Spring Extreme Precipitation Days in North China and Their Reliance on Atmospheric Circulation Patterns During 1979–2019

The interannual variation of springtime extreme precipitation (SEP) days in North China (NC) and their reliance on atmospheric circulation patterns are studied by using the continuous daily record of 396 rain gauges and the fifth generation of the European Centre for Medium-Range Weather Forecasts atmospheric reanalysis during 1979–2019. The SEP days are defined as the days when at least 10% of rain gauges in NC record daily precipitation no less than 10.5 mm. Results show that the number of SEP days shows large interannual variability but no significant trend in the study period. Using the objective classification method of the obliquely rotated principal analysis in T-mode, we classify the atmospheric circulation into five different patterns based on the geopotential height at 700 hPa. Three circulation patterns all have fronts and are associated with strong southerly wind, leading to 88% of SEP days in NC. The strong southerly wind may provide moisture and dynamic forcing for the frontal precipitation. The interannual variation of SEP days is related with the number of the three above-mentioned dominant circulation patterns. Further analysis shows that the West Pacific pattern could be one of the possible climate variability modes related to SEP days. This study reveals that the daily circulation pattern may be the linkage between SEP days and climate variability modes in NC.

Daily Atmospheric Circulation Patterns and Their Influence on Dry/Wet Events in Iran

Analyzing atmospheric circulation patterns characterize prevailing weather in a region. The method of principal component analysis and clustering was used to classify daily atmospheric circulation patterns. The average daily geopotential height of 500 hPa with 0.5° resolution of the ECMWF (1990–2019) were extracted from the Middle East. The S array was used to identify air types, and k-means clustering was used to classify daily air types. All days were divided into eighteen groups. Then, the surface maps and moisture flux divergence at the 850-hPa level of each pattern were studied. The, the connection between circulation patterns and precipitation occurrence is investigated by the PI index. The existence of a variety of precipitation and temperature regimes and consequent dry/wet periods is related to the type and frequency of the circulation patterns. In patterns with south to southwesterly currents, the low-pressure surface center extends from the south of the Red Sea to southern Turkey and is associated with the mid-level trough, where the moisture fluxes converge in the south of the Red Sea, southwest/south of Iran, and east of the Mediterranean Sea. Therefore, according to the intensity of the patterns, the most precipitation falls in the country’s western half, and the Zagros Mountain’s wind side. With the eastward movement of the Cyclonic patterns, the rainfall area extends to the eastern half of the country. With the pattern that the thermal low surface pressure extends to 35 °N latitude and is associated with the mid-level subtropical high, almost no rain occurs in the country.

Quasi-steady circulation regimes in the Baltic Sea

Circulation plays an essential role in the creation of physical and biogeochemical fluxes in the Baltic Sea. The main aim of the work was to study the quasi-steady circulation patterns under prevailing forcing conditions. Six months of continuous vertical profiling and fixed-point measurements of currents, two monthly underwater glider surveys, and numerical modelling were applied in the central Baltic Sea. The vertical structure of currents was strongly linked to the location of the two pycnoclines: the seasonal thermocline and the halocline. The vertical movements of pycnoclines and velocity shear maxima were synchronous. The quasi-steady circulation patterns were in geostrophic balance and high-persistent. The persistent patterns included circulation features such as upwelling, downwelling, boundary current, and sub-halocline gravity current. The patterns had a prevailing zonal scale of 5–60 km and considerably higher magnitude and different direction than the long-term mean circulation pattern. Northward (southward) geostrophic boundary current in the upper layer was observed along the eastern coast of the central Baltic in the case of southwesterly (northerly) wind. The geostrophic current at the boundary was often a consequence of wind-driven, across-shore advection. The sub-halocline quasi-permanent gravity current with a width of 10–30 km from the Gotland Deep to the north over the narrow sill separating the Farö Deep and Northern Deep was detected in the simulation, and it was confirmed by an Argo float trajectory. According to the simulation, a strong flow, mostly to the north, with a zonal scale of 5 km occurred at the sill. This current is an important deeper limb of the overturning circulation of the Baltic Sea. The current is stronger with northerly winds and restricted by the southwesterly winds. The circulation regime has an annual cycle due to seasonality in the forcing. Boundary currents are stronger and more frequently northward during the winter period. The sub-halocline current towards the north is strongest in March–May and weakest in November–December.

Can synoptic patterns influence the track and formation of tropical cyclones in the Mozambique Channel?

<abstract> <p>The influence of large-scale circulation patterns on the track and formation of tropical cyclones (TCs) in the Mozambique Channel is investigated in this paper. The output of the hourly classification of circulation types (CTs), in Africa, south of the equator, using rotated principal component analysis on the T-mode matrix (variable is time series and observation is grid points) of sea level pressure (SLP) from ERA5 reanalysis from 2010 to 2019 was used to investigate the time development of the CTs at a sub-daily scale. The result showed that at specific seasons, certain CTs are dominant so that their features overlap with other CTs. CTs with synoptic features, such as enhanced precipitable water and cyclonic activity in the Mozambique Channel that can be favorable for the development of TC in the Channel were noted. The 2019 TC season in the Mozambique Channel characterized by TC Idai in March and TC Kenneth afterward in April were used in evaluating how the CTs designated to have TC characteristics played role in the formation and track of the TCs towards their maximum intensity. The results were discussed and it generally showed that large-scale circulation patterns can influence the formation and track of the TCs in the Mozambique Channel especially through (ⅰ) variations in the position and strength of the anticyclonic circulation at the western branch of the Mascarene high; (ⅱ) modulation of wind speed and wind direction; hence influencing convergence in the Channel; (ⅲ) and modulation of the intensity of cyclonic activity in the Channel that can influence large-scale convection.</p> </abstract>

Development of longitudinal dunes under Pangaean atmospheric circulation

As a result of the large difference in heat capacity between land and ocean, global climate and atmospheric circulation patterns in the supercontinent Pangaea were significantly different from today. Modelling experiments have suggested the seasonal overturning of cross-equatorial Hadley circulation; however, there are large discrepancies between model-generated surface wind patterns and the reported palaeo-wind directions from aeolian dune records. Here, we present the results of measurements of spatial distribution of dune slip-face azimuths recorded in Lower Jurassic aeolian sandstones over a wide area of the western United States (palaeolatitude: ~19°–27° N). The azimuth data of dune slip-faces reveal a bi-directional and oblique angular pattern that resembles the internal structures of modern longitudinal dunes. Based on the spatial pattern of slip-face directions and outcrop evidences, we suggest most of Lower Jurassic aeolian sandstones to be NNE–SSW- to NNW–SSE- oriented longitudinal dunes, which likely formed as the result of a combination of westerly, northwesterly, and northeasterly palaeo-winds. The reconstructed palaeo-wind pattern at ~19°–27° N appears to be consistent with the model-generated surface wind pattern and its seasonal turnover. The reconstructed palaeo-wind patterns also suggest an influence of orbitally induced changes in atmospheric pressure configuration in Pangaea.

Upper troposphere circulation anomalies over Asia-Pacific region associated with the interannual variation of Indian summer monsoon

Monthly mean vector wind and geopotential heights at 200 hPa of 67 radiosonde stations from Asia Pacific regions for the period 1963-1988 are used to examine the composite circulation anomaly patterns for the month of May and the monsoon season (June- September) with respect to good monsoon years and bad monsoon years (both associated with ENSO and not associated with ENSO). There are significant differences in the anomalous circulation features between good and bad monsoon years. During the month of May an anomalous anticyclonic (cyclonic) circulation over-central Asia and an anomalous cyclonic (anticyclonic) circulation over Pacific ocean were observed during good (bad) monsoon years. These anomalies persist in the subsequent monsoon season. The key mechanisms of the development of these anomalous circulation patterns and their consequences are discussed.