Cyclonic Circulation Research Articles

ВНУТРИГОДОВАЯ ИЗМЕНЧИВОСТЬ ЦИКЛОНИЧЕСКОЙ ЦИРКУЛЯЦИИ ВОД СРЕДНЕГО КАСПИЯ

Hydrophysical studies in 2004-2018 showed that the cyclonic gyre of the Middle Caspian Sea is not a year-round circulation. In the cold season, the current in the cyclonic gyre intensifies, and in the warm season, the circulation weakens and the directed current turns into a drift one, except for the western part, where the water is transferred from the Middle Caspian to the southern part of the sea due to the difference in the levels of the basins. It is shown that the current in the cyclonic circulation follows the contour above the slope of the Derbent Basin in the entire layer from the surface to the bottom. The seasonal boundaries of the annual intensification and weakening of the cyclonic circulation are revealed.

The Present-Day Structure of the Temperature and Current Fields in Lake Sevan

Long-term features of the vertical and horizontal structure of water temperature field in Lake Sevan are discussed. It is shown that the climate warming has led to an increase in epilimnion temperature in Bol’shoi Sevan in July by 2.0‒3.0°C. The increase in water temperature in hypolimnion was not greater than 1.1°C. In autumn (October), the epilimnion became 1.2°C warmer, while hypolimnion temperature practically has not changed on the average over years. Temperature fields were used to calculate the density currents in summer and autumn periods. A dominating cyclonic water circulation was revealed all over the lake, confirmed by chlorophyll distribution by satellite image data. In the case of large horizontal gradients of water density, the flow velocity can reach 50 cm/s. Autonomous buoy stations revealed a wide range of water temperature variations due to internal waves of different nature. The reversible vertical mixing of water mass by internal waves plays an important role in the distribution of nutrients and plankton within the water mass. The water level rise by ~3 m, unlike it drop by 1981 by 18.48, has not caused any significant changes in lake hydrological regime.

Intraannual Variability of the Cyclonic Circulation of Water in the Middle Caspian Sea

Intraannual Variability of the Cyclonic Circulation of Water in the Middle Caspian Sea

The Variability of Maximum Daily Precipitation and the Underlying Circulation Conditions in Kraków, Southern Poland

This article studies the intra-annual and long-term variability in the maximum daily precipitation totals and their association with atmospheric circulation in Kraków. It investigates daily precipitation maxima by year and by month. The research is based on daily precipitation totals in the years 1863–2021 and draws on the calendar of atmospheric circulation types by Niedźwiedź. It examines the frequency of precipitation maxima in individual months and their variation from one year to another. No statistically significant trend of change in precipitation over the study period has been found. All annual maximum daily precipitation totals in Kraków fall into the category of heavy precipitation (>10 mm), and almost 99% qualify as very heavy (>20 mm). In the summer months, these are about 3–4 times higher than in winter. The share of the daily precipitation maximum in the monthly total exceeds 30% in all months. The maximum daily precipitation occurring on 5 August 2021 was the highest in the period that extends from the start of instrumental measurements. The study period saw 12 cases of maximum precipitation that belong to ‘flood-inducing’ categories (over 70 mm/day). Such cases of the very heaviest precipitation occurred in cyclonic situations: Cc, Bc, Nc, NEc, Ec and SEc. Most spring and summer maxima were seen on days with a cyclonic circulation. The instances of high daily precipitation in the Kraków area led to the flooding of residential and historic buildings, as well as of municipal infrastructure.

Satellite sensed summer monsoon torrential rain events characteristics along the Himalayan regions of North India and their dynamics

In the past three decades, the total number of extreme rainfall events over northern India has increased, which majorly caused flooding and landslides over the Himalayan foothills and the north Indian region. Some extreme events are reported as torrential rainfall or cloudburst events; others are linked with large-scale weather systems. In this article, we identified extreme precipitation events over northern India during the summer monsoon season using satellite-sensed high-resolution precipitation. Historically noted cloudbursts were selected from these extreme events to study the mechanisms behind their development. The cloudburst events were classified into five primary classes, C1, C2, C3, C4, and C5, based on the large-scale weather systems in the lower, middle, and upper troposphere. A detailed understanding of each class of cloudburst has been carried out to know the precursors for their formation, the dynamical mechanism involved in their development, and the thermodynamic features responsible for causing extreme rainfall. It is found that cloudburst events of classes C1, C2, and C3 are seen during the active monsoon season. Classes C4 and C5 cloudburst events are linked with the monsoon hiatus/break period. In the active monsoon season, these cloudburst events are associated with lower atmosphere cyclonic circulation over the head Bay of Bengal in class C1, and cyclonic circulation centered over Central India in class C2, and East-West oriented low-pressure conveyor system over central India in class C3. Additionally, thermodynamic analysis of each class revealed that class C1, C2, and C3 cloudbursts were associated with the moist instability of the atmosphere during the cloudburst events, where orographic forcing created potential instability and made them intense and overwhelming.

Natural opening of a new inlet in Chilika Lagoon: A cause and impact analysis

Natural opening of inlet(s) is very common along the shorefront of Chilika Lagoon. Inlet openings in most of the cases have been triggered due to the narrow spit width and cyclonic storm/ depression (CS/D). Wave height and energy get further enhanced when the CS/D occurs concurrently with a full moon/ new moon with a lag/lead of 2–3 days. This has happened many times in the past. Recently, a new inlet was opened on 13th July 2022 due to deep depression (DD) over the northwestern Bay of Bengal from 09th to 12th July 2022. A cyclonic circulation prevailed over the northwest and adjoining west-central Bay of Bengal off the south Odisha-north Andhra Pradesh coast in lower and mid-tropospheric levels tilting southwestwards with height. Wave heights during the pre and post-inlet opening period spanning over one month suggest the peak wave condition coinciding with the period of the inlet opening. Geomorphological conditions of the spit before and after the inlet opening were monitored. Results indicate fast expansion of inlet width due to erosion of the spit on both sides of the inlet and the tidal incursion into the lagoon predominantly during the full-moon day/new moon day. After the opening of a narrow inlet of width 246 m on 13 July 2022, it was doubled within a month. Shoreline analysis using Sentinel 2A imagery for pre- and post-inlet opening was carried out and validated with observations. Oceanographic conditions near the new inlet were simulated and monitored by deploying a Seaguard RCM current meter with a pressure and temperature sensor. This study assessed the reasons for the inlet opening and monitored the geomorphological changes after the inlet opening. More importantly, the impact of the new inlet on lagoon ecosystem was assessed, which has severe implications for the management of the lagoon ecosystem.

Combined effects of Arctic tropospheric warming and La Niña events on enhanced Eurasian cold anomalies

This study investigates the combined effects of Arctic tropospheric warming and La Niña events on climate variations in Eurasia during boreal winter by utilizing the reanalysis data and the output of CMIP6 models. Results show that the “Warm Arctic-Cold Eurasia” pattern emerges with enhanced intensity under the combined effects compared with the individual effect of Arctic tropospheric warming or La Niña events. The significantly intensified Eurasian cold anomaly is observed in the north of Lake Baikal during the combined years. However, this cold anomaly shrinks substantially in warm Arctic years while shifting southwestward with a weaker intensity in La Niña years. Additionally, the winter atmospheric variability under the combined effects is much stronger than that under the individual effect of these two factors. Further investigation reveals that the La Niña event is probably crucial to dominating the basic distribution of winter temperature variability and the associated tropospheric circulation anomalies. Recent Arctic tropospheric warm anomalies are also essential, possibly amplifying the impact of the La Niña event on Eurasia through the weakened stratospheric polar vortex. These two factors jointly contribute to the enhanced “+−+” wave train that displays over mid-high latitudes under the combined effects. This “+−+” wave train exhibits alternative anticyclonic, cyclonic, and anticyclonic circulation anomalies over the Ural Mountains, Lake Baikal, and North Pacific, respectively. It guides wave energy from the Barents-Kara Seas (BKS) along the Ural Mountains to Lake Baikal and promotes the development of the Ural blocking anomaly that favors the intensified extreme cold temperature in Eurasia. The results of the CMIP6 models largely replicate some major features of the combined effects in the observations.

Contribution of lakes in sustaining the Sahara greening during the mid-Holocene

Abstract. The climate impact contribution of lakes to sustain the Green Sahara in the mid-Holocene (MH, 6000 years ago) is still under debate. To assess the lake-induced climate response over North Africa, we investigated the roles of western Sahara lakes and Megalake Chad using reconstructions of MH Sahara lake maps as surface boundary conditions for the isotope-enabled atmospheric model MIROC5-iso. Our results show that the western Sahara lakes pushed the West African monsoon northward and extended it eastward by expanding Megalake Chad. This lake–climate impact was caused by the cyclonic circulation response related to the weakened African Easterly Jet and enhanced Tropical Easterly Jet. According to the Budyko aridity index, the northwestern Sahara climate region shifted from hyper-arid to arid or semi-arid with the lake expansion. Moreover, precipitation scarcity could have been reduced by up to 13 % to sustain the semi-humid conditions. Such lake–climate impacts could alleviate Sahara aridity, relying on lake positions in the monsoon regions. Our findings are promising for understanding the contribution of lakes to sustaining the Green Sahara.

Severe dust storm/thunderstorm activity over Uttar Pradesh on 13th May, 2018 - A case study

Premonsoon season over Uttar Pradesh is characterized withthunderstorm accompanied with rain, dust storm, gale winds and hail storms etc. These storms generally develop locally in association with convection and moisture convergence and seen as single cells in Doppler Weather Radar, but sometime these thunderstorms are associated with synoptic scale systems viz. Western Disturbance as cyclonic circulation/trough, induced low/ cyclonic circulation or northwest-southeast oriented trough, thereby increasing the spatial extent and severity of these thunderstorms significantly. In the present study, Duststorm/Thunderstorm activity that occurred over the state on large scale on 13th May 2018 and which claimed more than 49 human lives and large number of livestock in Uttar Pradesh has been analyzed. The purpose of this study wasto find out probable dynamic and thermodynamic aspects of this activity. The study indicates that the environment was highly favourable thermodynamically for severe thunderstorm activity with high maximum temperatures (>40C), high CAPE(>1000), high Total Total Index(>50) and high negative Lifted Index values (<-5) over most parts of the northwest Indian plains. The moisture discontinuity line was clearly noticed over south Uttar Pradesh with high moisture contents towards its north. Also 00UTC GFS wind analysis of the day at 925hPa indicated strong southeasterlies of the order of 30-35Kts over Uttar Pradesh resulting high moisture incursion in the lower levels over this region. The Low level wind shear was also high and was about 25-30Kt as evident from Skew-T gram of Lucknow for 12UTC of the day taken from Wyoming site as well as 12UTC wind shear analysis using ERA Interim daily data of ECMWF on 13 May, 2018. These features together with synoptic conditions viz; Western Disturbance (WD) in mid and upper levels and a Cyclonic Circulation (cycir) over south Haryana & neighbourhood as well as an east-west trough extending from this cycir in the lower levels made the environment highly favourable for severe thunderstorm activity over the region.

Impact of intra‐decadal variability of meridional heat transport on the rainfall over Southern Africa during austral summer

AbstractThe present study examines the impact of intra‐decadal variability of Meridional Heat Transport (MHT) associated with Indian Ocean Shallow Meridional Overturning Circulation (SMOC) on the rainfall variability over Southern Africa (e.g., Zambia, Malawi, Mozambique, south‐eastern Congo, Zimbabwe, southern Tanzania and Madagascar) during austral summer (December to February). Century long ocean reanalysis, atmospheric reanalysis and rainfall observations are used for the study. Spectrum analysis of MHT index and rainfall anomaly displays intra‐decadal (5–7 years) variability significant at 95% confidence level. The oceanic and atmospheric conditions during strong and weak phases of intra‐decadal MHT variability are examined. Strong (weak) MHT years displayed reduction (increase) in Sea Surface Temperature (SST) in southwest Indian Ocean, which are out‐of‐phase with net heat flux but in phase with ocean circulation. The negative (positive) SST anomaly favours positive (negative) sea level pressure anomaly, which supports the low level moisture divergence (convergence) and anticyclonic (cyclonic) circulation in the lower troposphere. This circulation leads to anomalous subsidence (convection) and below (above) normal moist static energy over the study region resulting in below (above) normal convection, that causes negative (positive) rainfall anomaly over the Southern Africa. The association between intra‐decadal variability of MHT associated with SMOC and rainfall over Southern Africa during austral summer are reported for the first time. The study may pave the way for enhanced predictability of intra‐decadal Southern African austral summer rainfall.

Hydrography of Northern Thermaikos Gulf based on an integrated observational-modeling approach

The Northern Thermaikos Gulf is a semi-enclosed coastal region of the Aegean Sea, characterized by anthropogenic and natural stresses such as intense industrial and agricultural activities, urban outflows, and several river discharges facing severe pollution events. The main motivation of this study is to investigate the prevailing oceanographic conditions of Northern Thermaikos Gulf, which are associated with the quality of the semi-enclosed basin's water masses. The hydrography and the hydrodynamic circulation patterns are revisited, based on the findings of a multi-platform observational study, conducted during a recent annual cycle, from June 2021 until May 2022. The observational findings are supported by a three-dimensional high-resolution hydrodynamic model (Deltf3D-Thermaikos) with updated freshwater input from all important land sources. The physical connectivity between the sub-basins of Thermaikos, the renewal of its northern coastal areas and the seawater quality are strongly related to the variability of wind-induced circulation. Northerly and southerly winds affect the spreading of the nutrient-rich riverine waters, discharged at the west coast of the gulf. The prevailing northerly winds contribute to the southward removal of the polluted sea surface and riverine waters, enhancing the cyclonic circulation around the gulf, allowing the inflow of clearer Aegean Sea Waters along the eastern coasts of the gulf. Northerlies also promote a connectivity pathway between the environmentally stressed Thessaloniki Bay (urban seafront) and the commonly less polluted southeastern coasts of the central basin. Southerlies mainly confine riverine waters in the western and northern coastal regions, weaken the renewal ability of the enclosed basins, and impose the formation of anticyclonic, mesoscale, circulation eddies in the outer and central areas.

Stratosphere–troposphere synergetic effect on the extreme low-temperature event over China in late November 2022

In late November 2022, most regions in China were hit by a strong northwest-path cold wave, bringing a high-hazard extreme low-temperature event (ELTE). Several parts of Northwest China experienced extremely low temperatures and record-breaking snow depths. A stratosphere–troposphere synergetic effect was suggested to be closely related to the ELTE according to a diagnostic analysis. On the one hand, the concurrent establishment of two blockings in Europe–Northeast Atlantic and North Pacific led to a polar vortex split at the tropopause, and the Arctic Oscillation phase subsequently turned negative. An airflow with high potential vorticity (PV) was squeezed out of the Arctic. Meanwhile, a high-PV air that originated from the lower stratospheric Arctic was conveyed southwards to the western Siberian Plain along the sloping isentropic surface. This condition triggered a tropospheric response in which the East Asia trough deepened due to the intensified cyclonic circulation induced by the high-PV intrusion. On the other hand, downward propagation of stratospheric anomalies accompanied by stratospheric polar vortex displacement and split was observed in mid- and late November, respectively. Changes in stratospheric circulation contributed to enhanced blockings over Europe–Northeast Atlantic in the lower stratosphere or upper troposphere. As a result, the inverted omega-shaped circulation pattern was formed in the middle to upper troposphere, and it consisted of the intensified East Asia trough and two blockings in the upstream and downstream regions. The high-PV air upstream of the East Asia trough was advected to China, which directly led to the outbreak of the ELTE. The establishment of double blockings and the displacement or split of the stratospheric polar vortex can be efficient signals for cold-event prediction in China. This study provides novel insights into the cause of ELTEs under warming climates in the future.

Quasi-Biweekly Oscillation of Surface Sensible Heating over the Central-Eastern Tibetan Plateau and Its Relationship with Spring Rainfall in China

Abstract This study examines the characteristics and phase evolution of the quasi-biweekly oscillation of surface sensible heating (SH) over the central-eastern Tibetan Plateau (CETP) during spring. The mechanism connecting CETP SH to spring rainfall in China on the quasi-biweekly time scale is further investigated. Results show that the dominant mode of quasi-biweekly CETP SH presents a monopole pattern, in which the peak leads the maximum of the quasi-biweekly rainfall in the middle and lower reaches of the Yangtze River (MLYR) and South China by approximately 5 and 7 days, respectively. As an upper-level Rossby wave train propagates eastward, an anomalous center of convergence moves to the CETP, which leads to a strong downdraft and reduced cloud cover. The resultant elevated shortwave radiation input and drier soil conditions are favorable for the CETP SH quasi-biweekly oscillation to enter a positive phase. When reaching its peak, the CETP SH efficiently heats the lower atmosphere, resulting in a local updraft. Due to the “SH-driven air pump” effect, abundant water vapor is transported from the oceans to China. A lower-layer southerly anomaly on the east side of the TP develops into an anomalous cyclonic circulation via the effect of topographic friction, which leads to the expansion of the positive potential vorticity anomaly and the maximum of the quasi-biweekly rainfall in the MLYR. Further southeastward propagation of the wave train leads to a shift in the rainfall anomaly center to South China. These findings suggest that the CETP monopole SH warming could be a good indicator for predicting intraseasonal variations in spring rainfall over China.

Contribution of the wind, Loop Current Eddies, and topography to the circulation in the southern Gulf of Mexico

The Bay of Campeche, located in the southern Gulf of Mexico (GoM), is characterized by a semi-permanent cyclonic circulation commonly referred to as the Campeche Gyre (CG). Several studies documenting its upper layer structure have suggested a possible relationship between its seasonal variability and the wind stress, and that non-seasonal variability arises mainly from the interaction of the gyre with Loop Current Eddies (LCEs) that arrive in the region. Nevertheless, a partition of the contributions of these forcings to the circulation of the CG in a statistically consistent manner is still needed. This study examines the wind- and eddy-driven circulation with long-term numerical simulations of the GoM using the HYbrid Coordinate Ocean Model. Our results show that, in the absence of LCEs, the wind can sustain a seasonal-modulated circulation in the CG, confined within the upper 600 m. When considering LCEs, high fluctuations on the flow at intraseasonal time scales are imposed. We found that the LCEs influence the western Bay of Campeche circulation through two main mechanisms: (a) by decelerating and inhibiting the CG through a positive vorticity flux out of the bay, leading to reversals in the flow if LCE southward penetration is large, or (b) by strengthening the CG when a big cyclone, accompanying the LCE, enters the region. It is proposed that the second mechanism is responsible for inducing a net weak cyclonic circulation in the Bay in the absence of wind. Furthermore, past studies have shown that the CG behaves as an equivalent-barotropic flow, with topography acting to confine the CG to the west of the bay. In our modeling results, the role of topography manifests similarly among the different numerical experiments, resulting in closed geostrophic contours to the west of the bay that confine an upper-layer, nearly-symmetric, equivalent-barotropic CG.

Physical mechanisms for the dominant summertime high-latitude atmospheric teleconnection pattern and the related Northern Eurasian climates

Summertime atmospheric teleconnection patterns over Eurasia have a significant influence on regional weather and climate. Despite extensive studies on the subtropical patterns, the high-latitude counterpart has received relatively less attention. This study proposes physical mechanisms for the formation and maintenance of the dominant high-latitude teleconnection pattern. The formation of the pattern is associated with variability in synoptic-scale eddy activity due to the meridional gradient of sea surface temperature anomalies in the vicinity of the Gulf Stream, causing a meridional shift of the central axis of storm track at the exit of Atlantic jet. The resultant convergence of transient vorticity fluxes to the west of the British Isles induces low-frequency cyclonic circulation anomalies and continued propagation of Rossby waves downstream along northern Eurasia. Once these circulation anomalies are formed, the subsequent latent heat-related diabatic anomalies over the northern Eurasian landmass act as another source of Rossby waves to maintain the teleconnection pattern. Regional temperature and precipitation variability is closely linked to the wave pattern along a route through northern Eurasia, and even precipitation over the East Asian summer monsoon region is influenced by the teleconnection pattern.

Asymmetric influence of the Pacific meridional mode on tropical cyclone formation over the western North Pacific

AbstractThis study investigates the asymmetric response of western North Pacific (WNP) tropical cyclone (TC) formation during August–November to the Pacific meridional mode (PMM) from 1961 to 2021. Basinwide WNP TC frequency significantly increases (slightly decreases) during positive (negative) PMM years, implying a nonlinear PMM–TC frequency relationship. Only small spatial changes in TC formation occur during negative PMM years, while there is nearly a basinwide enhancement of TC formation during positive PMM years, particularly over the region of 5°–30°N and 135°–155°E. This region is characterized by significantly enhanced low‐level vorticity, mid‐level updrafts and upper‐level divergence during positive PMM years, all favouring TC development. By contrast, environmental anomalies are of a smaller magnitude and mostly insignificant over the WNP during negative PMM years. These distinct environmental changes during different PMM phases can be explained by differences in PMM strength. Positive PMM events are of a greater strength and are associated with a stronger wind–evaporation–sea surface temperature feedback than negative PMM events, leading to a notable anomalous large‐scale low‐level cyclonic circulation over the WNP in positive PMM events.

Enhanced impact of prolonged MHWs on satellite-observed chlorophyll in the South China Sea

Marine heatwaves (MHWs) are period of extreme ocean temperatures that can last from a few days to several years. The increasing frequency and intensity of MHWs have devastating impacts on marine ecosystems. This study explores the characteristics of MHWs from 1982 to 2021 and their effects on chlorophyll (Chl) from 1998 to 2021 in the South China Sea (SCS), utilizing satellite data and reanalysis products. MHWs were classified into short-duration MHWs (SMHWs) and long-duration MHWs (LMHWs) based on their duration to analyze their effects. High-intensity and frequent MHWs were observed in the coastal regions of China, the western SCS (WSCS, average depth more than 200 m), and southwest of the Luzon Strait (LS, average depth more than 200 m). MHWs have increased by 14 days (R2 = 0.42, p < 0.01) and 0.99 times (R2 = 0.36, p < 0.01) per decade over the past four decades. The per-decade rate of increase is 44 % of the average MHWs days, and 36 % of the frequency in entire SCS.LMHWs contributed the most to this increase at a rate of 6.5 days (R2 = 0.43, p < 0.01) and 0.25 times (R2 = 0.45, p < 0.01) per decade. The trend of MHWs in the Pearl River estuary (PRE, On the coastal continental shelf, average depth <200 m) is noteworthy for cause for concern, increasing by 19 days and 1.4 times per decade.During SMHWs, the Chl response was relatively insignificant, with roughly 3 % negative anomalies across the entire SCS. However, LMHWs had a more profound impact on Chl,which decliend 8 % over the SCS, especially in the WSCS, PRE, and southwest of LS. During summer in the WSCS and winter southwest of LS, the negative anomalies of Chl varied between 20 % and 40 % of its daily climatology for each grid. In contrast, a noteworthy positive Chl response was observed in the PRE and adjacent waters during summer. The duration and intensity of MHWs in these regions affected Chl, and the suppression of Chl was more prominent as the intensity and duration increased, except in the PRE, where MHWs promoted Chl during both spring and summer. We identified various mechanisms by which MHWs impacted Chl in these regions, including weakened upwelling and eddies in the WSCS, changes in plume distribution along the PRE, and shallow mixed layer and weakened cyclonic circulation in the LS during winter. Our study underscores the need for immediate conservation measures to mitigate the effects of MHWs on marine ecosystems, particularly in the PRE and adjacent regions where harmful algal blooms (HABs) are a frequent and ongoing threat.

Dominant modes and mechanisms of the interannual variability of atmospheric water vapor content during winter over Central Asia

Atmospheric water vapor content (WVC)—a key component of the water cycle—plays a crucial role as an indicator of precipitation changes and directly influences precipitation in the arid areas of Central Asia. These areas are extremely sensitive to global climate change because of their dry climates and fragile water resource systems, especially during winter. This study analyzes the main interannual variation patterns of the atmospheric WVC during winter over Central Asia and reveals the possible physical mechanism behind the changes in the dominant modes (EOF1 and EOF2) of water vapor. The results show that EOF1 (49.4%)—responsible for the interannual variation of the winter atmospheric WVC in Central Asia—has a uniform monopole mode, while EOF2 (23.7%) shows a reverse north–south dipole distribution (with the boundary being at approximately 44° N). The negative geopotential height anomaly over the high latitudes of Europe and the positive geopotential height anomaly over Central Asia are the major impact factors of EOF1, whereas Central Asia has a prevalence of westerly airflow during winter. By influencing the geopotential height anomaly in Central Asia, the winter Niño 3.4 index intensifies the convergence of Atlantic Ocean moisture with tropical Indian Ocean moisture and their eastward transport. In contrast, EOF2 is mainly affected by the positive geopotential height anomaly over the high latitudes of Europe and the negative geopotential height anomaly over the middle latitudes of this region. In particular, the negative anomaly center of the geopotential height on the eastern side of Central Asia extends westward into Central Asia, resulting in reverse moisture transport by westerly and easterly airflows in the southern and northern parts of Central Asia, respectively. This anomaly is correlated with the formation of anomalous cyclonic circulation over North Africa and Europe because of the negative phase of the Arctic Oscillation, leading to prevailing southerly winds in the south of Central Asia, which correspond to abnormal vertical upward movement; however, the situation in the northern part of Central Asia is the opposite. Therefore, the distribution of winter precipitation in Central Asia shows the following pattern: more in the south and less in the north. This study provides scientific and technological support for the formation of research mechanisms and the prediction of winter precipitation in Central Asia.

Atmospheric–Oceanic Processes over the Pacific Involved in the Effects of the Indian Summer Monsoon on ENSO

Abstract This study investigates the physical mechanisms responsible for the impacts of the Indian summer monsoon (ISM) on the evolution of El Niño–Southern Oscillation (ENSO), with a focus on understanding the monsoon-induced Pacific air–sea interactive processes. An observational analysis displays that a weaker-than-normal ISM can strengthen an ongoing El Niño event and weaken a La Niña event, and conversely for a stronger-than-normal ISM. A 1000-yr output from the Community Earth System Model version 2 is capable of reproducing this observed feature and is therefore used to explore the responsible ocean–atmosphere interactive processes involved. Results show that a weak ISM can cause a cyclonic circulation over the western North Pacific via stimulating atmospheric cold Kelvin waves, and conversely for a strong ISM. The westerly (easterly) wind anomalies on the southern flank of the anomalous cyclone (anticyclone) generate eastward (westward) current anomalies in the mixed layer and thus induce anomalous warm (cold) zonal advection. Furthermore, the wind anomalies excite oceanic downwelling (upwelling) Kelvin waves, which deepen (shoal) the thermocline in the equatorial eastern Pacific and result in anomalous warm (cold) vertical advection. A quantitative mixed layer heat budget analysis demonstrates that the influence of the monsoon-induced Pacific wind anomalies on ENSO is mainly achieved by changing the zonal advective feedback and thermocline feedback. This result is confirmed by model sensitivity experiments in which additional monsoon heating or cooling anomalies are imposed over the Indian region during the developing summer of an ENSO event. Significance Statement While previous studies have reported that the Indian summer monsoon (ISM) could influence the evolution of El Niño–Southern Oscillation (ENSO), this study is aimed at understanding the physical mechanisms for the Pacific air–sea interactive processes induced by the monsoon and their impact on ENSO. It is found that a weak and strong ISM can respectively induce an anomalous cyclonic and anticyclonic circulation over the western North Pacific. The wind anomalies on the southern flank of the anomalous circulation mainly change the zonal advective feedback and the thermocline feedback to affect ENSO development. This study provides a detailed physical explanation of how the ISM influences ENSO evolution.

On the Observed Wind-Driven Circulation Response in Small Semienclosed Bays

Abstract This study analyzes horizontal and vertical wind-driven circulation responses in small semienclosed bays, the associated offshore dynamic conditions, and the relative importance of each term in the momentum balance equations using a multiplatform observational system. The observational platform consists of three ADCPs and a land-based radar monitoring the velocity field within the bay and in the contiguous offshore area. The wind-driven patterns in the bay can switch from a barotropic cyclonic or anticyclonic circulation to a two-layer baroclinic mode response as a function of the wind regime (its direction and magnitude). For the baroclinic mode, the vertical location of the inflection point in the velocity profile can vary according to the proximity of the boundary current to the entrance of the bay. The influence of offshore combined meteorological and marine conditions on the inner-bay dynamics is evidenced under moderate to strong wind conditions and is almost nonexistent under negligible wind. The momentum balance analysis as well as the nondimensional numbers evidence the impact of wind stress, coastline shape, stratification, and the nonlinear advective terms. Advection can be at the same order of magnitude as pressure gradient, Coriolis, or wind stress terms and can be greater than the bottom stress terms. The nonlinear terms in the momentum equations are frequently neglected when analyzing wind-driven circulation by means of in situ data or analytical models.