Advection Of Relative Vorticity Research Articles

Vertical Velocity Dynamics in the North Atlantic and Implications for AMOC

Abstract The Atlantic meridional overturning circulation (MOC) is traditionally monitored in terms of zonally integrated transport either in depth space or in density space. While this view has the advantage of simplicity, it obscures the rich and complex three-dimensional structure, so that the exact physics of the downwelling and upwelling branch remains poorly understood. The near-equivalence of the depth- and density-space MOC in the subtropics suggests that vertical and diapycnal volumes transports are intimately coupled, whereas the divergence of these two metrics at higher latitudes indicates that any such coupling is neither instantaneous nor local. Previous work has characterized the surface buoyancy forcing and mixing processes which drive diapycnal volume transport. Here, we develop a new analytical decomposition of vertical volume transport based on the vorticity budget. We show that most terms can be estimated from observations and provide additional insights from a high-resolution numerical simulation of the North Atlantic. Our analysis highlights the roles of 1) relative vorticity advection for the sinking of overflow water at the northern subpolar North Atlantic boundaries and 2) the geostrophic β effect for the sinking of dense waters in the intergyre region. These results provide insights into the coupling between density- and depth-space overturning circulations. Significance Statement The purpose of this study is to better understand where and why dense water sinks in the North Atlantic. This is important because dense water sinking in the North Atlantic is a crucial component of the global thermohaline circulation. Our results reveal the primary controls on dense water sinking at a regional level and highlight the importance of mesoscale processes at high latitudes in shaping the circulation and heat distribution throughout the Atlantic Ocean.

Impact of the Summer Atmospheric Heat Source over the Tibetan Plateau on Interannual Variability of Meridional Circulation on the North Side of the Tibetan Plateau

Abstract The summer atmospheric heat source (AHS) over the Tibetan Plateau (TP) induces meridional circulations in TP and its surrounding areas. Previous studies mainly focused on the monsoon circulation on the south side of TP, while the formation and maintenance mechanisms of meridional circulation on its north side remain unclear. This study compared three calculation methods of the AHS, analyzed the spatial–temporal variability of the summer AHS over the TP, and discussed its influence on the interannual variability of meridional circulation on the north side of the TP based on the two-dimensional decomposition method of atmospheric circulation and sensitivity experiments. The results indicate that in the positive AHS anomalies years, the diabatic heating of condensation latent release in southeastern TP could motivate anomalous ascending motion. Simultaneously, the increased meridional temperature gradient between the mid- and high latitudes of East Asia leads to an enhanced southward westerly jet. In this context, the region on the north side of TP, located on the north side of the westerly jet entrance, is affected by negative anomalous relative vorticity advection, prevailing anomalous descending motion, which makes the descending branch of meridional circulation significantly presented. Unlike previous studies that considered the descending branch of meridional circulation as the compensation for upward flow, the results of the linear baroclinic model (LBM) verify that the descending branch is mainly influenced by the vorticity advection related to regional scale variability of the westerly jet. This study reveals the physical mechanism of meridional circulation on the north side of TP, which offers valuable implications for seasonal forecasting in TP and Northwest China.

Quantifying the dynamic and synergistic effects of low and mid-latitudes signals on regional extreme climate events

Unlike previous studies solely focused on the influences of low or mid-latitudes signals on regional extreme events, this paper used the three-pattern decomposition of global atmospheric circulation (3P-DGAC), taking the 2018 hot summer in East Asia for example, to quantitatively reveal the dynamic and synergistic effects of low and mid-latitudes signals on regional extreme events. By analyzing the anomalous three-dimensional structure of atmospheric circulations in 2018 hot summer, we discovered that the local meridional circulation at low latitudes and zonal circulation at mid-latitudes both produced abnormal downdrafts in East Asia, which were consistent with the anomalous anticyclone in this region. Then, we investigated the relative vorticity advection combined with 3P-DGAC to uncover the dominant circulation patterns, and found the horizontal circulation at mid-latitudes and local meridional circulation at low latitudes were the major contributor to the anomalous anticyclone, accounting for 56% and 44% respectively. This result indicated the 2018 hot summer might be influenced by the synergistic effect of low and mid-latitudes signals. Finally, by investigating sea surface temperature anomaly (SSTA) and mid-latitudes wave train, we considered the tripole-like SSTA pattern in the North Atlantic and positive SSTA in Kuroshio and its extension area (K-KE) as the main causes of horizontal circulation anomalies, and the anomalous meridional circulation was related to the extremely late onset of South China Sea summer monsoon (SCSSM). This study provided a novel way to quantitatively study synergistic effects of low and mid-latitudes signals on regional extreme events.

Variability in coastal downwelling circulation in response to high-resolution regional atmospheric forcing off the Pearl River estuary

Abstract. We investigated the variabilities in coastal circulation and dynamics in response to spatiotemporally variable high-resolution atmospheric forcing off the Pearl River estuary during the downwelling wind. Our investigation focused on the dynamics of coastal downwelling circulation in response to variable atmospheric forcing of (1) single-station observation, (2) global reanalysis data, and (3) a high-resolution regional atmospheric model. We found that the high-resolution atmospheric model significantly improved the representations of the near-surface wind and air temperature, and the ocean model driven by the high-resolution and spatially variable atmospheric forcing improved the circulation and associated hydrographic properties in the coastal ocean. Momentum and vorticity analyses further revealed that the cross-isobath water exchange was primarily governed by the along-isobath pressure gradient force (PGF), which was influenced by different components of the atmospheric forcing. The spatial–temporal variability in high-resolution wind forcing determined the strength and structure of coastal circulation and improved estimates of cross-isobath transport and the associated PGF by refining the net stress curl and nonlinear advection of relative vorticity in the simulation. The high-resolution heat forcing can greatly improve the sea surface temperature simulation and adjust the nonlinear advection of relative vorticity, resulting in changes in cross-isobath transport.

Propagation Diversity of Quasi-Biweekly Oscillation of Summer Precipitation in the Middle and Lower Reaches of the Yangtze River

Abstract Summer precipitation in the middle and lower reaches of the Yangtze River (MLYR) has obvious characteristics of intraseasonal oscillation, but there are different inferences about its propagation. Based on observed precipitation and ERA5 data, the propagation diversity of summer quasi-biweekly precipitation in the MLYR is analyzed. The results show that summer precipitation in the MLYR has obvious quasi-biweekly oscillation (QBWO) characteristics, and precipitation QBWO events can be classified into three categories: southward, northward, and local oscillation events. The frequency of southward events is significantly higher than that of northward or local oscillation events. For southward events, a wave train is observed in the mid–high latitudes and propagates southward in East Asia. The northward and local oscillation events are closely related to the QBWO of the northwest Pacific in low latitudes. The meridional asymmetry of quasi-biweekly divergence and background wind is responsible for the diversity of QBWO propagation. Quasi-biweekly convergence is usually located on the south of the quasi-biweekly cyclone, so the convergence interacts with geostrophic vorticity to guide the quasi-biweekly cyclone to move southward; in the southerly basic flow, north of the quasi-biweekly cyclone is positive relative vorticity advection, which leads the quasi-biweekly cyclone to move northward. Therefore, quasi-biweekly disturbances originating from north of the MLYR, with weak basic wind, mainly move southward under the influence of convergence. However, quasi-biweekly disturbances at low latitudes move northward under the guidance of relative vorticity advection due to the strong basic flow. A disturbance originating from the MLYR presents local oscillation under the coaction of quasi-biweekly relative vorticity advection and divergence.

Interannual and decadal variations of the Kuroshio axis in the East China Sea

Much work has been focused on the variability and climatic effect of the Kuroshio Extension, but little focus on the Kuroshio variation in the East China Sea(ECS). This study, investigates the interannual and decadal migrations of Kuroshio axis(KA) in the ECS and its relationships to large-scale climate signals. On interannual timescale, variability of the KA in the ECS is closely related to El Niño-Southern Oscillation (ENSO) cycle. The shifting of the main axis in ECS presents a dipolar pattern with the offshore (onshore) shift of the south (north) part KA in ECS under the El Niño events and onshore (offshore) shift under the La Niña events. The interannual variation (IVAR) is modulated dominantly by wind stress curl (WSC) and surface relative vorticity advection associated with the ENSO. On the decadal timescale, the Pacific Decadal Oscillation (PDO) and North Pacific Gyre Oscillation (NPGO) could cause the WSC and relative vorticity (RV) anomalies, and in turn modulates the fluctuation of KA. The PDO is the main factor and the NPGO is secondary factor for the decadal variation (DVAR) of the KA in ECS.

A Theory of Standing Meanders of the Antarctic Circumpolar Current and Their Response to Wind

Abstract Standing meanders of the Antarctic Circumpolar Current (ACC) and associated eddy hotspots play an important role for the meridional heat flux and downward momentum transfer in the Southern Ocean. Previous modeling studies show that the vorticity balance characterizing standing meanders in the upper ocean is dominated by advection of relative vorticity and stretching. Through the adjustment of this vorticity balance, standing meanders have been suggested to provide a pathway for the transfer of the momentum input by the wind from the surface to the bottom, leading to stronger bottom flows and energy dissipation. However, the dynamics governing the meander formation and its adjustment to wind remain unclear. Here we develop a quasigeostrophic theory and combine it with a regional model of the Macquarie Ridge region and an idealized channel model to explore the dynamics and vertical structure of standing meanders of the ACC. The results show that the entire vertical structure of the meander, including its dynamics in the upper ocean, is controlled by the bottom flow interacting with topography. Based on our results, we suggest a novel mechanism for the response of the ACC to wind in which “flexing” of the meander, or change in its curvature, is a response to changes in the bottom (barotropic) flow. Stronger bottom flow in response to stronger wind interacts with topography and generates a larger-amplitude Rossby wave propagating into the upper ocean. The ACC mean shear aloft amplifies the Rossby wave and leads to a larger-amplitude meander in the upper ocean dominated by advection of relative vorticity and stretching.

Impact of the mid-latitude zonal circulation on dynamic mechanism of anomalous precipitation over China in summer 2021

China was hit by extreme precipitation in July and August of 2021 along with significant monthly variation. The positive precipitation anomalies observed in July were mainly located in eastern China, but these anomalies shifted to the middle and lower reaches of the Yangtze River (MLYR) in August. In this study, we investigated the July and August precipitation anomaly dynamics based on the three-pattern decomposition of global atmospheric circulation (3P-DGAC) method. The results showed that the mid-latitude zonal circulation played a dominant role in July 2021, causing anomalous positive relative vorticity advection over the center of the positive precipitation anomalies, thus providing strong convective activity for the anomalous precipitation. Nevertheless, the August anomalies could be better explained from a climate perspective. The anomalous relative vorticity distribution patterns recorded in August were often accompanied by extreme precipitation in the MLYR, and the negative relative vorticity anomalies identified in Northeast Asia also played a crucial role. In conclusion, the mechanisms of precipitation anomalies in July and August 2021 were almost completely disparate. In this study, we emphasized the role of the mid-latitude zonal circulation in extreme precipitation events, and suggested that the impact of mid-latitude zonal circulation will be further accentuated under global warming.

Upper‐Troposphere Saddle‐Like Response to Springtime Surface Sensible Heating Over the Tibetan Plateau: Combined Effect From Baroclinic and Barotropic Process

AbstractThe surface sensible heating over the Tibetan Plateau (TP) is a significant, elevated heating source in spring (March‐April‐May, MAM). It exerts considerable thermal effects on both regional and global climate. During MAM, the surface sensible heating over the TP triggers a giant saddle‐like circulation anomaly in the upper troposphere at 200 hPa. Although this structure was observed a long time ago and it contributes to the stationary wave, its formation mechanism remains unclear. Using the Linear Baroclinic Model, we reveal the evolution of this structure in detail. In the initial state, a typical baroclinic anticyclone with a vertical phase tilt forms around the heating source. When the wave propagates downstream due to strong relative vorticity advection and favorable lower level circulation, an anticyclone with a barotropic structure forms downstream. Our analysis indicates that both the barotropic process and baroclinic process contribute to the formation of the saddle‐like structure, and the latter is associated with the spring unique background circulation and vertical heating structure over the TP.

Dynamics of a Standing Meander of the Subantarctic Front Diagnosed from Satellite Altimetry and Along-Stream Anomalies of Temperature and Salinity

Abstract Meanders formed where the Antarctic Circumpolar Current (ACC) interacts with topography have been identified as dynamical hot spots, characterized by enhanced eddy energy, momentum transfer, and cross-front exchange. However, few studies have used observations to diagnose the dynamics of ACC standing meanders. We use a synoptic hydrographic survey and satellite altimetry to explore the momentum and vorticity balance of a Subantarctic Front standing meander, downstream of the Southeast Indian Ridge. Along-stream anomalies of temperature in the upper ocean (150–600 m) show along-stream cooling entering the surface trough and along-stream warming entering the surface crest, while warming is observed from trough to crest in the deeper ocean (600–1500 m). Advection of relative vorticity is balanced by vortex stretching, as found in model studies of meandering currents. Meander curvature is sufficiently large that the flow is in gradient wind balance, resulting in ageostrophic horizontal divergence. This drives downwelling of cooler water along isopycnals entering the surface trough and upwelling of warmer water entering the surface crest, consistent with the observed evolution of temperature anomalies in the upper ocean. Progressive along-stream warming observed between 600 and 1500 m likely reflects cyclogenesis in the deep ocean. Vortex stretching couples the upper and lower water column, producing a low pressure at depth between surface trough and crest and cyclonic flow that carries cold water equatorward in the surface trough and warm water poleward in the surface crest (poleward heat flux). The results highlight gradient–wind balance and cyclogenesis as central to dynamics of standing meanders and their critical role in the ACC momentum and vorticity balance. Significance Statement The Antarctic Circumpolar Current (ACC) in the Southern Ocean is a nearly zonal current that encircles Antarctica. It acts as a barrier between warmer water equatorward and colder water poleward. In a few regions where the current encounters strong topographic changes, the current meanders and opens a pathway for heat to travel across the ACC toward Antarctica. We surveyed a meander in the ACC and examined the along-stream change of temperature. In the upper ocean, temperature changes are caused by a vertical circulation, bringing cool water down when entering the surface trough (the part of the meander closest to the equator), and warm water up when exiting the surface trough and entering the surface crest. At depth, cold water is transported equatorward in the surface trough and warm water poleward in the surface crest, leading to a net transport of heat poleward. This study highlights the importance of the secondary circulation within a meander for generating cross-ACC flows and moving heat toward Antarctica.

Monsoon Surges Enhance Extreme Rainfall by Maintaining the Circulation of Landfalling Tropical Cyclones and Slowing Down Their Movement

Extreme rainfall induced by landfalling tropical cyclones (ERLTCs) in China can cause flash floods and other disastrous impacts, so investigating their genesis and mechanism of enhancement has been attracting considerable attention. This study demonstrates that the extreme rainfall of landfalling tropical cyclones (LTCs) possesses two key properties—namely, maintenance of the LTC circulation and a lagging (slowing down or looping) of its movement, and the monsoon surge can provide a positive contribution to these properties. Specifically, diagnostics show that the low-level cyclonic vorticity and upper-level divergence of ERLTCs are significantly stronger than those of NERLTCs (non-extreme-rainfall-producing LTCs). The continuous intensification of the cyclonic rotation in the lower troposphere before the occurrence of extreme rainfall is a significant feature that distinguishes ERLTCs from NERLTCs. Vorticity budget analysis further shows that the relative vorticity advection term contributes the most to the local increase and maintenance of vorticity in the middle and lower troposphere of ERLTCs under the influence of the southwest monsoonal surge, thus demonstrating that the monsoonal surge favors the maintenance of LTC circulation. On the other hand, the activity of the southwest monsoonal surge is mainly manifested in the zonal wind anomaly, and the corresponding strong westerly transport can significantly reduce the zonal component of the steering flow. As a result, the total steering flow can be weakened, which decreases the northwestward translation speed of ERLTCs, and thus the monsoonal surge favors a lagging (slowing down or looping) of LTC movement. These results reveal the mechanism of influence through which the monsoonal surge affects ERLTCs via its direct impacts on the maintenance of their circulation and lagging of their movement—two distinct evolutionary characteristics.

The influence of upper level surrounding winds on tropical cyclone motion: a case study of the Bay of Bengal region

The study is an attempt to classify the observed tropical cyclone (TC) track patterns over the Bay of Bengal (BoB) region. It is hypothesized that winds at 200 hPa level can determine the direction of TC motion. We found that the mean TC genesis locations were shifted southward from October to December with the shifting of the regional wind circulation patterns. The wind circulation pattern during the straight moving TCs were very similar to the recurving TCs. Straight moving TCs were embedded in a deep easterly flow whereas the recurving TCs significantly moved further northward compared to the straight tracks which brought them under the influence of mid-latitude westerlies and changed their paths towards northeast. A TC track prediction scheme is presented to evaluate the effectiveness of 200 hPa level surrounding winds in TC motion prediction. Sea level pressure fall and relative vorticity advection also have been used as supportive factors. We found results comparable with other TC track prediction models which did not consider the upper level surrounding winds as a TC track predictor. Hence, the use of 200 hPa level winds might be beneficial to improve the existing TC track prediction models over the BoB region.

Dynamics of cross-shelf water exchanges off Pearl River Estuary in summer

Dynamics regulating the cross-isobath momentum and water exchanges over the shelf in the vicinity of the Pearl River Estuary (PRE) in July 2015 and 2017 are examined using comprehensive observations and multiple-level nested high-resolution numerical simulations. Although driven by the upwelling- and downwelling-favorable winds in July 2015 and 2017, respectively, the shelf currents and the cross-isobath exchanges illustrate great similarity during these two periods. This study finds that the southwesterly-induced flow downstream in the Taiwan Strait maintained the northeastward upwelling currents in the vicinity of the PRE that caused the extensive upslope invasion of the deep shelf waters. The cross-shelf exchanges of the shelf and coastal waters mainly occur over the irregular topography off the entrance of the HuangMaoHai estuary, to the west of the Dangan Island, and over a shallow trough to the east of Hong Kong. It is also rationalized that the intensified exchange flow is established by an along-shelf pressure gradient force that notably exceeds the contributions from the bottom and surface Ekman transport as well as the nonlinearity of the shelf current in the water column. The baroclinicity of the shelf and coastal waters plays minimal roles in altering this along-shore pressure gradient force, especially when upwelling-favorable winds prevail. The exchange flows in the water column thereby show great similarity with those in the bottom layer, where the bottom along-shore pressure gradient force is jointly imposed by the nonlinear advection of relative vorticity and frictional stress curl in both the surface and bottom layers. The spatial variation of these terms is regulated by the intensity of the shelf current and along-shelf variation of bottom topography.

On a Flood-Producing Coastal Mesoscale Convective Storm Associated with the Kor’easterlies: Multi-Data Analyses Using Remotely-Sensed and In-Situ Observations and Storm-Scale Model Simulations

A flood-producing heavy rainfall event occurred at the mountainous coastal region in the northeast of South Korea on 5–6 August 2018, subsequent to extreme heat waves, through a quasi-stationary mesoscale convective system (MCS). We analyzed the storm environment via a multi-data approach using high-resolution (1-km) simulations from the Weather Research and Forecasting (WRF) and in situ/satellite/radar observations. The brightness temperature, from the Advanced Himawari Imager water vapor band, and the composite radar reflectivity were used to identify characteristics of the MCS and associated precipitations. The following factors affected this back-building MCS: low-level convergence by the Korea easterlies (Kor’easterlies), carrying moist air into the coast; strong vertical wind shear, making the updraft tilted and sustained; coastal fronts and back-building convection bands, formed through interactions among the Kor’easterlies, cold pool outflows, and orography; mid-level advection of cold air and positive relative vorticity, enhancing vertical convection and potential instability; and vigorous updraft releasing potential instability. The pre-storm synoptic environment provided favorable conditions for storm development such as high moisture and temperature over the coastal area and adjacent sea, and enhancement of the Kor’easterlies by expansion of a surface high pressure system. Upper-level north-northwesterly winds prompted the MCS to propagate south-southeastward along the coastline.

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.

The 10–30‐day oscillation of winter rainfall in southern China and its relationship with circulation patterns in different latitudes

AbstractIntraseasonal oscillation (ISO) of winter rainfall in southern China and its relationship with the circulation patterns in different latitudes are investigated using ERA‐Interim reanalysis data and observations. The results show that the rainfall in southern China has obvious ISO characteristics, with a dominant period of 10–30 days. The convergence of the whole layer integral water vapour flux can satisfactorily represent the rainfall rate, and the intraseasonal (10–30‐day filtered) rainfall is converted into a function of different time scale wind and water vapour. At the intraseasonal time scale, rainfall (10–30‐day filtered) is determined by both the 10–30‐day wind and the background (greater than 30 days) moisture. Nine days before the peak day of the intraseasonal rainfall in southern China, there is an abnormal low‐pressure system (10–30‐day filtered) in the polar region (east to Novaya Zemlya), and the low‐pressure system gradually strengthens because of the intraseasonal relative vorticity advection at high levels. The intraseasonal relative vorticity advection guides the intraseasonal geopotential anomaly to move from the high latitude to the low latitude, and the low‐pressure system at the surface also moves southward. There is no intraseasonal precipitation until the low‐pressure system at the surface moves to the region near southern China, because there is little water vapour in northern China and ample water vapour in the south. After the occurrence of precipitation, the vorticity advection from the high latitude no longer affects the strength of the low‐pressure system, but the intraseasonal diabatic heating accompanied by precipitation causes a second drop in pressure from −3 days to −1 day and makes the low‐pressure system strongest here. In addition, the geopotential anomaly (10–30‐day filtered) in low latitudes moves eastward along the southern foothills of the Tibetan plateau, which can guide the low‐pressure system to move southward until it reaches southern China.

Connection between Two Leading Modes of Autumn Rainfall Interannual Variability in Southeast China and Two Types of ENSO-Like SSTA

The interannual variability of autumn rainfall in Southeast China (SEC) is significant, with two major modes, namely, monopole and dipole modes. It is found that the monopole mode is closely related to EP ENSO-like sea surface temperature anomaly (SSTA), and the dipole mode is related to CP ENSO-like SSTA. During the warm phase of EP ENSO-like SSTA, an anomalous anti-Walker circulation emerges in the tropical Pacific, with an anomalous subsiding during 110°E–120°E and an anomalous ascending branch in SEC. These two branches of anomalous current are in the same longitude and form a closed meridional circulation. Besides, there also exists an anticyclone anomaly in the Northwest Pacific (NWP), transporting water vapor into SEC. These circulation configurations induced by the warm phase of EP ENSO-like SSTA are consistent with those of monopole mode positive anomaly year. The good correspondence between EP ENSO warm event and the positive monopole mode also helps to support the corresponding relationship between the EP ENSO-like SSTA and monopole mode of SEC autumn rainfall. After the diagnosis of the perturbation omega equation, the anomalous subsiding branch over SEC, as the key link of EP ENSO-like warm phase SSTA exerting impact on the monopole mode during positive anomaly year, is mainly related with the anomalous relative vorticity advection transported by basic zonal wind and temperature advection transported by meridional wind anomaly. As for the dipole mode, it is related to the CP ENSO-like SSTA, but the corresponding relationship is weaker than that of the monopole mode and EP ENSO-like SSTA. In special, during the warm phase of CP ENSO-like SSTA, an anomalous cyclone appears in the NWP and prevailing sinking motion over SEC, both of which favors the appearance of positive anomaly of the dipole mode. Specially, the local anomalous vertical motion mainly depends on anomalous relative vorticity transported by basic meridional wind. Generally speaking, the monopole (dipole) mode is closely associated with the EP (CP) ENSO-like SSTA, demonstrating some correspondence.

Evaluation of the Quasi-Biweekly Oscillation over the South China Sea in Early and Late Summer in CAM5

Abstract Low-frequency intraseasonal oscillations in the tropical atmosphere in general circulation models (GCMs) were studied extensively in many previous studies. However, the simulation of the quasi-biweekly oscillation (QBWO), which is an important component of the intraseasonal oscillations, in GCMs has not received much attention. This paper evaluates the QBWO features over the South China Sea in early [May–June (MJ)] and late [August–September (AS)] summer in the National Center for Atmospheric Research (NCAR) Community Atmosphere Model, version 5.3 (CAM5), using observations and reanalysis data. Results show that the major features of the spatial distribution of the QBWO in both MJ and AS are simulated reasonably well by the model, although the amplitude of the variation is overestimated. CAM5 captures the local oscillation in MJ and the westward propagation in AS of the QBWO. Although there are important biases in geographical location and intensity in MJ, the model represents the QBWO horizontal and vertical structure qualitatively well in AS. The diagnosis of the eddy vorticity budget is conducted to better understand the QBWO activities in the model. Both horizontal advection of relative vorticity and that of planetary vorticity (Coriolis parameter) are important for the local evolution of the QBWO in MJ in observations as well as model simulation, whereas advection of planetary vorticity contributes to the westward propagation of QBWO vorticity anomalies in AS. Since the Coriolis parameter f only changes with latitude, this suggests that the correct simulation of anomalous meridional wind is a key factor in the realistic simulation of the QBWO in the model.

Modeling the intrusion of the Loop Current into the Gulf of Mexico

The Loop Current (LC) intrusion into the Gulf of Mexico is analyzed using steady, 2-D nonlinear shallow water equations expressed in Cartesian and natural coordinates. The model suggests that the LC moves with constant relative vorticity along isopleths of the Bernoulli function B such that its surface area (A) has a steady dA/dt and frontal velocity. The intrusion is envisioned as an initial transport imbalance that creates volume storage within the Gulf. Because of the advection of relative vorticity, the intruding inflow splits into a northward flow on the west and a southward flow on the east. The storage creates a sea level difference across the LC that induces a northward force due to Coriolis. This force reinforces the northward flow but opposes the southward flow, helping store water inside the Gulf. This sea level difference is in addition to the one in the B field. The storage implies loss of kinetic energy, which creates a speed shear across the LC and gives rise to the small speed of the LC front. The relative vorticity equation in natural coordinates reveals three aspects of the LC: (1) a velocity difference across the LC, (2) a parabolic shape of the sea surface; and (3) a negative curvature of the LC meaning the velocity changes direction anticyclonically along the B-contours.

Coupled Summer Circulation and Dynamics Between a Bay and the Adjacent Shelf Around Hong Kong: Observational and Modeling Studies

AbstractObservational and numerical modeling studies are conducted to investigate the coupled circulation between Mirs Bay to the east of Hong Kong and the adjacent shelf sea during an upwelling season. Long‐ and short‐term observations are synthesized to characterize the circulations in the bay‐shelf region. A three‐dimensional‐coupled bay‐shelf‐estuary circulation model was developed with realistic topography and forced with time‐dependent wind, tides, and lateral fluxes to investigate the processes and physics in the circulation of the coupled bay‐shelf regime. Based on the validated model, it was found that a strong northeastward coastal upwelling jet persisted over the shelf with highly variable topography outside the bay, and a strong upslope current occurred where the topography was sharply convex. This upslope current intruded into the bay in the lower layer (>10 m) as a cold‐water stream. A horizontal anticyclonic circulation formed inside the bay with a seaward outflow in the upper layer (<10 m). Momentum and vorticity analyses showed that a southwestward along‐isobath pressure gradient force over the convex isobaths off the bay intensified that bay‐ward intrusion. Negative relative vorticity advection from the jet was responsible for this pressure gradient force. The horizontal anticyclonic circulation and elevation fluctuation inside the bay were determined by the interaction between the intruding shelf current and the topographic trough inside the bay, and they were also baroclinically modulated by the intrusion of denser shelf waters. Winds over Mirs Bay intensified exchange flow across its entrance but suppressed the anticyclonic circulation inside the bay.