Recurrent Rossby waves during Southeast Australian heatwaves and links to quasi-resonant amplification and atmospheric blocks

In the Northern Hemisphere, recurrence of transient Rossby wave packets over periods of days to weeks, termed RRWPs, may repeatedly create similar weather conditions. This recurrence leads to persistent surface anomalies and high-impact weather events. Here, we demonstrate the significance of RRWPs for persistent heatwaves in the Southern Hemisphere (SH). We investigate the relationship between RRWPs, atmospheric blocking, and amplified quasi-stationary Rossby waves with two cases of heatwaves in Southeast Australia (SEA) in 2004 and 2009. This region has seen extraordinary heatwaves in recent years. We also investigate the importance of transient systems such as RRWPs and two other persistent dynamical drivers: atmospheric blocks and quasi-resonant amplification (QRA). We further explore the link between RRWPs, blocks, and QRA in the SH using the ERA-I reanalysis dataset (1979–2018). We find that QRA and RRWPs are strongly associated: 40 % of QRA days feature RRWPs, and QRA events are 13 times more likely to occur with an RRWPs event than without it. Furthermore, days with QRA and RRWPs show high correlations in the composite mean fields of upper-level flows, indicating that both features have a similar hemispheric flow configuration. Blocking frequencies for QRA and RRWP conditions both increase over the south Pacific Ocean but differ substantially over parts of the south Atlantic and Indian Ocean.

African monsoons Part 1 : Climatological structure and circulation

A recent (1979-1996) NCEP/NCAR reanalysis dataset is utilized to study the structure and characteristics of the time-mean meteorological fields over the continent of Africa and surrounding oceans during February and August when the seasonal monsoons are normally at their peak intensity. The vertical structure, revealed in zonal and meridional sections over different parts of the continent, bring out the dominant influences of the neighbouring oceans and topography in the formation and distribution of monsoon troughs and ridges and associated dry and wet zones. Vertical circulations reveal a symbiotic relationship between the tropical monsoons and the subtropical deserts of the two hemispheres. Some salient features of the African monsoons, such as the formation of double equatorial troughs and their movement following change of season, are discussed and compared with similar features observed elsewhere over the world.

Characteristics of the Wind and Wave Climate along the European Seas Focusing on the Main Maritime Routes

European seas have a strong economic role both in terms of transport and tourism. Providing more knowledge, regarding the mean and extreme values of the wind and sea state conditions in the areas characterized by high maritime traffic, helps to improve navigational safety. From this perspective, six zones with high maritime traffic are studied. ERA5 database, a state-of-the-art global reanalysis dataset provided by ECMWF (European Centre for Medium-Range Weather Forecasts), is used to assess the average values and the percentiles for the wind speed and the main wave parameters in the target areas considering the period 2001–2020. The main European routes and the extreme conditions along them as well as the areas characterized by high values of wind speed and high waves were also identified. A more comprehensive picture of the expected dynamics of the environmental matrix along the most significant shipping routes is useful because in this way the most dangerous areas could be avoided by ships for the safety of passengers and transported goods.

Seasonal Variability of The Mixed Layer Depth in The Sulawesi Sea

The Sulawesi Sea is a semi-enclosed basin located in the Indonesian Seas and considered as the one of location in the west route of Indonesian Throughflow (ITF). There is less attention on the mixed layer depth investigation in the Sulawesi Sea. Concerning that the mixed layer plays an important role in influencing the ocean in air-sea interaction and affects biological activity, the estimation of mixed layer depth (MLD) in the Sulawesi Sea is important. Seasonal variation of the mixed layer in the Sulawesi Sea between 115°-125°E and 0°-8°N is estimated by using World Ocean Atlas 2013. Forcing elements on the mixed layer in terms of surface-forced turbulent mixing from mechanical forcing of wind stress and buoyancy forcing (from heat flux as well as freshwater flux) in the Sulawesi Sea is provided by using a reanalysis dataset. The MLD is estimated directly on grid profiles with interpolated levels based on chosen density fixed criterion of 0.03 kg.m^-3 and temperature criterion of 0.5°C difference from the surface. The results show that mixed layer depth in the Sulawesi Sea varies both spatially and temporally. Generally, the deepest MLD was occurred during the southwest monsoon (JJA), and the lowest MLD was occurred during the first transition (MAM) and second transition monsoon (SON). Strengthening and weakening MLD are influenced by mechanical forcing from wind stress and buoyancy flux. In the Sulawesi Sea, the mixed layer deepening coincides with the occurrence of a maximum in wind stress, and low buoyancy flux at the surface. This condition is the opposite when mixed layer shallowing occurs.

Seasonal Evolution of Supraglacial Lakes on Baltoro Glacier From 2016 to 2020

The existence of supraglacial lakes influences debris-covered glaciers in two ways. The absorption of solar radiation in the water leads to a higher ice ablation, and water draining through the glacier to its bed leads to a higher velocity. Rising air temperatures and changes in precipitation patterns provoke an increase in the supraglacial lakes in number and total area. However, the seasonal evolution of supraglacial lakes and thus their potential for influencing mass balance and ice dynamics have not yet been sufficiently analyzed. We present a summertime series of supraglacial lake evolution on Baltoro Glacier in the Karakoram from 2016 to 2020. The dense time series is enabled by a multi-sensor and multi-temporal approach based on optical (Sentinel-2 and PlanetScope) and Synthetic Aperture Radar (SAR; Sentinel-1 and TerraSAR-X) remote sensing data. The mapping of the seasonal lake evolution uses a semi-automatic approach, which includes a random forest classifier applied separately to each sensor. A combination of linear regression and the Hausdorff distance is used to harmonize between SAR- and optical-derived lake areas, producing consistent and internally robust time series dynamics. Seasonal variations in the lake area are linked with the Standardized Precipitation Index (SPI) and Standardized Temperature Index (STI) based on air temperature and precipitation data derived from the climate reanalysis dataset ERA5-Land. The largest aggregated lake area was found in 2018 with 5.783 km2, followed by 2019 with 4.703 km2, and 2020 with 4.606 km2. The years 2016 and 2017 showed the smallest areas with 3.606 and 3.653 km2, respectively. Our data suggest that warmer spring seasons (April–May) with higher precipitation rates lead to increased formation of supraglacial lakes. The time series decomposition shows a linear increase in the lake area of 11.12 ± 9.57% per year. Although the five-year observation period is too short to derive a significant trend, the tendency for a possible increase in the supraglacial lake area is in line with the pronounced positive anomalies of the SPI and STI during the observation period.

Using and Evaluating the Efficiency of GRIMs Model in Making Some Simulations of Weather Variables in Three Countries in Middle East: A Snowfall Case Study

Predicting weather by numerical models have been used extensively in research works for Middle East, mostly for dust storms, rain showers, and flash floods with a less deal of interest on snow precipitation. In this study, the Global/Regional Integrated Model System (GRIMs) that was developed in South Korea was used to predict a rare snowfall event occurred in three countries in Middle East (Syria, Jordan and Iraq) located between (25-65 oE; 12-42 oN) in year 2008. The main aim of this study was to test GRIMs efficiency, which would be used for the first time in Middle East, to make predictions of weather parameters such as pressure, temperature, and relative humidity especially in the selected area. In addition, the study would investigate the conditions that caused the snowfall event. GRIMs model was installed, compiled, and run on a Linux platform by using NCEP-NCAR reanalysis dataset as initial conditions on 0.5 × 0.5 grid resolution to make simulations for three days at intervals of three hours. The output of the model was evaluated by making comparisons with actual data obtained from the GFS Agency dataset and the model showed its efficiency. The snowfall event was synoptically discussed in details. It was found that the snowfall event was a result of fast succession systems of a strong cold high pressure and then a deep warm low pressure. The high instability in the region had led to form large cumuliform clouds with snow precipitation as a rare event in very long period.

Rapid increase of explosive cyclone activity over the midwinter North Pacific in the late 1980s

Long-term changes in the activity of explosively developing “Bomb” cyclones over the wintertime North Pacific are investigated by using a particular version of a global atmospheric reanalysis dataset into which only conventional observations have been assimilated. Bomb cyclones in January are found to increase rapidly around 1987 in the midlatitude central North Pacific. Some of the increased “Bomb” cyclones formed over the East China Sea and then moved along the southern coast of Japan before developing explosively in the central North Pacific. The enhanced cyclone activity is found to be concomitant with rapid warming and moistening over the subtropical western Pacific, the South and East China Seas under the weakened monsoonal northerlies, leading to the enhancement of lower-tropospheric Eady growth rate and equivalent potential temperature gradient, setting a condition favorable for cyclone formation in the upstream of the North Pacific storm track. Along the storm track, poleward moisture transport in the warm sector of a cyclone and associated precipitation along the warm and cold fronts tended to increase and thereby enhance its explosive development. After the transition around 1987, a Bomb cyclone has become more likely to develop without a strong upper-level cyclonic vortex propagating from Eurasia than in the earlier period. The increased Bomb cyclone activity in January is found to contribute to the diminished midwinter minimum of the North Pacific storm track activity after the mid-1980s.

General overestimation of ERA5 precipitation in flow simulations for High Mountain Asia basins

Precipitation is one of the most important input to hydrological models, although obtaining sufficient precipitation observations and accurate precipitation estimates in High Mountain Asia (HMA) is challenging. ERA5 precipitation is the latest generation of reanalysis dataset that is attracting huge attention from various fields but it has not been evaluated in hydrological simulations in HMA. To remedy this gap, we first statistically evaluated ERA5 precipitation with observations from 584 gauges in HMA, and then investigated its potential in hydrological simulation in 11 HMA basins using the Variable Infiltration Capacity (VIC) hydrological model. The ERA5 precipitation generally captures the seasonal variations of gauge observations, and the broad spatial distributions of precipitation in both magnitude and trends in HMA. The ERA5 exhibits a reasonable flow simulation (RB of 5%–10%) at the Besham hydrological station of the UI basin when the contribution from glacier runoff is added to the simulated total runoff. But it overestimates the observations in other HMA basins by 33%–106% without considering glacier runoff, mostly due to the overestimates in the ERA5 precipitation inputs. Therefore, a bias correction is definitely needed before ERA5 precipitation is used for hydrological simulations in HMA basins.

Abundant Precipitation in Qilian Mountains Generated from the Recycled Moisture over the Adjacent Arid Hexi Corridor, Northwest China

The observed precipitation was suggestive of abundant precipitation in upstream Qilian mountains and low precipitation in the downstream oasis and desert in an endorheic basin. However, precipitation in mountains generated from the recycled moisture over oasis and desert areas has rarely been studied. The climatological patterns of water vapor from 1980 to 2017 in the Qilian Mountain Region (QMR) and Hexi Corridor Region (HCR) were investigated by the European Centre for Medium-Range Weather Forecasts Interim reanalysis dataset and the Modern-Era Retrospective Analysis for Research and Application, Version 2 reanalysis dataset. The results suggest that the precipitable water content decreases from the adjacent to the mountain areas. There are two channels that transport water vapor from the HCR to the QMR in the low troposphere (surface—600 hPa), suggesting that parts of recycled moisture generated from evapotranspiration over the oasis and desert of the HCR is transported to the QMR, contributing to the abundant precipitation in the QMR. This indicates that the transport mechanism is probably because of the “cold and wet island effect” of the cryosphere in QMR. This is likely one of the essential mechanisms of the water cycle in endorheic river basins, which has rarely been reported.