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.

Slow-down in summer warming over Greenland in the past decade linked to central Pacific El Niño

Greenland warming and ice loss have slowed down since the early 2010s, in contrast to the rest of the Arctic region. Both natural variability and anthropogenic forcing contribute to recent Greenland warming by reducing cloud cover and surface albedo, yet most climate models are unable to reasonably simulate the unforced natural variability. Here we show that a simplified atmospheric circulation model successfully simulates an atmospheric teleconnection from the tropics towards Greenland, which accounts for Greenland cooling through an intensified cyclonic circulation. Synthesis from observational analysis and model experiments indicate that over the last decade, more central Pacific El Niño events than canonical El Niño events have generated the atmospheric teleconnection by shifting the tropical rainfall zone poleward, which led to an intensified cyclonic circulation over Greenland. The intensified cyclonic circulation further extends into the Arctic Ocean in observations, whereas the model does not show a direct remote forcing from the tropics, implying the contribution of an indirect atmospheric forcing. We conclude that the frequent occurrence of central Pacific El Niño events has played a key role in the slow-down of Greenland warming and possibly Arctic sea-ice loss.

Evolution of Warm Season Intense Rainfall in Yaan Against a Cold-Anomaly Background

This study investigates the rainfall characteristics during intense rainfall over Yaan against a cold-anomaly background, aiming to refine the understanding of different kinds of rainfall events across complex terrain. Hourly rain gauge records, ERA5 reanalysis data and the black body temperature of cloud tops derived from FY-2E were used. The results show that against a cold-anomaly background, the regional rainfall events (RREs) in Yaan exhibit west-to-east propagation, which is different from the north-to-south evolution of warm RREs. The middle and upper troposphere is dominated by a negative geopotential height anomaly corresponding to the cold anomaly. The cyclonic circulation at the higher level associated with the negative geopotential height anomaly bends the high-level jet to the south, forming a divergent zone over the Tibetan Plateau (TP) and guiding mid-level systems to move eastward. The cyclonic circulation at the mid-level produces a wind shear zone over the TP, generating anomalous vorticity that continuously moves eastward and develops to influence the rainfall over Yaan. The cold Yaan RREs are closely related to the TP low-pressure systems (both vortex and shearline). The anomalous vorticity over the TP can influence the local vortex over the eastern periphery of the TP at a distance mainly by the horizontal advection of anomalous vorticity by the mean flow and then enhance the local vortex mainly by anomalous convergence when it moves near Yaan.

Relative contribution of synoptic systems to monsoon rainfall over Orissa

ABSTRACT. The low/depression over northwest (NW) Bay of Bengal is the largest contributor to seasonal monsoon rainfall over all stations in Orissa and Orissa as a whole. The Low Pressure Systems (LPS) and cyclonic circulation (cycir) extending upto 500 hPa level over NW Bay of Bengal alone contribute about 22% to the seasonal monsoon rainfall through about 12 days. The monsoon trough without any significant embedded systems over Orissa and adjoining regions contributes about 28% to seasonal rainfall through about 55 days. All types of LPS including low, depression and cyclonic storm yield maximum rainfall in their left forward (southwest) sectors. The maximum rainfall belt lies more southward due to a depression compared to that due to a low. The spatial distribution of rainfall due to cycir is less systematic. The interaction due to Eastern Ghat plays a significant role in spatial distribution of rainfall over western and eastern sides of the Eastern Ghat due to monsoon lows and depressions over Orissa and adjoining Bay and land regions. The orographic interaction due to Eastern Ghat with the cycirs over Orissa and adjoining Bay and land regions is significantly less leading to no significant difference in spatial distribution of rainfall over eastern and western sides of the Eastern Ghat.

Observational Analysis of a Wind Gust Event during the Merging of a Bow Echo and Mini-Supercell in Southeastern China

The merging of a fast-moving bow echo with a convective cell of a hook-echo signature was studied by using polarimetric radar detections. Gusts with wind speeds near 35 m s–1 were recorded by the surface station, which caused significant damage. A convective cell with a mesovortex signature, which is hereafter referred to as a mini-supercell, was observed over the northeast of the bow echo before the convective merging. It was found that the mesovortex possessed cyclonic circulation and resembled a supercell-like feature. The merging of the bow echo and the mini-supercell strengthened the updraft near the apex of the bow echo. The enhanced updraft was also demonstrated by the appearance of a differential reflectivity (ZDR) column with a topmost height of 4 km above the melting layer (~4 km). The bow was separated into northern and southern sectors after merging with the mini-supercell, leading to the gusty wind over the surface of the south sector.

Lag impacts of the anomalous July soil moisture over Southern China on the August rainfall over the Huang–Huai River Basin

The effect of soil moisture (SM) on precipitation is an important issue in the land–atmosphere interaction and shows largely regional differences. In this study, the SM of the ERA-Interim reanalysis and precipitation data of the weather stations were used to investigate their relationship over eastern China during July and August. Moreover, the WRF model was applied to further validate the effect of SM on rainfall. In the observations, a significantly negative relationship was found that, when the soil over southern China is wet (dry) in July, the rainfall decreases (increases) over the Huang–Huai–River basin (hereafter HHR) in August. In the model results, the soil can “memorize” its wet anomaly over southern China from July to August. In August, the wet soil increases the latent heat flux at surface and the air moisture at lower levels of the atmosphere, which is generally unstable due to the summer monsoon. Thus, upward motion is prevailing over southern China in August, and the increased surface air moisture is transported upwards. After that, the condensation of water vapor is enhanced at the middle and upper levels, increasing the release of latent heat in the atmosphere. The heat release forms a cyclonic circulation at the lower levels over eastern China, and induces the transport and convergence of water vapor increased over southern China in August. This further strengthens the upward motion over southern China and the cyclonic circulation at the lower levels. Therefore, positive feedback appears between water vapor transport and atmospheric circulation. Meanwhile, the cyclonic circulation over southern China results in a response of water vapor divergence and a downward motion over HHR. Consequently, the negative anomalies of precipitation occur over HHR in August. When the July soil is dry over southern China, the opposite results can be found through the similar mechanism.

Cyclonic circulation and climatology of SST, CHL and wind stress curl in a semi-enclosed bay (Bahía de La Paz, Gulf of California)

The first direct current observations (with LADCP and surface drifters) in Bahía de La Paz, a bay in the southwestern Gulf of California (GC), concur with previous reports that the main dynamical feature during summer is a closed cyclonic circulation. However, we found that geostrophic calculations overestimate the speed of the orbital velocity: actual speeds (0.20-0.25 m s-1) were ~25-40% lower than those estimated from geostrophic balance (0.25-0.35 m s-1). The reason is that the centrifugal force cannot be neglected in this case. The mean rotation period during ship-borne observations in August 2004 was ~1.4 days, but it varied during the time that surface drifters were inside the bay, from ~1-2 days in June-July to ~2.5-3 days in September-October. The analysis of satellite data (wind velocity, sea surface temperature and chlorophyll) show that from May to September the wind stress curl is strong and cyclonic, and the surface of the bay is cooler and richer than the adjacent Gulf of California waters, which could be attributed to the positive wind stress curl. This positive wind stress curl on the bay is part of a larger-scale positive wind stress curl distribution that surrounds the southern part of the Baja California Peninsula during summer, probably enhanced in the bay by local topography features. Although there is an exchange of water between the bay and the GC, its effect on the dynamics is poorly known.

Surface-Layer Circulations in Suruga Bay Induced by Intrusions of Kuroshio Branch Water

The water mass structure in Suruga Bay is strongly influenced by open-ocean water. In particular, it is suggested that intermittent intrusions of the Kuroshio water generate characteristic circulations in the surface layer of the bay. In this study, we investigated the processes of the intrusions of open-ocean water into the bay and related generation of bay-scale cyclonic and anti-cyclonic circulation patterns. In doing so, we used an ocean simulation product with observational data constraint on meso and larger scales and with a resolution fine enough to resolve the smaller-scale intrusion structure. Cyclonic and anti-cyclonic circulation patterns as suggested by previous observational studies were detected as positive and negative first leading empirical orthogonal function (EOF) modes of the velocity field in Suruga Bay. The time scale of occurrences of these patterns was estimated as about 1 month, which was consistent with short-term Kuroshio fluctuations as reported in previous studies. Conditions favorable for generating these patterns were analyzed for three typical Kuroshio path periods individually. As suggested by previous studies, relatively strong northward flow to the west of Zeni-su generally promoted the open-ocean water intrusions into the eastern bay mouth, leading the cyclonic circulation in Suruga Bay. Our results showed that the correlation of this relation was significant for each Kuroshio path period. The open-ocean water intrusion increased the surface-layer temperature in Suruga Bay by about 0.7°C on average. On the other hand, the anti-cyclonic circulation pattern in Suruga Bay tended to be generated with relatively weak northward flow to the west of Zeni-su during the large meander Kuroshio path period, whereas this relation was rather weak during other periods. These results were mostly supported by available observations and would be useful for integrating our understanding of the influences of the western boundary current fluctuations on the circulation and temperature variations in proximal bays.

Pacific and Atlantic Multidecadal Variability Relations with the Choco and Caribbean Low-Level Jets during the 1900–2015 Period

This study analyzes the variability of the Choco jet (CJ) and Caribbean low-level jet (CLLJ) with consideration of the simultaneous Pacific interdecadal oscillation (PDO) and Atlantic multidecadal oscillation (AMO) low-frequency mean states and their effects on the atmospheric circulation and rainfall in northwestern South America and Central America for the 1900–2015 period, during the seasons with the highest intensities of the CJ (September–November (SON)) and the CLLJ (June–August). Variations in the sea surface temperature (SST) anomaly positioning in the eastern Pacific, tropical North Atlantic (TNA)/Caribbean Sea during different mean states restrict the anomalous circulation, and, consequently, the intensity of the CJ and CLLJ. During the warm AMO (WAMO)/cold PDO (CPDO), the SST gradient from the tropical Pacific into the TNA, accompanied by a cyclonic circulation near the east coast of the Americas, intensifies the west–east circulation in the region, strengthening the CJ and weakening the CLLJ during SON such that rainfall increases over Colombia, Central America and in adjacent oceans. During the cold AMO (CAMO)/warm PDO (WPDO) phase, a relative east/west SST gradient occurs in TNA, consistent with a cyclonic circulation in western TNA, establishing an anomalous southwest–northwestward circulation from the eastern Pacific into the Caribbean basin, forming a well-configured CJ, increasing precipitation over Central America and its adjacent oceans. For the CLLJ, during CAMO phases, the anticyclonic circulations extended over most of the TNA favor its intensification from 30° W to the Caribbean Sea. In contrast, during WAMO, the cyclonic circulation near the east coast of the United States restricts its intensification to the Caribbean Sea region. To the best of our knowledge, the results presented here are new and might be useful in atmospheric modeling and extreme event studies.

Subsidence Warming in the Tropical Cyclogenesis of Cindy (2017): CPEX observations and Coupled Modeling

The formation of tropical cyclones (TC) in unfavorable large-scale environments remains a challenge for TC forecasting. Tropical Storm (TS) Cindy (2017) formed at 1800 UTC 20 June in the Gulf of Mexico despite strong vertical wind shear, low mid-tropospheric relative humidity, and poorly organized convection. A key to TC genesis is the initial development of a warm core within an emergent cyclonic vortex, a process which occurs on small spatial scales and is often difficult to observe. TS Cindy was observed during the Convective Processes Experiment (CPEX) field campaign in 2017 by the NASA DC-8 aircraft, equipped with a Doppler wind lidar, precipitation radar, and GPS dropsondes. This study combines CPEX observations and a cloud-resolving, fully-coupled atmosphere-wave-ocean numerical simulation to investigate the formation of TS Cindy. Prior to TC genesis, a shallow cyclonic circulation was embedded in a deep layer of west-southwesterly flow associated with an upper-level trough. Within the disturbance, a warm and dry anomaly was observed by dropsondes near the center of the cyclonic circulation, with a maximum at about the 2.5 km level. The temperature perturbation reaches 5°C along with a dew point temperature depression of 8°C in the coupled model simulation. Backward trajectory analysis shows that subsidence is primarily associated with a thermally indirect circulation along the western flank of the storm. Air parcels descend more than 1000 m towards the lower troposphere while warming up by 9-12°C. The subsidence-induced virtual temperature perturbation in the 1.5-3.5 km layer accounts for 50 % of the sea-level pressure depression. Subsidence warming therefore played a key role in the genesis of TS Cindy.