The Pacific decadal oscillation as a modulator of summertime North Atlantic Rossby wave breaking

Climate Dynamics - Investigation into the Pacific decadal oscillation’s (PDO’s) role in modulating the intra-basin frequency of North Atlantic Rossby wave breaking (RWB) is carried out...     

A modeling study on the effects of MJO and equatorial Rossby waves on tropical cyclone genesis over the western North Pacific in June 2004

This study investigates the influence of the Madden Julian oscillations (MJO) and equatorial Rossby (ER) waves on tropical cyclone (TC) formation in western Pacific during June 2004 through one control and three wave experiments for each of the five TCs. The control experiment reasonably simulates the formation of five TCs. In the corresponding wave experiments, the MJO, ER waves, and both the MJO and ER waves are removed, from the initial fields and lateral boundary conditions, respectively. The differences of simulated TC intensity between the control and corresponding wave experiments provide a quantitative assessment of the relative contribution of each wave to TC formation.In the wave experiments with the MJO removed, three of the five TCs are weakened, and the remaining two (TC A and B) grow stronger due to an altered background flow that steered the TCs into more favorable oceanic regions. For the wave experiments with ER waves removed, three of the five simulated TCs become weaker (TC A, C, and E). TC D develops into a tropical storm because of a dominant influence from active synoptic-scale disturbance. The results indicate that both the MJO and ER waves have an important modulating effect on TC formation. In addition to the influence from the MJO, ER and synoptic-scale waves, local processes may dominate in TC formation; for the example of TC B, none of the waves positively influence the formation in significant ways. The present modeling approach provides a quantitative assessment of the relative contribution of tropical wave disturbances to TC formation.     

Oscillatory Rossby Solitary Waves in the Atmosphere

The linear Rossby wave frequency expression is expanded at higher accuracy based on the scale difference char?acteristics of atmospheric long waves in the and directions. That the nature of the waves represented by the expan?sion is identical to that of the original ones is demonstrated both in phase velocity and wave energy dispersion speed , followed by the derivation of the nonlinear expression describing atmospheric long wave behaviors with the associated approximate analytic solution obtained. Then, for the first time atmospheric’ oscillatory Rossby solitary wave’ with its dispersion relation is obtained by numerical calculation with the aid of physical parameters of the real atmosphere. The solitary wave is found to be very close to such longwave systems as blocking highs and cut-off de?pressions in the actual atmosphere.     

Dissipation process of summer tropical easterly waves in Western North Pacific

Tropical easterly waves are common features in the trade wind zones and they are important sources of tropical cyclogenesis. Despite numerous studies have analyzed the genesis and maintenance of easterly waves in the Western North Pacific, few had examined their dissipation processes. Focusing on tropical easterly waves during May-September of 1979–2017, this study shows that most of the easterly waves (∼70 %) eventually dissipate when encountering the monsoon trough and associated westerlies, while 22 % were carried northward by the monsoonal southwesterly flows and became recurving disturbances. Less than 10 % of easterly waves propagate across the South China Sea against the prevailing monsoon westerlies and into the Indochina peninsula. The vorticity budget analysis illustrates that total vortex stretching in the lower troposphere is the key factor in propelling the small number of easterly waves westward, suggesting that stronger and more convectively active easterly waves tend to move further into the developed monsoon trough. This echoes the previous observation that tropical disturbances alone have a limited probability in developing into a typhoon, for those disturbances or easterly waves almost always need to interact with the monsoon trough or a monsoon gyre, as well as other intraseasonal features to sustain the organized convection and rotation.     

北太平洋700毫巴月平均环流形势与海温的相互关系

北太平洋700 mb环流形势具有相当鲜明的季节性与地域性特征.北太平洋的海、气相互作用是这些特征以及东亚季风形成的一个原因.从海、气相互作用的角度出发,对比分析北太平洋700 mb环流形势和海表温度距平的分布,可以看出:有些情况下,大气对海洋的影响比较明显;在另一些情况下,海洋对700 mb环流形势的影响较为明显.它们之间的矛盾与矛盾的主要方面是不断地、经常地在转化以及相互影响着.它们的转化、演变对我国副热带高压及旱涝天气的变化有一定的关系。     

北太平洋冬季西部发展型天气尺度涡旋对平均流的影响

在中纬度北太平洋大气强斜压区,存在频繁的天气尺度涡旋活动,通过水分、动量和能量输送维持大气环流。为了进一步研究天气尺度涡旋发生发展与大尺度环流之间的联系,利用1981—2013年再分析资料,筛选出西部发展型天气尺度涡旋114个偏强日和87个偏弱日,给出了西部发展型天气尺度涡旋异常导致的动力和热力强迫的变化,同时从能量转换的角度分析了西部发展型天气尺度涡旋与平均流之间的相互作用,并探讨了其与西太平洋遥相关型的关系。结果表明:西部发展型天气尺度涡旋通过动力强迫和热力强迫影响平均流,其中动力强迫主要造成北太平洋中纬度上空的西风气流加速并向北移动;热力强迫的作用则是减弱中纬度大气斜压性。同时,强西部发展型天气尺度涡旋有利于西北太平洋上空对流层低层斜压有效位能向扰动动能的转化增大和扰动动能向平均流的转化增大,有利于中纬度地区对流层高层平均流向扰动动能的转化增大。此外,西部发展型天气尺度涡旋通过与平均流的作用,对维持西太平洋遥相关型的负位相有一定影响。     

On the contribution of Rossby waves driven by surface buoyancy fluxes to low-frequency North Atlantic steric sea surface height variations

Previous studies have shown that wind-forced baroclinic Rossby waves can capture a large portion of low-frequency steric sea surface height (SSH) variations in the North Atlantic. In this paper, the classical wind-driven Rossby wave model derived in a 1.5-layer ocean is extended to include surface buoyancy forcing, and the new model is then used to assess the contribution from buoyancy-forced Rossby waves to low-frequency North Atlantic steric SSH variations. Buoyancy forcing is determined from surface heating as freshwater fluxes are negligible. It is found that buoyancy-forced Rossby waves are important in only a few regions belonging to the subtropical-to-midlatitude and eastern subpolar North Atlantic. In these regions, the new Rossby wave model accounts for 25%–70% of low-frequency steric SSH variations. Furthermore, as part of the analysis it is also shown that a simple static model driven by local surface heat fluxes captures 60%–75% of low-frequency steric SSH variations in the Labrador Sea, which is a region where Rossby waves are found to have no influence on the steric SSH.     

Salinity minimum in the North Pacific

Analyzed is the variability of seasonal salinity in the North Pacific. It is demonstrated that the formation and disappearance of salinity minimum in subsurface layers depends on the freshwater budget variability of different time scales. In general, the salinity minimum is a temporary phenomenon formed during the negative phase of freshwater budget, when the evaporation exceeds the precipitation. Seasonal variability of fresh water budget leads to the seasonal formation of salinity minimum on the boundary between the climatic zones. The existence of salinity minimum at intermediate depths in the tropics is caused by the negative phase of long-period variability of freshwater budget.     

Extreme waves in the North Atlantic

Analyzed are the reasons of extreme waves with the height of more than 14 m formed in the North Atlantic in 2002–2011 and the evolution of atmospheric processes causing the extreme waves. It is revealed that the extreme waves in the North Atlantic during that period were mainly (in 81% of cases) formed under the influence of “explosive” cyclones. The height of waves in these cyclones can reach 20 m and more.     

On the stable stratification of the nocturnal lower troposphere inferred from lidar observations over Pune,India

The nocturnal structure of the lower troposphere is studied using aerosol profile data (50–2800 m AGL) obtained with a bistatic, continuous wave, Argon ion lidar system during October 1986–September 1989 at Pune (18°32 N, 73°51 E, 559m AMSL), India. The top of the nocturnal groundbased inversion is taken as the height above ground where the negative vertical gradient in aerosol concentration first reaches a maximum. During the post-sunset period over this station, this height is as low as 160m and frequently lies around 550m. Greater heights are observed in pre-monsoon months and smaller ones during the southwest monsoon season. Positive vertical gradients in aerosol concentration, indicative of stable/elevated layers, appear frequently around 750m. Temporal variations of aerosol concentration gradients in two adjacent air layers, 920–1000m and 100–1100m, provide evidence that stability increases downward in the early night hours.     

Two modes and their seasonal and interannual variation of the baroclinic waves/storm tracks over the wintertime North Pacific

In this study, a newly developed method, termed moving empirical orthogonal function analysis (MEOF), is applied to the study of midlatitude baroclinic waves over the wintertime North Pacific from 1979 to 2009. It is shown that when the daily, high-pass filtered (2-10 days) meridional wind at 250 hPa is chosen as the variable of the MEOF analysis, typical features of baroclinic waves/storm tracks over the wintertime North Pacific can be well described by this method. It is found that the first two leading modes of the MEOF analysis, MEOF1 and MEOF2, assume quite different patterns. MEOF1 takes the form of a single wave train running in the east-west direction along 40°N, while MEOF2 is a double wave train pattern running in the east-west direction along 50°N and 30°N, respectively. The shift composites of various anomalous fields based on MEOF1 and MEOF2 assume typical baroclinic wave features. MEOF1 represents a primary storm track pulsing with an intrinsic time scale of two days. It shows significant "midwinter suppression" and apparent interannual variability. It is stronger after the mid-1990s than before the mid-1990s. MEOF2 represents a double-branch storm track, also with an intrinsic time scale of approximately two days, running along 50°N and 30°N, respectively. It shows no apparent seasonal variation, but its interannual and decadal variation is quite clear. It oscillates with larger amplitude and longer periods after the mid-1990s than before the mid-1990s, and is heavily modulated by El Niño-Southern Oscillation (ENSO).     

Interdecadal changes in the storm track activity over the North Pacific and North Atlantic

Analysis of NCEP-NCAR I reanalysis data of 1948–2009 and ECMWF ERA-40 reanalysis data of 1958–2001 reveals several significant interdecadal changes in the storm track activity and mean flow-transient eddy interaction in the extratropics of Northern Hemisphere. First, the most remarkable transition in the North Pacific storm track (PST) and the North Atlantic storm track (AST) activities during the boreal cold season (from November to March) occurred around early-to-mid 1970s with the characteristics of global intensification that has been noticed in previous studies. Second, the PST activity in midwinter underwent decadal change from a weak regime in the early 1980s to a strong regime in the late 1980s. Third, during recent decade, the PST intensity has been enhanced in early spring whereas the AST intensity has been weakened in midwinter. Finally, interdecadal change has been also noted in the relationship between the PST and AST activities and between the storm track activity and climate indices. The variability of storm track activity is well correlated with the Pacific Decadal Oscillation and North Atlantic Oscillation prior to the early 1980s, but this relationship has disappeared afterward and a significant linkage between the PST and AST activity has also been decoupled. For a better understanding of the mid-1970s’ shift in storm track activity and mean flow-transient eddy interaction, further investigation is made by analyzing local barotropic and baroclinic energetics. The intensification of global storm track activity after the mid-1970s is mainly associated with the enhancement of mean meridional temperature gradient resulting in favorable condition for baroclinic eddy growth. Consistent with the change in storm track activity, the baroclinic energy conversion is significantly increased in the North Pacific and North Atlantic. The intensification of the PST and AST activity, in turn, helps to reinforce the changes in the middle-to-upper tropospheric circulation but acts to interfere with the changes in the low-tropospheric temperature field.     

Dynamic Linkage between the North Pacific and the Tropical Pacific: Atmosphere-Ocean Coupling

In this study, dynamic linkage of atmosphere-ocean coupling between the North Pacific and the tropical Pacific was demonstrated using a large number of ensemble perturbed initial condition experiments in a fully coupled fast ocean-atmosphere model (FOAM). In the FOAM model, an idealized mixed layer warming was initiated in the Kuroshio-Oyashio extension region, while the ocean and atmosphere remained fully coupled both locally and elsewhere. The modeling results show that the warm anomalies are associated with anomalous cyclonic winds, which induce initial warming anomalies extending downstream in the following winter. Then, the downstream warming spreads southwestward and induces SST warming in the equatorial Pacific via surface wind-evaporation-SST feedback. Warming in the tropical Pacific is further reinforced by Bjerknes’ feedback.     

北太平洋的年代际振荡与全球变暖

通过回顾和总结前人工作,特别是有关北太平洋年代际振荡的研究,针对近50年来北太平洋中纬度海温变冷的现象进行了分析与讨论。从全球气候变化的角度,总结了影响北太平洋中纬度海温变冷现象的几种可能机制,推测了全球气温变暖可能会对北太平洋的直接或间接影响,归纳指出了研究该问题的复杂性与目前面临的困难。     

Properties of unstable waves in the lower St Lawrence Estuary

Abstract

The lower St Lawrence Estuary is an interesting case amongst estuaries in that it is wide enough to accommodate the development of mesoscale unstable waves and eddies. These features are generated by the runoff‐driven jet along this body's south shore. We present data yielding estimates of the length, time and velocity scales of these unstable disturbances. To relate these quantities to the dynamics we employ a 2‐layer quasigeos‐trophic instability model featuring realistic lateral shear. All model runs show short time and length scales, e‐folding periods of less than 10 days and wavelengths less than 50 km.     

Multi-century variability in the Pacific North American circulation pattern reconstructed from tree rings

We here present a reconstruction (1725–1999) of the winter Pacific North American (PNA) pattern based on three winter climate sensitive tree ring records from the western USA. Positive PNA phases in our record are associated with warm phases of ENSO and PDO and the reorganization of the PNA pattern towards a positive mode is strongest when ENSO and PDO are in phase. Regime shifts in our PNA record correspond to climatic shifts in other proxies of Pacific climate variability, including two well-documented shifts in the instrumental period (1976 and 1923). The correspondence breaks down in the early 19th century, when our record shows a prolonged period of positive PNA, with a peak in 1800–1820. This period corresponds to a period of low solar activity (Dalton Minimum), suggesting a ‘positive PNA like’ response to decreased solar irradiance. The distinct 30-year periodicity that dominates the PNA reconstruction in the 18th century and again from 1875 onwards is disrupted during this period.     

Scavenging of Insoluble Particles from the Marine Atmosphere over the Sub-Arctic North Pacific

Direct deposition measurements ofatmospheric insoluble particles over the sub-ArcticNorth Pacific indicate that the number of particles(6.4 ± 3.8) × 10⁴ cm^-2 per rain eventwas fairly constant compared with the large variationof the precipitation rate, which ranged from 0.16 to18 mm per event for 43 rain days during the period ofMarch–September (total 109 days) 1996. Thissuggests that insoluble particles larger than 0.4 min area equivalent diameter are primarilyremoved at the margin of the rainfall area by wetscavenging processes below clouds in the marineatmosphere in sub-Arctic regions. The frequency ofrain events controls the fate of the troposphericaerosols larger than sub-micrometer in diameter. Reflecting the seasonal long-range transport of Asiandust, mineral particles were dominant as insolubleparticles in the spring rains, but carbonaceousparticles were dominant in the summer rains. Theatmospheric deposition of insoluble particles could besignificant as a source of sediment particles over thesub-Arctic North Pacific.     

A snapshot of the Labrador current inferred from ice‐floe movement in NOAA satellite imagery

Abstract

Ice floes along the Labrador Coast were tracked using visible NOAA satellite images on two consecutive days (26 and 27 April, 1984) when the ice‐pack extended beyond the Labrador Current, and winds were weak. The resulting “snapshot” of the velocity field reveals strong topographic steering of the Labrador Current, such that the current speed and width in different areas are dependent on the steepness of the continental slope, and the current deflects into and out of Hopedale Saddle. Between 55 and 58°N, the main core of the current is 60–90 km wide, with speeds of 30–55 cm s^?1. The overall circulation pattern is in good agreement with historical water mass analyses over the shelf and slope, and with estimates of the speed of the Labrador Current obtained by other methods.     

北大西洋臭氧极小值和北太平洋极大值及其相互关系

利用1979～2002年TOMS卫星观测资料,采用臭氧总量纬向偏差和区域强迫的分析方法,研究北大西洋东北部大气臭氧低值与北太平洋西北部臭氧高值的季节变化过程和相互关系.研究表明,(1)北大西洋东北部存在一个大气臭氧极小值,年平均臭氧总量比纬向平均值低20 DU以上,冬季低50 DU以上; 北太平洋西北部存在一个大气臭氧极大值,年平均臭氧总量比纬向平均值高35 DU以上,冬季高70 DU以上.(2)上述两个地区大气臭氧的季节变化具有很强的区域特征,区域大气动力学输送和化学过程对上述两个地区大气臭氧季节变化的强迫分别为50.3%和42.6%.(3)上述两个地区大气臭氧纬向偏差的季节变化间存在很好的反相关,相关系数达到-0.98,说明其臭氧区域强迫之间存在良好的关系.