Biennial Oscillation Research Articles

Long‐Term Variability and Tendencies in Non‐Migrating Diurnal Tide From WACCM6 Simulations During 1850–2014

AbstractThis study reports the long‐term variability and tendencies in important non‐migrating (DE3, DW2, DW3) and stationary (DS0) diurnal tides using a three‐member ensemble of historical simulations of National Center for Atmospheric Research Whole Atmospheric Community Climate Model latest version 6 for 1850–2014 (165 years). The climatological features of the tide in temperature (T), zonal wind (U) and meridional wind (V) are presented for equinoxes/solstices and winter/summer. The response due to solar cycle (SC), quasi biennial oscillation at 10 hPa and 30 hPa, El Niño‐Southern Oscillation (ENSO), ozone depleting substances (ODS), carbon dioxide (CO2), and stratospheric sulfate aerosol (volcanic eruptions) to change in tidal amplitudes with respect to 1850–1860 are analyzed using multiple linear regression. The annual mean amplitudes increase in the upper mesosphere and lowermost thermosphere (0.0001–0.01 hPa) in the three components predominantly due to increasing CO2 and the ODSs play secondary role. However, the increasing trends are not pronounced until 1950 in the case of DE3 and DW3 amplitudes. The contribution due to ENSO shows decreasing trend in DE3, DW2, and DW3 while it causes increasing trends for DS0. Furthermore, the seasonal and latitude‐pressure variations of the tidal responses to the above seven indices are presented in T and V. All the responses, except for the solar signal, are significant mostly in the upper mesosphere and lowermost thermosphere typically at equatorial/low latitudes.

Is Mesospheric Quasi Biennial Oscillation Ephemeral?

AbstractThe time evolution of mesospheric quasi biennial oscillation (MQBO) in the 82–98 km altitude region using long‐term meteor radar observations over low and equatorial latitudes and TIMED Doppler Interferometer (TIDI) observations is discussed for the first time. The wavelet spectra of monthly mean zonal winds over Serpog (6.4°S, 106.7°E; 1993–1998), Koto Tabang (0.2°S, 100.3°E; 2003–2012) and Thumba (8.5°N, 76.9°E; 2006–2015) show the transient nature of the MQBO amplitudes, thus, questioning its perpetuity in the mesosphere‐lower thermosphere (MLT). The time period of the MQBO as well as its amplitude show variability with time. The latitude‐height structures of the MQBO at four time periods (24, 26, 28, and 30 months) are constructed using TIDI measurements, which show diverse latitudinal structure. The significance of the present study lies in bringing out the time evolution of the MQBO and addressing a few outstanding issues on its ephemerality and discrepancies in its amplitudes.

Long‐Term Variability and Tendencies in Middle Atmosphere Temperature and Zonal Wind From WACCM6 Simulations During 1850–2014

AbstractLong‐term variability of middle atmosphere temperature (T) and zonal wind (U) is investigated using a three‐member ensemble of historical simulations of NCAR's Whole Atmospheric Community Climate Model latest version 6 (WACCM6) for 1850–2014 (165 years). The model reproduces the climatological features of T and U. The contributions of Quasi Biennial Oscillation (QBO) at 10 and 30 hPa, solar cycle (SC), El Niño‐Southern Oscillation (ENSO), ozone depleting substances (ODS), carbon dioxide (CO2), and stratospheric sulfate aerosol (volcanic eruptions) to change in monthly zonal mean T and U are analyzed using multiple linear regression. The signal due to CO2 increase dominates as a predictor of the net multidecadal global annual mean temperature change at all levels in the middle atmosphere. Contributions from ODS also affect the net multidecadal global mean temperature trend in the stratosphere. Because of similarities in the time evolution of the emissions of CO2 and ODS, the analysis of existing model output cannot accurately separate the attributions of cooling to these two dominant forcing processes. On shorter time scales, solar flux variations are the largest source of variability in the mesosphere while volcanic eruptions are the largest in the stratosphere. In the stratosphere and mesosphere, both QBO and ENSO can significantly impact zonal mean temperature and zonal‐mean zonal wind depending on latitudes, but their impact on the multidecadal global mean temperature trend is very small.

Long‐Term Variability and Tendencies in Migrating Diurnal Tide From WACCM6 Simulations During 1850–2014

AbstractLong‐term variability and tendencies in migrating diurnal tide (DW1) are investigated for the first time using a three‐member ensemble of historical simulations by NCAR's Whole Atmosphere Community Climate Model, latest Version 6 (WACCM6) for 1850–2014 (165 years). The model reproduces the climatological features of the tide in temperature (T), zonal wind (U), and meridional wind (V). The amplitudes peak in the upper mesosphere and lower thermosphere (above ~0.001 hPa) at the equator forT(~10 K) and over 20–30°N and S latitudes forU(~15 m/s) andV(~25 m/s). The contributions of solar cycle (SC), quasi biennial oscillation (QBO) at 10 and 30 hPa, El Niño–Southern Oscillation (ENSO), ozone depleting substances (ODS), carbon dioxide (CO2), and stratospheric sulfate aerosols (volcanic eruptions) to change in annual mean amplitudes are analyzed using multiple linear regression. The tidal amplitudes in three components show a long‐term increase in the upper stratosphere (0.95–10.7 hPa) and the upper mesosphere (0.0001–0.01 hPa), predominantly due to increasing CO2with a smaller contribution from the trend in ENSO. Interestingly, the global mean tidal amplitude inTdecreases sharply after 1950–1960 until 1995 and then increases in association with changes in ODSs. The seasonal differences in tidal responses to the above indices can be as large as the overall signals. All the responses are stronger in the upper mesosphere; however, there is also a pronounced negative response of temperature tide to ODSs over middle to high latitudes around the stratopause (~1 hPa) during all seasons.

Forecasting extreme stratospheric polar vortex events

Extreme polar vortex events known as sudden stratospheric warmings can influence surface winter weather conditions, but their timing is difficult to predict. Here, we examine factors that influence their occurrence, with a focus on their timing and vertical extent. We consider the roles of the troposphere and equatorial stratosphere separately, using a split vortex event in January 2009 as the primary case study. This event cannot be reproduced by constraining wind and temperatures in the troposphere alone, even when the equatorial lower stratosphere is in the correct phase of the quasi biennial oscillation. When the flow in the equatorial upper stratosphere is also constrained, the timing and spatial evolution of the vortex event is captured remarkably well. This highlights an influence from this region previously unrecognised by the seasonal forecast community. We suggest that better representation of the flow in this region is likely to improve predictability of extreme polar vortex events and hence their associated impacts at the surface.

Long Period Waves and Oscillations Over Costa Rica, a Tropical Station

Seasonal, annual and interannual variations of Ozone Mixing Ratio (OMR) have been studied for 12 years, from balloonsondes and Ozonesondes that were launched over Costa Rica (10oN, 83.4oW), a tropical station, as part of Southern Hemisphere Additional Ozonesondes (SHADOZ) network. It was found that near tropopause region there exists prominent peak of OMR during July month for most of the years. Lomb-Scargle Periodogram (LSP) analysis has been applied for the years 2005-2017 to identify the wave activities and found that Quasi Biennial Oscillations (QBO) with period of 2-2.5 years and even higher period oscillations of 3-4 years are prominent in middle troposphere and lower stratosphere regions which could cause large annual variabilities of OMR fluctuations. Fluctuations of OMR were subjected to Morlet wavelet transform over a year, 2008, to study seasonal variabilities. The wavelet analysis confirm that Madden Julian Oscillations (MJO) with periods 49-60 days are prominent during summer near tropopuase and in lower stratosphere regions, while lower period equatorial Kelvin waves of 14-20 days periods dominate during winter and spring in troposphere region, which could be responsible for maximum seasonal variability.

Multiscale Variability of Meiyu and Its Prediction: A New Review

AbstractFollowing the description of the basic climatic characteristics of Meiyu in East Asia, we present a comprehensive review of multi–time scale variabilities of the Meiyu together with the long‐term change of extreme precipitation during the Meiyu in this paper. The dynamic prediction skills of the Meiyu on subseasonal to seasonal time scales are further described and illustrated by using present numerical prediction models. For the multi–time scale variabilities of the East Asian Meiyu, they are closely related with the impacts of both quasi‐biweekly and 30–60 day oscillation from both the tropics and the middle and high latitudes on intraseasonal time scale, the influences of El Niño–Southern Oscillation and the Tropospheric Biennial Oscillation on interannual time scale, the variations of East Asian summer monsoon on decadal time scale, and the continued impacts of global warming, Pacific Decadal Oscillation and the Atlantic Multidecadal Oscillation on interdecadal scale and long‐term trend. Under the background of the global warming and the influences of urbanization and aerosol effects, the Meiyu shows some specific characteristics including significantly weakened continuous rain, uneven spatial distribution of precipitation, and increased (decreased) number of strong (weak) precipitation days. Improvements of prediction on rain belt location, total precipitation amount, persistent heavy precipitation events, and low‐frequency variability oscillation during Meiyu season have been achieved with the improved dynamic models of the Beijing Climate Center since 2010.

Nearly Century-scale Variation of the Sun’s Radius

Abstract The Kodaikanal Archive Program (India) is now available to the scientific community in digital form as daily digitized solar white light pictures, from 1923 to 2011. We present here the solar radius data, obtained after a painstaking effort to remove all effects that contribute to the error in their measurements (limb darkening, distortion of the objective lens, refraction, other instrumental effects, etc.). These data were analyzed to reveal any significant periodic variations, after applying a multi-taper method with red noise approximation and the Morlet wavelet transform analysis. After removing obvious periodic variations (such as solar rotation and Earth annual rotation), we found a possible cycle variation at 11.4 yr, quasi biennial oscillations at 1.5 and 3.8 yr, and Rieger-type periodicity at ≈159, 91, and 63 days. Another yr periodicity (as a mean) resulting from two other main periodicities detected at 6.3–7.8 yr can be identified as an atmospheric component. The detrending data show, over a mean radius of , a residual of less than ≈(−)1 mas over the time period of analysis: if not spurious, this estimate indicates a faint decline, but probably confirms more the constancy of the solar diameter during the considered ranging time, within instrumental and methodological limits. The Kodaikanal long quality observations contribute to international efforts to bring past solar data measurements to the community to further explore issues, for instance, those of the luminosity/radius properties that could be used to pinpoint the “seat of the solar cycle.”

Delineation of possible influence of solar variability and galactic cosmic rays on terrestrial climate parameters

The present paper has investigated the associations of solar activity (SA), represented by total solar irradiance (TSI), galactic cosmic rays (GCR) and terrestrial climate parameters in particular the global cloudiness and global surface temperature. To that end, we have analysed thirty five years (1983–2018) data of these parameters and have applied the Granger-causality test in order to assess whether there is any potential predictability power of one indicator to the other. The correlations among the involved parameters are tested using Vector Auto Regression (VAR) model and variance decomposition method. As a result of the above analysis, we have found that the TSI is an important factor and has contributed about 8.77 ± 0.42% in the cosmic ray intensity variations. In case of cloud cover variations, the other three parameters (TSI, cosmic ray and global surface temperature) have played a significant role. Further, the TSI changes have contributed 1.68 ± 0.03% fluctuations in the variance of the cloud cover while the cosmic ray intensity and global surface temperature have contributed about 4.89 ± 0.08% and 10.87 ± 1.41% respectively. In case of the global surface temperature anomaly both TSI and cloud covers have contributed about 5.07 ± 0.47% and 14.42 ± 2.13% fluctuations respectively. Additionally, we have also assessed the impact of internal climate oscillations like multivariate ENSO index (MEI), north Atlantic oscillations (NAO) and quasi biennial oscillations (QBO) on cloud cover variations. The contribution of these internal oscillations e.g. ENSO, NAO and QBO in cloud cover variation were reported as 7.48 ± 1.02%, 5.51 ± 0.16% and 1.36 ± 0.43% respectively.

Periodicity in stromatolitic lamination: A potential record of ENSO, NAO, and SUNSPOT in the Miocene lacustrine record of the Ebro Basin, Spain

The spectral decomposition of time series obtained from ancient rock records can be used to study the similarity between the dynamics of present-day and past climate systems. A high-resolution periodicity analysis of luminance and lamina thickness of lacustrine stromatolites in the Ebro Basin (northeast of Iberian Peninsula) reveals a significant signal of interannual and decadal climatic variability in the Miocene. This is one out of very few works that presents the use of spectral analysis to estimate the potential of stromatolite lamination as multiple-scale recorders of climate parameters. The effects of precipitation and evaporation variations on stromatolite lamination have been detected at three orders of cyclicity based on textural and high-resolution stable-isotope analyses (C and O) obtained in prior studies. These analyses also revealed that the light and dark simple lamina couplets are identified with annual cycles (third-order isotopic cycles). In the present study, the spectral analysis results obtained through different paths in five stromatolite specimens reveal significant periods in the power spectrum at around 2.5, 3.7, 5, 7, 10, and 22 years. These cycles can be correlated with the typical oscillation bands of different climate-related agents. The 2.5-year period corresponds to the Quasi Biennial Oscillation (QBO), or to the biennial component of the El Niño-Southern Oscillation (ENSO), or to the North Atlantic Oscillation (NAO). The 3 to 5 and 5 to 7-year bands can be linked to ENSO or NAO variability (second-order isotopic cycles). The 8 to 11-year bands fit the 11-year Schwabe (first-order isotopic cycles) and 22 to 23-year fit the 22-year Hale sunspot cycles. Thus, the stromatolite growth was controlled by ENSO, NAO, and solar activity cycles. The close relationship between these climate-related agents makes it difficult to specify the dominant agent controlling the stromatolite growth. Nevertheless, the significant periods obtained from this study, within interannual (2.5, 3.7, 5, 7 years), decadal (10, 22 years), and even multidecadal bands (37–42 years), support the existence, and concurrence, of ENSO and NAO precursors during the early and middle Miocene.

Observations of middle atmospheric seasonal variations and study of atmospheric oscillations at equatorial regions

The equatorial atmospheric regions are unique and one of the most important parts of the Earth's atmosphere. These regions receive the maximum energy from the Sun, and are highly turbulent and influenced by the processes of the Northern Hemisphere as well as by the Southern Hemisphere. Atmospheric temperature is one the most sensitive parameters and having imprints of dynamical, radiative and chemical processes. Thermal structure, seasonal variability and atmospheric oscillations occurring in the equatorial middle atmosphere have been studied here using temperature profiles retrieved from the Sounding of the Atmosphere using the Broadband Emission Radiometry (SABER) instrument on-board Thermosphere-Ionosphere-Mesosphere Energetics and Dynamics (TIMED) satellite during January 2002 to December 2015 (14 years). In the present work, three geographical locations over the Equatorial latitudes and ±120° longitude have been chosen, considering 5° × 15° latitude-longitude grid between the altitudes ranging from 20 to 100 km. A detailed time series and climatology analyses have been performed over these regions and significant longitudinal differences have been observed. Characteristics of Semi Annual oscillation (SAO), Annual Oscillation (AO) and Quasi Biennial Oscillation (QBO) have been investigated using Lomb Scargle Periodogram and wavelet transform analysis techniques. It has been noted the longitudinal differences in the spectral amplitudes of equatorial oscillations relative to the zonal mean spectrum. The longitudinal asymmetry might be due to the vertically propagating planetary waves. From the analysis, it is also observed that SAO is showing the highest amplitude at 45 km and 75–85 km, and QBO is showing significant amplitude in the lower stratosphere. At higher altitudes, amplitudes of different oscillations (viz. SAO, AO, and QBO) are very weak, and temperatures variations are not revealing any significant pattern. Moreover, from the climatology and time series of the temperature measurements, high temperature (∼260 K) has been noticed at 55–65 km height range. In addition to that, seasonal variations over equatorial regions have been studied and the strength of seasonal differences are found to be about 9 K (summer-winter) at higher altitudes.

Sources of the seasonal-trend behaviour and periodicity modulation of 7Be air concentration in the atmospheric surface layer observed in southeastern Spain

The components and the periodicity characterising airborne 7Be monthly concentration collected in Granada (SE Spain) between 2000 and 2018 are studied. These 19 years of measurements are particularly significant for the 7Be analysis as they cover the complete 24th solar cycle, 2008–2018.A Time Series Decomposition (TSD) technique has been applied to determine the different components present in the dataset: the trend, seasonal and irregular components. Then, the atmospheric parameters (Precipitation (Pp), Relative Humidity (RH), Temperature (Temp), Wind speed (WS), Total Cloudiness (TC), Low Cloudiness (LC)), teleconnection indices and the solar activity (Sun Spot Number, SSN) were used to justify the variability of each component.SSN and the teleconnection indices (Western Mediterranean Oscillation (WeMO), Atlantic Multidecadal Oscillation (AMO) and Quasi Biennial Oscillation (QBO)) are found to have a major impact on the trend component of 7Be. In turn, all the atmospheric parameters were found to have strong impact on the seasonal component.In order to observe the change in the impact of the parameters from the 24th solar cycle (from 2008 to 2018) to the complete time series (starting in 2000), the comparison between the two periods has been carried out. The results show that, over the 24th solar cycle, the SSN impacted predominantly on the trend component, whereas atmospheric parameters showed a slightly higher impact on the seasonal component. In addition, some atmospheric factors (Temp, RH, Pp, TC, LC) appeared to partially affect the irregular component.The analysis of the two phases of the 24th solar cycle shows that the influence of SSN is higher during the descending phase than during the ascending one. This behaviour is opposed to the one found for the 23rd solar cycle, which clarifies the special characteristics of the 24th cycle.Besides that, the Continuous Wavelet Transform (CWT) analysis was applied in order to extract the possible periodicities characterising 7Be data. The analysis revealed three sets of periodicities. Then, the Wavelet Coherence Analysis (WCA) method was particularly useful to study coherences between 7Be data and teleconnection indices. The 7Be concentrations in the large period (11–14 years) detected by CWT was found to be mainly modulated by WeMO, AMO and QBO, while the NAO modulates the smaller periods.

Global Patterns of Carbon Dioxide Variability from Satellite Observations

Advanced satellite technology has been providing unique observations of global carbon dioxide (CO2) concentrations. These observations have revealed important CO2 variability at different timescales and over regional and planetary scales. Satellite CO2 retrievals have revealed that stratospheric sudden warming and the Madden-Julian Oscillation can modulate atmospheric CO2 concentrations in the mid-troposphere. Atmospheric CO2 also demonstrates variability at interannual timescales. In the tropical region, the El Niño–Southern Oscillation and the Tropospheric Biennial Oscillation can change atmospheric CO2 concentrations. At high latitudes, mid-tropospheric CO2 concentrations can be influenced by the Northern Hemispheric annular mode. In addition to modulations by the large-scale circulations, sporadic events such as wildfires, volcanic eruptions, and droughts, which change CO2 surface emissions, can cause atmospheric CO2 concentrations to increase significantly. The natural variability of CO2 summarized in this review can help us better understand its sources and sinks and its redistribution by atmospheric motion. ▪ Global satellite CO2 data offer a unique opportunity to explore CO2 variability in different regions. ▪ Atmospheric CO2 concentration demonstrates variations at intraseasonal, seasonal, and interannual timescales. ▪ Both large-scale circulations and variations of surface emissions can modulate CO2 concentrations in the atmosphere.

Evaluating the Brewer–Dobson circulation and its responses to ENSO, QBO, and the solar cycle in different reanalyses

This study compares the climatology and long-term trend of northern winter stratospheric residual mean meridional circulation (RMMC), as well as its responses to El Nino-Southern Oscillation (ENSO), stratospheric Quasi Biennial Oscillation (QBO), and solar cycle in ten reanalyses and a stratosphere-resolving model, CESM1-WACCM. The RMMC is a large-scale meridional circulation cell in the stratosphere, usually referred to as the estimate of the Brewer Dobson circulation (BDC). The distribution of the BDC is generally consistent among multiple reanalyses except that the NOAA twentieth century reanalysis (20RC) largely underestimates it. Most reanalyses (except ERA40 and ERA-Interim) show a strengthening trend for the BDC during 1979–2010. All reanalyses and CESM1-WACCM consistently reveal that the deep branch of the BDC is significantly enhanced in El Nino winters as more waves from the troposphere dissipate in the stratospheric polar vortex region. A secondary circulation cell is coupled to the temperature anomalies below the QBO easterly center at 50 hPa with tropical upwelling/cooling and midlatitude downwelling/warming, and similar secondary circulation cells also appear between 50–10 hPa and above 10 hPa to balance the temperature anomalies. The direct BDC response to QBO in the upper stratosphere creates a barrier near 30°N to prevent waves from propagating to midlatitudes, contributing to the weakening of the polar vortex. The shallow branch of the BDC in the lower stratosphere is intensified during solar minima, and the downwelling warms the Arctic lower stratosphere. The stratospheric responses to QBO and solar cycle in most reanalyses are generally consistent except in the two 20CRs.

Comparison of the QBO and F10.7 Solar Flux Effects on Total Mass Density

The comparison of the Quasi Biennial Oscillation (QBO) and F10.7 solar flux effects on Total Mass Density (TMD) obtained from NRLMSIS-00 model for 90 km altitude of ionosphere known as Mesosphere-Lower Thermosphere (MLT) region was made statistically. In the results of calculations, it was observed that QBO and F10.7 solar flux have an effect on TMD. It was determined that about 69% of the variations in TMD could be explained by F10.7 and QBO. Also, it was seen that an increase/a decrease of 1 meter per second occurred in QBO gave rise to an increase/a decrease of 2.36 × 10–9 kg/m3 in TMD while an increase/a decrease of 1 s.f.u. in F10.7 gave rise to a decrease/increase of 1.02 × 10–9 kg/m3 in TMD. Based on these results, it was observed that the effects of stratospheric QBO, which is one of the meteorological processes, and F10.7, which is one of the indicators of the solar processes, were nearly at the same rate on TMD of MLT region. Furthermore, the results show that the stratospheric QBO may be a suitable candidate for increasing the anomalous density reductions observed in TMD.

Short‐Term and Interannual Variations of Migrating Diurnal and Semidiurnal Tides in the Mesosphere and Lower Thermosphere

AbstractAmong the broad spectrum of vertically propagating tides, migrating diurnal (DW1) and semidiurnal (SW2) are prominent modes of energetic and dynamical coupling between the mesosphere and lower thermosphere and the upper thermosphere and ionosphere. DW1 and SW2 tides are modulated on time scales ranging from days to years. NASA Thermosphere‐Ionosphere‐Mesosphere Energetic and Dynamics (TIMED) is the first observational platform to perform global synoptic observations of these fundamental tides (for nearly two decades) overcoming previous observational limitations. Here we utilize the extensive archive of TIMED Doppler Interferometer wind measurements and exploit the capabilities of tidal theory to estimate short‐term (<1 month), seasonal (intra‐annual), long‐term (>1 year), and climatological variability in DW1 (1,1), SW2 (2,2), and SW2 (2,3) modes and then compare with tidal estimates derived from the Navy Global Environmental Model‐High Altitude version data assimilation system. Overall, the tidal estimates from TIMED Doppler Interferometer and Navy Global Environmental Model‐High Altitude version are similar and exhibit significant short‐term and intra‐annual variability. The short‐term variability can induce ∼64% change in the DW1 amplitude. Statistically, the short‐term variability in DW1 (1,1), SW2 (2,2), and SW2 (2,3) modes is of the order of ∼9, 33, and 20 m/s, respectively. The biennial oscillations in DW1 and SW2 modes suggest a systematic correlation with the equatorial quasi‐biennial oscillation in the stratosphere and are more apparent in DW1 amplitudes. Although there is significant interannual variability in addition to the apparent biennial signal, there is no clear evidence of any solar cycle dependence or long‐term trend in either DW1 or SW2 modes.

On the QBO modulation in the frequency of occurrence of pre noon F3 layers over the dip equatorial location of Thiruvananthapuram

Using the ionosonde observations, over Thiruvananthapuram (8.5° N, 77oE, 0.5oN dip lat.), India, for a period of 20 years (1990–2009), the present study reveals the presence of quasi biennial oscillation (QBO) in the pre noon F3 layer occurrence percentage. The presence of a strong QBO modulation in the occurrence of pre noon F3 layers is observed during the period from 1992 to 2000 beyond which it is rather weak/nonexistent. The wavelet periodogram analysis indicates the fact that F3 occurrences are more during the eastward phase of MQBO and less during the westward phase of MQBO.

Relationships between intensity of the Kuroshio current in the East China Sea and the East Asian winter monsoon

Based on satellite altimeter and reanalysis data, this paper studies the relationships between the intensity of the Kuroshio current in the East China Sea (ECS) and the East Asian winter monsoon (EAWM). The mechanisms of their possible interaction are also discussed. Results indicate that adjacent transects show consistent variations, and on an interannual timescale, when the EAWM is anomalously strong (weak), the downstream Kuroshio in the ECS is suppressed (enhanced) in the following year from February to April. This phenomenon can be attributed to both the dynamic effect (i.e., Ekman transport) and the thermal effect of the EAWM. When the EAWM strengthens (weakens), the midstream and downstream Kuroshio in the ECS are also suppressed (intensified) during the following year from October to December. The mechanisms vary for these effects. The EAWM exerts its influence on the Kuroshio's intensity in the following year through the tropospheric biennial oscillation (TBO), and oceanic forcing is dominant during this time. The air-sea interaction is modulated by the relative strength of the EAWM and the Kuroshio in the ECS. The non-equivalence of spatial scales between the monsoon and the Kuroshio determines that their interactions are aided by processes with a smaller spatial scale, i.e., local wind stress and heating at the sea surface.

Surface impacts of the Quasi Biennial Oscillation

Abstract. Teleconnections between the Quasi Biennial Oscillation (QBO) and the Northern Hemisphere zonally averaged zonal winds, mean sea level pressure (mslp) and tropical precipitation are explored. The standard approach that defines the QBO using the equatorial zonal winds at a single pressure level is compared with the empirical orthogonal function approach that characterizes the vertical profile of the equatorial winds. Results are interpreted in terms of three potential routes of influence, referred to as the tropical, subtropical and polar routes. A novel technique is introduced to separate responses via the polar route that are associated with the stratospheric polar vortex, from the other two routes. A previously reported mslp response in January, with a pattern that resembles the positive phase of the North Atlantic Oscillation under QBO westerly conditions, is confirmed and found to be primarily associated with a QBO modulation of the stratospheric polar vortex. This mid-winter response is relatively insensitive to the exact height of the maximum QBO westerlies and a maximum positive response occurs with westerlies over a relatively deep range between 10 and 70 hPa. Two additional mslp responses are reported, in early winter (December) and late winter (February/March). In contrast to the January response the early and late winter responses show maximum sensitivity to the QBO winds at ∼ 20 and ∼ 70 hPa respectively, but are relatively insensitive to the QBO winds in between (∼ 50 hPa). The late winter response is centred over the North Pacific and is associated with QBO influence from the lowermost stratosphere at tropical/subtropical latitudes in the Pacific sector. The early winter response consists of anomalies over both the North Pacific and Europe, but the mechanism for this response is unclear. Increased precipitation occurs over the tropical western Pacific under westerly QBO conditions, particularly during boreal summer, with maximum sensitivity to the QBO winds at 70 hPa. The band of precipitation across the Pacific associated with the Inter-tropical Convergence Zone (ITCZ) shifts southward under QBO westerly conditions. The empirical orthogonal function (EOF)-based analysis suggests that this ITCZ precipitation response may be particularly sensitive to the vertical wind shear in the vicinity of 70 hPa and hence the tropical tropopause temperatures.

Tropospheric biennial oscillation and south Asian summer monsoon rainfall in a coupled model

In this study Tropospheric Biennial Oscillation (TBO) and south Asian summer monsoon rainfall are examined in the National Centers for Environmental Prediction (NCEP) Climate Forecast System (CFSv2) hindcast. High correlation between the observations and model TBO index suggests that the model is able to capture most of the TBO years. Spatial patterns of rainfall anomalies associated with positive TBO over the south Asian region are better represented in the model as in the observations. However, the model predicted rainfall anomaly patterns associated with negative TBO years are improper and magnitudes are underestimated compared to the observations. It is noted that positive (negative) TBO is associated with La Niña (El Niño) like Sea surface temperature (SST) anomalies in the model. This leads to the fact that model TBO is El Niño-Southern Oscillation (ENSO) driven, while in the observations Indian Ocean Dipole (IOD) also plays a role in the negative TBO phase. Detailed analysis suggests that the negative TBO rainfall anomaly pattern in the model is highly influenced by improper teleconnections allied to IOD. Unlike in the observations, rainfall anomalies over the south Asian region are anti-correlated with IOD index in CFSv2. Further, summer monsoon rainfall over south Asian region is highly correlated with IOD western pole than eastern pole in CFSv2 in contrast to the observations. Altogether, the present study highlights the importance of improving Indian Ocean SST teleconnections to south Asian summer rainfall in the model by enhancing the predictability of TBO. This in turn would improve monsoon rainfall prediction skill of the model.