Meridional Pressure Gradient Research Articles

Rapid Dynamical Evolution of ITCZ Events over the East Pacific

Abstract The latitudinal location of the east Pacific Ocean intertropical convergence zone (ITCZ) changes on time scales of days to weeks during boreal spring. This study focuses on tropical near-surface dynamics in the days leading up to the two most frequent types of ITCZ events, nITCZ (Northern Hemisphere) and dITCZ (double). There is a rapid daily evolution of dynamical features on top of a slower, weekly evolution that occurs leading up to and after nITCZ and dITCZ events. Zonally elongated bands of anomalous cross-equatorial flow and off-equatorial convergence rapidly intensify and peak 1 day before or the day of these ITCZ events, followed 1 or 2 days later by a peak in near-equatorial zonal wind anomalies. In addition, there is a wide region north of the southeast Pacific subtropical high where anomalous northwesterlies strengthen prior to nITCZ events and southeasterlies strengthen before dITCZ events. Anomalous zonal and meridional near-surface momentum budgets reveal that the terms associated with Ekman balance are of first-order importance preceding nITCZ events, but that the meridional momentum advective terms are just as important before dITCZ events. Variations in cross-equatorial flow are promoted by the meridional pressure gradient force (PGF) prior to nITCZ events and the meridional advection of meridional momentum in addition to the meridional PGF before dITCZ events. Meanwhile, variations in near-equatorial easterlies are driven by the zonal PGF and the Coriolis force preceding nITCZ events and the zonal PGF, the Coriolis force, and the meridional advection of zonal momentum before dITCZ events.

Wind–SST Dipole Mode in the Caribbean and Gulf of Mexico: large-scale features and drivers

The Dipole Mode (DM) is the leading pattern of springtime wind–SST coupled interannual variability in the Intra-Americas Seas, characterized by SST anomalies of opposite sign between the Caribbean Sea and the Gulf of Mexico. Using the standard deviation (STD) of the wind in a Maximum Correlation Analysis (MCA), this study aims to provide a more dynamic view of the role of the atmosphere in its coupling with the SST. The MCA reveals that the positive phase of the DM is associated with an increase in atmospheric instability, while the negative phase emerges under more stable atmospheric conditions. The DM is preceded by changes in the subtropical high-pressure belt during the previous winter, particularly in the North Atlantic Subtropical High (NASH), and reflects shifts in the latitudinal position of the subtropical jet stream. The DM positive phase tends to occur after an El Niño winter, under negative North Atlantic Oscillation (NAO) conditions. El Niño modulates the DM through a weakening in the meridional pressure gradient and a southward shift of the jet stream. A negative NAO implies a weaker NASH and, therefore, a more irregular circulation over the region. Both El Niño and negative NAO conditions favor the increase in wind STD during the DM positive phase, consistent with an increment in atmospheric disturbances. The DM negative phase responds more to a positive NAO in the previous winter, revealing a stronger NASH acting as an atmospheric block, which justifies the decrease in STD and a more stable circulation.

A dissection of the topographic effects from Eurasia and North America on the isentropic meridional mass circulation in Northern Winter

The topographic dynamical effect from Eurasia (EA_Topo) and North America (NA_Topo) on the winter isentropic meridional mass circulation (IMMC) is investigated using the WACCM. The independent effect of EA_Topo and that of NA_Topo, with the former much stronger, are both to strengthen the IMMC that is composed of the lower equatorward cold air branch (CB) and the upper poleward warm air branch in the extratropical tropopshere (WB_TR) and stratosphere (WB_ST). Further investigation of the individual contributions from changes in stationary vs. transient and zonal-mean flow vs. waves reveals that, due to the topography-forced mass redistribution, changes in the low-level meridional pressure gradient force a zonal-mean counter-clockwise/clockwise meridional cell in the southern/northern side of topography. This weakens/strengthens the IMMC south/north of 30° N from the troposphere to lower stratosphere, acting as a dominant contributor to the IMMC changes south of 50° N. Meanwhile, the EA/NA_Topo-forced amplification of stationary waves constructively interacts with those determined by land-sea contrast, making the dominant/minor contributions to the strengthening of CB and WB_TR north of 50° N. The related increase in the upward wave propagation further dominates the WB_ST strengthening in the subpolar region. Meanwhile, transient eddy activities are depressed by EA/NA_Topo along with the weakened background westerly, which partly-offset/dominate-over the contribution from stationary flow in midlatitudes and subpolar region. The coexistence of the other topography (NA/EA_Topo) yields destructive mutual interferrence, which can weaken/offset the independent-EA/NA_Topo-forced meridional mass transport mainly via changing the zonal-mean as well as the downstream wave pattern of mass and meridional wind.

Evolution of large‐scale factors influencing extreme rainfall over south western coast of India

AbstractThe life cycle and the large‐scale factors driving extreme heavy rainfall events over the south west coast of India are studied. The extreme rainfall events are linked to the development of monsoon depressions and the associated large‐scale dynamics. Strengthening of these parameters intensifies the monsoon low‐level circulation over the Arabian Sea and the west coast via steepened meridional pressure gradient. The intensification of the low‐level jet stream speed and its extension in the vertical causes an increase in the humidity flux in the lower and midtroposphere. The consequent ascending motion is from the midtroposphere to the upper troposphere, resulting in the formation of deep convective cloud clusters over the west coast and eastern parts of the Arabian Sea. This results in the incidence of extreme heavy rainfall over the south west coast of India. It is observed that during days of extreme rainfall, the direction of wind in the lower troposphere tends to be almost perpendicular to the Western Ghats favouring a strong orographic lift. The extreme rainfall events over the south west coast do not necessarily occur during the active cycle of monsoon intraseasonal oscillation, but are linked to the north westwards propagating monsoon depressions. We show that the signatures of extreme rainfall can be observed in several meteorological variables developing over different parts of the monsoon region. A synergistic analysis of these variables may help in the accurate and timely prediction of these events.

The Canary Intermediate Poleward Undercurrent: Not Another Poleward Undercurrent in an Eastern Boundary Upwelling System

AbstractPoleward undercurrents are well-known features in eastern boundary upwelling systems. In the California Current upwelling system, the California poleward undercurrent has been widely studied, and it has been demonstrated that it transports nutrients from the equatorial waters to the northern limit of the subtropical gyre. However, in the Canary Current upwelling system, the Canary intermediate poleward undercurrent (CiPU) has not been properly characterized, despite recent studies arguing that the dynamics of the eastern Atlantic Ocean play an important role in the Atlantic meridional overturning circulation, specifically on its seasonal cycle. Here, we use trajectories of Argo floats and model simulations to characterize the CiPU, including its seasonal variability and its driving mechanism. The Argo observations show that the CiPU flows from 26°N, near Cape Bojador, to approximately 45°N, near Cape Finisterre and flows deeper than any poleward undercurrent in other eastern boundaries, with a core at a mean depth of around 1000 dbar. Model simulations manifest that the CiPU is driven by the meridional alongshore pressure gradient due to general ocean circulation and, contrary to what is observed in the other eastern boundaries, is still present at 1000 dbar as a result of the pressure gradient between the Antarctic Intermediate Waters in the south and Mediterranean Outflow waters in the north. The high seasonal variability of the CiPU, with its maximum strength in autumn and minimum in spring, is due to the poleward extension of AAIW, forced by Ekman pumping in the tropics.

Contrasting northern and southern European winter climate trends during the Last Interglacial

Abstract The Last Interglacial (LIG; 130–115 ka) is an important test bed for climate science as an instance of significantly warmer than preindustrial global temperatures. However, LIG climate patterns remain poorly resolved, especially for winter, affected by a suite of strong feedbacks such as changes in sea-ice cover in the high latitudes. We present a synthesis of winter temperature and precipitation proxy data from the Atlantic seaboard of Europe, spanning from southern Iberia to the Arctic. Our data reveal distinct, opposite latitudinal climate trends, including warming winters seen in the European Arctic while cooling and drying occurred in southwest Europe over the LIG. Climate model simulations for 130 and 120 ka suggest these contrasting climate patterns were affected by a shift toward an atmospheric circulation regime with an enhanced meridional pressure gradient and strengthened midlatitude westerlies, leading to a strong reduction in precipitation across southern Europe.

Role of unusual moisture transport across Equatorial Indian Ocean on the extreme rainfall event during Kerala flood 2018

Southwestern Indian state, Kerala experienced extreme devastating statewide flood event of the century during 2018 monsoon season. In this study, an attempt has been made to bring out the salient dynamical factors contributed to the Kerala flood. There were 3 active spells over Kerala during 2018 Monsoon season. All the three spells were accustomed with the intrinsic factors of low frequency components of the active spells such as strength of monsoon Low Level Jet (LLJ), Monsoon depressions in the Bay of Bengal, favorable Madden-Julian oscillation (MJO) phases and Western Pacific systems. Though all the common ingredients remain same, the third spell is distinct with the less evaporation flux over Western and Central Arabian Sea and unusual moisture transport from maritime continent through South Equatorial Indian ocean (SEIO) towards the Kerala coast across Equator. Strong meridional pressure gradient force created by the combined effect of high pressure anomaly oriented Northwest-Southeast direction across tropical Indian ocean and anomalous low pressure along monsoon trough might have contributed to this unusual moisture transport across SEIO originating from west of Australia. The anomalous high pressure in South Indian ocean was greatly influenced by the position of the Mascarene high. Subtropical Indian ocean dipole (SIOD) also exhibits an influential role by altering tropical Southern Indian ocean dynamics in favor of the unusual moisture transport. The position of the monsoon depression and presence of typhoons in Western Pacific might have aided to this moisture transport. However, the normal moisture transport from Central Arabian sea towards Kerala coast by virtue of the strong LLJ along with additional moisture transport directly from South of maritime continent through SEIO across the Equator towards Kerala coast might have played a dominant role in the historical flood event over entire Kerala state.

Understanding the variability of the rainfall dipole in West Africa using the EC-Earth last millennium simulation

There is a well-known mode of rainfall variability associating opposite hydrological conditions over the Sahel region and the Gulf of Guinea, forming a dipole pattern. Previous meteorological observations show that the dipole pattern varies at interannual timescales. Using an EC-Earth climate model simulation for last millennium (850–1850 CE), we investigate the rainfall variability in West Africa over longer timescales. The 1000-year-long simulation data show that this rainfall dipole presents at decadal to multidecadal and centennial variability and long-term trend. Using the singular value decomposition (SVD) analysis, we identified that the rainfall dipole present in the first SVD mode with 60% explained variance and associated with the variabilities in tropical Atlantic sea surface temperature (SST). The second SVD mode shows a monopole rainfall variability pattern centred over the Sahel, associated with the extra-tropical Atlantic SST variability. We conclude that the rainfall dipole-like pattern is a natural variability mode originated from the local ocean–atmosphere-land coupling in the tropical Atlantic basin. The warm SST anomalies in the equatorial Atlantic Ocean favour an anomalous low pressure at the tropics. This low pressure weakens the meridional pressure gradient between the Saharan Heat Low and the tropical Atlantic. It leads to anomalous northeasterly, reduces the southwesterly moisture flux into the Sahel and confines the Gulf of Guinea's moisture convergence. The influence from extra-tropical climate variability, such as Atlantic multidecadal oscillation, tends to modify the rainfall dipole pattern to a monopole pattern from the Gulf of Guinea to Sahara through influencing the Sahara heat low. External forcing—such as orbital forcing, solar radiation, volcanic and land-use—can amplify/dampen the dipole mode through thermal forcing and atmosphere dynamical feedback.

Characteristics and causes of surface wind speed variations in Northwest China from 1979 to 2019

New characteristics of surface wind speed variations in Northwest China and the possible explanations for them have remained undetermined over the recent decades. This study presents the characteristics and possible mechanisms of surface wind speed variations in Northwest China from 1979 to 2019. The results show that the surface wind speed in Northwest China underwent a two-stage change rather than a persistent decline from 1979 to 2019; specifically, it declined significantly before the early 2000s and then increased gradually thereafter. The meridional pressure gradient decrease caused by the meridional temperature gradient reduction corresponded to a decrease in the winter surface wind speed in Northwest China during 1979–2002. The meridional pressure gradient increase caused by the meridional temperature gradient increase was consistent with an increase in the winter surface wind speed in Northwest China during 2003–2019. This suggests that the spatially inhomogeneous variations in surface air temperature in Eurasia's mid-high latitudes may be the main reason for the surface wind speed variations in Northwest China. Furthermore, the decadal strengthening of the East Asian winter monsoon (EAWM) may have resulted in an upward trend of the surface wind speed in Northwest China after the early 2000s by strengthening the Siberian High (SH). Although the variations in the surface wind speed in Northwest China are inconsistent with those in the winter Arctic Oscillation (AO) index, there is a significant negative correlation between them on the interannual timescale. With a negative (positive) AO phase, the surface wind speed in Northwest China increases (decreases) in winter.

Synoptic pattern of the Red Sea trough associated with spring dust over the northern and western Arabian Peninsula

From 1979 to 2006, the Red Sea trough (RST) in spring was objectively detected and classified into three categories (short, moderate and long) depending on its northward extension. The results indicated that most short RSTs appeared during a cold spring (larger by 7% in March than May), while most long RSTs appeared during a warm spring, where reached 71.7% in April and May. The dust events during the study period were identified using a threshold value of the Total Ozone Monitoring Satellite (TOMS) aerosol index (AI). The identified dust cases were used to classify each RST category into two classes, dust RSTs and non-dust RSTs. The synoptic characteristics of the RSTs associated with dust events that influenced the northern and western Arabian Peninsula (AP) were studied using each dust category (i.e., comparing dust and non-dust cases) and were compared between the different categories (i.e., comparing the dust cases among short, moderate and long categories). The synoptic study shows that the condition favorable for transporting dust westward into the AP occurs when an interaction is produced between the low pressure over the southern AP and Sudan low and forms a wavy zonal pressure gradient. When the RST is relatively strong, i.e., the situation has a pronounced wavy zonal and meridional pressure gradient area, it produces conditions favorable for transporting dust around the Red Sea. Commonly, dust RSTs are atmospherically integrated systems with pronounced circulation (downward motion in the eastern AP and upward motion in the western AP), a vertical westward trough tilt, horizontal tropical–extratropical interactions and north–south orientations of pressure/geopotential isolines.

Changes in the characteristics of \u2018wet\u2019 and \u2018dry\u2019 Red Sea Trough over the Eastern Mediterranean in CMIP5 climate projections

The Eastern Mediterranean resides on the border between the temperate and semi-arid and arid climate zones, and is thus influenced by both mid-latitude and sub-tropical weather systems. Precipitation and extreme weather in this region are mainly associated with either Cyprus Lows or the “wet” Red Sea Troughs. Current regional climate projections indicate that the region may become warmer and drier in future decades. Here, we analyze the influence of enhanced greenhouse gas forcing on the climatological properties of the ‘wet’ and ‘dry’ Red Sea Trough (WRST & DRST, respectively). With this aim, a regional synoptic classification and a downscaling algorithm based on past analogs are applied to eighteen rain stations over the main ground water basins in Israel. The algorithms are applied to the NCEP/NCAR reanalysis data for 1986–2005 and to eight CMIP5 model simulations for the historical (1986–2005) and end of the century (2081–2100) climate conditions according to the RCP8.5 scenario. For the historical period, the CMIP5 models are largely able to represent the characteristics of the Red Sea Trough. Based on the multi-model mean, significant changes are found for WRST and DRST for the late XXI Century. First, an increase in the meridional pressure gradient is found for both the WRST and the DRST, implying stronger horizontal winds. Furthermore, a significant decrease in the occurrence of the WRST (− 20%) and a significant increase in the frequency of the DRST (+ 19%) are identified. Accordingly, the persistence of the WRST decreases (− 9%), while for DRST increases (+ 9%). The decline in the frequency of WRST occurs primarily in the transition seasons, while the increase for DRST is found throughout the wet season. In total, the daily rainfall associated with the WRST system is projected to significantly decline (− 37%) by the end of the XXI century. These results document the projected changes in a dominant synoptic system in this area, which can facilitate a better estimation of the arising challenges, e.g., related to shortage of water resources and associated political unrest, reduced agricultural potential, and increased air pollution and forest fires. Such a pathway can ultimately foster novel mitigation strategies for water resources management and regional climate change adaptation.

Summer Westerly Jet in Northern Hemisphere during the Mid-Holocene: A Multi-Model Study

The upper-level jet stream, a narrow band of maximum wind speed in the mid-latitude westerlies, exerts a considerable influence on the global climate by modulating the transport and distribution of momentum, heat and moisture. In this study by using four high-resolution models in the Paleoclimate Modelling Intercomparison Project phase 3, the changes of position and intensity of the northern hemisphere westerly jet at 200 hPa in summer during the mid-Holocene (MH), as well as the related mechanisms, are investigated. The four models show similar performance on the westerly jet. At the hemispheric scale, the simulated westerly jet has a poleward shift during the MH compared to the preindustrial period. The warming in arctic and cooling in the tropics during the MH are caused by the orbital changes of the earth and the precipitation changes, and it could lead to the weakened meridional temperature gradient and pressure gradient, which might account for the poleward shift of the westerly jet from the thermodynamic perspective. From the dynamic perspective, two maximum centers of eddy kinetic energy are simulated over the North Pacific and North Atlantic with the north deviation, which could cause the northward movement of the westerly jet. The weakening of the jet stream is associated with the change of the Hadley cell and the meridional temperature gradient. The largest weakening is over the Pacific Ocean where both the dynamic and the thermodynamic processes have weakening effects. The smallest weakening is over the Atlantic Ocean, and it is induced by the offset effects of dynamic processes and thermodynamic processes. The weakening over the Eurasia is mainly caused by the dynamic processes.

Interdecadal enhancement in the interannual variability of the summer monsoon meridional circulation over the South China Sea around the early 1990s

The interdecadal enhancement in the interannual variability of summer monsoon meridional circulation (SMMC) over the South China Sea around the early 1990s is investigated. Results show the change in the SMMC variability may arise from the interdecadal shift in the leading modes of low-level geopotential height over East Asia–Australia and Indo–Pacific sea surface temperature anomalies (SSTAs) in boreal summer. Before the early 1990s, the leading mode of Indo–Pacific SSTAs shows a zonal tripole pattern, with abnormally warm eastern Pacific and northern Indian Ocean and cold western Pacific. At the lower level, the western North Pacific cooling and northern Indian Ocean warming generate an anticyclonic anomaly over western North Pacific, while the cooling over the Maritime Continent and east of Australia favors an abnormal anticyclone over Australia. Hence region-wide positive geopotential height anomalies cover East Asia–Australia, which resemble the major mode of geopotential height and generate weak south–north pressure gradient and SMMC variability. After the early 1990s, the leading SSTAs mode shifts to a zonal dipole with abnormally cold western Pacific and warm equatorial central–eastern Pacific. The central Pacific warming induces an anomalous low-level cyclone over Philippines and it is further maintained by the Maritime Continent cooling. Meanwhile, the cooling over the east of Australia and Maritime Continent favors an abnormal Australian anticyclone. The low-level geopotential height thus shows south–north dipole anomalies over East Asia–Australia, resembling its major mode and generating obvious meridional pressure gradient and SMMC variability. The atmospheric responses to different SSTAs modes are confirmed by CAM4 experiments.

On the Formation Dynamics of the North Equatorial Undercurrent

AbstractBased on a physics-oriented modeling study, we investigate the underlying forcing processes of the North Equatorial Undercurrent (NEUC). Made up of large-scale (~90%) and mesoscale (~10%) components, the NEUC weakens eastward with a longitude-independent seasonality. The large-scale component reflects the effect of the meridional baroclinic pressure gradient force (PGF_BC). The vertical velocity shear forms the eastward NEUC, when the PGF_BC exceeds the meridional barotropic pressure gradient force (PGF_BT). The mesoscale variability with alternating jets is linked to the wind stress curl in different regions of the tropical North Pacific. Spatially, the NEUC has a northern (NEUC_N) and a southern branch (NEUC_S), which are mainly attributed to the transports from Luzon Undercurrent (LUC) and Mindanao Undercurrent (MUC), respectively. The LUC of ~3 Sv (1 Sv ≡ 106 m3 s−1) feeds the NEUC_N in summer, while the MUC of ~4 Sv fuels the NEUC_S in autumn and the two branches do not coexist. The total NEUC transport peaks in August/September, and there exist three distinct periods in a 1-yr cycle: the non-NEUC period in winter, the LUC-driven period in summer, and the MUC-driven period in autumn. Based on the layer-integrated vorticity equation, we diagnose quantitatively that the variation of the NEUC is dominated by the lateral planetary vorticity influx from the LUC and the MUC. These external influxes interact with the internal dynamics of pressure torques and stress curls in the NEUC layer, to jointly govern the NEUC and its variability. Meanwhile, the nonlinearity due to relative vorticity advection near the coast modulates the strength of the NEUC.

A pole-to-pole pressure–temperature map of Saturn’s thermosphere from Cassini Grand Finale data

Temperatures of the outer planet thermospheres exceed those predicted by solar heating alone by several hundred degrees. Enough energy is deposited at auroral regions to heat the entire thermosphere, but models predict that equatorward distribution is inhibited by strong Coriolis forces and ion drag. A better understanding of auroral energy deposition and circulation are critical to solving this so-called energy crisis. Stellar occultations observed by the Ultraviolet Imaging Spectrograph instrument during the Cassini Grand Finale were designed to map the thermosphere from pole to pole. We analyze these observations, together with earlier observations from 2016 and 2017, to create a two-dimensional map of densities and temperatures in Saturns thermosphere as a function of latitude and depth. The observed temperatures at auroral latitudes are cooler and peak at higher altitudes and lower latitudes than predicted by models, leading to a shallower meridional pressure gradient. Under modified geostrophy, we infer slower westward zonal winds that extend to lower latitudes than predicted, supporting equatorward flow from approximately 70 to 30 degrees latitude in both hemispheres. We also show evidence of atmospheric waves in the data that can contribute to equatorward redistribution of energy through zonal drag.

Remote Thermodynamic Impact of the Kuroshio Current on a Developing Tropical Cyclone Over the Western North Pacific in Boreal Fall

AbstractTo examine the remote linkage between tropical cyclone (TC) development and moisture transport from the Kuroshio Current toward the TC, we conducted sensitivity experiments that modified the surface latent heat flux (LHF) over the Kuroshio, applying a cloud‐resolving regional model and Lagrangian diagnostics to Typhoon Chaba as it approached Japan in October 2010. The synoptic environment was characterized by a combination of an anticyclone of continental origin around Japan and the northward‐migrating TC, bringing about low‐level northeasterlies over the Kuroshio through an enhanced meridional pressure gradient. When the LHF over the Kuroshio was artificially reduced or removed under this environmental flow, the intensity of the TC, which was fully away from the domain in which the LHF is modified, appreciably attenuated during its developing stage, implying the importance of moisture influx due to the environmental flow in TC development. Since dry air of midlatitude origin could receive little of the moisture supply from the Kuroshio in the sensitivity experiments, the values of equivalent potential temperature within the inflow layer tend to decrease, thus facilitating the weakening of convective updrafts and downdrafts around the eyewall of the TC. It is suggested that such changes act as an inhibitory factor for TC development. The Kuroshio has the potential to remotely influence the TC intensity over the western North Pacific if favorable synoptic conditions are satisfied.

The Spatial Pattern of Midsummer Drought as a Possible Mechanistic Response to Lower-Tropospheric Easterlies over the Intra-Americas Seas

Abstract Following the idea that large-scale wind perturbations cause repeatable rainfall patterns over small tropical islands, the spatial pattern of the midsummer drought (MSD) is investigated as a repeatable rainfall pattern over the Intra-Americas Seas (IAS). For that, statistical techniques, including linear regressions, canonical correlation analysis, and variance budgets, were applied to the Tropical Rainfall Measuring Mission and ERA-Interim datasets to assess 1) the MSD pattern repeatability and 2) its explained variance in different time scales. As shown by the results, the MSD pattern is not a unique feature of the boreal summer intraseasonal variability in the IAS: it is more robust during summer but it exists in all rainy seasons on daily, intraseasonal, and interannual time scales. On diurnal time scales, the MSD pattern explains a negligible part of the total variance during summer (<2%), but on interannual scales it explains up to 20% and it captures the spatial features of “El Niño” rainfall anomalies. On all time scales, the MSD pattern is accompanied by repeatable wind and pressure patterns: anomalous lower-tropospheric (925 hPa) easterlies over a domain-wide meridional northward pressure gradient. These results provide evidence for the hypothesis that the MSD pattern manifests an underlying geographically determined, mechanistic pattern. Also, they suggest that the repeatable MSD-shaped rainfall and wind patterns could be extrapolated in time to better understand the climatic conditions behind droughts and pluvials, and to diagnose the causes behind rainfall trends.

The Effects of Spring and Winter Blocking on PM10 Concentration in Korea

Atmospheric blocking is known to be related to locally developed abnormal climate of the Korean Peninsula, such as heat and cold waves. However, little attention has been devoted to the effects of blocking on the fine dust concentration over Korea. In this study, we analyze the connection between the monthly frequency of blocking occurrence and high particulate matter smaller than 10 μm in diameter (PM10) concentration events in Korea. Our analyses are limited to winter (DJF) and spring (MAM) when both the blocking and high PM10 events tend to take place most frequently. During winter, significant and positive correlations are found in the North Pacific, while significantly negative values are seen over Ural-Siberia. A high PM10 concentration tends to be accompanied by negative (positive) pressure anomalies over Siberia (North Pacific). However, while examining atmospheric fields associated with the high PM10 events, we find that the high PM10 events show two different types of atmospheric fields depending on the strength of wind speed over Korea. When the local wind speed over Korea is weaker than normal, the connection with the North Pacific blocking is strengthened, whereas, the connection with the Ural-Siberia blocking is enhanced when the wind speed is stronger than normal. However, to increase the occurrence of the high PM10 events, both types feature a strong meridional pressure gradient caused by a high pressure system forming over Siberia. This is accompanied by a strong upper tropospheric zonal wind from China and Mongolia to Korea.

Sill-Influenced Exchange Flows in Ice Shelf Cavities

AbstractBathymetric sills are important features in the ocean-filled cavities beneath a few fast-retreating ice shelves in West Antarctica and northern Greenland. The sills can be high enough to obstruct the cavity circulation and thereby modulate glacial melt rates. This study focuses on the idealized problem of diabatically driven, sill-constrained overturning circulation in a cavity. The circulation beneath fast-melting ice shelves can generally be characterized by an inflow of relatively warm dense water (with temperatures of a few degrees Celsius above the local freezing point) at depth and cold, less-dense, outflowing water, which exhibits an approximately two-layer structure in observations. We use a two-layer isopycnal hydrostatic model to study the cross-sill exchange of these waters in ice shelf cavities wide enough to be rotationally dominated. A quasigeostrophic constraint is determined for the transport imposed by the stratification. Relative to this constraint, the key parameters controlling the transport and its variability are the sill height relative to the bottom layer thickness and the strength of the friction relative to the potential vorticity (PV) gradient imposed by the sill. By varying these two key parameters, we simulate a diversity of flow phenomena. For a given meridional pressure gradient, the cross-sill transport is controlled by sill height beyond a critical threshold in the eddy-permitting, low-friction regime, while it is insensitive to friction in both the low-friction and high-friction regimes. We present theoretical ideas to explain the flow characteristics: a Stommel boundary layer for the friction-dominated regime; mean–eddy PV balances and energy conversion in the low-friction, low-sill regime; and hydraulic control in the low-friction, high-sill regime, with various estimates for transport in each of these regimes.

Assessment of the changes in the Etesians in the EURO‐CORDEX regional model projections

The northerly Etesian winds blow persistently over the Aegean Sea and eastern Mediterranean (EM) during summer months (June, July and August). This study investigates the July–August (JA) characteristics of the Etesians for a present time period (1981–2000) from both ECMWF ERA‐Interim Reanalysis data set (observations) and 11 EURO‐CORDEX regional climate model (RCM) simulations and assesses future changes using the associated RCP8.5 simulations. An EOF analysis applied to observations near surface meridional wind component (v10) over eastern Mediterranean demonstrates its strength as the leading mode, which explains 43.6% of the total variance, as the principal component (PC) time series of EOF1 has a very high correlation (r = 0.98) with an index showing the intensity of the Etesians. A composite analysis involving the opposite phases in the PC time series reveals that the strong Etesians develop when high pressure takes place towards Balkans and the Black Sea, which increases the meridional pressure gradient over the Aegean Sea. Some of the RCMs reproduce quite realistically pressure system and its associated Etesian winds. However, the intensity of the Etesians in these simulations is usually not as high as that in observations despite their higher horizontal resolutions. In general, the Etesians are projected to be stronger in the future periods as a result of the increasing pressure gradient over the Aegean Sea. This result suggests that the wind energy potential of the region that already attracts investment from the energy sector will increase in the future.