Variability of seasonal-mean fields arising from intraseasonal variability. part 2, application to nh winter circulations

A method for studying patterns of interannual variability arising from intraseasonal variability has been applied to the extratropical Northern Hemisphere wintertime 500 hPa geopotential height, using data from the NCEP-NCAR. These patterns describe the effects predominantly of intraseasonal variability and blocking. Removing this component from the sample interannual covariance matrix, one can define a residual, or slow, component of interannual variability that is more closely related to external forcings and very slowly varying (interannual/supra-annual) internal dynamics. For the Northern Hemisphere NCEP-NCAR reanalysis data, there are considerable differences between the intraseasonal patterns and the total patterns. The intraseasonal patterns are more spatially localized and more closely related to known intraseasonal variability, especially blocking events and the Madden-Julian Oscillation. Although the slow patterns and the total patterns look similar, they have some important differences. The slow patterns are more closely related to the slowly varying external forcing and very low-frequency internal dynamics than those derived by the sample covariance matrix. This is evidenced by the fact that the principal component time series of the slow patterns have a larger proportion of variability related to these factors. Where tropical SST forcing is important, the slow patterns tended to be more highly correlated with the interannual variations in the forcing. Three slow modes, related to the Tropical Northern Hemisphere, East Atlantic and Western Pacific teleconnections, are all significantly related to tropical SST variability associated predominantly with the El Nino-Southern Oscillation, in the case of the first two, and Indian Ocean variability, in the third case. The derived slow patterns and intraseasonal patterns may help to better understand the long-range predictability, uncertainty, and forcing of climate variables, for the wintertime circulation.     

Variability of seasonal-mean fields arising from intraseasonal variability. Part 3: Application to SH winter and summer circulations

A study has been made, using the National Centers for Environmental Prediction and National Center for Atmospheric Research re-analysis 500 hPa geopotential height data, to determine how intraseasonal variability influences, or can generate, coherent patterns of interannual variability in the extratropical summer and winter Southern Hemisphere atmospheric circulation. In addition, by separating this intraseasonal component of interannual variability, we also consider how slowly varying external forcings and slowly varying (interannual and longer) internal dynamics might influence the interannual variability of the Southern Hemisphere circulation. This slow component of interannual variation is more likely to be potentially predictable. How sea surface temperatures are related to the slow components is also considered. The four dominant intraseasonal modes of interannual variability have horizontal structures similar to those seen in both well-known intraseasonal dynamical modes and statistical modes of intraseasonal variability. In particular, they reflect intraseasonal variability in the high latitudes associated with the Southern Annular Mode, and wavenumber 4 (summer) and wavenumber 3 (winter) patterns associated with south Pacific regions of persistent anomalies and blocking, and possibly variability related to the Madden-Julian Oscillation (MJO). The four dominant slow components of interannual variability, in both seasons, are related to high latitude variability associated with the Southern Annular Mode, El Nino Southern Oscillation (ENSO) variability, and South Pacific Wave variability associated with Indian Ocean SSTs. In both seasons, there are strong linear trends in the first slow mode of high latitude variability and these are shown to be related to similar trends in the Indian Ocean. Once these are taken into account there is no significant sea surface temperature forcing of these high latitude modes. The second and third ENSO related slow modes, in each season, have high correlations with tropical sea surface temperature variability in the Pacific and Indian Oceans, both contemporaneously and at one season lag. The fourth slow mode has a characteristic South Pacific wave structure of either a wavenumber 4 (summer) or wavenumber 3 (winter) pattern, with strongest loadings in the South Pacific sector, and an association simultaneously with a dipole SST temperature gradient in the subtropical Indian Ocean.     

Pathways of intraseasonal variability in the Indonesian Throughflow region

The recent INSTANT measurements in the Indonesian archipelago revealed a broad spectrum of time scales that influence Indonesian Throughflow (ITF) variability, from intraseasonal (20–90 days) to interannual. The different time scales are visible in all transport and property fluxes and are the result of remote forcing by both the Pacific and Indian Ocean winds, and local forcing generated within the regional Indonesian seas. This study focuses on the time-dependent three-dimensional intraseasonal variability (ISV) in the ITF region, in particular at the locations of the INSTANT moorings at the Straits of Lombok, Ombai and Timor. Observations from the INSTANT program in combination with output from the Bluelink ocean reanalysis provide a comprehensive picture about the propagation of ISV in the ITF region. The reanalysis assimilates remotely sensed and in situ ocean observations into an ocean general circulation model to create a hindcast of ocean conditions. Data from the reanalysis and observations from the INSTANT program reveal that deep-reaching subsurface ISV in the eastern Indian Ocean and ITF is closely linked with equatorial wind stress anomalies in the central Indian Ocean. Having traveled more than 5000 km in about 14 days, the associated Kelvin waves can be detected as far east as the Banda Sea. ISV near the Straits of Ombai and Timor is also significantly influenced by local wind forcing from within the ITF region. At the INSTANT mooring sites the ocean reanalysis agrees reasonably well with the observations. Intraseasonal amplitudes are about ±1.0 °C and ±0.5 m/s for potential temperature and velocity anomalies. Associated phases of ISV are very similar in observations and the reanalysis. Where differences exist they can be traced back to likely deficits in the reanalysis, namely the lack of tidal dissipation, insufficient spatial resolution of fine-scale bathymetry in the model in narrow straits or errors in surface forcing.     

Interactions between synoptic, intraseasonal and interannual convective variability over Southern Africa

After removing the annual cycle, a principal component analysis is applied to the daily outgoing longwave radiation anomaly field, used here as a proxy for atmospheric convection. The analysis is carried out over the southern African region (7.5°E–70°E, 10°S–40°S) for austral summer (November through February) for the period 1979–1980 to 2006–2007. The first five principal components (PC) are retained. The first two PCs describe spatial patterns oriented north-west to south-east from tropical southern Africa (SA) to the mid-latitudes. They are interpreted to be different possible locations for synoptic-scale tropical–temperate troughs (TTT), one dominant rainfall-producing synoptic system in the region. The phase relationship between these two PCs describes a tendency for these TTT to propagate eastwards from SA to the Mozambique Channel and southern Madagascar. The next three PCs describe convective fluctuations, respectively, located over the north-west, the south and the centre of SA. Their time series are significantly associated with Madden–Julian oscillation (MJO) activity in the tropics. However, we find that TTT systems are statistically independent of the MJO, i.e. they are equally liable to occur during any phase of the MJO. Three PCs out of five also show a significant association with El Niño southern oscillation, confirming that El Niño years mostly coincide with suppressed convection at the intraseasonal time-scales, a result consistent with its impact on seasonal averages diagnosed in previous studies.     

Seasonal evolution of the intraseasonal variability of China summer precipitation

Climate Dynamics - Seasonal cycle of China summer precipitation has significant impacts on its subseasonal predictability; yet current understanding of seasonal evolution of the intraseasonal...     

Relationships between intraseasonal rainfall variability of coastal Tanzania and ENSO

Summary The influence of ENSO on intraseasonal variability over the Tanzanian coast during the short (OND) and long (MAM) rainy seasons is examined. In particular, variability in the rainfall onset, peak and end dates as well as dry spells are considered. In general, El Niño appears to be associated with above average rainfall while La Niña is associated with below average rainfall over the northern Tanzanian coast during OND, and to lesser extent MAM. Over the southern coast, the ENSO impacts are less coherent and this region appears to be a transition zone between the opposite signed impacts over equatorial East and southern Africa. The increased north coast rainfall during El Niño years is generally due to a longer than normal rainfall season associated with early onset while reduced rainfall during La Niña years tends to be associated with a late onset, and thus a shorter than average rainfall season. Wet conditions during El Niño years were associated with enhanced convection and low-level easterly anomalies over the equatorial western Indian Ocean implying enhanced advection of moisture from the Indian Ocean while the reverse is true for La Niña years. Hovmöller plots for OLR and zonal wind at 850 hPa and 200 hPa show eastward, westward propagating and stationary features over the Indian Ocean. It was observed that the propagating features were absent during strong El Niño years. Based on the Hovmöller results, it is observed that the convective oscillations over the Tanzanian coast have some of the characteristic features of intraseasonal oscillations occurring elsewhere in the tropics.     

AGCM simulations of intraseasonal variability associated with the Asian summer monsoon

The intraseasonal variability associated with the Asian summer monsoon as simulated by a number of atmospheric general circulation models (AGCMs) are analyzed and assessed against observations. The model data comes from the Monsoon GCM Intercomparison project initiated by the CLIVAR/Asian–Australian Monsoon Panel. Ten GCM groups, i.e., the Center for Ocean–Land–Atmosphere Studies (COLA), Institute of Numerical Mathematics (DNM), Goddard Space Flight Center (GSFC), Geophysical Fluid Dynamics Laboratory (GFDL), Institute of Atmospheric Physics (IAP), Indian Institute of Tropical Meteorology (IITM), Meteorological Research Institute (MRI), National Center for Atmospheric Research (NCAR), Seoul National University (SNU), and the State University of New York (SUNY), participated in the intraseasonal component of the project. Each performed a set of 10 ensemble simulations for 1 September 1996–31 August 1998 using the same observed weekly SST values but with different initial conditions. The focus is on the spatial and seasonal variations associated with intraseasonal variability (ISV) of rainfall, the structure of each model's principal mode of spatial-temporal variation of rainfall [i.e. their depiction of the Intraseasonal Oscillation (ISO)], the teleconnection patterns associated with each model's ISO, and the implications of the models' ISV on seasonal monsoon predictability. The results show that several of the models exhibit ISV levels at or above that found in observations with spatial patterns of ISV that resemble the observed pattern. This includes a number of rather detailed features, including the relative distribution of variability between ocean and land regions. In terms of the area-averaged variance, it is found that the fidelity of a model to represent NH summer versus winter ISV appears to be strongly linked. In addition, most models' ISO patterns do exhibit some form of northeastward propagation. However, the model ISO patterns are typically less coherent, lack sufficient eastward propagation, and have smaller zonal and meridional spatial scales than the observed patterns, and are often limited to one side or the other of the maritime continent. The most pervasive and problematic feature of the models' depiction of ISV and/or their ISO patterns is the overall lack of variability in the equatorial Indian Ocean. In some cases, this characteristic appears to result from some models forming double convergence zones about the equator rather than one region of strong convergence on the equator. This shortcoming results in a poor representation of the local rainfall pattern and also significantly influences the models' representations of the global-scale teleconnection patterns associated with the ISO. Finally, analysis of the model ensemble shows a positive relationship between the strength of a model's ISV of rainfall and its intra-ensemble variability of seasonal monsoon rainfall. The implications of this latter relation are discussed in the context of seasonal monsoon predictability.     

Influence of the intraseasonal variability on heat waves in subtropical South America

The influence of the intraseasonal variability on heat wave development over subtropical South America during austral summer is analyzed. The role of the South Atlantic Convergence Zone (SACZ) on this development is documented. Results show that intraseasonal variability can explain on average at least 32% of summer temperature variance. Moreover, 73% of the heat waves in subtropical South America develop in association with an active SACZ. The analysis of pentad maps shows that warm conditions in the region under study develop in association with the strengthening of an anticyclonic anomaly, which is discernible over the subtropical regions at least 15?days before temperature peak occurrence. That circulation anomaly is embedded in a large-scale Rossby wave train extending along the South Pacific Ocean that is linked to convection anomalies at the equatorial western and central Pacific Ocean. In addition, the development of the anticyclonic circulation over subtropical South America appears to be strengthened by the subsidence conditions promoted by the active SACZ, which result in temperature rise in the subtropical region under relatively dry conditions. On the other hand, during the last 2?days of evolution, SACZ activity weakens and the progressive temperature rise in the region is dominated by warmer and moister air being anomalously advected from the north. Results confirm the important role that SACZ activity on intraseasonal time scales has in inducing persistent circulation anomalies at the subtropical regions that can result in the development of persistent heat waves, and very extreme daily temperature.     

Revisiting the intraseasonal,interannual and interdecadal variability of tropical cyclones in the western North Pacific

This paper reviews the recent progress and research on the variability of tropical cyclones(TCs) at different time scales. Specific focus is placed on how different types of external forcings or climatic oscillations contribute to TC variability in the western North Pacific(WNP). At the intraseasonal scale, recent advances on the distinctive impacts of the Madden–Julian Oscillation(MJO), the Quasi-biweekly Oscillation, and the asymmetric MJO modulation under different El Ni?o–Southern Oscillation(ENSO) states, as well as the influences of the Pacific–Japan teleconnection, are highlighted. Interannually, recent progress on the influences of the ENSO cycle, different flavors of ENSO, and impacts of Indian Ocean warming is presented. In addition, the uncertainty concerning interdecadal TC variations is discussed, along with the recently proposed modulation mechanisms related to the zonal sea surface temperature gradient, the North Pacific Gyre Oscillation, and the Pacific Decadal Oscillation(PDO). It is hoped that this study can deepen our understanding and provide information that the scientific community can use to improve the seasonal forecasting of TCs in the WNP.     

Simulation of the intraseasonal variability over the Eastern Pacific ITCZ in climate models

During boreal summer, convective activity over the eastern Pacific (EPAC) inter-tropical convergence zone (ITCZ) exhibits vigorous intraseasonal variability (ISV). Previous observational studies identified two dominant ISV modes over the EPAC, i.e., a 40-day mode and a quasi-biweekly mode (QBM). The 40-day ISV mode is generally considered a local expression of the Madden-Julian Oscillation. However, in addition to the eastward propagation, northward propagation of the 40-day mode is also evident. The QBM mode bears a smaller spatial scale than the 40-day mode, and is largely characterized by northward propagation. While the ISV over the EPAC exerts significant influences on regional climate/weather systems, investigation of contemporary model capabilities in representing these ISV modes over the EPAC is limited. In this study, the model fidelity in representing these two dominant ISV modes over the EPAC is assessed by analyzing six atmospheric and three coupled general circulation models (GCMs), including one super-parameterized GCM (SPCAM) and one recently developed high-resolution GCM (GFDL HIRAM) with horizontal resolution of about 50?km. While it remains challenging for GCMs to faithfully represent these two ISV modes including their amplitude, evolution patterns, and periodicities, encouraging simulations are also noted. In general, SPCAM and HIRAM exhibit relatively superior skill in representing the two ISV modes over the EPAC. While the advantage of SPCAM is achieved through explicit representation of the cumulus process by the embedded 2-D cloud resolving models, the improved representation in HIRAM could be ascribed to the employment of a strongly entraining plume cumulus scheme, which inhibits the deep convection, and thus effectively enhances the stratiform rainfall. The sensitivity tests based on HIRAM also suggest that fine horizontal resolution could also be conducive to realistically capture the ISV over the EPAC, particularly for the QBM mode. Further analysis illustrates that the observed 40-day ISV mode over the EPAC is closely linked to the eastward propagating ISV signals from the Indian Ocean/Western Pacific, which is in agreement with the general impression that the 40-day ISV mode over the EPAC could be a local expression of the global Madden-Julian Oscillation (MJO). In contrast, the convective signals associated with the 40-day mode over the EPAC in most of the GCM simulations tend to originate between 150°E and 150°W, suggesting the 40-day ISV mode over the EPAC might be sustained without the forcing by the eastward propagating MJO. Further investigation is warranted towards improved understanding of the origin of the ISV over the EPAC.     

Stable boundary layers: Observations,models and variability part I: Modelling and measurements

Selected field measurements of evening stable boundary layers are presented in detail comparable with published Large Eddy Simulation results. Such models appear to match idealized theories more closely than do some boundary-layer observations. Any attempt to compare detailed observations with idealized models therefore highlights the variability of the real boundary layer.Here direct turbulence measurements across the stable boundary layer from a heterogeneous and an ideal site are contrasted. Recommendations are made for the information needed to distinguish heterogeneous and ideal cases.The companion paper (Part II) discusses further the issues of data, analysis in the presence of variability, and the effects of averaging over heterogeneous terrain.Part of UK Meteorological Office Atmospheric Process Research Division.     

The dynamical effects of divergent wind on the intraseasonal variability of the East Asian circulation

In this paper, the dynamical effects of divergent wind on the intraseasonal variability of atmospheric circulation over East Asia are analyzed by using the function of Rossby-wave source and the energy exchanging function be-tween divergent component and rotational component of the flow.The results analyzed from the observed data show that the advection of vorticity by divergent wind caused by the heating due to the monsoon rainfall in the south to the Yangtze River and the strong convective activities around the Philippines may play an important role in the northward jump of westerly jet stream during the seasonal transition from spring circulation to summer circulation over East Asia. Due to the northward movement of the advection of vorticity by the divergent wind, the energy transformation from divergent component into rotational component can be caused over the Yellow River basin and Northwest China and will cause the intensification of the zonal flow there. Thus, the jet stream abruptly shifts northward to North China.Moreover, the analysed results also show that the advection of vorticity by divergent wind caused by the heating due to the strong convective activities around the Philippines also plays an important role in the intraseasonal varia-bility of the circulation over East Asia during the seasonal transition from summer to winter. With the southward movement of the advection of vorticity by the divergent wind, the energy transformation from divergent component into rotational component can be caused over East Asia, especially over the Yangtze-Huaihe River basin. Therefore, the jet stream gradually moves southward from North China to the Yangtze River basin.     

Influence of sea surface temperature on the intraseasonal variability of the South China Sea summer monsoon

The objective of this study is to examine, based on recently available high resolution satellite and observational data, the evolution and role of sea surface temperature (SST) in influencing the intraseasonal variability of the South China Sea (SCS) summer monsoon (SM). The study focuses on the 30–60?day timescale when the northward propagating anomalies are dominant over the SCS. Composite analysis of the SST maximum events during SCS SM shows that increased SST anomalies over the SCS are significantly influenced by the downward shortwave radiation flux anomalies, with the suppressed surface latent heat flux anomalies supplementing to it. A thermal damping of the positive SST anomalies induces positive upward heat fluxes, which then destabilize the lower atmosphere between 1,000 and 700?hPa. The positive SST anomalies lead the positive precipitation anomalies over the SCS by 10?days, with a significant correlation (r?=?0.44) between the SST-precipitation anomalies. The new findings here indicate an ocean-to-atmosphere effect over the SCS, where underlying SST anomalies tend to form a favorable condition for convective activity and sustain enhanced precipitation during the SCS SM. It is also argued, based on our observations, that the negative sea level pressure anomalies induced by the positive SST anomalies play a role in enhancing the northward propagation of the intraseasonal anomalies over the SCS.     

Boreal summer intraseasonal variability simulated in the NCEP climate forecast system: insights from moist static energy budget and sensitivity to convective moistening

The NCEP Climate Forecast System (CFS) with the relaxed Arakawa Schubert (RAS, hereafter referred to as CTRL) convection scheme of Moorthi and Suarez exhibits better performance in representing boreal summer tropical intraseasonal variability as compared with a simulation using simplified Arakawa–Schubert scheme. The intraseasonal moist static energy (MSE) budget is analyzed in this version of the CFS model (CTRL), which produces realistic eastward and northward propagation characteristics. The moist and thermodynamic processes involved in the maintenance and propagation of the poleward moving intraseasonal oscillation (ISO) disturbances are examined here. Budget diagnostics show that horizontal MSE advection is the principal component of the budget, contributing to the poleward movement of the convection. The injection of MSE moistens the atmosphere north of the convective area causing the poleward movement of convection by destabilization of the atmosphere. The moistening process is mainly contributed by the climatological wind acting on the anomalous moisture gradient as confirmed from the examination of moisture advection equation. While surface enthalpy fluxes (consisting of radiative and surface turbulent heat fluxes) maintain the ISO anomalies, they oppose the MSE tendency due to horizontal advection thus regulating the poleward propagation characteristics. In addition, the model results show that wind–evaporation feedback dominates over cloud–radiation feedback for ISO propagation; this is in contrast to our estimates using the newly available European Centre for Medium Range Weather Forecasts Interim reanalysis. Sensitivity experiments suggest that intraseasonal variability in the CFS model with the RAS scheme is highly sensitive to the parameterization of both the shallow convection and the convective rain evaporation and downdrafts. Removal of these components adversely affects the propagation characteristics and greatly reduces the amplitude of intraseasonal variability. Our results support the primary importance of the moisture preconditioning ahead of the ISO and the physical relationship between moisture and precipitation. For realistic ISO simulations, models need to represent these features appropriately.