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Chunzai Wang and Liping Zhang

Abstract

The Atlantic multidecadal oscillation (AMO) is characterized by the sea surface warming (cooling) of the entire North Atlantic during its warm (cold) phase. Both observations and most of the phase 5 of the Coupled Model Intercomparison Project (CMIP5) models also show that the warm (cold) phase of the AMO is associated with a surface warming (cooling) and a subsurface cooling (warming) in the tropical North Atlantic (TNA). It is further shown that the warm phase of the AMO corresponds to a strengthening of the Atlantic meridional overturning circulation (AMOC) and a weakening of the Atlantic subtropical cell (STC), which both induce an anomalous northward current in the TNA subsurface ocean. Because the mean meridional temperature gradient of the subsurface ocean is positive because of the temperature dome around 9°N, the advection by the anomalous northward current cools the TNA subsurface ocean during the warm phase of the AMO. The opposite is true during the cold phase of the AMO. It is concluded that the anticorrelated ocean temperature variation in the TNA associated with the AMO is caused by the meridional current variation induced by variability of the AMOC and STC, but the AMOC plays a more important role than the STC. Observations do not seem to show an obvious anticorrelated salinity relation between the TNA surface and subsurface oceans, but most of CMIP5 models simulate an out-of-phase salinity variation. Similar to the temperature variation, the mechanism is the salinity advection by the meridional current variation induced by the AMOC and STC associated with the AMO.

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Liping Wang and Rui Xin Huang

Abstract

An analytical solution is sought for a wind-driven circulation in the inviscid limit in a linear barotropic channel model of the Antarctic Circumpolar Ocean in the presence of a bottom ridge. There is a critical height of the ridge, above which all geostrophic contours in the channel are blocked. In the subcritical case, the Sverdrup balance does not apply and there is no solution in the inviscid limit. In the supercritical case, however, the Sverdrup balance applies and an explicit form for the zonal transport in the channel is obtained.

In the case with a uniform wind stress, the transport in the β-plane channel is independent of the width of the ridge, linearly proportional to the wind stress and the length of the channel, while inversely linearly proportional to the ridge height. In the f plane with β = 0, the transport is even independent of the width of the channel. In the case with a nonuniform wind stress τx = τ0(1-−cosπy/D), the Sverdrup flow driven by the vorticity input always induces a form drag against the mean wind stress. Now, the transport depends on the width of the ridge but not on the length of the channel.

The model clearly demonstrates how the topographic form drag is generated in a linear barotropic model, which is fundamentally different from the nonlinear Rossby wave drag generation. Here, in this linear model, the presence of a supercritical high ridge is essential in the inviscid limit. The form drag is generated regardless of the flow direction. Besides, the model demonstrates that most of the potential vorticity dissipation occurs at the northern boundary where the ridge is located.

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Liping Wang and Chester J. Koblinsky

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Using sea surface height data collected by Geosat and Topex/Poseidon, the seasonal (annual) gyre circulations in the regions of the Gulf Stream and the Kuroshio Extension were studied. The seasonal gyre circulation is roughly confined to the regions where the annual mean subtropical recirculations exist. Associated with this seasonal gyre circulation, the surface transports of the Kuroshio and the Gulf Stream are found to be maximum in the late fall and minimum in the late spring. Using historical data, the authors demonstrated that these seasonal gyre circulations are mostly confined to the mixed layer. A simple diagnostic calculation of the buoyancy balance associated with the seasonal gyre circulations shows that they are driven primarily by local buoyancy flux (heating and cooling), while contribution from advection by large-scale ocean circulation is negligible. Even though the seasonal gyre circulation is primarily driven by local buoyancy forcing, it is in the opposite sense to that originally proposed by Worthington for the annual mean subtropical recirculation. The buoyancy balance within the study region suggests that dynamics associated with the mean recirculation and the seasonal gyre are fundamentally different.

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Liping Wang and Rui Xin Huang

Abstract

A simple barotropic model of abyssal circulation in a circumpolar ocean basin is constructed. In the presence of a sufficiently high ridge, the classical Stommel and Arons theory applies here with very substantial modifications In the case with a point source at one side of the channel and a point sink at the other side of the channel, there is a through-channel recirculation in addition to the flow from the source to the sink. The volume flux of this recirculation is critically determined by the supercriticality of the ridge height. In the case with a uniform sink and point sources and sinks, the circulation is essentially in the Stommel and Arons sense with one major novelty; that is, a through-channel recirculating flow is generated. Both its strength and direction depend critically upon the model parameters. This suggests that the Antarctic Bottom Water formation could drive a substantial amount of westward flow that counterbalances the wind-driven eastward flow. Last, a schematic picture of the abyssal circulation in an idealized Southern Ocean is obtained. The most significant feature is the narrow current along the northern boundary of the circumpolar basin, which feeds the deep western boundary currents of the Indian Ocean and Pacific Ocean and connects all the oceanic basins in the Southern Ocean.

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Hongyan Wang, Gaili Wang, and Liping Liu

Abstract

The vertical refractivity gradient (VRG) is critical to weather radar beam propagation. The most common method of calculating beam paths uses the 4/3 Earth radius model, which corresponds to standard refraction conditions. In the present work, to better document propagation conditions for radar electromagnetic waves, which is essential for hydrology and numerical weather forecast models to more fully benefit from observations taken from the new-generation weather radar network in China, VRG spatial and temporal variations in the first kilometers above the surface are explored using 6-yr sounding observations. Under the effects of both regional climatic and topographic conditions, VRG values for most of the radars are generally smaller than those of the standard conditions for much of the year. There are similar or slightly larger values at only a few radar sites. Smaller VRG values are more frequent and widespread, especially during rainy seasons when weather radar observations are important. In such conditions, beam heights estimated using standard atmospheric refraction are overestimated relative to actual heights for most of the radars. Underestimates are much less common and of much shorter duration. However, height deviations are acceptable for being well within the uncertainty of radar echo height owing to the ~1° beamwidth. In coastal areas and the middle and lower reaches of the Yangtze River, radar observations should be applied with much more caution because of the greater risk of beam blockage and clutter contamination.

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Xidong Wang, Chunzai Wang, Liping Zhang, and Xin Wang

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This study investigates the variation of tropical cyclone (TC) rapid intensification (RI) in the western North Pacific (WNP) and its relationship with large-scale climate variability. RI events have exhibited strikingly multidecadal variability. During the warm (cold) phase of the Pacific decadal oscillation (PDO), the annual RI number is generally lower (higher) and the average location of RI occurrence tends to shift southeastward (northwestward). The multidecadal variations of RI are associated with the variations of large-scale ocean and atmosphere variables such as sea surface temperature (SST), tropical cyclone heat potential (TCHP), relative humidity (RHUM), and vertical wind shear (VWS). It is shown that their variations on multidecadal time scales depend on the evolution of the PDO phase. The easterly trade wind is strengthened during the cold PDO phase at low levels, which tends to make equatorial warm water spread northward into the main RI region rsulting from meridional ocean advection associated with Ekman transport. Simultaneously, an anticyclonic wind anomaly is formed in the subtropical gyre of the WNP. This therefore may deepen the depth of the 26°C isotherm and directly increase TCHP over the main RI region. These thermodynamic effects associated with the cold PDO phase greatly support RI occurrence. The reverse is true during the warm PDO phase. The results also indicate that the VWS variability in the low wind shear zone along the monsoon trough may not be critical for the multidecadal modulation of RI events.

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Liping Wang, Chester J. Koblinsky, and Stephan Howden

Abstract

Using both altimetry data (TOPEX/Poseidon and Geosat) and Levitus climatology and a linear reduced-gravity model, the authors studied the annual Rossby waves in the southern Indian Ocean from 19° to 9°S. The most striking feature from the data analysis is that the westward phase propagation of the annual variability appears to break up in the midocean, which results in two local maxima for annual variability in both the sea level and the depth of the 18°C isotherm, with one in the eastern basin and the other in the western basin. Separating the two maxima is a midocean minimum. Decomposition of the annual variability into Rossby waves and localized response indicates that the two local maxima of the annual variability simply result from the constructive interference between the localized response and the Rossby waves in the eastern and western basin. On the other hand, the midocean local minimum results from the destructive interference between the Rossby waves and the localized response. Modeling results suggest that the bulk of the annual variability in the study domain is driven by wind forcing, while forcing by throughflow through eastern boundary radiation is of minor importance. Dissipation is found to have a much greater impact on the Rossby waves than on the local response.

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Liping Wang, Chester J. Koblinsky, and Stephen Howden

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Using 4.2 years of sea surface height data collected by the TOPEX/POSEIDON altimeter and 2.3 years of data collected by the Geosat altimeter, the authors study the annual, intra-annual, and linear-trend sea level height variability in the region of the Kuroshio Extension. Both the annual and intra-annual variabilities are separated spatially into large (≥2000 km), intermediate (∼1500 km), and short scales (∼800 km). The intermediate and short-scale annual and intra-annual sea level height fluctuations in both periods have large components with two-dimensional phase propagation. Close correspondence between the characteristic spatial structure of both the intermediate and short-scale annual and intra-annual sea level height fluctuations and major bottom topographic features suggests that bottom topography plays a critical role in the generation of those low-frequency variabilities. On the other hand, large-scale annual variability is primarily a standing oscillation, and bottom topography does not appear to be involved in its generation. The linear trend of the sea level change in the 4.2-yr TOPEX/POSEIDON period suggests that both the northern and the southern recirculations have been weakening considerably in the last 4.2 years, and that there exists a strong intradecadal to decadal sea level height change.

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Chunzai Wang, Liping Zhang, and Sang-Ki Lee

Abstract

The response of freshwater flux and sea surface salinity (SSS) to the Atlantic warm pool (AWP) variations from seasonal to multidecadal time scales is investigated by using various reanalysis products and observations. All of the datasets show a consistent response for all time scales: A large (small) AWP is associated with a local freshwater gain (loss) to the ocean, less (more) moisture transport across Central America, and a local low (high) SSS. The moisture budget analysis demonstrates that the freshwater change is dominated by the atmospheric mean circulation dynamics, while the effect of thermodynamics is of secondary importance. Further decomposition points out that the contribution of the mean circulation dynamics primarily arises from its divergent part, which mainly reflects the wind divergent change in the low level as a result of SST change. In association with a large (small) AWP, warmer (colder) than normal SST over the tropical North Atlantic can induce anomalous low-level convergence (divergence), which favors anomalous ascent (decent) and thus generates more (less) precipitation. On the other hand, a large (small) AWP weakens (strengthens) the trade wind and its associated westward moisture transport to the eastern North Pacific across Central America, which also favors more (less) moisture residing in the Atlantic and hence more (less) precipitation. The results imply that variability of freshwater flux and ocean salinity in the North Atlantic associated with the AWP may have the potential to affect the Atlantic meridional overturning circulation.

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Chuanfeng Zhao, Liping Liu, Qianqian Wang, Yanmei Qiu, Wei Wang, Yang Wang, and Tianyi Fan

Abstract

This study describes the microphysical properties of high ice clouds (with bases above 5 km) using ground-based millimeter cloud radar cirrus-mode observations over the Naqu site of the Tibetan Plateau (TP) during a short period from 6 to 31 July 2014. Empirical regression equations are applied for the cloud retrievals in which the parameters are given on the basis of a review of existing literature. The results show a unimodal distribution for the cloud ice effective radius r e and ice water content with maximum frequencies around 36 μm and 0.001 g m−3, respectively. Analysis shows that clouds with high ice r e are more likely to occur at times from late afternoon until nighttime. The clouds with large (small) r e mainly occur at low (high) heights and are likely orographic cumulus or stratocumulus (thin cirrus). Further analysis indicates that ice r e decreases with increasing height and shows strong positive relationships between ice r e (μm) and depth h (m), with a regression equation of r e = 35.45 + 0.0023h + (1.7 × 10−7)h 2. A good relationship between ice r e and temperature T (°C) is found, r e = 44.65 + 0.1438T, which could serve as a baseline for retrieval of characteristic ice r e properties over the TP.

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