Anticyclonic Flow Research Articles

Impacts of MJO on the Intraseasonal Temperature Variation in East Asia

AbstractThe Madden–Julian oscillation (MJO) is closely related to the intraseasonal variability of surface temperature in East Asia. It has been shown that significant cold surface temperature anomalies are observed in East Asia during MJO phase 3. However, the cooling tendency develops prior to phase 3, suggesting that the cold surface anomalies in East Asia are a delayed and accumulated response to the MJO forcings prior to phase 3. Here, using a thermodynamic equation, it is shown that both meridional advection and adiabatic cooling terms associated with the MJO flow are the dominant contributors to the cooling tendency. The meridional cold advection initially manifests in East Asia in the form of northerly wind anomalies in the eastern part of anticyclonic circulation anomalies that are centered over eastern Europe and develop before the establishment of the cold anomalies. It is suggested that the enhanced convection in the western North Pacific Ocean is responsible for the anomalous anticyclonic flow over eastern Europe with about a 10-day lag via a meridionally propagating Rossby wave train. Further, cooling by the vertical wind component in East Asia is a result of adiabatic cooling interpreted as a reversed local overturning circulation, with downward motion in the tropics and upward motion in the subtropics. This anomalous meridional overturning circulation process initiated from suppressed convection spanning the tropical Indian Ocean to East Asia also takes about 10 days. Therefore, both the Rossby wave propagation and a local overturning circulation induced by the tropical convections play an important role in driving the lagged response of cold surface anomalies in East Asia. Interestingly, these tropical convection forcings are similar to the typical dipole pattern in convection during MJO phase 7, with suppressed Indian Ocean convection and enhanced western North Pacific convection. This implies that the dipole convective forcing during MJO phase 7 possibly leads to the cold anomalies in East Asia following phase 3.

Transport From the Asian Summer Monsoon Anticyclone Over the Western Pacific

AbstractThe subseasonal scale dynamics of the Asian summer monsoon (ASM) upper troposphere (UT) anticyclone has been identified as a primary mechanism for convectively lofted Asian boundary layer air to leave the confinement of the anticyclone and impact the global UT and lower stratosphere (UTLS). This work quantifies eastward eddy shedding associated with the subseasonal scale oscillation of the anticyclone and associated chemical transport. Using reanalysis data together with satellite trace gas data, we examine the correlation between enhanced tropospheric trace gas species and the presence of a secondary anticyclone over the western Pacific Ocean. To diagnose the role of the western Pacific anticyclone (WPA) in the transport of ASM air into the global UTLS, transport pathways and transit times of air found within the WPA are analyzed using Lagrangian kinematic trajectories. Results show that about two thirds of the air within the WPA originates from, or is influenced by, the greater region of the Tibetan anticyclone. After leaving the WPA, air favors two pathways forward: eastward following the subtropical jet and southwestward following UTLS anticyclonic flow. Approximately 60% of the air parcels cross the tropopause into the lower stratosphere within 30 days, though most cross within a few days. These results highlight opportunities for investigating the impact of the ASM on global UTLS chemical and aerosol composition over the western Pacific via research aircraft.

The relationship between atmospheric circulation patterns and extreme temperature events in North America

AbstractExtreme temperature events (ETEs) pose a significant risk to society, especially vulnerable populations with limited access to shelter and water and those with pre‐existing respiratory and cardiovascular ailments. This research examines the relationship of atmospheric circulation with a myriad of metrics related to ETEs to better understand which synoptic‐scale circulations are likely to have negative health/thermal comfort outcomes. Daily sea‐level pressure (SLP) and 500‐hPa geopotential height (z500) data from the North American Regional Reanalysis (NARR) were used to identify circulation patterns over North America. Self‐organizing maps were used to partition the variability in circulation patterns over five distinct domains covering North America for both variables. Daily 2‐m temperature, 2‐m dewpoint temperature, and 10‐m wind data from the NARR were used to derive five major categories of ETEs based on 95th percentiles: temperature events, apparent temperature events, dew point events, and excess heat and excess cold temperature events. The relationship of circulation pattern frequencies (SOM nodes) leading up to ETEs were assessed using point biserial correlations, accounting for spatial and temporal autocorrelation. The results show that z500 has a stronger association with ETEs than does SLP. A great deal of spatial variability exists in the strength of relationship for many ETE variables with circulation patterns likely due to the local geographical influence (e.g., leeside mountain adiabatic warming and low‐level maritime flow). Generally, high extremes are associated with broad ridging and anticyclonic flow and cold extremes are associated with high amplitude trough patterns with low‐level flow originating from the continental interior. The use of self‐organizing maps presents a unique way of examining the potential for human health risks related ETEs and may be an effective method for statistically downscaling climate model data to assess the potential for ETEs in the future.

An Island Mass Effect Resolved Near Mo’orea, French Polynesia

We sought to resolve the extent, variability, and magnitude of productivity enrichment around a high tropical island consistent with the phenomenon of an Island Mass Effect (IME). Key biogeochemical constituents and physical oceanographic parameters were measured offshore over the upper 500 m from July 27 to August 7, 2014 around the Society Island of Mo’orea in French Polynesia in association with the nearshore measurements of the Mo’orea Coral Reef Long Term Ecological Research program. High-resolution synoptic sampling in a rectangular grid around the island revealed vertical and horizontal patterns in hydrographic conditions, inorganic nutrients, rates of productivity, and concentrations of organic matter that are characteristic of oligotrophic gyre ecosystems. Within the upper euphotic zone (0 – 75 m), levels of net primary productivity (NPP), chlorophyll a (Chl), heterotrophic bacterioplankton productivity (BP), and particulate organic carbon (POC) exhibited concurrent enhancement at stations located within 5 to 15 km of shore, relative to stations farther offshore. These observations of enhanced productivity near an island are consistent with an IME. Particulate organic matter nitrogen isotopes (POM- δ15N) were significantly lower near the island than at stations farther offshore, further emphasizing spatial differences in water column biogeochemistry consistent with an IME. Vertical profiles suggest thermocline shoaling and mixing associated with the pycnocline impinging on the island’s submerged flanks and coral reef slope may have contributed to the decreasing depth and increasing intensity of chlorophyll-a concentration in the DCM at nearshore stations relative to farther offshore. Shipboard measurements of an anticyclonic flow within the upper 75 m of the water column in the vicinity of Mo’orea suggest that retention of inorganic nutrients and organic matter near Mo’orea may also have contributed to the patterns in NPP, Chl, BP, POC and POM- δ15N, providing a potential mechanistic understanding of the processes driving an IME.

A Methodology for the Estimation of Microplastic Concentration in Relation to the Meteorological Forcing and WWTPs Effluents in Urban Coastal Areas

The study of the urban coastline is of great significance for understanding the impacts of human activities on the marine environment. Only recently the emission of microplastics is starting to be considered as a threat to aquatic and terrestrial ecosystems and currently, little information on this topic is available. The present study focuses on the development of a methodology based on the implementation of the hydrodynamic circulation model ELCOM (Estuary and Lake Computer Model) for the estimation of microplastics distribution in Kavala Gulf coastline. The approach aims to include the effects of the high seasonal touristic activity and consequent large fluctuations in wastewater treatment plants (WWTPs) discharge rates. Parameters such as microplastic concentration, effluent flow rate and temperature, meteorological forcing and scalars at the boundaries were applied and the simulation was designed to cover the entire 2006 in order to include an approximation of the seasonal 3D dispersion patterns. The microplastic concentration was estimated based on the suspended particulate matter (SPM) concentration measurements applying the approach of [1]. The physical and chemical parameters of the particles were assumed to conform with the assumptions of [2]. The results showed a strong correlation between the microplastics dispersion patterns, the wind climate and the seasonal increase in population during the summer. Microplastic concentrations reached up to 0.185 μg/l in the WWTPs adjacent coastline in correspondence to the beaches with the highest touristic activity. The periodical formation of anticyclonic flows resulted in a net transport towards the center of the gulf and an increase in microplastic concentration in the bottom layer. On the other hand, the simulation showed that considerable quantities of microplastics tend to be transported outside the study area and far from the coastline in deeper waters.

Circulation in Tasman-Golden bays and Greater Cook Strait, New Zealand

ABSTRACT Analyses are presented of model simulations and Lagrangian drifter experiments designed to quantify circulation and dispersal in Tasman and Golden Bays, and their connection to Cook Strait, New Zealand. Observations made in autumn 2017 and summer 2018 are compared with numerical model results. The drifter experiments were focussed on three repeat experiments in each bay designed to measure the local horizontal eddy diffusivity. An additional experiment investigated the connection with, and flow through, Greater Cook Strait. Point by point velocities extracted from the model show low correlation with, but on average are in the same direction as observed velocities. Eddy diffusivities computed from the drifter experiments are highly variable, but the observations support the modelling. We find that the mean velocity in Tasman and Golden Bays is weak and the circulation is dominated by wind and tidal flows. The mean depth-averaged velocity shows anticyclonic flows in the south of both Tasman and Golden Bays, with cyclonic flow in the north of Golden Bay. The mean near-surface velocity is generally out of each bay and is compensated for by inward circulation at depth.

Transport of the 2017 Canadian wildfire plume to the tropics via the Asian monsoon circulation

Abstract. We show that a fire plume injected into the lower stratosphere at high northern latitudes during the Canadian wildfire event in August 2017 partly reached the tropics. The transport to the tropics was mediated by the anticyclonic flow of the Asian monsoon circulation. The fire plume reached the Asian monsoon area in late August/early September, when the Asian monsoon anticyclone (AMA) was still in place. While there is no evidence of mixing into the center of the AMA, we show that a substantial part of the fire plume is entrained into the anticyclonic flow at the AMA edge and is transported from the extratropics to the tropics, and possibly the Southern Hemisphere particularly following the north–south flow on the eastern side of the AMA. In the tropics the fire plume is lifted by ∼5 km in 7 months. Inside the AMA we find evidence of the Asian tropopause aerosol layer (ATAL) in August, doubling background aerosol conditions with a calculated top of the atmosphere shortwave radiative forcing of −0.05 W m−2. The regional climate impact of the fire signal in the wider Asian monsoon area in September exceeds the impact of the ATAL by a factor of 2–4 and compares to that of a plume coming from an advected moderate volcanic eruption. The stratospheric, trans-continental transport of this plume to the tropics and the related regional climate impact point to the importance of long-range dynamical interconnections of pollution sources.

Seasonality and El Niño Diversity in the Relationship between ENSO and Western North Pacific Tropical Cyclone Activity

Abstract Both the impacts of two types of El Niño on the western North Pacific (WNP) tropical cyclone (TC) activity and the seasonality in the relationship between genesis potential index (GPI) and El Niño–Southern Oscillation (ENSO) are investigated. The ENSO-induced GPI change over the northwestern (southeastern) part of the WNP is mostly attributed to the relative humidity (absolute vorticity) term, revealing a distinct meridional and zonal asymmetry in summer and fall, respectively. The seasonal change in ENSO (background states) from summer to fall is responsible for the seasonal change in GPI anomalies south of 20°N (over the northeastern part of the WNP). The downdraft induced by the strong upper-level convergence in the eastern Pacific (EP)-type El Niño and both the northwestward-shifted relative vorticity and northward-extended convection over the southeastern part of the WNP in the central Pacific (CP)-type El Niño lead to distinct TC impacts over East Asia (EA). The southward movement of genesis location of TCs and increased westward-moving TCs account for the enhanced strong typhoon activity for the EP-type El Niño in summer. In fall the downdraft and anomalous anticyclonic steering flows over the western part of the WNP remarkably decrease TC impacts over EA. The enhanced moist static energy and midlevel upward motion over the eastern part of the WNP under the northern off-equatorial sea surface temperature warming as well as longer passage of TCs toward EA are responsible for the enhanced typhoon activity for the CP-type El Niño. It is thus important to consider the seasonality and El Niño pattern diversity to explore the El Niño–induced TC impacts over EA.

Influence of Wintertime Polar Vortex Variation on the Climate over the North Pacific during Late Winter and Spring

The effects of wintertime stratospheric polar vortex variation on the climate over the North Pacific Ocean during late winter and spring are analyzed using the National Centers for Environmental Predictions, version 2 (NCEP2) reanalysis dataset. The analysis revealed that, during weak polar vortex (WPV) events, there are noticeably lower geopotential height anomalies over the Bering Sea and greater height anomalies over the central part of the North Pacific Ocean than during strong polar vortex (SPV) events. The formation of the dipolar structure of the geopotential height anomalies is due to a weakened polar jet and a strengthened mid-latitude jet in the troposphere via geostrophic equilibrium. The mechanisms responsible for the changes in the tropospheric jet over the North Pacific Ocean are summarized as follows: when the stratospheric polar westerly is decelerated, the high-latitude eastward waves slow down, and the enhanced equatorward propagation of the eddy momentum flux throughout the troposphere at 60° N. Consequently, the eddy-driven jet over the North Pacific Ocean also shows a southward displacement, leading to a weaker polar jet but a stronger mid-latitude westerly compared with those during the SPV events. Furthermore, anomalous anti-cyclonic flows associated with the higher pressure over the North Pacific Ocean during WPV events induce a warming sea surface temperature (SST) over the western and central parts of the North Pacific Ocean and a cooling SST over the Bering Sea and along the west coast of North America. This SST pattern can last until May, which favors the persistence of the anti-cyclonic flows over the North Pacific Ocean during WPV events. A well-resolved stratosphere and coupled atmosphere-ocean model (CMCC-CMS) can basically reproduce the impacts of stratospheric polar vortex variations on the North Pacific climate as seen in NCEP2 data, although the simulated dipole of geopotential height anomalies is shifted more southward.

On the Structure and Formation of UTLS PV Dipole/Jetlets in Tropical Cyclones by Convective Momentum Surges

AbstractThe structure and origin of mesoscale jets and associated potential vorticity (PV) dipoles in the upper troposphere and lower stratosphere (UTLS) in tropical cyclones (TCs) are investigated. UTLS PV dipole/jetlets, which occurred in Talas (2011), Edouard (2014), and Ita (2014), are simulated with the University of Wisconsin Nonhydrostatic Modeling System (UWNMS). PV dipoles are confined to the UTLS, where the jetlets oppose the ambient anticyclonic flow. They form ~100–250 km from the eye in convective asymmetries and are characterized by surges of air that accelerate in the updraft, overshoot, and extend radially outward. In these cases, the outflow jet merges with the subtropical westerly jet. Analysis of the structure of UTLS PV dipole/jetlets led to a new physical interpretation for their formation, based on the difference in momentum between the updraft and air in the UTLS: the convective momentum transport hypothesis. This view is complementary to the vorticity tilting hypothesis. A jetlet will form whenever an updraft carries horizontal winds to a level with different wind. Schematic diagrams show how to predict jetlet orientation based on horizontal speeds in the updraft and UTLS ambient air. In TCs, horizontal winds in the updraft are cyclonic, so a UTLS jetlet will be cyclonic and oppose the ambient flow. Each jetlet creates an anticyclonic, inertially unstable PV member, which lies radially outward. Estimates of terms in the PV conservation equation support the hypothesis that the dipoles arise from the curl of shear stress. Convective asymmetries associated with PV dipole/jetlets can significantly modify TC evolution by local thermodynamic acceleration.

Investigating the sensitivity of glaciers to climate variability since the MIS-2 in the upper Ganga catchment (Saraswati valley), Central Himalaya

Moraines, outwash gravel terraces, fluvial drapes and lacustrine sequences are used to infer the pattern of glacial fluctuations in the Saraswati valley (upper Ganga catchment). Located in transitional climatic zone between dry steppe of the Tibetan plateau in the north and sub-humid higher Himalaya in the south, the Saraswati valley has preserved evidence of four glacier advances. These are identified as the Saraswati Glacial Stage (SGS)-1 (oldest) to SGS-4 (youngest). Based on the relative dating and luminescence ages obtained on younger advances, the SGS-1 is ascribed to pre-Marine Isotopic Stage (MIS)-2. The SGS-2 is dated to the MIS-2 (24.5 ± 2.8–21.2 ± 2.0 ka); the SGS-3 is speculatively ascribed to the Younger Dryas (YD) and the SGS-4 is suggested to be of the mid-Holocene (~6 ka) age. The bifurcated moraine ridges of SGS-2 represent standstill conditions during the post-Last Glacial Maximum (LGM) period. The deglaciation is represented by outwash gravel terraces, impounded sedimentary (lacustrine) sequences and fluvial drapes overlying and abutting the moraines are dated to early-mid (11.9 ± 0.9–7.5 ± 0.6 ka) and late Holocene (3.3 ± 0.2–1.7 ± 0.3 ka) intensified/moderate Indian Summer Monsoon. Considering the timing of glacial advances and stand-still condition, it is proposed that across the orographic barrier (rain shadow valleys), glaciers responded sensitively to the intensified anticyclonic flow of the cooler Mid-latitude Westerlies; implying that in a monsoon dominated transient climatic zone, even the rain shadow valleys responded sensitively to temperature changes.

Dominant large-scale parameters responsible for diverse extreme rainfall events over vulnerable Odisha state in India

The exorable climate changes leads to changes in the frequency and intensity of extreme rainfall events worldwide; however, the amendment in extreme rainfall events is not uniform over space, precisely it is more localized and a great threat to the society. Thus, study of rainfall extremes at a finer spatial scale is essential and identifying the large-scale parameters that are responsible factors is highly needed. Odisha state in India is one of the most vulnerable to weather extremes and considered as a study region. The present study is bi-fold, firstly examining the changes in the extreme rainfall (≥ 204.5 mm/day) over Odisha and exploring the foremost large-scale meteorological parameters responsible for heterogeneous characteristics of extreme rainfall within Odisha during 1980–2017 summer monsoon period. India Meteorological Department gridded high-resolution (0.25° × 0.25°) rainfall analysis and ERA-Interim (0.25° × 0.25°) reanalysis data at daily scale are used for the analysis. The study region has an increasing trend in extreme rainfall events and it is evident that the Indian Ocean is warmer during extreme rainfall events compared to the dry events, particularly near the seashore of Odisha. The stronger (weak) and cyclonic (anti-cyclonic) flows at 850-hPa exhibit during extreme rainfall (dry) events. The moisture flux is convergent during extreme rainfall events, while it is reverse during dry events. The monsoon trough has been shifted to south (north) from its normal position during extreme rainfall (dry) events. A detailed investigation is carried out for extreme rainfall events over five different regions in Odisha. It reveals that the wind at 850 hPa, omega at 500 hPa, and SST play the important role for Region I, while OLR and omega at 500 hPa are dominating for the Region IV in the occurrence of extreme rainfall. Moreover, the role of the dominant climatic parameters for the extreme rainfall occurrence varies for the other three regions. Analysis confirms that the role of main meteorological parameters is statistically significant for the extreme rainfall events over the respective region. Although the Odisha is a small state in India, not only the long-term trend in extreme rainfall varies region-to-region but also responsible factors associated with the climatic conditions differ for the occurrence of the extreme rainfall.

The effect of atmosphere–ocean coupling on the prediction of 2016 western North Pacific tropical cyclones

AbstractWe examine the merit of atmosphere–ocean coupled models for tropical cyclone (TC) predictions in the western North Pacific (WNP), where accurate TC predictions remain challenging. The UK Met Office operational atmospheric global numerical weather prediction (NWP) model is compared with two trial coupled configurations, in which the operational atmospheric model is coupled to a one‐dimensional mixed‐layer ocean model and a three‐dimensional dynamical ocean model. Reforecasts for the 2016 TC season show that the coupled models outperform the NWP model for TC location predictions, with a systematic improvement of 50–100 km over the seven‐day forecasts, but the coupled models amplify the underestimation of TC intensity in the NWP model. Nearly identical TC predictions (for both location and intensity) are found in the two coupled models, indicating the dominance of thermodynamic processes at the air–sea interface for TC predictions on these timescales. The improved prediction of the TC position in the coupled models is associated with an enhanced Western North Pacific Subtropical High (WNPSH), which introduces an anticyclonic steering flow anomaly that shifts TC tracks further west in the southern part of the region and further east in the northern part. Based on sensitivity experiments, we show that these improvements in the coupled models are due mainly to colder initial sea‐surface temperatures (SSTs). Air–sea feedbacks do not change the WNPSH or TC tracks noticeably. Apart from the effect of the initial SSTs, tropical ocean warming due to air–sea interaction in the coupled forecasts can also reduce the predicted TC intensity, presumably due to a stronger regional Hadley circulation with increased subtropical subsidence.

Vosproizvedenie glubokovodnoy tsirkulyatsii Chernogo morya s pomoshch'yu modeli INMOM i sopostavlenie rezul'tatov s dannymi buev ARGO

Purpose. The present paper is focused on theoretical analysis of hydrophysical characteristics of the Black Sea in 2011. Methods and Results. The Black Sea hydrophysical characteristics are calculated using a version of the marine circulation σ -model (Institute of Numerical Mathematics Ocean Model – INMOM) for the Black, Azov and Marmara seas (BAMS) with high spatial 1 km resolution and 20 σ -levels distributed non-uniformly over the depth. Quality of the simulated salinity and temperature fields is assessed by their comparison with the data of the ARGO deep-profiling floats available for the period under consideration. Degree of correspondence of the modeled current fields to the observed ones is estimated based on the average deep-sea current velocities calculated from the data on the Argo profiling floats’ movements. Conclusion. The comparative analysis results show that the INMOM adequately reproduced vertical distribution of the Black Sea hydrophysical characteristics. Comparison of the simulated temperature and salinity fields with those derived from the ARGO buoys data demonstrates the fact that the strong-est deviations of the characteristics under study are observed in the sea upper layers (0–100 m); whereas in the deep-water layers (300–1500 m), the degree of consistency between the simulated results and the field data is much more higher. On the depths below 800 meters, the deep-sea anticyclonic flows with the velocities attaining 1.5 cm/s are present. The revealed feature is not typical of the generally accepted scheme of the Black Sea cyclonic circulation.

Composite analysis of the tropopause inversion layer in extratropical baroclinic waves

Abstract. The evolution of the tropopause inversion layer (TIL) during cyclogenesis in the North Atlantic storm track is investigated using operational meteorological analysis data (Integrated Forecast System from the European Centre for Medium-Range Weather Forecasts). For this a total of 130 cyclones have been analysed during the months August through October between 2010 and 2014 over the North Atlantic. Their paths of migration along with associated flow features in the upper troposphere and lower stratosphere (UTLS) have been tracked based on the mean sea level pressure field. Subsets of the 130 cyclones have been used for composite analysis using minimum sea level pressure to filter the cyclones based on their strength. The composite structure of the TIL strength distribution in connection with the overall UTLS flow strongly resembles the structure of the individual cyclones. Key results are that a strong dipole in TIL strength forms in regions of cyclonic wrap-up of UTLS air masses of different origin and isentropic potential vorticity. These air masses are associated with the cyclonic rotation of the underlying cyclones. The maximum values of enhanced static stability above the tropopause occur north and northeast of the cyclone centre, vertically aligned with outflow regions of strong updraft and cloud formation up to the tropopause, which are situated in anticyclonic flow patterns in the upper troposphere. These regions are co-located with a maximum of vertical shear of the horizontal wind. The strong wind shear within the TIL results in a local minimum of Richardson numbers, representing the possibility for turbulent instability and potential mixing (or air mass exchange) within regions of enhanced static stability in the lowermost stratosphere.

Critical Role of Continental Slopes in Halocline and Eddy Dynamics of the Ekman‐Driven Beaufort Gyre

AbstractThe Beaufort Gyre (BG) is a large‐scale bathymetrically constrained circulation driven by a surface Ekman convergence that creates a bowl‐shaped halocline and stores a significant portion of the Arctic Ocean's freshwater. Theoretical studies suggest that in the gyre interior, the halocline is equilibrated by a balance between Ekman pumping and counteracting mesoscale eddy transport energized by baroclinic instability. However, the strongest anticyclonic flows occur over steep continental slopes, and, despite bathymetric slopes being known to influence baroclinic instability, their large‐scale impacts on BG halocline remain unexplored. Here we use an idealized eddy‐resolving BG model to demonstrate that the existence of continental slopes dramatically affects key gyre characteristics leading to deeper halocline, stronger anticyclonic circulation, and prolonged equilibration. Over continental slopes, the magnitude of the Eulerian mean circulation is dramatically reduced due to the Ekman overturning being compensated by the eddy momentum‐driven overturning. The eddy thickness flux overturning associated with lateral salt transport is also weakened over the slopes, indicating a reduction of eddy thickness diffusivity despite the isopycnal slopes being largest there. Using a theoretical halocline model, we demonstrate that it is the localized reduction in eddy diffusivity over continental slopes that is critical in explaining the halocline deepening and prolonged equilibration time. Our results emphasize the need for observational studies of eddy overturning dynamics over continental slopes and the development of slope‐aware mesoscale eddy parameterizations for low‐resolution climate models.

Temporal and spatial evolution of a deep-reaching anticyclonic eddy in the South China Sea

The temporal and spatial evolution of a deep-reaching anticyclonic eddy (AE) is studied using a combination of satellite measurements, moored observations and ocean model reanalysis data in the South China Sea (SCS). Three evolutionary stages in eddy’s lifecycle are identified from changes in eddy dynamical characteristics estimated from satellite altimetry: birth (22 days), growth (64 days), and decay (47 days). Similar patterns are also distinguished from dynamic signals in HYCOM. Further, flows reversal and upwelling of cold water below 1500 m were captured by the in-situ records when this energetic, highly nonlinear and long-lived (over 19 weeks) AE passed by our mooring position. Its detailed vertical structure is examined through temperature anomalies, vertical shear of horizontal velocities, and horizontal streamlines estimated from ocean model reanalysis data. Results from the model reveal a mesoscale AE with first-mode baroclinic structure: a bowl-shaped anticyclonic flow in the upper ocean connected to a slant-cylinder cyclonic flow at depth, with a transition layer at depths between 400 and 700 m. It is in good agreement with moored observations but showing a shallower transition depth, suggesting a slight deficiency in the model due to limited deep-sea observations. Last, we estimate eddy heat transport at different depths and stages along the AE’s path based on the model data. The result reveals that pronounced heat fluxes occur during growth stage (depths 400 m), counting for 73.03% of the total value. In the decay stage, major heat transport occurs at deeper depth (depths > 700–1500 m). Dynamical characteristics suggest that the vertical structure and temporal evolution of the eddy play significant roles in basin-scale movement and heat transferring. Considering that mesoscale eddies are ubiquitous in the SCS, our results support a recently-proposed mechanism, whereby upper ocean flows produce changes in the deep-sea circulation, potentially influencing boundary layer dynamics. For the first time to track and link an individual AE observed by satellite altimetry and ocean model, comparisons indicate that assimilative HYCOM outputs may be useful for examining the deep ocean properties within the SCS, especially under the impact of such an intensified surface-detected eddy.

Linking the North American Dipole to the Pacific Meridional Mode

AbstractThe North American dipole (NAD) represents a meridional dipole of sea level pressure anomalies over the western tropical North Atlantic and northeastern North America. This study demonstrates that the NAD is intimately linked to the development of the Pacific meridional mode (PMM). In addition to the North Pacific Oscillation, the NAD provides another important remote forcing source to trigger the PMM. The NAD influences the PMM through both direct and indirect pathways. The direct effect is that the winter NAD influences the sea surface temperature (SST) and surface winds over the northeastern subtropical Pacific (NESP) through concurrent anticyclonic flow associated with the NAD, which tends to generate a weak initial warming over the NESP region during late winter and early spring. The indirect effect is that the NAD first induces SST cooling over the northern tropical Atlantic during spring, and the northern tropical Atlantic SST cooling then generates a low‐level anticyclonic flow anomaly over the NESP, which further strengthens the surface warming over the NESP, thereby causing the development of the PMM in the following months. The NAD can also exert an influence on the El Niño–Southern Oscillation through its effects on the PMM. In particular, El Niño episodes led by the combined NAD‐PMM events tend to take the form of the central Pacific El Niño, rather than the canonical eastern Pacific El Niño. We suggest that a better understanding of the NAD‐PMM‐El Niño–Southern Oscillation dynamic link could be useful for the prediction of different types of El Niño event.

Impacts of various types of El Niño–Southern Oscillation (ENSO) and ENSO Modoki on the rainy season over the Huaihe River basin

Precipitation is a significant parameter in many aspects such as agriculture, water management and climate variability. To characterize rainy season variations is important to understand precipitation variability under the effect of climate change. In this study, rainy season features (i.e., onset, retreat and rainy‐season precipitation) over the Huaihe River basin (HRB) and the response to different types of El Niño–Southern Oscillation (ENSO), that is, central Pacific warming (CPW), eastern Pacific warming (EPW), eastern Pacific cooling (EPC), conventional ENSO and ENSO Modoki in the developing and decaying phases are evaluated. The multi‐scale moving t test was used to capture onset and retreat of rainy season. The possible dynamic causes of ENSO‐induced precipitation over the HRB, such as monsoon and atmospheric circulation, were also explored. Results show that (a) onset (retreat) of rainy season progressed northwards (westwards), with rainy‐season precipitation increasing from north to south; (b) rainy‐season precipitation showed a strong correlation to Sea Surface Temperature (SST) in Niño regions. Dry and wet signals were identified in different regions of the HRB in the developing and decaying phases of CPW, EPC and EPW. Special attention should be paid on decaying EPW, where totally dry signals were found, which can reach down to 25% below average precipitation; (c) developing El Niño Modoki and decaying El Niño showed totally dry signals, with decaying El Niño Modoki and conventional La Niña demonstrating overall increasing precipitation; (d) different performances of ENSO‐induced precipitation during rainy season over the HRB are attributable to the combined influences of the monsoon from the India Ocean and the anticyclonic flow in the western North Pacific (WNP). Stronger anticyclone and monsoon are generally associated with increasing rainy‐season precipitation. These results can improve predictability of rainy season features and ENSO‐induced precipitation over the Huaihe River basin.

The Northeast Winter Monsoon over the Indian Subcontinent and Southeast Asia: Evolution, Interannual Variability, and Model Simulations

AbstractThe northeast monsoon (NEM) brings the bulk of annual rainfall to southeastern peninsular India, Sri Lanka, and the neighboring Southeast Asian countries. This October–December monsoon is referred to as the winter monsoon in this region. In contrast, the southwest summer monsoon brings bountiful rainfall to the Indo-Gangetic Plain. The winter monsoon region is objectively demarcated from analysis of the timing of peak monthly rainfall. Because of the region’s complex terrain, in situ precipitation datasets are assessed using high-spatiotemporal-resolution Tropical Rainfall Measuring Mission (TRMM) rainfall estimates, prior to their use in monsoon evolution, variability, and trend analyses. The Global Precipitation Climatology Center’s in situ analysis showed the least bias from TRMM.El Niño–Southern Oscillation’s (ENSO) impact on NEM rainfall is shown to be significant, leading to stronger NEM rainfall over southeastern peninsular India and Sri Lanka but diminished rainfall over Thailand, Vietnam, and the Philippines. The impact varies subseasonally, being weak in October and strong in November. The positive anomalies over peninsular India are generated by anomalous anticyclonic flow centered over the Bay of Bengal, which is forced by an El Niño–related reduction in deep convection over the Maritime Continent.The historical twentieth-century climate simulations informing the Intergovernmental Panel on Climate Change’s Fifth Assessment (IPCC-AR5) show varied deficiencies in the NEM rainfall distribution and a markedly weaker (and often unrealistic) ENSO–NEM rainfall relationship.