In this study we investigate the Indian Ocean Dipole/Zonal Mode (IODZM) in a 250-yr simulation of the GFDL coupled global general circulation model (CGCM). The CGCM successfully reproduces many fundamental characteristics of the climate system of the Indian Ocean. We explore the character of the IODZM, and relationships between positive IODZM and El Niño events through a composite analysis. The IODZM events in the CGCM grow through feedbacks between heat-content anomalies and SST-related atmospheric anomalies, particularly in the eastern tropical Indian Ocean. The composite IODZM events that co-occur with El Niño have stronger anomalies and sharper east-west SSTA contrast than those that occur without El Niño. IODZM events, whether or not they occur with El Niño, are preceded by distinctive Indo-Pacific warm pool anomaly patterns in boreal spring: in the central Indian Ocean easterly surface winds, and in the western equatorial Pacific an eastward shift of deep convection, westerly surface winds and warm sea surface temperature. However, delayed onset of the anomaly patterns (e.g. boreal summer) are often not followed by IODZM events. The same anomaly patterns often precede El Niño, suggesting that the warm pool conditions favorable for both IODZM and El Niño are similar. Given that IODZM events can occur without El Niño, we propose that the observed IODZM-El Niñ o relation arises because the IODZM and El Niñ o are both large-scale phenomena in which variations of the Indo-Pacific warm pool deep convection plays a central role. Yet each phenomenon has its own dynamics and lifecycle, allowing each to develop without the other.

The NCEP regional Eta model was nested in the NCEP atmospheric general circulation model (AGCM) to provide a general assessment of the dynamic downscaling method in seasonal climate simulations over South America (SA). Both models were initialized with same initial conditions taken from the NCEP/NCAR reanalysis. The results showed that the nested model improved the simulation of seasonal mean lower- and upper-level circulations in both warm and cold seasons when compared to the AGCM alone. The nested model also produced better simulations of seasonal mean 2-meter temperature and precipitation. The improved resolution topography in the nested model substantially enhanced the SA low-level jet circulation, resulting in a better prediction of daily precipitation over the subtropics and eastern Brazil. However, the simulation of the ITCZ by the nested model was weak due to weak moisture convergence at low-levels. A possible cause for this deficiency is presented. The interannual and intraseasonal variability of the wind circulation, surface temperature, and precipitation were also examined. The nested model significantly improved the differences in the lower- and upper-level circulations between the summers of 1997-98 and 1988-89.   Furthermore, it captured the differences in precipitation over eastern and southern SA, which were neither simulated by the AGCM, nor depicted in the NCEP/NCAR reanalysis data.

The focus of this presentation will be on the estimation of Lagrangian entropy sources as a means to assess both the accuracies and uncertainties of models in the simulation of weather and climate. In an 1960 lecture at the Princeton Institute for Advanced Studies, von Neumann commented that numerical weather prediction was less difficult than modeling climate, while the most difficult challenge would be to bridge time and space scales in the modeling of weather and climate. The underlying reason for this assessment was that success in numerical weather prediction was critically dependent on the initial state, success in simulating climate was critically dependent on specifying boundary layer processes, while bridging of time and scales of weather and climate would depend on the complementary linking of internal and boundary processes.

NCEP’s emphasis is that success in modeling climate depends on success in modeling weather, a position dear to my interests and focus. A key aim in this lecture is to utilize Carathéodory’s statement of the Second Law in motivating the relevance of entropy and its change as a fundamental property in numerical simulations of weather and climate, both theoretically and pragmatically. The theoretical motivation embeds Carathéodory’s perspective in a basis relation regarding initial and final states with respect to attainable states, which can be reached by isentropic processes, and also the difference between states that can only be reached by non isentropic processes from those which cannot be reached at all.

The pragmatic motivation utilizes Carathéodory’s perspective of attainable and unattainable states to examine and quantify relative to truth the accuracies of models as a function of numerics, parameterizations all or in part, resolution, as well as the interrelation of these factors and others. Accuracies relative to truth can then be ascertained from the relative capabilities of models to simulate thermodynamic processes involving reversibility and irreversibility.

Then the equivalence that of simulating climate regionally and globally with the initial value problem of weather will be supported by utilizing Carathéodory’s insight concerning attainable and unattainable states. Here the age old concept of air masses, their transport and modification in terms of potential and equivalent potential temperature as the proxies for dry and moist entropy will be briefly reviewed and brought to bear. The results of the overall analysis provide the means to; 1) study the issue of biases and statistical uncertainity in models, 2) investigate why modeling of hydrological processes is so difficult and 3) determine inaccuracies from numerics alone and then from the interaction of numerics and parameterizations.

An ocean general circulation model (OGCM) is used to
study the roles of equatorial waves and western
boundary reflection in the seasonal circulation of the
equatorial Indian Ocean. The western boundary
reflection is defined as the total Kelvin waves
leaving the western boundary, which include the
reflection of the equatorial Rossby waves as well as
the effects of longshore winds, off-equatorial Rossby
waves, and nonlinear processes near the western
boundary. The evaluation of the reflection is based
on a wave decomposition of the OGCM results and
experiments with linear models. It is found that the
longshore winds along the east coast of Africa and the
Rossby waves in the off equatorial areas contribute
significantly to the annual harmonics of the
equatorial Kelvin waves at the western boundary. The
semi-annual harmonics of the Kelvin waves, on the
other hand, originate primarily from a linear
reflection of the equatorial Rossby waves.

The dynamics of a dominant annual oscillation of sea
level co-existing with the dominant semi-annual
oscillations of surface zonal currents in the central
equatorial Indian Ocean are investigated. These sea
level and zonal current patterns are found to be
closely related to the linear reflections of the
semi-annual harmonics at the meridional boundaries.
Because of the reflections, the second baroclinic mode
resonates with the semi-annual wind forcing, i.e. the
semi-annual zonal currents carried by the reflected
waves enhance the wind-forced currents at the central
basin. Due to the different behavior of the zonal
current and sea level during the reflections, the
semi-annual sea level of the directly forced and
reflected waves cancels each other significantly at
the central basin. In the meantime, the annual
harmonic of the sea level remains large, producing a
dominant annual oscillation of sea level in the
central equatorial Indian Ocean.

The linear reflection causes the semi-annual harmonics
of the incoming and reflected sea level enhance each
other at the meridional boundaries. In addition, the
weak annual harmonics of sea level in the western
basin, resulting from a combined effect of the western
boundary reflection and the equatorial zonal wind
forcing, facilitate the dominance by the semi-annual
harmonics near the western boundary in spite of the
strong local wind forcing at the annual period.
The Rossby waves are found to have a much larger
contribution to the observed equatorial semi-annual
oscillations of surface zonal currents than the Kelvin
waves. The westward progressive reversal of seasonal
surface zonal currents along the equator in the
observations is primarily due to the Rossby wave
propagation.

Program in Atmospheric & Oceanic Sciences, Princeton University, GFDL, NOAA

The Atlantic thermohaline circulation (THC) is an important part of the Earth's climate system. Previous research has shown large uncertainties in simulating the dynamical behavior, future evolution and climate impact of the THC. The simulated response of the THC to freshwater perturbations and the associated climate changes have been intercompared among 14 climate models over the world. The robustness of particular simulation features has been evaluated across the model results.   In response to 0.1 Sv freshwater addition in the northern North Atlantic projected under realistic CO 2 scenarios , the multi-model ensemble mean THC weakens by 30% after 100 years. No model simulates a complete shutdown of the THC. The surface air temperature presents a complex anomaly pattern and the Atlantic ITCZ tends to shift southward.   In response to 1.0 Sv freshwater input that resembles the past melting water pulses from the paleo-glaciers , the THC shuts down rapidly in all models. The resultant global-scale climate changes are considerable. Models disagree in terms of the bi-stability of the THC. The THC is bi-stable in one model group but mono-stable in the other model group. The differing dynamical properties of the THC are closely related to the formation and stability of a reversed thermohaline circulation (RTHC) in the South Atlantic Ocean. The role of the RTHC in causing the differing simulations of the THC is analyzed in detail.

The increasing demand by the scientific community, policy makers, and the public for localized climate information has rendered the need for climate downscaling. High-resolution Regional Climate Models (RCMs) have been extensively used for climate downscaling over many regions around the world. This presentation summarizes the values added by dynamical downscaling using RCMs. The discussion focuses on subseasonal to interdecadal time scales, including 1) improvement of spatial patterns; 2) predictability at smaller spatial and temporal scales; 3) dynamical downscaling seasonal forecasts; and 4) interdecadal variability, and 5) a brief consideration of the coming decades that seeks to improve the climate dynamical downscaling.

The presentation will focus on recent advances in NCEP prediction services related to operational weather, climate and ocean forecasts systems and cover such topics as the push to dynamical modeling in the SI range, the increasing emphasis on multi model ensembles for an increasing array of forecast applications with examples shown from this past winter, the application of Test Beds and Joint Centers throughout NCEP and an update on the new NOAA Center for Weather and Climate Prediction near the University of Maryland.

The eddy- zonal flow feedback in SH winter and summer is investigated in this study. The persistence timescale of the leading principal components (PCs) of the zonal mean flow shows substantial seasonal variations. In SH summer, the persistence timescale of PC1 is significantly longer than that of PC2, while the persistence timescales of the two PCs are quite similar in SH winter. Storm track modeling approach is applied to demonstrate that seasonal variations of eddy-zonal flow feedback for PC1 and PC2 account for the seasonal variations of the persistence timescale. The eddy feedback timescale estimated from a storm track model simulation and a wave response model diagnostic quantitatively shows that PC1 in JJA and DJF, and PC2 in JJA, have significant positive eddy-mean flow feedback, while PC2 in DJF has no positive feedback. The consistency between the persistence and eddy feedback timescales for each PC suggests that the positive feedback increases the persistence of the corresponding PC, with stronger (weaker) positive feedback giving rise to a longer (shorter) persistence timescale.
EP-flux diagnostics have been performed to demonstrate the dynamics governing the positive feedback between eddies and anomalous zonal flow. The mechanism of the positive feedback, for PC1 in JJA and DJF and PC2 in JJA, is as follows: enhanced baroclinic wave source (heat fluxes) at low level in the region of positive wind anomalies, propagate upward and then equatorward from the wave source, thus giving momentum fluxes that reinforce the wind anomalies. The difference of PC2 between DJF and JJA is because of the zonal asymmetry of climatological flow in JJA. For PC2 in DJF, wind anomalies reinforce the climatological jet, thus increasing the barotropic shear of jet flow. The “barotropic governor” plays a role in suppressing eddy generations for PC2 in DJF and thus inhibiting the positive eddy-zonal flow feedback.

Presently, the direct effect of aerosols has been included in many GCMs and this helps to better simulate incoming surface solar radiation. In fact,even this parameterization requires "difficult-to-prescribe" space-time
distribution of particle sizes, mass-load, and optical properties of several key atmospheric aerosol species (that change with age). Nevertheless, the "indirect effect" of aerosols is even more important because it affects the cloud particulate number density, life-time, in-cloud water amounts, and precipitation efficiency, all of which
impact the simulated cloud-radiative forcing of water, mixed phase and ice clouds. Of these, only the physics of cloud-droplet formation and rainfall production in water clouds is well understood, however even this is
lacking in many GCMs. We have been developing a scheme to represent aerosol water-cloud interaction in our GCM. Our CCN activation scheme is due to Nenes and Seinfeld (2003) that was subsequently simplified for
log-normally distributed aerosols (Fountoukis and Nenes, 2005). It interacts with state-of-the-art microphysics algorithms due to Seifert and Beheng (2001, 2006). Simulation results of water-cloud aerosol interaction in the
ARM-SCM evaluation and validation with the ARM-SGP data will be shown. The parameterization of mixed-phase and ice clouds is even more challenging; some potentially useful constructs of Liu and Penner (2005) will be
discussed. Indeed, a number of modeling assumptions still need to be anchored to the observations as well as need reconfirmation through aerosol-chemistry-cloud microphysics models and performance evaluation
against analyzed clouds and aerosols climatologies.

Dust storm is a very severe disastrous weather system consisting of strong wind, soil erosion, air pollution by dust particles, and very low visibility. Satellite image can present the gigantic picture of dust storm. In order to make numerical-dynamical prediction of dust storm, we should develop a dust emission and transport model and couple it with a weather forecasting model with good enough treatments of atmospheric boundary layer and land surface processes. The mechanism of dust emission takes the central part in such a dust model. We discovered that wind gust and its relevant coherent disturbances in the atmospheric boundary layer play a crucial role in the dust emission and its penetration into the troposphere from the boundary layer. A parameterization scheme of gust effect was then developed and applied to the model. Furthermore, the climatic conditions favorable for the frequent occurrence of dust storms in the spring in East Asia were investigated, and preliminary results of dynamical prediction (for two seasons) of such climate situations were encouraging.

Climate varies across a wide range of temporal and spatial scales. Yet, climate modeling has long been approached using global models that can resolve only the broader scales of atmospheric processes and their interactions with land, ocean, and sea ice. Clearly, large-scale climate determines the environment for mesoscale and microscale processes that govern the weather and local climate, but, likewise, processes that occur at the regional scale may have significant impacts on the large scale circulation. Resolving such scale interactions will lead to much improved understanding of how climate both influences, and is influenced by, human activities.

Since the early 1990s, regional climate models have been used as a downscaling tool to study regional climate processes and provide regional climate information such as climate change projections and seasonal climate predictions for assessing climate impacts. In this presentation, I will summarize a number of regional climate modeling studies that we have performed to understand the effects of climate variability and change at the regional scale, with more focus on the western U.S. where the strong topographic influences on regional climate necessitate the need for regional climate information. I will also summarize some recent efforts on developing and applying the Weather Research and Forecasting (WRF) model to study regional climate. In particular, I will present preliminary results of applying WRF to simulate tropical convection and tropical modes using a channel model configuration. Future plans in using regional climate models to address both downscaling and upscaling issues will also be discussed.