Subseasonal Time Scales Research Articles

Characteristics and Mechanisms of the Subseasonal Eastward Extension of the South Asian High

Abstract This study investigates the features of eastward extension of the South Asian high (SAH) and its connection with diabatic heating and rainfall over eastern Asia on subseasonal time scales. The causes of SAH’s eastward extension are examined by potential vorticity (PV) diagnosis with emphasis on the joint role of diabatic heating feedback and midlatitude wave train. The SAH’s eastward extension features eastward propagation of a wave train across Eurasia. Among the wave train, the migration of weak high from the western flank of the Tibetan Plateau (TP) to the east of TP contributes to the SAH’s eastward extension at the early stage. When the SAH approaches its easternmost position, a strong negative PV (positive geopotential height) center prevails to the east of the TP at 200 hPa. The associated anomalies in diabatic heating and rainfall include the anomalous heating and above-normal rainfall over the South China Sea (SCS) and subtropical western Pacific occurring 12 days before the SAH’s easternmost stretch, and then anomalous cooling and below-normal rainfall over the southern foot of the TP and southern China and heating and above-normal rainfall over the northern TP and northern China a week later. The anomalous heating and ascending motion over the northern TP and northern China act to increase negative PV locally at 200 hPa. The cooling and descending induce positive PV over southern China. The north–south dipolar structure of PV anomaly with the climatological northerly flow is favorable to southward advection of a negative PV anomaly at 200 hPa. The anomalous heating over the SCS–western Pacific helps to develop a below-normal rainfall condition over southern China via inducing a lower-level anomalous cyclone over coastal region. These processes are conducive to the SAH’s eastward extension at its later stage.

NOAA’s African Desk: Twenty Years of Developing Capacity in Weather and Climate Forecasting in Africa

Abstract Drought is one of the leading causes of death in Africa because of its impact on access to sanitary water and food. This challenge has mobilized the international community to develop famine early warning systems (FEWS) to bring safe food and water to populations in need. Over the past several decades, much attention has focused on advance risk planning in agriculture and water and, more recently, on health. These initiatives require updates of weather and climate outlooks. This paper describes the active role of NOAA’s African Desk in FEWS and in enhancing the capacity of African institutions to improve forecasts. The African Desk was established in 1994 to provide services to U.S. agencies and African institutions. Emphasis is on the operational products across all time scales from weather to climate forecasts in support of humanitarian relief programs. Tools to provide access to real-time weather and climate information to the public are described. These include the downscaling of the U.S. National Multimodel Ensemble (NMME) to improve seasonal forecasts. The subseasonal time scale has emerged as extremely important to many socioeconomic sectors. Drawing from advances in numerical models, operational subseasonal forecasts are included in the African Desk product suite. These capabilities along with forecast skill assessment, verifications, and regional hazards outlooks for food security are discussed. Finally, the African Desk residency training program, an effort aimed at enhancing the capacity of African institutions to improve forecasts, and supported by this seamless approach to operational forecasting, is described.

Dynamic downscaling of summer precipitation prediction over China in 1998 using WRF and CCSM4

To study the prediction of the anomalous precipitation and general circulation for the summer (June–July–August) of 1998, the Community Climate System Model Version 4.0 (CCSM4.0) integrations were used to drive version 3.2 of the Weather Research and Forecasting (WRF3.2) regional climate model to produce hindcasts at 60 km resolution. The results showed that the WRF model produced improved summer precipitation simulations. The systematic errors in the east of the Tibetan Plateau were removed, while in North China and Northeast China the systematic errors still existed. The improvements in summer precipitation interannual increment prediction also had regional characteristics. There was a marked improvement over the south of the Yangtze River basin and South China, but no obvious improvement over North China and Northeast China. Further analysis showed that the improvement was present not only for the seasonal mean precipitation, but also on a sub-seasonal timescale. The two occurrences of the Mei-yu rainfall agreed better with the observations in the WRF model, but were not resolved in CCSM. These improvements resulted from both the higher resolution and better topography of the WRF model.

Using Temporal Changes in Drought Indices to Generate Probabilistic Drought Intensification Forecasts

Abstract In this study, the potential utility of using rapid temporal changes in drought indices to provide early warning of an elevated risk for drought development over subseasonal time scales is assessed. Standardized change anomalies were computed each week during the 2000–13 growing seasons for drought indices depicting anomalies in evapotranspiration, precipitation, and soil moisture. A rapid change index (RCI) that encapsulates the accumulated magnitude of rapid changes in the weekly anomalies was computed each week for each drought index, and then a simple statistical method was used to convert the RCI values into drought intensification probabilities depicting the likelihood that drought severity as analyzed by the U.S. Drought Monitor (USDM) would worsen in subsequent weeks. Local and regional case study analyses revealed that elevated drought intensification probabilities often occur several weeks prior to changes in the USDM and in topsoil moisture and crop condition datasets compiled by the National Agricultural Statistics Service. Statistical analyses showed that the RCI-derived probabilities are most reliable and skillful over the central and eastern United States in regions most susceptible to rapid drought development. Taken together, these results suggest that tools used to identify areas experiencing rapid changes in drought indices may be useful components of future drought early warning systems.

A numerical study on the relationships of the variations of volume transport around the China seas

Three dimensional oceanic circulations in the North Pacific Ocean are simulated using the Regional Ocean Model System (ROMS). The model very well reproduces the main oceanic circulation patterns and the hydrodynamic structures in the North Pacific Ocean, especially in the China seas. Eight transects around the China seas are selected to analyze volume transport and their relationships. The simulated mean volume transport and their standard deviations are in good agreement with previous studies and observations in the Taiwan Strait (TWS, 1.79±0.96Sv, 1Sv≡106m3/s), East of Taiwan (PCM, 20.03±5.19Sv), Tokara Strait (TKS, 19.22±3.22Sv), Tsushima Strait (TUS, 2.14±0.47Sv), Luzon Strait (LUS, 4.57±4.13Sv), Karimata Strait (KMS, 1.28±0.65Sv), East of Luzon Island (ELI, 13.93±5.31Sv), and 137°E across the North Equatorial Current from 10°N to 20°N (137E, 44.0±9.90Sv). Three kinds of time scale variations (seasonal, sub-seasonal, intra-seasonal) were found in the volume transport through these transects with over the 95% significance level. The power spectrum density of the seasonal variation was almost one order of magnitude larger than the others for transects around the China seas. Coherency relationships among the volume transport through transects were analyzed. The results indicate that the seasonal time scale variations in volume transport around the China seas were dominated by the monsoon winds, but the Kuroshio along its pathway was relatively stable with small seasonal variation; the sub-seasonal time scale variations were associated with strong monsoon reversal winds; and the intra-seasonal variations were complex and weak, associating with local small scale winds and mesoscale eddy activities along the Kuroshio.

Potential sea ice predictability and the role of stochastic sea ice strength perturbations

AbstractEnsemble experiments with a climate model are carried out in order to explore how incorporating a stochastic ice strength parameterization to account for model uncertainty affects estimates of potential sea ice predictability on time scales from days to seasons. The impact of this new parameterization depends strongly on the spatial scale, lead time and the hemisphere being considered: Whereas the representation of model uncertainty increases the ensemble spread of Arctic sea ice thickness predictions generated by atmospheric initial perturbations up to about 4 weeks into the forecast, rather small changes are found for longer lead times as well as integrated quantities such as total sea ice area. The regions where initial condition uncertainty generates spread in sea ice thickness on subseasonal time scales (primarily along the ice edge) differ from that of the stochastic sea ice strength parameterization (along the coast lines and in the interior of the Arctic). For the Antarctic the influence of the stochastic sea ice strength parameterization is much weaker due to the predominance of thinner first year ice. These results suggest that sea ice data assimilation and prediction on subseasonal time scales could benefit from taking model uncertainty into account, especially in the Arctic.

Modeling very large-fire occurrences over the continental United States from weather and climate forcing

Very large-fires (VLFs) have widespread impacts on ecosystems, air quality, fire suppression resources, and in many regions account for a majority of total area burned. Empirical generalized linear models of the largest fires (>5000 ha) across the contiguous United States (US) were developed at ∼60 km spatial and weekly temporal resolutions using solely atmospheric predictors. Climate−fire relationships on interannual timescales were evident, with wetter conditions than normal in the previous growing season enhancing VLFs probability in rangeland systems and with concurrent long-term drought enhancing VLFs probability in forested systems. Information at sub-seasonal timescales further refined these relationships, with short-term fire weather being a significant predictor in rangelands and fire danger indices linked to dead fuel moisture being a significant predictor in forested lands. Models demonstrated agreement in capturing the observed spatial and temporal variability including the interannual variability of VLF occurrences within most ecoregions. Furthermore the model captured the observed increase in VLF occurrences across parts of the southwestern and southeastern US from 1984 to 2010 suggesting that, irrespective of changes in fuels and land management, climatic factors have become more favorable for VLF occurrence over the past three decades in some regions. Our modeling framework provides a basis for simulations of future VLF occurrences from climate projections.

Subseasonal variability of North American wintertime surface air temperature

Using observational pentad data of the recent 34 Northern Hemisphere extended winters, subseasonal variability of surface air temperature (SAT) over North America is analyzed. The four leading modes of subseasonal SAT variability, that are identified with an empirical orthogonal function (EOF) analysis, account for about 60% of the total variance. The first (EOF1) and second (EOF2) modes are independent of other modes, and thus are likely controlled by distinct processes. The third (EOF3) and fourth (EOF4) modes, however, tend to have a phase shift to each other in space and time, indicating that part of their variability is related to a common process and represent a propagating pattern over North America. Lagged regression analysis is conducted to identify the precursors of large-scale atmospheric circulation for each mode a few pentads in advance, and to understand the processes that influence the subseasonal SAT variability and the predictability signal sources. EOF1 is found to be closely related to the Pacific-North American (PNA) circulation pattern and at least part of its variability is preceded by the East Asian cold surge. The cold surge leads to low-level convergence and enhanced convection in the tropical central Pacific which in turn induces the PNA. EOF2 tends to oscillate at a period of about 70 days, and is influenced by the low-frequency component of the Madden–Julian Oscillation (MJO). On the other hand, EOF3 and EOF4 are connected to the high-frequency part of the MJO which has a period range of 30–50 days. These findings would help understanding the mechanisms of subseasonal surface air temperature variability in North America and improving weather predictions on a subseasonal time scale.

Dynamical Forcing of Subseasonal Variability in the Tropical Brewer–Dobson Circulation

Abstract Upwelling across the tropical tropopause exhibits strong subseasonal variability superimposed on the well-known annual cycle, and these variations directly affect temperature and tracers in the tropical lower stratosphere. In this work, the dynamical forcing of tropical upwelling on subseasonal time scales is investigated using the European Centre for Medium-Range Weather Forecasts (ECMWF) Interim Re-Analysis (ERA-Interim) for 1979–2011. Momentum balance diagnostics reveal that transience in lower-stratospheric upwelling is linked to the effects of extratropical wave forcing, with centers of action in the extratropical winter stratosphere and in the subtropical upper troposphere of both hemispheres. The time-dependent forcing in these regions induces a remote coupled response in the zonal mean wind and the meridional circulation (with associated temperature changes), which drives upwelling variability in the tropical stratosphere. This behavior is observed in the reanalysis, consistent with theory. Dynamical patterns reflect distinctive forcing of the shallow versus deep branches of the Brewer–Dobson circulation; the shallow branch is most strongly correlated with wave forcing in the subtropical upper troposphere and lower stratosphere, while the deep branch is mainly influenced by high-latitude planetary waves.

The NOW regional coupled model: Application to the tropical Indian Ocean climate and tropical cyclone activity

AbstractThis paper presents the NOW regional coupled ocean‐atmosphere model built from the NEMO ocean and WRF atmospheric numerical models. This model is applied to the tropical Indian Ocean, with the oceanic and atmospheric components sharing a common ¼° horizontal grid. Long experiments are performed over the 1990–2009 period using the Betts‐Miller‐Janjic (BMJ) and Kain‐Fritsch (KF) cumulus parameterizations. Both simulations produce a realistic distribution of seasonal rainfall and a realistic northward seasonal migration of monsoon rainfall over the Indian subcontinent. At subseasonal time scales, the model reasonably reproduces summer monsoon active and break phases, although with underestimated rainfall and surface wind signals. Its relatively high resolution results in realistic spatial and seasonal distributions of tropical cyclones, but it fails to reproduce the strongest observed cyclone categories. At interannual time scales, the model reproduces the observed variability associated with the Indian Ocean Dipole (IOD) and the delayed basin‐wide warming/cooling induced by the El Niño Southern Oscillation (ENSO). The timing of IOD occurrence in the model generally matches that of the observed events, confirming the influence of ENSO on the IOD development (through the effect of lateral boundary conditions in our simulations). Although the KF and BMJ simulations share a lot in common, KF strongly overestimates rainfall at all time scales. KF also overestimates the number of simulated cyclones by a factor two, while simulating stronger events (up to 55 m s−1) compared to BMJ (up to 40 m s−1). These results could be related to an overly active cumulus parameterization in KF.

On using the seasonal cycle to interpret extratropical temperature changes since 1950

AbstractExtratropical near‐surface air temperature variability is explored on three different time scales: the seasonal cycle, observed changes in temperature since 1950, and the equilibrium response to increasing CO2 in an atmospheric general circulation simulation with fixed sea surface temperatures. Exploration is undertaken using an energy balance model (EBM) that parameterizes advective land‐ocean heat fluxes. The EBM is tuned only to the climatological seasonal cycle yet captures 47% of the variability in observed multidecadal temperature changes in the extratropics and 78% of the variability in the equilibrated model simulation. The subseasonal time scale of atmosphere‐surface heat fluxes explains, at least in the context of this EBM, the ability to infer patterns of multidecadal change using information primarily drawn from the seasonal cycle.

Examining the Relationship between Drought Development and Rapid Changes in the Evaporative Stress Index

AbstractIn this study, the ability of a new drought metric based on thermal infrared remote sensing imagery to provide early warning of an elevated risk for drought intensification is assessed. This new metric, called the rapid change index (RCI), is designed to highlight areas undergoing rapid changes in moisture stress as inferred from weekly changes in the evaporative stress index (ESI) generated using the Atmosphere–Land Exchange Inverse (ALEXI) surface energy balance model. Two case study analyses across the central United States revealed that the initial appearance of negative RCI values indicative of rapid increases in moisture stress preceded the introduction of severe-to-exceptional drought in the U.S. Drought Monitor (USDM) by more than 4 weeks. Using data from 2000 to 2012, the probability of USDM intensification of at least one, two, or three categories over different time periods was computed as a function of the RCI magnitude. Compared to baseline probabilities, the RCI-derived probabilities often indicate a much higher risk for drought development that increases greatly as the RCI becomes more negative. When the RCI is strongly negative, many areas are characterized by intensification probabilities that are several times higher than the baseline climatology. The highest probabilities encompass much of the central and eastern United States, with the greatest increase over climatology within regions most susceptible to rapid drought development. These results show that the RCI provides useful drought early warning capabilities that could be used to alert stakeholders of an increased risk for drought development over subseasonal time scales.

Modeled interaction between the subseasonal evolving of the East Asian summer monsoon and the direct effect of anthropogenic sulfate

AbstractA regional climate model is used to conduct four experiments to reveal the interaction between the subseasonal evolving of the East Asia summer monsoon (EASM) and the direct effect of anthropogenic sulfate. One of the experiments is conducted without the emission of anthropogenic SO2. The other three are carried out with the 1, 1.5, and 2 multiples of the SO2 emission of 2000, respectively. The model results suggest that in the rainfall, wind, and circulation of the EASM, the changes caused by the direct effect of anthropogenic sulfate on subseasonal time scale are within the model variability. The change of the EASM rainfall is not sensitive to sulfate forcing on subseasonal time scale but sensitive to the forcing on seasonal time scale. Moreover, rainfall change in the forcing experiments is significantly less (greater) than zero to the south (north) of the EASM rainfall centers on subseasonal time scale, but the EASM rainfall phase is not changed. The rainfall change associates with the correspondingly significant increase (decrease) of the atmospheric stability induced by the forcing of sulfate direct effect. The direct effect of sulfate plays different roles in different stages of the EASM subseasonal evolving. In the early stage, the EASM is significantly enhanced in terms of the interaction between tropical and midlatitude atmosphere. In the late stage, the EASM is weakened in terms of the large‐scale atmospheric circulation. Lastly, the responses of the rainfall, wind, and circulation of the EASM to the linear changes in the SO2 emission are nonlinear.

Variability of the Antarctic Circumpolar Current transport through the Fawn Trough, Kerguelen Plateau

The Kerguelen Plateau is a major topographic obstacle to the eastward flowing Antarctic Circumpolar Current (ACC). While approximately two-third of the ACC transport is diverted to the North, most of the remaining flow engulfs in the Fawn Trough, the only deep passage across the plateau. As part of the TRACK (TRansport ACross the Kerguelen plateau) project, three mooring lines of current meters were deployed in the Fawn Trough for one year in February 2009, underneath ground-track 94 of the Jason-2 satellite altimeter. Full depth CTD-LADCP casts carried out during the deployment cruise were previously analyzed to provide a comprehensive description of the regional circulation, featuring in particular a volume transport of ~43Sv across the Fawn Trough (Park et al., 2009).Here we present a time series of the transport in the Fawn Trough estimated from current meter observations, featuring a mean eastward transport of 34Sv (possibly biased low by at most 5Sv) and a root mean squared variability of 6Sv, consistent with LADCP estimates (43Sv in February 2009 and 38Sv in January 2010). In addition, we analyze to what extent the transport can be directly monitored from along-track satellite altimeter data, which would enable study of the variability of the Fawn Trough Current from a now 20-year long archive. The ability to reconstruct the flow from a limited set of moored instruments as well as from altimeter-derived surface geostrophic velocity is further assessed from synthetic data extracted from a high-resolution peri-Antarctic simulation. While a canonical method to derive transport from altimetry, previously applied to the Malvinas Current, gives here unsatisfactory comparisons with in situ estimates, an ad hoc approach using only the two northernmost mooring lines yields an estimate well correlated (~0.8) with in situ transport at subseasonal time scales during the one year period of observations. At interannual time scales, however, both methods provide significantly correlated (0.7) transport estimates, suggesting that long-term transport fluctuations across the Kerguelen Plateau can be confidently estimated from altimetry. These consistently indicate a measurable impact of the outstanding 1997–1998 El Niño Southern Oscillation (ENSO) event, yielding an increase of the annual mean transport of ~3Sv, possibly with a one year lag. The transport estimate based on the ad hoc approach is significantly correlated (0.6) with the Southern Annular Mode (SAM) index at interannual time scales, suggesting that an intensification of the circumpolar winds drives an increase in the transport across the Kerguelen Plateau.

Spatiotemporal variability and drivers of <i>p</i>CO<sub>2</sub> and air–sea CO<sub>2</sub> fluxes in the California Current System: an eddy-resolving modeling study

Abstract. We quantify the CO2 source/sink nature of the California Current System (CalCS) and determine the drivers and processes behind the mean and spatiotemporal variability of the partial pressure of CO2 (pCO2) in the surface ocean. To this end, we analyze eddy-resolving, climatological simulations of a coupled physical–biogeochemical oceanic model on the basis of the Regional Oceanic Modeling System (ROMS). In the annual mean, the entire CalCS within 800 km of the coast and from ∼33° N to 46° N is essentially neutral with regard to atmospheric CO2: the model simulates an integrated uptake flux of −0.9 ± 3.6 Tg C yr−1, corresponding to an average flux density of −0.05 ± 0.20 mol C m−2 yr−1. This near zero flux is a consequence of an almost complete regional compensation between (i) strong outgassing in the nearshore region (first 100 km) that brings waters with high concentrations of dissolved inorganic carbon (DIC) to the surface and (ii) and a weaker, but more widespread uptake flux in the offshore region due to an intense biological reduction of this DIC, driven by the nutrients that are upwelled together with the DIC. The air–sea CO2 fluxes vary substantially in time, both on seasonal and sub-seasonal timescales, largely driven by variations in surface ocean pCO2. Most of the variability in pCO2 is associated with the seasonal cycle, with the exception of the nearshore region, where sub-seasonal variations driven by mesoscale processes dominate. In the regions offshore of 100 km, changes in surface temperature are the main driver, while in the nearshore region, changes in surface temperature, as well as anomalies in DIC and alkalinity (Alk) owing to changes in circulation, biological productivity and air–sea CO2 fluxes dominate. The prevalence of eddy-driven variability in the nearshore 100 km leads to a complex spatiotemporal mosaic of surface ocean pCO2 and air–sea CO2 fluxes that require a substantial observational effort to determine the source/sink nature of this region reliably.

Probabilistic Ensemble Forecast of Summertime Temperatures in Pakistan

Snowmelt flooding triggered by intense heat is a major temperature related weather hazard in northern Pakistan, and the frequency of such extreme flood events has increased during the recent years. In this study, the probabilistic temperature forecasts at seasonal and subseasonal time scales based on hindcasts simulations from three state-of-the-art models within the DEMETER project are assessed by the relative operating characteristic (ROC) verification method. Results based on direct model outputs reveal significant skill for hot summers in February 3–5 (ROCarea=0.707with lower 95% confidence limit of 0.538) and February 4-5 (ROCarea=0.771with lower 95% confidence limit of 0.623) forecasts when validated against observations. Results for ERA-40 reanalysis also show skill for hot summers. Skilful probabilistic ensemble forecasts of summertime temperatures may be valuable in providing the foreknowledge of snowmelt flooding and water management in Pakistan.

Dynamic variability of the Asian monsoon anticyclone observed in potential vorticity and correlations with tracer distributions

The Asian summer monsoon is associated with strong upward transport of tropospheric source gases and isolation of air within the upper tropospheric anticyclone, with a high degree of dynamical variability. Here we study the anticyclone in terms of potential vorticity (PV) as derived from reanalysis data. The strength of the anticyclone, as measured by low PV area, varies on subseasonal time scales (periods of 30–40 days), driven by variability in convection. The convective forcing of low PV areas is associated with heating in the middle troposphere and divergent motion in the upper troposphere, and we find that upper level divergence is a good predictor of the anticyclone strength. Low PV air is often observed to propagate from the forcing region to the west, and occasionally to the east. Carbon monoxide (CO) measured by the Aura Microwave Limb Sounder is used to study the covariability of chemical tracers with the anticyclone strength and location. Concentrations of CO maximize within the upper tropospheric anticyclone, and enhanced CO is well correlated with the spatial distribution of low PV. Time variations of CO concentrations in the upper troposphere (around 360 K) are not strongly correlated with anticyclone strength, probably because CO transport also involves coupling with surface CO sources (unlike PV). Temporal correlations with PV are stronger for CO at higher levels (380–400 K), suggesting that advective upward transport is important for tracer evolution at these levels.

Persistent heavy rainfall over South China during May–August: Subseasonal anomalies of circulation and sea surface temperature

This study investigates the relationship between subseasonal variations of the circulation and sea surface temperature (SST) over the South China-East Asian coastal region (EACR) in association with the persistent heavy rainfall (PHR) events over South China during May-August through statistical analysis. Based on the intensity threshold and duration criterion of the daily rainfall, a total of 63 May-June (MJ) and 59 July-August (JA) PHR events are selected over South China from 1979 to 2011. The lower-level circulation anomalies on subseasonal timescale exhibit an anomalous cyclone over South China and an anomalous anticyclone shaped like a tongue over the South China Sea (SCS) during the PHR events for MJ group. The anomalous cyclone over South China in MJ originates from low-value systems in the mid-high latitudes before the rainfall. The anomalous anticyclone over the SCS is due to the westward extension of the western Pacific subtropical high (WPSH) and the southeastward propagation of the anomalous anticyclone from South China before the rainfall. For JA group, the lower-level anomalous circulation pattern is similar to that for MJ over the South China-EACR, but with different features of propagation. The subseasonal anomalous anticyclone is also related to the westward stretch of the WPSH, while the anomalous cyclone is traced back to the weak anomalous cyclone over the Philippine Sea several days before the rainfall events.

Quantifying tracer transport in the tropical lower stratosphere using WACCM

Abstract. The zonal mean transport of ozone and carbon monoxide (CO) near the tropical tropopause is investigated using the Whole-Atmosphere Community Climate Model version 4 (WACCM4). The variability in temperature, ozone and CO in the model shows good agreement with satellite and balloon observations. Modeled temperature and tracers exhibit large and closely coupled annual cycles in the tropical lower stratosphere, as in the observations. The thermodynamic and tracer budgets in the model are analyzed based on the Transformed Eulerian Mean (TEM) framework on log-pressure coordinates and also using the isentropic formulation. Results show that the coupled seasonal cycles are mainly forced by tropical upwelling over altitudes with large vertical tracer gradients, in agreement with previous observational studies. The model also allows explicit calculation of eddy transport terms, which make an important contribution to ozone tendencies in the tropical lower stratosphere. The character of the eddy fluxes changes with altitude. At higher levels (~2 km above the cold point tropopause), isentropic eddy transport occurs during winter and spring in each hemisphere in the sub-tropics, associated with transient Rossby waves acting on strong background latitudinal gradients. At lower altitudes, close to the tropical tropopause, there is a maximum in horizontal eddy transport during boreal summer associated with the Asian monsoon anticyclone. Sub-seasonal variability in ozone and CO, tied to fluctuations in temperature, is primarily driven by transient tropical upwelling. In isentropic coordinates, the overall tracer budgets are similar to the log-pressure results, highlighting cross-isentropic advection as the main term in the time-mean balance, with large seasonality above the tropopause. However, in isentropic coordinates the tracer variability is largely reduced on both seasonal and sub-seasonal timescales, because tracer fluctuations are highly correlated with temperature (as a response to upwelling).

Zonal Oscillation of Western Pacific Subtropical High and Subseasonal SST Variations during Yangtze Persistent Heavy Rainfall Events

AbstractThis study examines the relationship between the zonal oscillation of the western Pacific subtropical high (WPSH) and underneath sea surface temperature (SST) variation on a subseasonal time scale, associated with the persistent heavy rainfall (PHR) events over the middle and lower reaches of the Yangtze River valley (MLYRV) in China. A total of 76 PHR events and 45 break events in the summers of 1979–2011 are first identified over the MLYRV and divided into early and late summer groups. During the PHR events over the MLYRV for both groups, the WPSH stretches more westward, accompanied by the positive anomalies of the 500-hPa geopotential height field over East Asia and its coastal region south of 30°N and the subseasonal warmer SSTs beneath the WPSH western edge. The time-lagged composites suggest that the WPSH western edge exhibits westward-then-eastward migration on a subseasonal time scale for the PHR events. The zonal changes of the WPSH and anomalous circulation and SST anomaly (SSTA) signals for break events is almost the mirror image of that for the PHR events for the early summer group. Accompanied by the WPSH westward extension, the increased incident solar radiation and decreased latent heat flux over the coastal region of East Asia contribute to the positive SSTAs beneath the western part of the WPSH. The positive SSTAs construct a convective instability that provides an adverse condition for maintaining the anticyclonic anomalies in the mid–lower levels. The persistent SST warming is also favorable to the transition of low-level circulation from anticyclonic to cyclonic anomalies over the coastal region. As a result, the WPSH withdraws eastward after the peak of the rainfall events over the MLYRV.