A31D-01 INVITED
Insights into East Pacific ITCZ convection from EPIC observations and cloud-resolving modeling
During the EPIC 2001 project, a wealth of airborne and ship-based measurements (including a continuous scanning precipitation radar and vertically-pointing cloud radar) were made in a mesoscale region around 10 N, 95 W. Several studies based on this data have documented the vertical and horizontal structure of the convection and its connection to surface wind speed, convective inhibition, and easterly waves. Lower-tropospheric humidity, surface latent heat fluxes and mesoscale SST variability have been suggested as primary drivers modulating area-averaged rainfall over this area. We present the first EPIC cloud-resolving model (CRM) simulations of the EPIC ITCZ study area. These can be compared with the whole suite of atmospheric observations and allow exploration of the correlation of rainfall with the hypothesized drivers. We compare simulations conventionally forced with specified horizontal advection and vertical motion with ‘weak-temperature gradient' simulations in which the vertical motion is chosen to maintain the observed tropospheric temperature, and simulations including mesoscale SST variability, and use these to test and enhance our understanding of the thermodynamic control of ITCZ convection in the east Pacific.
A31D-02
Mesoscale Structure and Interannual Variability of Atmospheric Kelvin Waves Over the Eastern Tropical Pacific
Atmospheric Kelvin waves are prominent disturbances within the eastern tropical Pacific. Convectively coupled Kelvin waves modulate a substantial portion of the rainfall within the ITCZ. The convective envelopes of these waves move eastward at between 15-18 m/s on average, and are composed primarily of westward propagating features within them. Many of these westward disturbances move too quickly (greater than 20 m/s) to be explained solely by advection, and therefore must also be wavelike. We present evidence that some of these features are likely inertio-gravity waves. Using high temporal and spatial resolution satellite data, we show that Kelvin waves are much more common than previously realized. Many of these events are relatively short-lived, but they can still have a pronounced impact on ITCZ convection. Kelvin waves readily propagate unimpeded across the Andes barrier and control convection over Amazonia, however the connection between the Pacific and South America is strongly influenced on the interannual time scale by the El Nino/Southern Oscillation (ENSO) phenomenon. During warm ENSO events, when sea surface temperature (SST) is high in the eastern equatorial Pacific, the Kelvin wave activity in enhanced along the equator and they propagate readily from the Pacific into South America and on through the Atlantic to Africa. During cold events, the activity is confined to the ITCZ off the equator and the connection between the Pacific and Amazonia is suppressed, but there is still substantial Kelvin wave generation to the east of the Andes. In these cases Kelvin waves are frequently excited by incursions of mid-latitude air into Amazonia, characterized by lower tropospheric high pressure surges trapped along the eastern flank of the Andes. Strong upper tropospheric extratropical wavetrain signatures over the South Pacific precede the development of composite Kelvin waves over both the Pacific and South America, indicating that such events may be predictable in numerical models several days in advance.
A31D-03
Horizontal and Vertical Structure of Pacific and Atlantic Easterly Waves
Outgoing longwave radiation (OLR) and wind fields in the Atlantic and Pacific intertropical convergence zone (ITCZ) are dominated by variability on synoptic time scales primarily associated with easterly waves during boreal summer and fall. This study uses spectral filtering of observed OLR data to capture the convective variability coupled to easterly waves. Filtered OLR is then used as an independent variable to isolate easterly wave structure in wind, temperature and humidity fields from radiosondes and NCEP/NCAR reanalyses. Our previous work has isolated the structure of Atlantic and Pacific easterly waves. The current study documents the structure of these waves in the Caribbean region and their passage from the Atlantic into the east Pacific. The structure is found to vary according to surface conditions (i.e. land or water), as well as with latitude within the Caribbean region. Eddy energy conversion terms are also presented for key locations throughout the region and compared to Atlantic and Pacific wave energetics.
A31D-04
Surface convergence and vertical motion profiles in the central-eastern Pacific Intertropical Convergence Zone: why is the convection so "bottom-heavy?"
Recent work has shown that in parts of the eastern Pacific ITCZ, large-scale vertical motion profiles are "bottom- heavy" with strong boundary-layer horizontal convergence, maximum vertical motion around 850mb, and divergence above. This contrasts with rainy regions over the western Pacific, where boundary-layer horizontal convergence is weak and mean horizontal convergence extends to above 400mb. Thus, for a given time-mean conditional instability, e.g. as measured by the difference of boundary-layer and mid-tropospheric moist static energy (MSE), the convection tends to be more bottom-heavy where there is strong boundary-layer convergence. Our ultimate goal is to explain this correlation using column MSE budget analysis, but a preliminary step is to see if we can explain the climatological pattern of boundary-layer convergence across the Pacific with a simple model. We use a linear mixed layer model (Stevens et al. 2002) to diagnose the extent to which observed surface convergence can be explained by SST-driven pressure gradients and friction. The model is forced with 850mb horizontal winds and pressure gradients from the ERA40 reanalysis. We find that differences between surface convergence in the east and west Pacific are predominantly associated with boundary layer temperature gradients and SST, not deeper tropospheric processes.
A31D-05
The Tropical East Pacific as a Laboratory for Tropical Cyclones
The summertime tropical cyclogenesis rate per unit area in the eastern Pacific ocean is arguably higher than in any other location in the world. Many if not most of these cyclones form from African easterly waves which cross Central America into the Pacific. Of order 25% of these waves intensify into cyclones. A significant fraction of east Pacific tropical cyclones undergoes landfall on the Mexican coast. Those which do not, generally dissipate over cold ocean waters north of the east Pacific intertropical convergence zone, often not far from land. The layer of warm ocean water which supports the development of east Pacific cyclones is unusually shallow and is structured by anticyclonic vortices which form by various processes and propagate slowly to the west. These vortices locally deepen the oceanic mixed layer and support stronger convection than their surroundings, possibly promoting cyclogenesis and cyclone intensification. Cyclones in turn have an unusually large effect on the ocean mixed layer due to its shallowness. The east Pacific is thus a region of strong coupling between the atmosphere and the ocean, mediated in large part by the action of tropical cyclones. In most cases cyclogenesis, intensification, landfall, and decay over cold water occur within easy range of research aircraft launched from a number of Central American and Mexican bases such as San Jose, Huatulco, Acapulco, Puerto Vallarta, and Cabo San Lucas. The U. S. National Center for Atmospheric Research, the National Oceanic and Atmospheric Administration, and the National Aeronautics and Space Administration have all successfully operated aircraft-based research projects from one or more of these locations. The frequency with which cyclones form, develop, and decay in the east Pacific and their proximity to land bases with excellent facilities make the tropical east Pacific an ideal international laboratory for the study of tropical cyclones. Given the importance of these cyclones to global weather and climate, every effort should be made to take advantage of this natural laboratory to enhance our knowledge of this phenomenon.
A31D-06
The Tropical Eastern Pacific Seasonal Cycle: Assessment of Errors and Mechanisms in IPCC AR4 Coupled Ocean-Atmosphere General Circulation Models
The climate of the eastern tropical Pacific Ocean and atmosphere is analyzed in 15 coupled ocean-atmosphere models from 8 nations. Coupled models variously reproduce the observed meridional and seasonal march of sea surface temperature (SST) and intertropical convergence zone (ITCZ) precipitation. Year-round, warmer SST and more precipitation are observed in the northern hemisphere. Briefly during boreal spring two precipitation maxima straddle the equator. This "double ITCZ" error identified in coupled models more than 10 years ago persists in 3 of the models examined, while in 8 models the ITCZ alternates symmetrically between the hemispheres with the seasons. Only 3 models maintain stronger precipitation in the northern hemisphere year- round. Simple metrics are introduced to diagnose model fidelity. Multi-model ensemble spread elucidates the coupled physics of various model solutions. Year-round southerly wind on the equator follows the atmospheric heating, maximum in September when the northern tropics are warmer and minimum in March when the ITCZ straddles the equator. The seasonally alternating ITCZ error generates two wind speed maxima per year, one northerly and one southerly, resulting in a cool bias of the equatorial ocean. The equatorial cold tongue temperature among the models is correlated to the equatorial scalar wind speed at -0.6. Errors in the meridional wind within 1000 km of the South American coast explain warm SST errors. SST on the equator (80-90° W, 2° S-2° N) is correlated to meridional wind speed at -0.66 among models. Northeasterly wind jets blowing over Central American isthmus in winter cool the SST in the eastern Pacific warm pool, contributing to the early demise of the northern ITCZ relative to observations. The February-April northerly wind bias on the equator is correlated to the antecedent December-February Central American Pacific wind speed at -0.87, suggesting there is an atmospheric connection linking the Central American northwesterlies and the southward shift of convergence. The various representations of stratus clouds among models clearly affect the underlying SST, but their effect on the meridional atmospheric circulation is difficult to discern. This study suggests that both the double ITCZ error and equatorial SST errors in the eastern Pacific could be alleviated by reducing errors in the climatology of meridional wind.
A31D-07
The warm bias in the Southeastern Pacific in the NCEP CFS model
As in many contemporary coupled atmosphere-ocean general circulation models, there exists a sea surface temperature (SST) warm bias in the Southeastern Pacific (SEP) in the National Centers for Environmental Prediction (NCEP) coupled Climate Forecast System (CFS) model. This study examines the formation of this warm bias, its association with the deficiency of model-produced stratus clouds, and its impact on the model's time-mean state and interannual variability. Questions we will address include: (1) Where does the warm bias start, how does it evolve in space, and how quickly does it develop? (2) How does the warm bias relate to the deficiency of model-produced stratus clouds, and what is the contribution of the associated excessive surface radiation fluxes? (3) What is the role of oceanic dynamics in the evolution of the warm bias? (4) What are the impacts of the warm bias on the mean state of the model and on its interannual variability? These questions are addressed based on outputs from a suite of CFS integrations, including: (a) retrospective 9-month seasonal forecasts from realistic atmospheric and oceanic initial conditions from each month of 1981-2004, (b) long-term multi-decade free simulations, (c) experimental free simulations with mean surface radiation errors over the SEP area [30°S-0°; 90°W-68°W] corrected, and (d) simulations with the uncoupled oceanic component of the CFS with and without mean surface radiation errors over the SEP area.