A52B-01
Spatio-temporal Distribution of Latent Heating in the Southeast Asian Monsoon Region
The Latent Heat (LH), released as a result of deep convection, plays an important role in the vertical distribution of the diabatic energy budget from the surface to the atmosphere: the motor which drives the global atmospheric circulation, including the Southeast Asian Monsoon. In particular, knowing the spatio-temporal structure of the LH during the wet monsoon season could be a key factor to understand the interaction between seasonal features of the monsoon with the summer manifestation of the intra-seasonal oscillation in the Indian Ocean basin, and hence the distribution of the precipitation. Several studies have investigated how the structure of heating in the tropics has a direct influence in the dynamical response of the atmosphere to the large-scale dynamical forcing associated with tropical precipitating systems. However, these studies assume a uniform geographically distribution of the vertical diabatic heating profiles across the Tropics. The major objective of this study is to produce and to examine three-dimensional latent heating structures over the Indian Monsoon region for the three summer seasons of 1998-2000 period using TRMM-2A12 (GPROF algorithm) and TRMM-CSH (CSH algorithm) data. A specific goal in this work is to explore the differences in the distribution of the latent heating throughout the intraseasonal cycle. This intra- seasonal cycle not only generates wet and dry spells over the South-East Asian continent but also determines the spatial distribution of the climatological JJAS rainfall in the Indian Monsoon Region. Results show spatial distribution differences between the LH profiles during the suppressed and active phases of the oscillation as well as differences in the vertical. During an active phase of the oscillation over the Indian Ocean, the released latent heat is concentrated predominantly near the equator while during the suppressed phased the heating is concentrated in the Bay of Bengal and the continental South East Asia. In the vertical, a deep warm blob aloft over the Bay of Bengal contrast with lower heating peaks over equatorial regions, related with the characteristics of the cloud systems in these regions. These differences in profiles can be used as key inputs in the forcing of general circulation models and cloud resolving models to understand the main differences in the atmospheric response to the geographic location of the heating.
A52B-02
NAO-Ocean Circulation Interactions in a Coupled General Circulation Model
In the present study, an oscillatory mode in the North Altantic sector of the INGV/CMCC coupled general circulation model (Gualdi et al. 2006) with a typical subdecadal timescale is examined in detail. The oscillation involves coordinated changes in SST and atmospheric circulation, with a typical North Atlantic Oscillation (NAO)- like structure. The interplay between mid-to-high latitude SST, ocean circulation variability and the NAO is analysed. A major focus of this study is the role of ocean circulation on the NAO variability, with specific attention on the barotropic wind-driven component. In particular, the role of the Inter-Gyre Gyre (IGG; Marshall et al. 2001; hereafter M01) as a heat carrier, and its impact on the low frequency modulation of the North Atlantic SST tripole is investigated. A mechanism governing the oscillation is identified, bearing strong similarities with the mid-latitude delayed oscillator paradigm. An estimate of the essential parameters governing the oscillation in the coupled model within the frame of the simplified M01 conceptual model is also presented. In particular, the strength of the SST/NAO feedback (f), the IGG heat transport efficiency (g) and the damping of SST anomalies by air-sea interaction (λ) are evaluated. This will enable to determine the R = fg/λ factor, controlling the coupling strength of the system.
A52B-03
Sensitivity of Tropical Atlantic Sea-Surface Temperature Variability to Changes in the Atlantic Meridional Overturning Circulation
Coupled climate model simulations suggest that a slow-down in the Atlantic Meridional Overturning Circulation (MOC) can cause a substantial change in tropical Atlantic sea-surface temperature (SST) variability. However, it is still not clear how the underlying dynamics work in linking the MOC change to the tropics. In this study, we use a reduced gravity ocean model to examine the sensitivity of the tropical Atlantic SST variability to changes in the MOC strength by systematically varying the imposed constant northward mass transport at the open boundaries from 14 SV to 0 SV. The results show that the tropical Atlantic SST responds nonlinearly to changes in the MOC strength. A prominent equatorial warming occurs when the MOC is weakened below a threshold value. This nonlinear behavior is attributed to an interaction between the Atlantic thermohaline circulation and the wind driven subtropical cells. A dynamic mechanism is proposed to explain how the MOC change can affect coupled climate variability in the tropical Atlantic sector.
A52B-04
The Modulation of Potential Vorticity Streamers and Surface Weather Patterns by the Madden Julian Oscillation
The Madden Julian Oscillation (MJO), a large-scale coupled pattern between tropical deep convection and atmospheric circulation, is known to be the dominant source of intraseasonal variability in the tropics. Its impacts, however, are not limited to tropical regions: latent heat release associated with MJO convection can force planetary scale Rossby wave trains that propagate over the globe. On the synoptic scale, propagating Rossby wave trains can culminate in Rossby wave breaking, a process that generates so-called potential vorticity (PV) streamers. PV streamers have been linked to positive upper-level PV anomalies that can influence surface weather, extreme precipitation events and the enhanced exchange between the stratosphere and troposphere. The primary goal of the present study is to investigate the possible relationship between the MJO and PV streamers. Ten MJO indices (representing MJO activity at ten specific tropical longitudes) have been obtained from the Climate Prediction Center for the boreal winter between 1978 and 2001. The data are subsequently separated into terciles (allowing for the designation of convectively active and suppressed periods of the MJO) and compared with a unique PV streamer dataset that has been previously computed from ECMWF Re-analysis data (ERA-40). The streamer dataset allows for the dynamically useful separation of streamers into cyclonically and anticyclonically breaking waves, referred to as LC2 and LC1 streamers, respectively. Statistically significant differences are found regarding the amount, location and type of PV streamers during different phases of the MJO. When the MJO is convectively active over the Indian Ocean and Indonesia, the North Central Pacific is dominated by a positive 500 hPa height anomaly, a co-located reduction in LC2 streamers and increased LC1 streamers to the south and southeast. As the convection enters the Western Pacific warm pool and approaches the international dateline, a distinct regime shift occurs: over an approximately 10-15 day period, a negative 500 hPa height anomaly forms, supplanting the previous positive anomaly. Subsequent to this time, the negative height anomaly strengthens and there is a significant increase (decrease) in LC1 (LC2) streamers. To illustrate the connection between the MJO, Rossby wave breaking and surface weather in the extratropics, a comparison between PV streamers and surface cyclone frequency (also computed from ERA-40 data; see Wernli et al. 2006) is made. A fairly good agreement between LC2 streamer and surface cyclone frequency anomalies throughout the life cycle of the MJO is found in the North Pacific basin, further highlighting the ability of organized tropical convection to modulate extratropical weather.
A52B-05
Detecting Source Regions of Intraseasonal Oscillations in the Tropical Atmosphere by Applying Beamforming
Since the discovery of the tropical intraseasonal oscillations (ISO) in the early 1970s many studies have postulated and examined various mechanisms for the ISO. The fact that these mechanisms can explain many different aspects of observed ISO also suggests that multiple mechanisms may have existed and functioned simultaneously, and likely interacted, in development of ISO. What specific mechanisms are working and interacting to generate individual ISO, and under what conditions? What are the characteristics, e.g., wavelength and phase speed, of the waves generated from the different mechanisms and interactions? To address these questions requires knowing where the ISO are generated, because in such source regions these mechanisms and interactions are most distinctive and may be detected. By gathering and analyzing the data in the source regions the information that may lead to the answers to these questions can be extracted. This study proposes a fixed beamformer method. After being tested, it is applied to the ECMWF interpolated data grids as its sensor arrays to identify ISO source regions in the tropical Indian and Pacific Ocean region. Major results show three ISO source regions in the tropical Indian Ocean, the western tropical Pacific and the eastern tropical Pacific for the ISO events in 1974-2002, albeit thesource in the eastern ropical Pacific is much weaker than the other two and becomes more active during El Nino years. These sources could be active simultaneously and produce visually continuous eastward propagation ISO. In some situations the propagating signals were strongly strengthened over the downstream ISO source regions, a new explanation of the observed chains of embedded centers of strong wind and convection anomalies in most propagation ISO. When some or all these sources produced their own strong ISO in other situations the local ISO became less cohesive with each other and yielded disconnected quasi-stationary ISO. While improving understanding of the observed structures and differences of the ISO, these results show that different ISO could result from different mechanisms that play more important roles in different dynamic and thermodynamic conditions at different source regions. Understanding these conditions and ISO sources could lead to improving predictions of the tropical ISO.
A52B-06 [WITHDRAWN]
On the Causes of the Poor Simulation and Forecast of the Intraseasonal Oscillation by Numerical Models
Many attempts to simulate and forecast the Intraseasonal Oscillation (ISO) using numerical models have met with difficulties. In this work we present results from a series of extended forecasting simulations conducted jointly between Georgia Tech and the ECMWF aimed at improving our understanding of the problems that numerical models have in simulating and forecasting the ISO. This study evaluates the skill of a numerical model in simulating the processes that occur during the transition from suppressed to active convection which is considered key for skillful extended forecasts in the Indo-West Pacific region. Regional and local vertical structure of ISO-related anomalies from the numerical forecasts using the ECMWF model are compared to those in the ERA-40 data during different stages of the convective activity (suppressed, transition, and active). This analysis explores ISO numerical simulations during the TOGA COARE winter case, as well ISO events during the summers of 2002 and 2004. Results suggest that the skill of the model forecasting the vertical structure of the ISO strongly depends on the atmospheric thermodynamic state at the beginning of each forecast run. In addition, there are states of the system for which the skill of the forecast is always low associated with convective events for which the skill of the forecast decreases regardless of the starting date of the forecast. The forecast skill of circulation anomalies is higher than the skill of moist convective associated anomalies. The time scale of skillful forecasts during summer is half of that obtained for winter, indicating that the skill of the forecast is greater for winter ISO cases than for summer events. Analyses of the summer simulations indicate that the model is always predicting an active-like phase of the monsoon. Since the model is not able to forecast skillfully the generation of specific humidity anomalies in the equatorial Indian Ocean, convective anomalies do not propagate from the equator resulting in the lack of intraseasonal modulation of the monsoon.
A52B-07
Estimating the atmospheric and SST memory of tropical cyclones
This study examines the local memory of atmospheric and oceanic changes associated with a tropical cyclone (TC). The memory is quantified through anomalous maximum potential intensity (MPI) evolution for 20 days prior to the arrival of a TC through 60 days after the TC passage. The MPI weakens and is not restored to the evolving climatology until weeks after the TC has departed. Stabilization occurs through warming of the atmosphere and cooling of the ocean surface on different timescales. The timescale of MPI stabilization following TC passage is approximately 30-35 days for a tropical storm to 50-60 days for a category 3-5 hurricane, with significant storm-specific and basin-specific variability. The atmospheric stabilization begins with TC arrival and continues for approximately 7-10 days after passage, when the troposphere cools below normal. The rewarming of SST and the atmosphere to climatology occurs within approximately 35 days for all intensities, despite a positive (weakened) MPI anomaly through two months. This suggests that the atmosphere warms beyond what can be attributable to sensible heating from the rewarmed SST. The maintenance of a positive MPI anomaly beyond 35 days is thus attributed to a feedback on larger scales that requires considerable further research. A TC's passage through a region does not always lead to a weakening of the MPI. In regions poleward of the sharp SST gradient, the MPI one month after TC passage is often several millibars stronger than climatology. There are also mesoscale regions of destabilization one month after TC passage that may result partially from salinity changes driven by oceanic mixing as well as changes in precipitation and evaporation.
A52B-08
Oscillations of the Intertropical Convergence Zone and the Genesis of Easterly Waves
It is noted that in regions of strong cross-equatorial pressure gradient (CEPG), the climatological Intertropical Convergence Zone (ITCZ) is more intense than in regions where the CEPG is smaller. In the eastern tropical Pacific, where the CEPG during the northern summer is set up by the slowly varying SST gradient across the equator, there is a mean advection of anticyclonic vorticity across the equator denoting broad regions of inertial instability. Furthermore, in such regions, the maximum convection and convergence is located equatorward of the maximum sea-surface temperature and the minimum sea level pressure. Using a combination of diagnostic and modeling studies it is shown that the location of the mean convection is determined by the magnitude of the CEPG (which determines the magnitude of the divergent meridional wind field) and where the advection of absolute vorticity is matched by the vortex stretching term in the absolute vorticity equation. The ITCZ is shown to be highly transient with strong oscillations occurring in the 3-8 day period range. A series of modeling experiments using the Weather Research and Forecast Model (WRF) are performed to show that the observed transients arise from a continual state of inertial instability in which the ITCZ regime is continually destabilized and stabilized against a background large-scale destabilizing CEPG. The model response to the variations of SST and SST gradient is tested. Significant SST gradients which result in stronger pressure gradients generate greater advection of negative absolute vorticity across the equator, with enhancement of convection, north of the zero absolute vorticity line.
A52B-09
How ENSO Impacts Precipitation in Southwest and Central Asia
Hydroclimatological variability in parts of Southwest and Central Asia is very large. For instance, a very severe drought was experienced in a broad region centered around Iran, Afghanistan and Pakistan during the period 1998-2002, which has been associated with exceptionally prolonged La Nina-like conditions. Several studies have investigated the broader role of ENSO (El Nino Southern Oscillation) events in precipitation variability in this region. A late summer-early winter ENSO precipitation signal has consistently been reported by various authors, however the underlying cause has not been determined. In other seasons the ENSO precipitation signal is less well established. In this talk, the impact of ENSO events on interannual precipitation variability in parts of Southwest and Central Asia is described using state-of-the-art precipitation datasets and re-analyses. Decadal changes in this teleconnection are addressed.The underlying mechanism is discussed based on AMIP-type simulations for the autumn season with a model of intermediate complexity. In particular, the roles played by various oceanic regions in producing the observed signal are discussed.
A52B-10
The Relation of El Nino Southern Oscillation to Winter Tornado Outbreaks
Winter tornado activity (January, February, and March) between 1950 and 2003 was analyzed to determine the possible effect of seasonally averaged sea surface temperatures in the equatorial Pacific Ocean, the ENSO phase, on the location and strength of tornado outbreaks in the United States. Tornado activity was gauged through analyses of tornadoes occurring on tornado days (a calendar day featuring 6 or more tornadoes within the contiguous United States) and strong and violent tornado days (a calendar day featuring 5 or more tornadoes rated F-2 and greater within the contiguous United States). The tornado days were then stratified according to warm (37 tornado days, 14 violent days), cold (51 tornado days, 28 violent days), and neutral (74 tornado days, 44 violent days) winter ENSO phase. It is seen that during winter periods of neutral tropical Pacific sea surface temperatures, there is a tendency for United States tornado outbreaks to be stronger and more frequent than they are during winter periods of anomalously warm tropical Pacific sea surface temperatures (El Nino). During winter periods with anomalously cool Pacific sea surface temperatures (La Nina), the frequency and strength of United States tornado activity lies between that of the neutral and El Nino phase. ENSO related shifts in the preferred location of tornado activity are also observed. Historically, during the neutral phase, tornado outbreaks typically occurred from central Oklahoma and Kansas eastward through the Carolinas. During cold phases, tornado outbreaks have typically occurred in a zone stretching from southeastern Texas northeastward into Illinois, Indiana, and Michigan. During anomalously warm phases activity was mainly limited to the Gulf Coast States including central Florida. The data are statistically and synoptically analyzed to show that they are not only statistically significant, but also meteorologically reasonable. http://www.spc.noaa.gov/publications/schaefer/ensowntr.pdf
A52B-11
Effects of Climate Oscillators on January Monthly Extreme Temperatures in North America
The El Nino-Southern Oscillation (ENSO), the Pacific Decadal Oscillation (PDO), and the Polar Vortex Oscillation (PVO) produce conditions favorable for monthly extreme temperatures and precipitation. These climate modes produce conditions that favor regional cold and warm spells, and these extremes impact agriculture, energy, forestry, and transportation. The above sectors prefer the knowledge of the worst (and sometimes the best) case scenarios. This study examines the extreme scenarios for each phase and the combination of phases that produce the greatest monthly extremes. Data from Canada and the United States are gathered from the Historical Climatology Network (HCN). Monthly data are simulated by the utilization of a stochastic model in order to simulate joint-probability distributions of temperature with respect to phases of climate oscillators. This stochastic method simulates monthly data by the stochastic selection of daily data with identical ENSO, PDO, and PVO characteristics. In order to test the quality of the simulation, monthly simulations for PDO and PVO categories are compared with the observations. The simulated data are arranged, and the tenth and ninetieth percentiles are analyzed. The magnitudes of temperature anomalies are the greatest in the western Canada and the southeastern United States during January. Western Canada has its coldest (warmest) Januaries when the PDO and PVO are low (high). These conditions tend to occur during La Niņa (El Niņo) or neutral ENSO. The southeastern United States has its coldest Januaries during high PDO and low PVO and warmest Januaries during low PDO and high PVO. Although extremes occur during El Niņo or La Niņa, many stations have the highest or lowest temperatures during neutral ENSO. The magnitudes of the temperature anomalies and the corresponding phase combinations vary regionally.
A52B-12
Tempests in the Troposphere and in Tokamaks: New Insights From Comparative Physics
The persistent patterns of disturbance stream function contours that mark winter storm tracks in mid latitudes, computed from a two layer quasi geostrophic model (J. S. Frederiksen, J. Atm. Sci. 39, 969, 1982), bear a striking resemblance to the patterns of perturbed electrostatic potential contours in poloidal cross sections of a magnetic tokamak plasma (Y. Kishimoto et al, Phys. Plasma 3, 1289, 1996). At first sight these are two vastly different physical systems, yet in both the flows are structured by temperature gradient driven convection. Both types of flow are quasi two-dimensional, but for quite different reasons. In the case of the stratified, rotating flows comprising the atmosphere the horizontal scale is large compared with the vertical scale, while in magnetized plasmas the converse holds. Does this affect the location of enhanced eddy activity? In other words, are the similarities between the systems more important than the manifest differences? In this work we make a systematic comparison of the physics governing baroclinic instabilities in the atmosphere and ion temperature gradient instabilities in plasmas, and examine the stability properties of the equations of motion for both flow fields. We find that the evolution and location of storm tracks is a robust and resilient phenomenon that can be modeled and simulated using a relatively simple proxy. This allows us to predict and compare transport in both systems from much simpler models than have been used to date. The analogs of the plasma Bohm and gyro-Bohm diffusivities are given for the baroclinic instability.