Spectral characteristics of quasi-biennial oscillations of the equatorial stratospheric wind and the problem of synchronization

Spectral characteristics of the quasi-biennial oscillations (QBO) of the zonal velocity in the equatorial stratosphere are investigated in this work on the basis of data from the NCEP/NCAR and ERA40 reanalyses and numerical experiments with the atmospheric general circulation (GCM) model developed at the Institute of Numerical Mathematics, Russian Academy of Sciences (INM RAS). The problem of synchronizing QBO and semiannual oscillations (SAO) of the zonal velocity in the mesosphere is considered. It is shown that the process of synchronization to multiples of SAO periods is identifiable in the transition region between QBO and SAO. For all heights where QBO exist, their synchronization with SAO is expressed in the calculation of the period in terms of differences between the westerly maxima. The INM RAS GCM model is shown to satisfactorily reproduce the main spectral characteristics of QBO and SAO, as well as specific features of the variability of the QBO period obtained from reanalysis data. The possibility of synchronization with SAO or the annual cycle in the upper layers is shown on the basis of an investigation of QBO models with a small number of parameters, both for the absorption mechanism of planetary waves by the mean flow and for the breaking of short gravity waves. The QBO formation from different wave types, together with SAO and the annual cycle, can be considered a unified system of oscillations in the circulation of the equatorial upper atmosphere.     

Simulation of the quasi-biennial oscillations of the zonal wind in the equatorial stratosphere: Part I. Low-parameter models

The paper focuses on the simulation of the quasi-biennial oscillations (QBOs) of zonal velocity in the equatorial stratosphere. Low-parameter models are used to examine two mechanisms for excitation of the QBO: one through the interaction of planetary waves with the mean flow at critical levels and another through gravity-wave obliteration. The possible use of each of these mechanisms for generating the QBO is shown, the ranges of parameter values where this generation is possible are determined, and the dependences of the period and amplitude of the limit cycle on the model parameters are analyzed. A relative role of waves of different scales in the formation of the period of the oscillations of zonal wind is studied with a coupled model combining both mechanisms. The conditions that are required to reproduce the QBO in general circulation models are discussed.     

Quasi-biennial oscillation of wind in the low-latitude stratosphere and the winter wave activity of the atmosphere in the Northern Hemisphere

According to the Holton-Tan hypothesis [1], oscillations of the equatorial stratospheric wind change the conditions of the vertical and meridional propagation of planetary waves in extratropical regions, which can cause quasi-biennial oscillations (QBOs) at middle and polar latitudes. To verify the Holton-Tan hypothesis, the intensity of the winter wave activity of the atmosphere in the Northern Hemisphere was estimated at different phases of the quasi-biennial oscillation of the equatorial stratospheric zonal wind. As it turned out, a higher level of the wave activity expected at the easterly phase of the equatorial QBO is characteristic only of the period when the winter circulation is established. At the end of winter a higher level of the wave activity is observed at the westerly QBO phase, which contradicts the Holton-Tan hypothesis. Small but nevertheless noticeable distinctions in the wave activity at low tropospheric levels suggest that the quasi-biennial periodicity of the wave activity at middle latitudes can be caused by oscillations of synoptic processes between the predominantly zonal and meridional forms of the circulation, as was indicated by Pogosyan and Pavlovskaya [2, 3].     

Modeling the global circulation response and the regional response of the Arctic Ocean to the external forcing anomalies

The problem of numerical modeling and analysis of the large-scale World Ocean circulation variability under variations of the external forcing is considered. A numerical model was developed in the INM RAS and is based on the primitive equations of the ocean circulation written in a spherical generalized σ-coordinate system. The model’s equations are approximated on a grid with resolution of 2.5° × 2° × 33, and the North Pole is displaced to the continental point (60°E, 60.5°N). There are two stages for the numerical experiments. The quasi-equilibrium circulation of the World Ocean under the climatological atmospheric forcing is simulated at the first stage. The run is carried out over a period of 3000 years during which a quasi-equilibrium model regime is formed. At the second stage, the sensitivity of the model ocean circulation to the atmospheric forcing perturbations in the Southern Hemisphere is studied. According to the results, the strongest regional changes in the hydrography take place in the Arctic Ocean. Substantial changes of sea’s surface height and local anomalies of the temperature and salinity are formed there.     

Probability distributions for cyclones and anticyclones from the NCEP/NCAR reanalysis data and the INM RAS climate model

Analysis of statistical characteristics of cyclones and anticyclones in the latitudinal belt between 20° and 80°N has been performed with the NCEP/NCAR reanalysis data and simulations with the general circulation climate model of the Institute of Numerical Mathematics of the Russian Academy of Sciences (INM RAS GCCM). The model results have been analyzed for the second half of the 20th century against the NCEP/NCAR reanalysis data and for the 21st century with the SRES-A2 anthropogenic scenario. Overall for the 20th century, no statistically significant changes in the number of cyclones and anticyclones are obtained from either the NCEP/NCAR reanalysis data [1] or from simulations with the INM RAS GCCM [2]. It is found that the total number of cyclones and anticyclones decreased in the 20th century as compared to the 21st century. It is shown that cumulative distributions of the number of cyclones and anticyclones by their intensities and areas have an exponential form from both the reanalysis data and the model simulations, although the corresponding exponents are different.     

Modeling the water circulation in the North Atlantic in the scope of the CORE-II experiment

A numerical experiment on the reproduction of the variability in the state of North Atlantic water in 1948–2007 with a spatial resolution of 0.25° has been performed using the global ocean model developed at Institute of Numerical Mathematics, Russian Academy of Sciences (INM RAS), and the Shirshov Institute of Oceanology (IO RAS) (the INM–IO model). The data on the state of the atmosphere, radiation fluxes, and bulk formulas of the CORE-II protocol are used as boundary conditions. Five successive 60-year calculation cycles have been performed in order to obtain the quasi-equilibrium state of a model ocean. For the last 20 years, the main elements of large-scale ocean circulation have been analyzed and compared with the WOA09 atlas data and the results of other models.     

Analysis of the quasi-biennial variability of carbon monoxide total column

On the basis of satellite observations of column carbon monoxide (CO) and total ozone (TO), an analysis has been performed of the connection of the interannual variability of CO with the quasi-biennial oscillation (QBO) of the equatorial stratospheric wind and the QBO of total ozone. It is found that the CO total colomn over most of the globe in the westerly phase of the QBO is greater than that in the easterly phase. The global distribution of the CO QBO amplitudes exhibits a local maximum over Indonesia, where the peak-to-peak amplitude of the CO QBO signal averages 15% of the local annual mean CO in this region. Analysis shows that the QBOs of CO are well synchronized with the QBO of wind at 50 hPa. At the same time, a joint analysis of the characteristics of the CO QBO and TO QBO demonstrates no direct photochemical coupling between of the quasi-biennial variations of TO and CO.     

Quasi-biennial variations in ozone and meteorological parameters over western Europe from ozonesonde data

Quasi-biennial variations in vertical profiles of ozone, temperature, air pressure, and zonal and meridional wind velocities are analyzed from ozonesonde data obtained at the western European stations of Lindenberg, Hohenpeissenberg, and Payerne. The effect of quasi-biennial variations manifests itself variously in different variables and is nonuniform in altitude. The period of quasi-biennial variations is not constant, and the values of the mean period group mainly around 2 and 2.5 years. As in the North American region, the effects of quasi-biennial variations in different parameters of the stratosphere and troposphere over western Europe are due to a combination of the effects of the quasi-biennial oscillation (QBO) in the equatorial stratosphere, the El Niño-Southern Oscillation (ENSO), and the North Atlantic Oscillation (NAO). The observed 2.5-year variations in stratospheric ozone are related to the equatorial QBO to a larger extent in comparison with variations in other variables. It seems likely that a determining influence on variations in stratospheric wind and temperature is exerted by the ENSO. Variations in tropospheric and stratospheric parameters with a mean period of about 2 years are due to the ENSO and NAO effects.     

High resolution modeling of the monsoon circulation in the Indian Ocean

The goal of this paper is to present some results on the monsoon circulation in the Indian Ocean simulated with a σ-coordinate ocean model developed at the Institute of Numerical Mathematics, RAS. The model has a horizontal resolution of (1/8)° × (1/12)° and contains 21 σ-layers of uneven thickness. Realistic bottom topography and land geometry are used. The numerical experiments were carried out for 15 years starting from the Levitus climatology for January and monthly mean climatic atmospheric forcing from the NCEP reanalysis data. The annual cycle of the surface and subsurface currents and temperature and salinity fields were analyzed. The model reproduces well the Summer Monsoon and the Winter Monsoon currents and their time evolution and spatial structures. The Somali Current is adequately modeled. During the Summer Monsoon, the velocities of the current exceed 2 m/s, while the total mass transport is approximately 70 Sv. The model results show that a reversal of the Somali Current from the northern direction in the summer to the southern direction in the winter is accompanied by the generation of anticyclonic eddies, which drift westward owing to the β-effect and dissipate either near the Somali shore or in the Gulf of Aden. The monsoon variability of the equatorial surface current and equatorial subsurface countercurrent system are analyzed. It is shown that these currents are generated predominantly by the zonal component of wind stress, in which the half-year harmonic dominates. This leads to the fact that the equatorial surface current also changes its direction with a half-year periodicity almost in phase with the wind. The oppositely directed subsurface compensational countercurrent changes its direction with a time lag of approximately one month. Gradient currents, which appear in the Bay of Bengal due to the riverine runoff, make an important contribution to the circulation. This effect manifests itself especially strongly in the summer during the peak of the Ganges River runoff, which transports fresh turbid waters. The principal features of the large-scale quasi-stationary gyre structure of the Indian Ocean such as the Great Whirl, Socotra high, and Laccadive high and low are simulated.     

Direct measurements of deep current at 162°E south of the equator in the Melanesian Basin: a trial to detect a cross-equatorial deep western boundary current

We conducted 1-year-long mooring observations four times below 2000?m, slightly south of the equator (2°39?? to 4°35??S) at 162°E in the Melanesian Basin in order to detect the southward deep western boundary return current crossing the equator. Contrary to our initial expectation of the deep flow scheme in the equatorial western boundary region, the observed results indicated a fairly complicated flow configuration. We analyzed the results with the help of a high-resolution model simulation. The ensemble average of the horizontal flow at each level near the deep western boundary indicates a significant westward flow at 2000 and 2250?m, with an insignificant southward component at 2500 and 2750?m. The annual mean meridional transports are very small (>1?Sv) and insignificant, with an ensemble-averaged value of 0.3?Sv (southward) ±0.4?Sv at most. Combining this with high-resolution model results, it is deduced that the southward transport of the deep western boundary current (DWBC) leaving the equator may be smaller than those obtained by low-resolution models, because of trapping of its fairly large fraction in the equatorial zone. Annual-scale flow patterns are classified into several categories, mainly based on the meridional-flow dominating or the zonal-flow dominating pattern. A case of the meridional-flow dominating patterns may possibly capture an annual-scale variability of DWBC, because its meridional transport variation, though somewhat weak, is consistent with that simulated. The zonal-flow dominating regime includes two types: long-lasting, almost steady westward flows and long-term zonal flow oscillations. The former seems to comprise well-known zonally elongated and meridionally narrow structures of the zonal flow beneath the thermocline in the equatorial region. The ensemble-averaged flow mentioned above is dominated by this type at the upper two levels 2000 and 2250?m, with total westward transport of 1.6?±?0.7?Sv. The latter type seems to be a manifestation of the vertically propagating equatorial annual Rossby waves.     

Preliminary study on the dynamic mechanism of the deep equatorial jets

Ｐｒｅｌｉｍｉｎａｒｙｓｔｕｄｙｏｎｔｈｅｄｙｎａｍｉｃｍｅｃｈａｎｉｓｍｏｆｔｈｅｄｅｅｐｅｑｕａｔｏｒｉａｌｊｅｔｓ￥ＷｕＤｅｘｉｎｇ（ＲｅｃｅｉｖｅｄＤｅｃｅｍｂｅｒ１５，１９９３；ａｃｃｅｐｔｅｄＭａｒｃｈ７，１９９４）（Ｉｎｓｔｉｔｕｔｅｏ...     

Validating and assessing the sensitivity of the climate model with an ocean general circulation model developed at the Institute of Atmospheric Physics,Russian Academy of Sciences

A new version of the Institute of Atmospheric Physics, Russian Academy of Sciences (IAP RAS), climate model (CM) has been developed using an ocean general circulation model instead of the statistical-dynamical ocean model applied in the previous version. The spatial resolution of the new ocean model is 3° in latitude and 5° in longitude, with 25 unevenly spaced vertical levels. In the previous version of the oceanic model, as in the atmospheric model, the horizontal resolution was 4.5° in latitude and 6° in longitude, with four vertical levels (the upper quasi-homogeneous layer, seasonal thermocline, abyssal ocean, and bottom friction layer). There is no correction for the heat and momentum fluxes between the atmosphere and ocean in the new version of the IAP RAS CM. Numerical experiments with the IAP RAS CM have been performed under current initial and boundary conditions, as well as with an increasing concentration of atmospheric carbon dioxide. The main simulated atmospheric and oceanic fields agree quite well with observational data. The new version’s equilibrium temperature sensitivity to atmospheric CO₂ doubling was found to be 2.9 K. This value lies in the mid-range of estimates (2–4.5 K) obtained from simulations with state-of-the-art models of different complexities.     

西风爆发、次表层暖水东移与厄尔尼诺现象

利用最近20 a的大气海洋资料,分析了厄尔尼诺事件与赤道太平洋西风异常以及赤道太平洋次表层海温之间的关系.结果表明,赤道西太平洋(5°S～5°N,120°～160°E)和赤道中东太平洋(5°S～5°N,160°E～160°W)西风异常都存在着与厄尔尼诺周期一致的年际变化,但前者还包含有显著的2～3个月季节内振荡.赤道西太平洋次表层冷暖水东移也呈现年和年际时间尺度的振荡周期.在厄尔尼诺发生前,赤道西太平洋次表层海水出现持续性增暖,赤道西太平洋西风异常频率加快,强度增强.随后赤道中太平洋(160°E～160°W)出现持续性(3个月以上)强西风异常(即西风爆发),并进一步向东扩展,同时次表层暖水沿着赤道波导东移到赤道东太平洋混合层,导致赤道东太平洋海表大面积异常增暖,形成一次厄尔尼诺现象.最后,模式模拟了1980～1984年赤道太平洋海温的变化,进一步证实了赤道纬向西风异常对暖水东移起着重要的作用.     

Zonal Wavelengths of Planetary Rossby Waves Derived from Hydrographic Transects in the Northeast Atlantic Ocean?

Using hydrographic data of three extended zonal sections, which cover the upper 1000 dbar layer along 10°, 21°, and 32°N in the North-East Atlantic between 20° and 45°W, observational evidence is presented for zonal wavelengths of resonantly excited, first mode, long, baroclinic Rossby waves. The amplitudes of associated anomalies in the mass field decrease with increasing offshore distance. The associated zonal wavelengths reach several hundred kilometres and decrease with increasing latitude. Due to the Rossby dispersion, the detected wave patterns slowly propagate westward, somewhat faster in the south than in the north. The results obtained confirm the data sets remotely sensed by satellites, as well as the outcomes of analytical and numerical models.     

Zonal variability of plankton and particle export flux in the equatorial Pacific upwelling between 165° E and 150° W

Observations made during a “La Niña” situation (April–May 1996) in the equatorial Pacific upwelling, between 165° E and 150° W, show the classic deepening of hydrological isolines from east to west, resulting in zonal gradients for surface temperature and macronutrients. However, contrasting with such a gradient, no clear zonal variation could be seen for integrated planktonic biomasses and carbon fluxes, namely: chlorophyll a, bacterial abundances, particulate organic phosphorus, mesozooplankton ash-free dry weight, primary production, and the sinking flux of particulate organic carbon (POC). Moreover, mean values of these parameters along the zonal equatorial transect, are not significantly different from those of a 7-day-long time series station made at 0°, 150° W in October 1994 during an El Niño period. Such a steady zonal distribution of planktonic parameters seems to be characteristic of equatorial Pacific upwelling west of the Galapagos Islands so that the spatial distributions of nutrient concentrations and planktonic biomass appear to be uncoupled. This is consistent with the High Nutrient-Low Chlorophyll (HNLC) concept, in which primary production is not controlled directly by macronutrient concentrations. The lack of zonal gradient also suggests that carbon budget of the equatorial Pacific is primarily controlled by oscillations in the zonal and meridian extension of the HNLC area, rather than by values of planktonic biomasses and carbon fluxes within the upwelled water, which are quite constant.     

Short-term variabilities of upper ocean current in the warm pool region during TOGA/COARE IOP

Oceanic current data in the warm pool region of the western equatorial Pacific measured by upward-looking moored Acoustic Doppler Current Profilers at two equatorial sites (147°E and 154°E) and two off-equatorial sites (2°N and 2°S, 156°E) during TOGA/COARE Intensive Observing Period (IOP) from November 1992 to February 1993 are used to examine short-term variabilities in the upper layer above 160–240 m. In time series of the zonal and meridional currents in many layers, spectral peaks are found at periods around 2 days and 4 days in addition to high energies in a period range longer than 10 days. The signal with the period of about 2 days has significantly high energies at all sites, and its magnitude is higher for the meridional current than for the zonal one. This signal is especially active in the first half of IOP from November to December in 1992. In this period, the quasi-2-day signal in the current field is coherent between northern (2°N) and southern (2°S) stations, but it has no evident relationship with that in the surface wind field around the stations. The quasi-4-day signal with the period of about 4 days has highest energies in layers above 160 m at the southern station, and is coherent between northern and southern stations. Besides, the signal at the station of 2°S has a significantly high coherence with that in the wind at the southern station, suggesting that it is a local phenomenon.     

海洋的季节变化及风应力异常对海洋影响的数值试验

对一个6层5°×4°网格的全球海洋模式作了一些改进,建立了10层5°×4°网格的全球海洋模式,进行了季节变化数值模拟,积分250a,取得稳定的结果.除了高纬度海洋外,模拟的季节变化与实际观测十分接近.在此基础上,作了热带太平洋海温场对热带季风异常响应的3组敏感性实验,第1组为赤道西太平洋异常西风向东传播的试验;第2组为整个赤道太平洋风应力振荡异常试验;第3组为赤道西太平洋异常西风、东风交替向东传播的敏感性试验.模拟结果表明:(1)第1组风应力敏感性实验结果揭示出,西太平洋西风异常的向东传播的风应力异常可以产生类似厄尔尼诺的赤道东太平洋变暖;(2)第2组试验结果表明,热带太平洋风应力的局地振荡首先在中太平洋东西部激发出海温扰动,然后海温扰动分别向东太平洋和西太平洋传播,从而引起东、西太平洋海温的异常;(3)第3组试验验证风应力QBO可以产生海洋中类似的QBO振荡.     

Numerical Simulation of North Indian Ocean State Prior to the Onset of SW Monsoon Using SSM/I Winds

The Sea Surface Temperatures (SST) and currents are simulated over the north Indian Ocean, during the onset phase of southwest monsoon for the three years 1994, 1995, and 1996, using daily Special Sensor Microwave/Imager (SSM/I) winds and National Center for Environmental Prediction (NCEP) heat fluxes as forcings in the 2½ layer thermodynamic numerical ocean model. The results are discussed for the 30-day period from 16 May to 13 June for all the three years, to determine the ocean state during the onset phase of SW monsoon. The maximum variability in the simulated SST is found along the Somali coast, Indian coasts, and equatorial regions. The maximum SST in the North Arabian Sea is found to be greater than 30°C and minimum SST in the west equatorial region is 25°C during the onset phase of all three years. Model SSTs are in agreement with Reynolds SST. SST gradients in the north-south as well as in the east-west directions, west of 80°E are found to change significantly prior to the onset. It can be inferred from the study that the SST gradient of 2.5°C/2000 km is seen due north and due west of the region 2° - 7°S, 60° - 65°E, about 8 to 10 days prior to the arrival of SW monsoon near Kerala coast. Upper and lower layer circulation fields do not show prominent interannual variability.     

Simulation of the spatiotemporal variability of the World Ocean sea surface hight by the INM climate models

The results of simulations of the World Ocean sea surface hight (SSH) in by various versions of the Climate Model of the Institute of Numerical Mathematics, Russian Academy of Sciences, are compared with the CNES-CLS09 fields of the mean dynamic topography (deviation of the ocean level from the geoid). Three models with different ocean blocks are considered which slightly differ in numerical schemes and have various horizontal spatial resolution, i.e., the INMCM4 model, which participated in the Climate Model Intercomparison Project (CMIP Phase 5, resolution of 1° × 1/2°); the INMCM5 model, which participates in the next project, CMIP6 (resolution of 1/2° × 1/4°); and the advanced INMCM-ER eddy-resolving model (resolution of 1/6° × 1/8°). It is shown that an increase in the spatial resolution improves the reproduction of ocean currents (with Agulhas and Kuroshio currents as examples) and their variability. A probable cause of relatively high errors in the reproduction of the SSH of Southern and Indian oceans is discussed.