A43C-01
Interaction between the Equatorial Atlantic and Pacific
Both the tropical Pacific and Atlantic host an equatorial mode of interannual variability called the Pacific El Nino and the Atlantic Nino, respectively. Although the Pacific El Nino does not correlate with the Atlantic Nino, anomalous warming or cooling of the two equatorial oceans can form an inter-Pacific-Atlantic sea surface temperature (SST) gradient variability that induces surface zonal wind anomalies over equatorial South America and over some regions of both ocean basins. The zonal wind anomalies act to bridge the interaction of the two ocean basins, reinforcing the inter-Pacific-Atlantic SST gradient through atmospheric Walker circulations and oceanic dynamics. Thus, a positive feedback seems to exist for climate variability of the tropical Pacific-Atlantic Oceans and atmosphere system, in which the inter-basin SST gradient is coupled to the overlying atmospheric wind. Rainfall responds to the inter-Pacific-Atlantic SST gradient by showing an anti-symmetric configuration between the two equatorial oceans, suggesting that rainfall is sensitive to the equatorial inter-basin SST gradient, regardless of which ocean is anomalously warm or cold.
A43C-02
African aerosol and interannual variability of precipitation in the Atlantic ITCZ
The interannual variability of precipitation in the Atlantic marine ITCZ (AMI) is affected by many factors. Possible
aerosol effects have, however, not been thoroughly investigated, although huge quantities of African aerosol are
transported over tropical Atlantic each year. Using satellite observations of aerosol (TOMS) and precipitation
(GPCP) over the period 1979-2000, this study elucidates possible large-scale, interannual relationships between
the absorbing aerosol (primarily mineral dust and carbonaceous aerosol) and precipitation in the AMI. For each
calendar month, a composite of precipitation difference between years of anomalously high and low aerosol was
made. It is found that possible influence of aerosol on precipitation in the AMI have a strong seasonality. Averaged
over the tropical Atlantic, the aerosol-related variability in precipitation explains 5-25% of the variance of
precipitation after effects from ENSO, zonal and meridional modes of tropical Atlantic SST and North Atlantic
Oscillation (NAO) are removed. This fractional variance related to aerosol can be as high as 40-50% in some
regions of the AMI in particular months (such as January and February). From the anomalously low to
anomalously high aerosol years, the precipitation is generally suppressed in boreal autumn and winter. A
northward shift of the AMI is observed in winter and a southward shift in spring. Based on a Monte Carlo
simulation and the Kolmogorov-Smirnov test, the precipitation changes related to aerosol are significantly
different from those due to random sampling at 95% confidence level. Aerosol therefore might play an important
role in modifying spatial distribution of precipitation in the AMI in addition to known climatic factors (e.g., ENSO,
tropical Atlantic SST modes, NAO). Further studies are needed to explore whether the precipitation changes
related to aerosol are indeed caused by aerosol effects or by other associated factors, such as dry air and large-
scale circulation.
http:metofis.rsmas.miami.edu/~jhuang/ami/index.htm
A43C-03
The Role of the tropical Atlantic in the Dynamics of Persistent US Droughts
The role of the Atlantic in US hydroclimate variability has often been discussed in the literature. In most past studies the focus has been on East Coast precipitation. Recently however, there has been increased interest in the effect of long-term variability in Atlantic SST on the dynamics of droughts in the Great Plains (GP) and the US West (USW). Most of these areas are semi-arid and prone to sever droughts. As recent modeling studies show, the major external forcing of these droughts are low-frequency fluctuations of tropical Pacific SST. Here however, we study the dynamics of an apparent causal association between tropical Atlantic SST variability and the frequency of occurrences and intensity of these droughts. In particular, we are interested in finding out how an anomalously warm tropical Atlantic Ocean induces, as it seems, dry conditions in the GP and USW. We use an atmospheric GCM (NCAR CCM3) forced with observed SST and find that such warm SST conditions are associated with a local response within the basin that entails a weakening of the subtropical high, with non-local, upstream high-pressure anomaly in the Pacific. The low-pressure response in the Atlantic occurs year-round but the Pacific response is a wintertime phenomenon. To understand the dynamics of these influences we utilize a series of controlled experiments with a linear, primitive equation model where the diabatic heating field from the full GCM is used. The linear model mimics the overall winter and summer effects of tropical Atlantic heating on the Azores high and the upstream Pacific response. These results confirm that weak but persistent forcing from the tropical Atlantic can modulate the influence of tropical Pacific SST on western US precipitation variability.
A43C-04
The barrier layer and its role in maintaining the tropical Atlantic warm pool
In the western tropical Atlantic Ocean, the barrier layer (BL), which is defined as the distance between the bottom of the mixed layer and the bottom of the isothermal layer, is often associated with a temperature inversion. When this occurs, the vertical entrainment brings warmer water from the BL into the mixed layer, causing an increase in the sea surface temperature (SST). We explore the role of this warming mechanism in the maintenance of the western Tropical Atlantic warm pool using the latest Simple Ocean Data Assimilation (SODA) product. During boreal fall and winter, the BL in the northwestern tropical Atlantic reaches a depth of 40 m, due to the combined effects of ITCZ rainfall, Amazon River discharge and the subsurface salinity maximum. An average temperature inversion of 0.1°C is common during these seasons. We estimate that the BL-induced temperature inversion can contribute to a warming of the surface mixed layer at a rate of 0.1°C per month, which accounts for about 30 % of total SST change. This result suggests that in the northwestern tropical Atlantic Ocean the BLs and the associated temperature inversion play an important role in maintaining the warm sea-surface temperature and need to be properly simulated to reduce a cold bias which is common in coupled climate model simulations.
A43C-05
Negative ocean--atmosphere feedback in the South Atlantic Convergence Zone
The temporal evolution of the coupled variability between the South Atlantic Convergence Zone (SACZ) and the underlying sea surface temperature (SST) during austral summer is investigated using monthly data from the NCEP/NCAR reanalysis. A maximum covariance analysis shows that the SACZ is intensified [weakened] by warm [cold] SST anomalies in the beginning of summer, drifting northward. This migration is accompanied by the cooling [warming] of the underlying oceanic anomalies. The results confirm earlier analyses using numerical models, and suggest the existence of a negative feedback between the SACZ and the underlying South Atlantic SST field. A linear regression of daily anomalies of SST and omega at 500 hPa to the equations of a stochastic oscillator reveals a negative ocean--atmosphere feedback in the western South Atlantic, stronger during January and February and directly underneath the oceanic band of the SACZ.