Dynamics of Atmospheres and Oceans最新文献

In this study, temperature changes and its concentration distribution in the period of 1984–2015 and 2015–2100 were investigated under CanESM5 climate model and SSP126, SSP245 and SSP585 scenarios. By confirming the correlation (more than 0.96) and the efficiency coefficient of the model (more than 0.82), the trend of temperature values using modified Mann-Kendall test and temperature concentration index (TCI) values in the sub-basins of Zayanderood Dam, Iran was estimated. The results indicated a non-significant upward trend in the base period (1984–2015) and a significant increasing trend at the level of 5% in the future period (2015–2100) produced by the mentioned scenarios. According to the slope of the trend line, an increase of 1.45, 4 and 9.8 degrees Celsius is predicted during the period of 2015–2100 according to the SSP126, SSP245 and SSP585 scenarios, respectively. The evaluation of changes in TCI values in the studied area showed that in the future period, the distribution of rainfall patterns will be regular and the uniformity of temperature distribution in the SSP585 scenario is more than in the other two scenarios. The results of the temperature pattern distribution in the study area showed that according to the upcoming climate changes and under the studied scenarios, it is expected that while the study area is warming in the future, the uniformity of the temperature distribution will also appear in the months of the year. This shows the reduction of temperature fluctuations and the uniformity of the average temperature in the months of the year. The reduction of TCI values shows the equalization of average temperature changes in the seasons. The results of the investigations showed that the combination of climate change scenarios with the TCI can well show the concentration and distribution of the temperature in different periods.

The Indian Summer Monsoon Rainfall (ISMR) plays a critical role in agriculture, thereby significantly affecting the economy of India. Yet, there is a large spread in the ISMR variability for future projections (by the end of 21st century) as simulated by coupled general circulation models. Gaining insight into the variations of the ISMR during warm periods could enhance our ability to understand ISMR variability in the future. In this study, we have selected the mid-Pliocene warm period from 3.0 to 3.3 million years ago (Ma), which has similar external forcing (orbital parameters) comparable to the end of the 21st century. To evaluate the ISMR mean state during the mid-Pliocene, we have used six available Coupled Model Intercomparison Project phase 6 (CMIP6) model simulations and their multi-model ensemble mean. Our analysis suggests that the ensemble of CMIP6 models is better than individual models in capturing the ISM rainfall patterns and its characteristics for the historical period of 1914–2013. During the mid-Pliocene, we find an increase in the JJAS rainfall over most parts of India in comparison to the pre-industrial period with an increase of 34% in seasonal precipitation. This higher precipitation conditions during the mid-Pliocene is accompanied by thermo dynamical (higher CO₂ forcing led to higher tropospheric temperature and higher precipitable water) and dynamical (larger tropospheric temperature gradient between Indian landmass and southern Indian Ocean corresponds to enhanced moisture transport, enhanced low-level cross-equatorial flow and intensified Monsoon Hadley Circulation) aspects.

Current perspectives on lower frequency variations and secular warming have predominantly been shaped by traditional anomalies that assume an annual cycle (AC) with a time-invariant amplitude. However, this anomaly framework falls short in capturing the complexity of multiple periodic modes with intricate waveforms and time dependent amplitude—the traits, in general, shared by externally forced responses of complex dynamical systems. By allowing interannual amplitude modulation of ACs, we show that the monotonic amplitude increases of the first AC of Sea Surface Temperature (SST) are manifested as the basin-wide secular warming of the ocean surface. Notably, the first SST-AC exhibits significant interannual variances and the largest linear warming rates in the Pacific Warm Pool. While the linear warming pattern mirrors that of a long-term mean SST, it depicts an entirely different warming pattern on the surface of the tropical Pacific Ocean compared to those reported so far. Moreover, all interannual warm (El Niño) and cold (La Niña) events in tropical Pacific regions are abnormal interannual modulations in the third and fourth ACs of SST, respectively. Specifically, a strong El Niño event occurs when a positive amplitude modulation leads to the overlap of two consecutive positive phases of the third AC of SST. Conversely, the absence of such overlaps during negative amplitude modulations significantly contributes to the positive skewness of SST anomalies. No systematic decadal changes in the zonal propagation characteristics of SST in the eastern Pacific (EP) and central Pacific (CP) regions were detected. These findings underscore that the secular warming and low-frequency events in EP and CP are intrinsic to three distinct ACs.

This paper aims to assess the mean precipitation and precipitation extremes over Iran as simulated by the Regional Climate Model (RegCM4). A simulation spanning 20 years (1991–2010) at a horizontal resolution of 20 Km is driven by the NCEP / NCAR reanalysis. We evaluated the model by comparing simulated precipitation with observations using Bias, Root-Mean-Square Error, and Index of Agreement metrics. We examined the extreme precipitations based on a set of extreme indices recommended by the Expert Team on Climate Change Detection and Indices (ETCCDI) in three categories of intensity (Rx1day and SDII), duration (CDD and CWD), and frequency (R10mm and R20mm). The linear trends are calculated using the Theil–Sen estimator method, and the statistical significance (95% confidence level) is determined by using a modified Mann-Kendall (MMK) trend test. The RegCM4 model satisfactory captured the spatial distribution of precipitation and precipitation extremes, although high bias remained in small parts of Iran, including the northwest and southeast. The northwest bias is due to spring convectional precipitation and the southeast bias could be caused by precipitation from the Asian summer monsoon system, which both of them may not be well simulated by the applied Grell convective scheme. Results indicate that the model reasonably captures Rx1day, SDII, CWD, R10mm, and R20mm Indices over Iran. In good agreement with precipitation observations, the southern coast of the Caspian Sea represents the second-highest extreme precipitation, except for SDII, which is probably due to the high frequency of rainy days in this region. The highest CDD of more than 200 days is found in the arid and semi-arid regions of the southeast. In general, precipitation decreased in most regions of Iran, especially the western, southern, and interior regions. In addition, the results reveal that heavy (R10mm) and very heavy (R20mm) precipitation events have also decreased in the same regions. Results also emphasize an increase in consecutive dry days (CDD) in most parts, especially in the southeast, which deserves more attention in future research. The decreasing trend of precipitation and the increasing trend of CDD show that Iran has become drier in the 2000 s compared to the 1990 s

Maximum potential intensity (MPI), which a TC may reach in certain environment conditions, can be affected by microphysical processes. Latent heat released in the process of TC development plays a significant role in it. However, the impacts of hail added both to single-moment and double-moment microphysics parameterization scheme on the MPI remain unclear. In this study, high-resolution sensitivity experiments are conducted in the Weather Research and Forecasting (WRF) model by using four bulk microphysics schemes belonging to a family, namely, WRF Single-Moment 6-Class (WSM6) scheme, WRF Double-Moment 6-Class (WDM6) scheme, WRF Single-Moment 7-Class (WSM7) scheme, WRF Double-Moment 7-Class (WDM7) scheme. Results show that SM schemes simulate the greater MPI than DM schemes. Adding hail in SM scheme increases the MPI while in DM scheme makes less difference. There is a close relationship between the MPI and the radial peak location and intensity of latent heat. The closer the latent heat peak is to the TC center and the greater the peak intensity is, the greater the MPI can be achieved. Though the presence of hail plays a cooling effect thermally, it may affect the TC structures due to the larger sedimentation speed. WSM7 scheme including hail microphysics simulates the TC with smaller size and eye wall inclination, and thus the latent heating efficiency in the eye wall is higher, which is more conducive to TC intensification. However, the larger content of hail resulting from the accretion of liquid water in WDM7 scheme brings a stronger cooling effect and probably offsets the dynamic advantage.

Chang, Wolfe, Stewart, and McWilliams commented on my recent work regarding the horizontal gravity disturbance vector in atmospheric and oceanic dynamics. Their comments are incorrect. They ignored the horizontal pressure gradient error, claimed the validity of the spheroidal geopotential approximation subjectively based only on small metric error, and decomposed gravity into gravitational and centrifugal accelerations, which should never have been done. Here, I explain further why the spheroidal geopotential approximation is invalid and why gravity cannot be decomposed into gravitational and centrifugal accelerations in atmospheric and oceanic dynamics. Physically, the horizontal gravity disturbance vector vanishes in the horizontal momentum equation using the true gravity g_t in the true geopotential coordinates but does occur in the horizontal momentum equation using the true gravity g_t in the spheroidal geopotential coordinates due to the horizontal pressure gradient error in the coordinate transformation. The error of horizontal pressure gradient force in transforming true geopotential to spheroidal geopotential coordinates equals to the horizontal gravity disturbance vector. The spheroidal geopotential approximation claimed by Chang, Wolfe, Stewart, and McWilliams is invalid.

This study aims to investigate the role of tropical-extratropical interactions in the formation of atmospheric rivers as an important source of moisture in extreme-widespread precipitation of Iran. Extreme precipitation events are extracted based on the 95th percentile index from 1989 to 2019 in Iran. Then, the threshold of widespread precipitation is determined. A day is defined as having extreme-widespread precipitation if one-third of the selected stations have mostly extreme precipitation. Finally, 9 days with the highest extreme precipitation and spatial continuity are selected. The upper air data of the 9 selected days are extracted and, accordingly, synoptic charts are plotted. The used data include ERA5, which are extracted from the lower (800 and 850 hPa) and middle (500, 600 and 700 hPa) levels. The results indicate an interaction with tropical circulation patterns by penetration of extratropical circulation patterns into tropical and subtropical regions. The interaction between patterns could lead to the formation of atmospheric rivers with tropical origin from ITCZ, their transport to subtropical and extratropical regions and their moisture supply along the path by different moisture sources in tropical, subtropical and extratropical regions. The formed atmospheric rivers are divided into two categories, namely continental and oceanic, based on their formation origin. The continental atmospheric river is formed at the lower level and, in some cases, at the middle level, while the oceanic atmospheric river is only formed at the middle level. With the emergence of atmospheric rivers in Iran, there have been extreme-widespread precipitation events due to unstable conditions and rising atmosphere.

On September 29, 2021, Cyclone Shaheen attained significant development in the Arabian Sea. It proceeded to cross the Gulf of Oman on October 3, causing substantial economic damage and casualties due to the heavy rain and high waves it generated. Using meteorological and satellite data, measuring the physical and chemical properties of the water column by CTD a few days before and after the cyclone, and coastal observations, we analyzed the impact of Cyclone Shaheen on the northern shelf of the Gulf of Oman, Iran. High sea surface temperature in the Arabian Sea favored to strengthen the cyclone toward the Gulf of Oman. Strong winds over 20 m s^-1 caused a dust storm on the Iranian Makran coast followed by heavy rains of 72 mm during a day with extensive flooding. Before the passage of cyclone Shaheen, the surface water temperature in the northern shelf of the Gulf of Oman was about 32°C and the dissolved oxygen concentration was 6 mg l^-1, which reached the hypoxia threshold at a depth of about 60 m. The cyclone intensified the vertical mixing in the upper layer, leading to a decrease in surface water temperature by approximately 2–5 °C. Additionally, it pushed the hypoxia boundary down to a depth of 110 m, thereby causing the oxygenated upper layer to become thicker. The current research demonstrates that the Gulf of Oman stratified waters above the oxygen minimum zone could benefit from the passage of the tropical cyclone during the warm season in terms of temperature, dissolved oxygen, and probably dissolved nutrients.

Recent state transition of the Beaufort Gyre has drawn great interest in the Arctic research community, but how the upper ocean hydrographic structure varies with this transition remains poorly understood. The upper ocean mixed layer plays an important role in climatic and ecological processes. Therefore, we analyze the Ice-Tethered Profiler (ITP) observations over the last two decades (2004–2022) to investigate the long-term trend of the mixed layer in the Arctic Ocean’s Beaufort Gyre (BG) from an observational perspective. Results show that the linear trend of the BG surface mixed layer depth (MLD) before and after 2015 has changed significantly, characterized by the vanishing or even reversal of the significant deepening trend. This transition is most pronounced in winter. The BG winter mixed layer is significantly cooler, saltier and denser in the mid-transition period (2013–2017) compared to the pre-transition period (2004–2012), but becomes significantly warmer, fresher and lighter in the post-transition period (2018–2022). The transition feature of the depth of maximum buoyancy frequency in the upper BG is similar to that of MLD, while this maximum decreases significantly in both the mid- and post-transition period when compared to their previous period. The deepening signal of MLD is propagated eastward, which coincides with the recent transition of BG position and freshwater distribution. Mechanism analysis further reveals that the reversal of winter MLD trend before and after 2015 may be due to changes in surface wind stirring and Ekman pumping. This study extends the investigation of the recent state transition of BG considering the upper hydrographic structure.