A42B-01 INVITED
Observations of Air-sea Interaction in the Northeast Tropical Atlantic
The NOAA Earth System Research Laboratory (ESRL) air-sea interaction group has initiated a planned multi-year
study of climate processes in the E. Equatorial Atlantic that follows a (now ended) 5-year study in the E. Equatorial
Pacific. The goal is to collect observations over several years to characterize the balance of turbulent and
radiative fluxes at the surface and the role of clouds and precipitating systems in variations of that balance. The
observations will also provide a venue to statistically examine model physics and satellite retrievals. The project
was inaugurated in the summer of 2006 as part of a joint ESRL, University of Miami, and NOAA/AOML study
during the Special Observing Period (SOP1) of the African Monsoon Multidisciplinary Analyses project (AMMA).
The air-sea interaction group at ESRL contributed to the field campaign by measuring traditional near-surface
meteorological data aboard the R/V Ronald H. Brown in the northeast tropical Atlantic during May 27- July 16. The
observations included direct turbulent fluxes, radiative fluxes, cloud properties, aerosols, and atmospheric
boundary layer profiles. This presentation will focus on flux comparisons using the direct covariance and inertial-
dissipation methods, and the NOAA TOGA-COARE bulk flux algorithm (version 3.0), connecting clear-sky radiative
fluxes with near-surface aerosol concentrations, and comparisons of cloud radiative forcing characteristics to
those obtained previously in the Pacific study.
http:www.esrl.noaa.gov/psd/psd3/air-sea/
A42B-02
Observed Influence of Amazon rainfall on the Atlantic ITCZ and Atlantic Nino
Most of previous studies on climate variabilities of the tropical Atlantic Ocean have been focused on remote and internal oceanic processes or atmosphere-ocean interaction. In comparison, relatively few studies have examined the influences from adjacent continents, especially the influence of rainfall over the South American continent. Using the Tropical Rainfall Measuring Mission (TRMM) daily rain-rate dada, the QuikSCAT ocean surface wind and PIRATA buoy data, we have found that convection developed over the Amazonia appears to propagate eastward across the Atlantic and then into Africa. Such changes modulate the intensity and location of the convection within the Atlantic ITCZ and result in a zonal oscillation of the ITCZ between the west and east equatorial Atlantic Ocean. The eastward propagating disturbances appear to be an atmospheric Kelvin wave with a period of 6 to 7 days and a phase speed of around 12 m s-1. Such convectively coupled Kelvin wave is particularly strong during boreal spring and dominates the synoptic variations of the lower and upper troposphere winds. Our results further suggest that the interannual changes of these convective coupled Kelvin waves have an important influence on trigging the onset of Atlantic Ninos. In particular, anomalously late northward withdraw of the South American rainfall in boreal spring lead to stronger Kelvin wave activities and stronger westerly wind anomalies in the western equatorial Atlantic. The latter triggers a change of the slope of the thermocline in the equatorial Atlantic Ocean and induces sea surface temperature anomalies in the eastern Atlantic. These changes contribute to the onset of the Atlantic Nino in earlier boreal summer.
A42B-03
Statistical Characteristics of Atlantic Hurricanes From Time Series Observations
In the last decade there has been a series of active seasons for hurricanes, including that of 2005. The recent increase in hurricane frequency and intensity may be linked to the trend of increasing global temperatures. Several studies have shown that the rise in global temperatures is correlated with upward trends in sea surface temperature (SST). Such trends have been documented in all ocean regions where hurricanes are formed. The skill in forecasting the seasonal or inter-annual activity of Atlantic hurricanes (AH) with dynamical models is limited, due to a rapid decay of predictability in such models. Nevertheless, time series of data accumulated over more than 100 years yield some insight into the general dynamical features of AH systems. In particular, nonlinear time-series analysis is becoming a reliable tool for the study of complex dynamics from measurements. We present the AH characteristics by analyzing the statistics of long-term time series, illustrating the inter-annual and decadal variability. Hurricane trajectory data, peak wind speed and the "eye" or minimum pressure all display significant variability over the last century. Power spectrum density (PSD) analysis of time series of annual AH numbers shows periods of ~2-3 years and ~ 5 years. These periods are confirmed by PSDs of the annual SST anomaly, between latitudes of 30 S and 30 N, and of the annual CAR Index (the SST anomaly in the Caribbean region). The analysis presented here suggests that part of AH inter-annual variability can be directly linked to SST variations in the same region. The study underlines the potential role of time-series analysis to complement other statistical and dynamic methods used in the description and forecasting of hurricanes.
A42B-04
Forced and Natural Atlantic Multidecadal Variabilities in Observations and Coupled Ocean- Atmosphere Models
The recent increase in both the intensity and frequency of the Atlantic hurricane activities and the rapid increase in Greenland ice sheet melting have raised an extremely urgent question as to what causes these abrupt changes in climate. The most relevant scientific question is the relative contribution of those caused by the natural climate variabilities and those due to anthropogenic forcing. If both effects are important, then the next question is to determine the phase of the natural oscillation, so one can correctly predict what we can expect for the next decades given the anthropogenic climate change. This study examines the different methods to detect and separate the natural and forced components of the most prominent multidecadal variabilities in observations: the so called Atlantic Multdecadal Oscillation or AMO.
A42B-05 INVITED
Tropical Atlantic Variability in the Future
Tropical Atlantic Variability (TAV) is shaped by atmospheric processes, such as the trade winds and remote patterns of variability, and oceanic processes, such as upwelling on the equator and the transfer of water masses in the upper branch of the meridional overturning circulation (MOC). Climate change affects these processes. In the 21st century the meridional temperature gradients will reduce, the African monsoon will be affected and the MOC is expected to reduce. Abrupt climate change may even occur when the MOC collapses. In this talk the impact of global warming and changes in the MOC on TAV will be discussed. These changes will affect the predictability of the TAV and regional and remote impacts of TAV. For instance, a reduction of the MOC reduces upwelling on the equator and deepens the thermocline, causing a weakening of the thermocline feedback in the eastern cold tongue region. A shift in the Intertropical Convergence Zone will affect thermodynamic feedbacks between the atmosphere and the ocean in the tropics. Results from IPCC-class climate models and a regional coupled ocean-atmosphere model will be used to discuss projections for Tropical Atlantic Variability in the future.
A42B-06
Increased Tropical Atlantic Wind Shear in Model Projections of Global Warming
To help understand possible impacts of anthropogenic greenhouse warming on hurricane activity, we assess model-projected changes in large-scale environmental factors tied to variations in hurricane statistics. This study focuses on vertical wind shear (Vs) over the tropical Atlantic during hurricane season, the increase of which has been historically associated with diminished hurricane activity and intensity. A suite of state-of-the-art global climate model experiments is used to project changes in Vs over the 21st century. Substantial (up to 30 percent) increases in tropical Atlantic and East Pacific shear are robust features of these experiments, and are shown to be connected to the model-projected decrease in the Pacific Walker circulation. The relative changes in shear are found to be comparable to those of other large-scale environmental parameters associated with Atlantic hurricane activity. The influence of these Vs changes should be incorporated into projections of long-term hurricane activity.
A42B-07
Statistical Analysis of Environmental Factors Affecting the Genesis of Tropical Atlantic Cyclones in Climate Model Simulations
Tropical cyclogenesis is affected by several large-scale environmental factors, such as the vertical wind shear, relative humidity, etc. It is therefore very important to understand how these factors are affected by climate variations, both natural and anthropogenic. Although coarse-resolution climate models cannot be used to study the actual genesis of tropical cyclones, they can be used to investigate the trends in the environmental factors that affect them. In this study, we perform a statistical analysis of the ensemble of climate model simulations carried out for the Intergovernmental Panel on Climate Change (IPCC) 4th Assessment. We consider both model simulations of the 20th century and model projections for the 21st century. Multivariate analysis is used to identify statistically robust indicators of tropical cyclone genesis for the 20th century simulations, by comparing the simulated large-scale environmental factors to observed cyclone frequencies. The focus is on the Main Development Region (MDR) for tropical cyclones in the Atlantic, but the remote influence of phenomena such as the El Nino-Southern Oscillation (ENSO) and West African rainfall is also considered. Our results show that climate model integrations for the 20th century have considerable difficulty in simulating the environmental factors that affect tropical cyclogenesis, such as the climatological vertical shear in the MDR region and correlations with the El Nino-Southern Oscillation. Model simulations for the 21st century reveal trends in the environmental vertical shear that can have a dynamical effect on tropical cyclogenesis, which may partially offset the thermodynamic effect of increased sea surface temperatures.