Zonal Transport Research Articles

Large-scale background and role of quasi-biweekly moisture transport in the extreme Yangtze River rainfall in summer 2020

A severe flooding hit southern China along the Yangtze River in summer 2020. The floods were induced by extreme rains, and the associated dynamic and thermodynamic conditions are investigated using daily gridded rainfall data of China and NCEP-NCAR reanalysis. It is found that the June–July rainfall over the Yangtze River Basin (YRB) experienced pronounced subseasonal variation in 2020, dominated by a quasi-biweekly oscillation (QBWO) mode. The southwestward-moving anomalous QBWO circulation was essentially the fluctuation of cold air mass related to the tropospheric polar vortex or trough-ridge activities over the mid-high latitude Eurasian in boreal summer. The southwestward-transport of cold air mass from mid-high latitudes and the northeastward-transport of warm and moist air by the strong anomalous anticyclone over the western North Pacific provided important large-scale circulation support for the extreme rainfall in the YRB. The analysis of streamfunction of water vapor flux demonstrates that a large amount of water vapor eastward zonal transport from the Bay of Bengal and Indo-China and northward transport from the South China Sea provided the background moisture supply for the rainfall. The quasi-biweekly anomalies of potential and divergent component of vertically integrated water vapor flux played an important role in maintaining the subseasonal variability of rainfall in June–July of 2020. The diagnosis of moisture tendency budget shows that the enhanced moisture closely related to the quasi-biweekly fluctuated rainfall was primarily attributed to the moisture convergence. Further analysis of time-scale decomposition in the moisture convergence indicates that the convergence of background mean specific humidity by the QBWO flow and convergence of QBWO specific humidity by the mean flow played dominant roles in contributing to the positive moisture tendency. In combination with adiabatic ascent over the YRB induced by the warm temperature advection, the boundary layer moisture convergence strengthened the upward transport of water vapor to moisten the middle troposphere, favoring the persistence of rainfall during June–July. The vertical moisture transport associated with boundary layer convergence was of critical importance in causing low-level tropospheric moistening. By comparison, the horizontal moisture advection played a secondary important role in the quasi-biweekly oscillation of rainfall in June–July 2020.

Effects of Typhoon In‐Fa (2021) and the Western Pacific Subtropical High on an Extreme Heavy Rainfall Event in Central China

AbstractAn extreme heavy rainfall event in Henan Province (HnHRE) of central China in July 2021, which is jointly affected by the western Pacific subtropical high (WPSH) and Typhoon In‐Fa (2021), is comprehensively studied through five experiments conducted using the Weather Research and Forecasting Model (WRF). The patterns of the WPSH, In‐Fa, and precipitation are well reproduced in the control experiment. The magnitude of potential vorticity (PV) anomaly associated with the WPSH (In‐Fa) is increased and decreased by 50% in the initial condition based on the piecewise PV inversion method, namely, SH150 and SH050 (TC150 and TC050), respectively. The analysis shows that the 54‐hr rainfall accumulations in TC150 and TC050 are reduced by 39.5% and 31.8%, respectively, more than 28.8% in SH150 and 20.1% in SH050, indicating that Typhoon In‐Fa plays a more critical role in the rainfall amplification than the WPSH. This result also confirms that the HnHRE occurs within the quite favorable configuration of WPSH and In‐Fa circulations. The WPSH and In‐Fa majorly control the meridional and zonal moisture transports, respectively. The diagnosis of moist ageostrophic turns out that diabatic heating is closely related to the vertical motion during the HnHRE. Besides, the heavy rainfall region is also closely attributed to the distributions of equivalent potential temperature and divergence.

Moisture Sources of Precipitation in the Great Lakes Region: Climatology and Recent Changes

AbstractGiven the critical role of precipitation on hydroclimate, we quantified the contributions of moisture source regions to precipitation in the Great Lakes Region (GLR) using multiple reanalysis data sets. Results show that the Great Plains (GPs) and the GLR itself are the primary sources of moisture. The moisture sources for the double peaks in the GLR precipitation that occur in June and September are identified, which is caused by a shift in the peak timing of moisture contribution from the GLR and GPs. In particular, moisture from the GPs contributes more to the heavy precipitation, while moisture from the GLR contributes more to the light precipitation. We also found a statistically significant (p < 0.05) increasing trend in the moisture contribution from the mid‐Pacific, caused by an intensified zonal moisture transport from the mid‐Pacific through changes in atmospheric circulation.

Novel Insights Into the Zonal Flow and Transport in the Luzon Strait Based on Long‐Term Mooring Observations

AbstractThe zonal flow and volume transport in the Luzon Strait play a crucial role in modulating the circulation, heat and material balance, and biogeochemical processes in the South China Sea (SCS), but the quantitative values remain unclear due to lack of long‐term direct observations. Here, this knowledge gap is bridged by analyzing 4‐year velocity measurements from a mooring array along 119.9°E. Based on the novel data, the mean value of the upper‐layer Luzon Strait transport (i.e., LST_up) is estimated at −4.54 ± 1.69 Sv. Seasonally, the westward LST_up attains its peak and trough in January and June with values of −6.80 ± 1.46 Sv and −2.59 ± 0.76 Sv, respectively. At the interannual time scale, the LST_up was strongest in 2016–2017 but weakest in 2017–2018. Further analysis suggested that local winds and the combination of local winds and upstream Kuroshio transport are likely the dominant modulation mechanisms for its seasonal and interannual variations, respectively. In the middle layer, a quasi‐steady cyclonic flow structure is identified and the volume transport is therefore trivial. We further found that seasonal variation of the middle‐layer transport is dominated by the eastward flow of this cyclonic structure. Corresponding to the gravest‐mode response in the Luzon Strait, seasonal‐to‐interannual variations of this middle‐layer eastward flow are nearly in‐phase with the upper‐layer westward flow. Overall, the above observational results provide a benchmark for the flow and transport in the Luzon Strait which can help understand the dynamics of the SCS circulation and validate and improve regional numerical simulations.

Recent Warming Trends in the Arabian Sea: Causative Factors and Physical Mechanisms

In recent years, and particularly from 2000 onwards, the North Indian Ocean (NIO) has been acting as a major sink of ocean heat that is clearly visible in the sub-surface warming trend. Interestingly, a part of the NIO—the Arabian Sea (AS) sector—witnessed dramatic variations in recent sub-surface warming that has direct repercussion on intense Tropical Cyclone (TC) activity. This study investigated the possible causative factors and physical mechanisms towards the multi-decadal warming trends in surface and sub-surface waters over the AS region. Responsible factors towards warming are examined using altimetric observations and reanalysis products. This study used ORAS5 OHC (Ocean Heat Content), derived meridional and zonal heat transport, currents, temperature, salinity, Outgoing Longwave Radiation (OLR), and air-sea fluxes to quantify the OHC build-up and its variability at water depths of 700 m (D700) and 300 m (D300) during the past four decades. The highest variability in deeper and upper OHC is noticed for the western and southern regions of the Indian Ocean. The warming trend is significantly higher in the deeper regions of AS compared to the upper waters, and relatively higher compared to the Bay of Bengal (BoB). Increased OHC in AS show good correlation with decreased OLR in the past 20 years. An analysis of altimetric observations revealed strengthening of downwelling Kelvin wave propagation leading to warming in eastern AS, mainly attributed due to intrusion of low saline water from BoB leading to stratification. Rossby wave associated with deepening of thermocline warmed the southern AS during its propagation. Heat budget analysis reveals that surface heat fluxes play a dominant role in warming AS during the pre-monsoon season. Increasing (decreasing) trend of surface heat fluxes (vertical entrainment) during 2000–2018 played a significant role in warming the southeastern sector of AS.

СРАВНИТЕЛЬНЫЙ АНАЛИЗ МУЛЬТИДЕКАДНОЙ ИЗМЕНЧИВОСТИ ГИДРОМЕТЕОРОЛОГИЧЕСКИХ ПАРАМЕТРОВ В ПОНТО-КАСПИЙСКИХ МОРЯХ

NCEP/NCAR reanalysis data for the period of 1948–2020 were used to detect, describe (in geographical sense) and compare the patterns of multidecadal variability of the hydrometeorological parameters (thermal, dynamic and humidity) in the Ponto-Caspian region (Black Sea, Sea of Azov and Caspian Sea). Polynomial approximations of the time series of the annual mean values indicated the non-linear nature and periodicity of the long-term variability of considered parameters. Being geographically closely located, these seas reveal significantly different patterns of variability. In the Black Sea two temporal intervals of an average duration of 20 and 40 years with multidirectional trends of hydrometeorological parameters changes have been detected, while in the Caspian Sea three such intervals lasting 10–25 years are identified. The North-Eastern wind regime prevails over the both basins. However, if in the Black Sea wind components act unidirectionally (air temperature rise/cooling coincides with the weakening/ strengthening of both components), then in Caspian they act the opposite way, with the major portion of heat advection provided by zonal (Eastern) transport. Phases of air warming/cooling in both basins coincide with a weakening/strengthening of this transport. The time lag between the shift of dynamic and thermal (as well as humidity) regimes is about 6–8 years, suggesting a leading role of the large-scale atmospheric forcing variations in the regional variability. Specific humidity is positively correlated with air temperature and decreases/increases during periods of cooling/warming. In contrast, relative humidity and precipitation rate variations are negatively correlated with air temperature trends. Over the Caspian Sea intensification of Eastern transport and associated cooling coincide with a situation when the North Atlantic Oscillation (NAO) index decreases to negative values, and the East Atlantic-West Russian pattern (EAWR) index is significantly positive. On the contrary, weakening of the Eastern transport and warming occur during a period of sharp strengthening of the NAO, coinciding with strongly negative values of the EAWR index. As for the Black Sea, the situation is controversial: the general pattern of variability in 1948–1992 was similar to that in the Caspian Sea and consistent with described combinations of indexes. However, after 1992 no pronounced trend in variability of NorthEastern transport have been observed.

Global coarse-grained mesoscale eddy statistics based on integrated kinetic energy and enstrophy correlations

Abstract. Recently, Jánosi et al. (2019) introduced the concept of a “vortex proxy” based on an observation of strong correlations between integrated kinetic energy and integrated enstrophy over a large enough surface area. When mesoscale vortices are assumed to exhibit a Gaussian shape, the two spatial integrals have particularly simple functional forms, and a ratio of them defines an effective radius of a “proxy vortex”. In the original work, the idea was tested over a restricted area in the Californian Current System. Here we extend the analysis to global scale by means of 25 years of AVISO altimetry data covering the (ice-free) global ocean. The results are compared with a global vortex database containing over 64 million mesoscale eddies. We demonstrate that the proxy vortex representation of surface flow fields also works globally and provides a quick and reliable way to obtain coarse-grained vortex statistics. Estimated mean eddy sizes (effective radii) are extracted in very good agreement with the data from the vortex census. Recorded eddy amplitudes are directly used to infer the kinetic energy transported by the mesoscale vortices. The ratio of total and eddy kinetic energies is somewhat higher than found in previous studies. The characteristic westward drift velocities are evaluated by a time-lagged cross-correlation analysis of the kinetic energy fields. While zonal mean drift speeds are in good agreement with vortex trajectory evaluation in the latitude bands 30–5∘ S and 5–30∘ N, discrepancies are exhibited mostly at higher latitudes on both hemispheres. A plausible reason for somewhat different drift velocities obtained by eddy tracking and cross-correlation analysis is the fact that the drift of mesoscale eddies is only one component of the surface flow fields. Rossby wave activities, coherent currents, and other propagating features on the ocean surface apparently contribute to the zonal transport of kinetic energy.

Spatio-temporal signature of surface biophysical parameters in response to extreme IOD events using remote sensing

The Indian Ocean (IO) evidenced significant effects on regional and global ocean-atmospheric coupling variability in recent decades. Anomalous negative (positive) Indian Ocean Dipole (IOD) events witnessed along tropical IO during 2016 (2019) are one of them. The spatial signature of anomalous IOD events was studied using sea surface temperature anomaly (SSTA), sea surface height, sea surface salinity, chlorophyll-a, zonal and meridional wind components obtained from remote sensing data. The strong easterly wind anomaly blowing along the Sumatra coast causes strong coastal upwelling pioneer for the positive IOD event in 2019. The westward horizontal advection of seawater leads to low (high) sea surface height anomaly along eastern (western) tropical IO. The eastern (western) tropical IO becomes gradually cold (warm) during the mature phase (September–October–November) of positive IOD. Due to strong upwelling high and low sea surface salinity were observed along the eastern and western tropical IO respectively. The prolonged stronger upward (downward) Ekman pumping velocity in east (west) results in divergence (convergence) of seawater responsible for the increase (decrease) of chlorophyll-a and marine primary productivity. The reverse phenomena were characterized by warmer (cooler) SSTA, higher (lower) sea surface height, low (high) sea surface salinity, and decrease (increase) of chlorophyll-a in the eastern (western) tropical IO in the 2016 negative IOD year. The westward (eastward) zonal Ekman mass transport is possible fueling the extreme condition of positive (negative) IOD events. A strong eastward propagating downwelling Kelvin wave was observed due to the reflection of the upwelling Rossby wave from the African coast. Dominated picoplankton (microplankton) to other phytoplankton size classes are responsible for the reoccurrence of open ocean bloom along western (eastern) tropical IO during positive IOD.

Impact of a long-lived anticyclonic mesoscale eddy on seawater anomalies in the northeastern tropical Pacific Ocean: a composite analysis from hydrographic measurements, sea level altimetry data, and reanalysis model products

Abstract. Using observational data, satellite altimeters, and reanalysis model products, we have investigated eddy-induced seawater anomalies and heat and salt transport in the northeastern tropical Pacific Ocean. An eddy detection algorithm (EDA) was used to identify eddy formation at the Mexican Tehuantepec Gulf (TT) in July 2018 during an unusually strong summer wind event. The eddy separated from the coast with a mean translation velocity of 11 cm s−1 and a mean radius of 115 km and traveled 2050–2400 km westwards off the Central American coast, where it was followed at approx 114∘ W and 11∘ N for oceanographic observation between April and May 2019. The in situ observations show that the major eddy impacts are restricted to the upper 300 m of the water column and are traceable down to 1500 m water depth. In the eddy core at 92 m water depth an extreme positive temperature anomaly of 8.2 ∘C, a negative salinity anomaly of −0.78 psu, a positive fluorescence anomaly of +0.8 mg m−3, and a positive dissolved oxygen concentration anomaly of 137 µmol kg−1 are observed. Compared with annual climatological averages in 2018, the water trapped within the eddy is estimated to transport an average positive westward zonal heat anomaly of 85×1012 W and an average westward negative salt anomaly of -2.1×106 kg s−1. The heat transport is the equivalent of 1 % of the total annual zonal eddy-induced heat transport at this latitude in the Pacific Ocean. Understanding the dynamics of long-lived mesoscale eddies that may reach the seafloor in this region of the Pacific Ocean is especially important in light of potential deep-sea mining activities that are being targeted on this area.

Large-Scale Meteorological Drivers of the Extreme Precipitation Event and Devastating Floods of Early-February 2021 in Semarang, Central Java, Indonesia

Unusually long duration and heavy rainfall from 5 to 6 February 2021 caused widespread and devastating floods in Semarang, Central Java, Indonesia. The heavy rainfall was produced by two mesoscale convective systems (MCSs). The first MCS developed at 13Z on 5 February 2021 over the southern coast of Sumatra and propagated towards Semarang. The second MCS developed over the north coast of Semarang at 18Z on 5 February 2021 and later led to the first peak of precipitation at 21Z on 5 February 2021. These two MCSs eventually merged into a single MCS, producing the second peak of precipitation at 00Z on 6 February 2021. Analysis of the moisture transport indicates that the strong and persistent north-westerly wind near the surface induced by CENS prior to and during the event created an intensive meridional (southward) tropospheric moisture transport from the South China Sea towards Semarang. In addition, the westerly flow induced by low-frequency variability associated with La Nina and the tropical depression over the North of Australia produced an intensive zonal (eastward) tropospheric moisture transport from the Indian Ocean towards Semarang. The combined effects of the zonal and meridional moisture transport provided favorable conditions for the development of MCSs, and hence extreme rainfall over Semarang. These results provide useful precursors for extreme weather-driven hazard prediction in Semarang and the surrounding regions in the future.

A Laboratory Model for a Meandering Zonal Jet

AbstractThe meandering jet streams of the Northern Hemisphere influence the weather for more than half of Earth's population, so it is imperative that we improve our understanding of their behavior and how they respond to climate change. Here, we describe a novel laboratory model for a meandering zonal jet. This model comprises a large rotating annulus with a series of topographic ridges, and an imposed radial vorticity flux. Flow interactions with the topographic ridges operate to concentrate the zonal transport into a narrow jet, which supports the development and propagation of Rossby waves. We investigate the dynamics of the jet for a range of rotation rates, imposed radial vorticity fluxes, and topographic ridge configurations. The circulations are classified into two distinct regimes: predominantly zonal or predominantly meandering. The flow regime can be quantified by the ratio of the Ekman dissipation and jet advection timescales, which gives an indication of whether disturbances arising from the flow‐topography interaction are dissipated faster than the time taken to circuit the annulus; if not, these disturbances will reencounter the topography, and thus be reinforced and amplified. For predominantly zonal flows, the radial vorticity flux is split equally between the standing meanders and transient eddies. For predominantly meandering flows, standing meanders perform 79% of the radial vorticity flux, with 18% accommodated by the transient eddies. Our experiments indicate that the Arctic amplification associated with climate change will tend to favor predominantly zonal flow conditions, suggesting a reduced occurrence of atmospheric blocking events caused by the jet streams.

Three-dimensional structure and transport pathways of dust aerosols over West Asia

AbstractThis study investigates the seasonal climatology of the three-dimensional distribution and transport pathways of dust aerosols over West Asia (WA). Dust column loading over WA exhibits strong seasonality, with markedly high (weak) loading during summer (winter). The summer dust features over WA include the (i) dust reaching up to the 500 hPa level between the Red Sea (RS) and the west coast of the Indian subcontinent (IS); (ii) a slantwise advection of dust aerosols between 850 and 700 hPa levels over the Arabian Peninsula (AP) and Arabian Sea (AS); and (iii) a prominent mid-tropospheric zonal transport of AP dust toward the IS. Maximum column integrated horizontal dust mass flux (DMF) over WA is observed in summer. The intraday changes in the intensity and spatial spread of the DMF over the AP are mediated by the out-of-phase evolution of the surface winds and low-level Shamal jets. Furthermore, the diurnal changes in the strength of the inversion layers located above the monsoon boundary layer and associated wind shear regulate the spatial patterns and intensity of the DMF over the AS. The findings will support future studies aiming at quantifying the radiative effects of dust on the regional climate.

Zonal and vertical transports of Fukushima-derived radiocesium in the subarctic gyre of the North Pacific until 2014

The Fukushima Dai-ichi Nuclear Power Plant (FNPP1) accident in March 2011 resulted in serious radiocesium contamination of the North Pacific Ocean. Most of the radiocesium was dissolved in seawater and transported by surface currents and subduction of mode waters. Within several years after the accident, a high-concentration water plume of the FNPP1-derived radiocesium at the sea surface had been transported from Japan to the North American continent across the subarctic gyre of the North Pacific Ocean. We measured vertical profiles of dissolved radiocesium along the nominal 47°N zonal line across the North Pacific subarctic gyre twice, in summer 2012 and summer 2014. Using these data and published data, we quantitatively discussed the zonal and vertical transports of the water plume until 2014. The FNPP1-derived radiocesium remained in the surface layer shallower than 200 m, which is the approximate winter mixed-layer depth in the western subarctic gyre. The mean penetration depth did not change between 2012 and 2014. The highest concentration was observed at 180°W in 2012 and at 151°W in 2014, which suggests that the zonal transport speed of the water plume in the eastern subarctic gyre was about 3.8 cm s−1. By combining the data from the zonal line in 2014 and a nominal 152°W meridional line in 2015, we elucidated the three-dimensional size of the high-concentration water plume in summer 2014. The total inventory of the FNPP1-derived radiocesium in the subarctic North Pacific Ocean, decay-corrected to the accident date, was estimated to be 12.0 ± 2.4 PBq.

Bioaerosols as Evidence of Atmospheric Circulation Anomalies over the Okhotsk Sea and Shantar Islands in the Late Glacial–Holocene

Allochthonous biofossil distribution in the blanket peat bog of Bolshoy Shantar Island was used to analyze atmospheric circulation anomalies in the north-western Okhotsk Sea over the last 12.6 ka. The main aim of this study was to determine periods of intensification of deep cyclones and extreme storms. The composition of bioaerosols is significantly influenced by atmospheric zonal and meridional transport anomalies associated with anomalies of the monsoon system of Northeast Asia, atmospheric fronts and cyclone trajectories. Marine diatoms enter the peatland from the sea during extreme storms and record the passage of sea cyclones in the autumn-winter, whereas the distribution of allochthonous pollen indicates the intensity of continental cyclones. We used Pinus pumila pollen as an indicator of heavy snowfalls and winter cyclone activity. Fifteen phases of extreme storms were identified. Changes in ice coverage also played an important role in bioaerosol emission. During cold periods, emissions of bioaerosols mainly occurred in the open sea, whereas during warm periods, emissions occurred near the coast. The recurrence and intensity of cyclones during the cold seasons depends on displacement of the Siberian High and Aleutian Low. Periods of continental cyclones intensified in spring-summer and coincided with periods of active winter cyclogenesis.

Moisture origin, transport pathways, and driving processes of intense wintertime moisture transport into the Arctic

Abstract. A substantial portion of the moisture transport into the Arctic occurs in episodic, high-amplitude events with strong impacts on the Arctic's climate system components such as sea ice. This study focuses on the origin of such moist-air intrusions during winter and examines the moisture sources, moisture transport pathways, and their linkage to the driving large-scale circulation patterns. For that purpose, 597 moist-air intrusions, defined as daily events of intense (exceeding the 90th anomaly percentile) zonal mean moisture transport into the polar cap (≥70∘ N), are identified. Kinematic backward trajectories combined with a Lagrangian moisture source diagnostic are then used to pinpoint the moisture sources and characterize the airstreams accomplishing the transport. The moisture source analyses show that the bulk of the moisture transported into the polar cap during these moist-air intrusions originates in the eastern North Atlantic with an uptake maximum poleward of 50∘ N. Trajectories further reveal an inverse relationship between moisture uptake latitude and the level at which moisture is injected into the polar cap, consistent with ascent of poleward-flowing air in a baroclinic atmosphere. Focusing on intrusions in the North Atlantic (424 intrusions), we find that lower tropospheric moisture transport is predominantly accomplished by two types of airstreams: (i) cold, polar air warmed and moistened by surface fluxes and (ii) air subsiding from the mid-troposphere into the boundary layer. Both airstreams contribute about 36 % each to the total transport. The former accounts for most of the moisture transport during intrusions associated with an anomalously high frequency of cyclones east of Greenland (218 intrusions), whereas the latter is more important in the presence of atmospheric blocking over Scandinavia and the Ural Mountains (145 events). Long-range moisture transport, accounting for 17 % of the total transport, dominates during intrusions with weak forcing by baroclinic weather systems (64 intrusions). Finally, mid-tropospheric moisture transport is invariably associated with (diabatically) ascending air and moisture origin in the central and western North Atlantic, including the Gulf Stream front, accounting for roughly 10 % of the total transport. In summary, our study shows that moist-air intrusions into the polar atmosphere result from a combination of airstreams with predominantly high-latitude or high-altitude origin, whose relative importance is determined by the underlying driving weather systems (i.e., cyclones and blocks).

Analyzing Variations in the Association of Eurasian Winter–Spring Snow Water Equivalent and Autumn Arctic Sea Ice

Because Eurasian snow water equivalent (SWE) is a key factor affecting the climate in the Northern Hemisphere, understanding the distribution characteristics of Eurasian SWE is important. Through empirical orthogonal function (EOF) analysis, we found that the first and second modes of Eurasian winter SWE present the distribution characteristics of an east–west dipole and north–south dipole, respectively. Moreover, the distribution of the second mode is caused by autumn Arctic sea ice, with the distribution of the north–south dipole continuing into spring. As the sea ice of the Barents–Kara Sea (BKS) decreases, a negative-phase Arctic oscillation (AO) is triggered over the Northern Hemisphere in winter, with warm and humid water vapor transported via zonal water vapor flux over the North Atlantic to southwest Eurasia, encouraging the accumulation of SWE in the southwest. With decreases in BKS sea ice, zonal water vapor transport in northern Eurasia is weakened, with meridional water vapor flux in northern Eurasia obstructing water vapor transport from the North Atlantic, discouraging the accumulation of SWE in northern Eurasia in winter while helping preserve the cold climate of the north. The distribution characteristics of Eurasian spring SWE are determined primarily by the memory effect of winter SWE. Whether analyzed through linear regression or support vector machine (SVM) methods, BKS sea ice is a good predictor of Eurasian winter SWE.

Cases of mass development of intrusions in Lake Baikal and the correlation of intrusions with atmospheric circulation processes

Abstract. The cases of the development of mass intrusions in some basins of Lake Baikal were reviewed based on the measurements of water temperature and dissolved oxygen in water in 1993-2022. It was shown that in Southern, Middle and Northern Baikal during the development of intrusions in the spring-summer period, the heat deficit value was within 0.5-103.0, 3.1-15.8, 0.2-3.6 MJ/m2, dissolved oxygen amount was 0.2-161.3, 14.7-46.0, 0.6-6.3 g/m2 and the renewed bottom layer thickness was 36-318, 48-176, 38-94 m respectively. In winter in Southern Baikal, the heat deficit value varied within 0.8-5.2 MJ/m2, the amount of dissolved oxygen was 9.9-19.2 g/m2, and the thickness of the renewed bottom layer was 24-124 m. Analysis of the correlation between the heat deficit values and the circulation indices revealed that mass intrusions in Southern and Northern Baikal during the spring-summer period were associated with the prevailing meridional transport of air masses from the Eastern Atlantic and blocking zonal transport of air masses from the Atlantic and Pacific Oceans. On the contrary, in Middle Baikal, mass intrusions during the spring-summer period were developing under the conditions of zonal transport of air masses simultaneously from the Eastern Atlantic and northern and western areas of the Pacific Ocean. The development of mass intrusions in Southern Baikal in winter was related to the intensification of circumpolar circulation and zonal transport of air masses from the Atlantic Ocean.

Tying policy to system: Does the Ross Sea region marine reserve protect transport pathways connecting the life history of Antarctic toothfish?

A central objective of the Ross Sea region Marine Protected Area (MPA) is to protect areas important to the life cycle of Antarctic toothfish (Dissostichus mawsoni), a top fish predator and by far the region’s most important commercial species. Juvenile toothfish predominate in deep basins along the inner continental shelf, whereas adults are found mostly along the continental slope and spawning areas on the Pacific-Antarctic Ridge. The inner basins connect to the continental slope via glacial troughs and predictable transport along each trough results in exchange with the Antarctic Slope Current as it flows westward. From the slope, two transport pathways, an eastern one from Iselin Bank and a western one that turns cyclonically along the flank of the Southeast Indian Ridge, connect northward to the Pacific-Antarctic Ridge, where the northern arm of the Ross Gyre and the Antarctic Circumpolar Current flow eastward. Using a circulation model to compare transport pathways connecting toothfish life history areas, we consider which inshore basins are likely most important in contributing to adult spawning aggregations; how transport pathways from each may be expected to influence distributions along the continental slope and Pacific-Antarctic Ridge; and how zonal transport pathways may promote export to areas downstream of the marine reserve. Although the MPA protects some critical life history pathways for toothfish, others remain vulnerable to commercial fishing, and we argue that those in adjacent areas along the Iselin Bank, Pacific-Antarctic Ridge and the Amundsen Sea might usefully be protected, discussing the range of policy instruments available. We also recommend consideration of transport pathways in deliberations for a proposed network of Southern Ocean MPAs, introducing a system-based tool using chemical tracers in otoliths that can test for toothfish movement between areas connected along the Antarctic Slope Current and Antarctic Circumpolar Current.

Impacts of multi‐timescale circulations on meridional heat transport

AbstractRelative contributions to the zonal mean meridional heat transport by the climatological annual mean, climatological annual variation, synoptic, intra‐seasonal and lower‐frequency motions were examined based on the ERA‐Interim reanalysis data for the period of 1981–2015. The meridional heat transport analysed in this study only includes the component related to meridional wind and temperature. In the tropics, the climatological annual mean circulations dominate the long‐term mean meridional heat transport, while the interaction between the climatological annual mean temperature and the seasonal anomalous flow largely contributes to the seasonal variation of the meridional heat transport. In the middle latitudes, the climatological annual mean circulations and transient eddies (mostly synoptic and intra‐seasonal eddies) are of roughly equal importance in the poleward heat transport, leading to the maximum poleward heat transport around 50°N/S. The upper‐ and lower‐tropospheric heat transports by the climatological annual mean circulations appear opposite, with the magnitude of the lower‐tropospheric transport being greater. The preferred maximum zonal mean heat transport at 50°N by the climatological mean flow is attributed to the maximum zonal mean low‐level southerly in situ. The preferred peak latitude of the mid‐latitude poleward heat transport by synoptic eddies near 50°N arises from the combined effect of the strong synoptic‐scale meridional wind and temperature variabilities in situ and their in‐phase relationship. The heat transport by tropical cyclones (TCs) was estimated by applying a statistical relationship between TC intensity and the vertically integrated temperature averaged over the TC core region derived from high‐resolution Weather Research and Forecasting (WRF) model simulations. For northern hemisphere summer, TCs contribute about 35% of the total heat transport in the active TC regions, suggesting that TCs play a critical role in the regional meridional heat transport.

The impact of wind corrections and ocean-current influence on wind stress forcing on the modeling of Pacific North Equatorial Countercurrent

Bias correction of reanalysis-based wind stress using scatterometer derived equivalent neutral wind has been a common practice in producing the forcing datasets used in recent global ocean model intercomparisons (OMIPs). Here we systematically evaluate the effect of this wind correction procedure on the simulation of the Pacific North Equatorial Countercurrent (NECC) with multiple sets of model experiments. The weak NECC evident in earlier OMIPs employing the Coordinated Ocean-ice Reference Experiments (COREs) forcing dataset persists with the new JRA55-do (Japanese 55-year Reanalysis) forcing dataset. Two factors appear to significantly affect the Pacific NECC in forced ocean simulations: i) the bias correction procedure using QuikSCAT derived winds and ii) whether or not the ocean current is considered in the bulk formula. In the forced ocean simulations, the QuikSCAT correction weakens the averaged NECC transports by about 60%. Taking the ocean currents into account in the bulk formula may weaken the averaged NECC transports by about 26%–30%. Under the current OMIP protocol the above two procedures are used together to force the ocean model resulting in a double-counting of ocean surface current feedback on wind stress because the QuikSCAT estimates the equivalent 10-m neutral winds relative to surface current. We further systematically verify and investigate the impacts of this double-counting of the ocean surface currents on the modeled Pacific NECC using offline linear Sverdrup transport analysis, in which the observational data of vector wind and surface current are used to calculate the surface wind stress. It shows that including the ocean current in the bulk formula may reduce the zonal Sverdrup transport (ZST) by about 6.6 Sv (33%) and the further double-counting of the ocean current leads to an additional reduction of 6.4 Sv (48%). Next, using “perfect model” experiments with output from a coupled ocean–atmosphere model we further identify that the double-counting of current feedback in the bulk formula results in approximately 21% weakened volume transport. The built-in nonlinear processes in the model, such as the advection and friction terms, may partly damp the reduction due to the double-counting of the ocean current. However, the double-counting bias can only explain 26%–30% of the Pacific NECC simulation bias and the other part of the bias, around 30%, caused by the correction with QuikSCAT has not been explained. We speculate that this part may be explained by the retrieval biases in QuikSCAT wind data and the use of annual mean climatological wind adjustment factors.