heat, and pollution levels. It is also influenced by meteorological events, such as short-term moderate to strong wind conditions. This study investigated the sensitivity of İzmit Bay Water Circulation to wind speed, direction, and duration using the MITgcm model with Orlanski boundary conditions and process-oriented modeling. The simulations showed that under weak forcing conditions, seawater temperature, salinity, and stratification do not significantly vary. However, strong forcing and wind speeds (statistically defined by percentiles of observation data) of 4.9 m/s (75%), 6.7 m/s (90%), and 10.1 m/s (99%) generate significant mesoscale and sub-mesoscale processes, depending on the direction. Westerly component winds cause downwelling at the eastern coastline, while easterly component winds bring sub-surface water to the surface. Strong winds from N, NE, and E sectors lead to the rise in lower-layer waters in the western basin, forcing them to overflow through the Hersek Delta sill into the central basin. Overall, severe wind events greater than 4.9 m/s (75%) significantly affect the bay’s hydrography by transforming the upper layer, with a decrease in temperature up to 5 °C and an increase in salinity up to 10 ppt.

In December 2021 and March 2022, synoptic-scale weather systems triggered blockages in the Istanbul Strait, resulting in a notable alteration of the water dynamics of the upper layer of the northeastern Marmara Sea. The blockages resulted in the disruption of water mass exchanges between İzmit Bay and the eastern Marmara basin, driven by intensified hydrodynamic forces and severe wind conditions. The blockage of the two-layer flow in the strait led to an intensification of upper layer dynamics, resulting in an elevation of nutrient levels through mixing with nutrient-rich lower waters and/or the redistribution of nutrient fluxes via an enhanced jet current. This study demonstrates the occurrence of rapid changes in the biochemical structure of İzmit Bay, as evidenced by ADCP measurements and near-weekly temperature and salinity profiles. The analyses demonstrate significant shifts in flow dynamics, with salinity serving as an indicator of enhanced mixing. This mixing enabled the transport of nutrient-rich deep waters to the upper productive layer, thereby stimulating increased biological activity. The findings highlight that the altered dynamics resulted in a notable surge in nutrient concentrations and biochemical activity, potentially fostering the expansion of mucilage-forming species in İzmit Bay.