Date of Award

Spring 1-1-2011

Document Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Department

Atmospheric & Oceanic Sciences

First Advisor

Weiqing Han

Second Advisor

David Gochis

Third Advisor

Tom Warner

Abstract

Intraseasonal variability (10-100 day periods) of surface wind and convection in the tropical Atlantic is analyzed using QuikSCAT satellite wind, outgoing longwave radiation (OLR), and precipitation for the period of 2000-2008. Similar analyses have also been performed using the European Centre for Medium-Range Weather Forecasts (ECMWF) 40-year Re-Analysis (ERA40) data from 1960-2001 and ERA-interim reanalysis products for 1990-2008. Case studies show that the MJO propagated eastward from the Indo-Pacific Ocean to the Atlantic during winter and spring of 2002, causing the observed 40-60-day wind variations in the equatorial Atlantic basin. The Isthmus of Panama is a dominant pathway for these surface wind anomalies to propagate into the Atlantic, where they can produce important climate impacts. This pathway is statistically significant based on the analysis using multi-year data. Further analyses have been carried out to assess the relative importance of dominant atmospheric intraseasonal convective processes over the tropical Atlantic Ocean and African Monsoon region: the Madden-Julian Oscillation (MJO, which dominates the eastward-propagating signals at 20-100-day periods), quasi-biweekly (10-25 days) Kelvin waves, and 10-100-day westward propagating Rossby waves. The results show that contribution from each process varies in different regions of the tropical Atlantic Ocean and African monsoon region. In general, the eastward-propagating MJO and quasi-biweekly Kelvin wave more frequently dominate strong convective events than Rossby waves in the African monsoon region. The westward-propagating Rossby waves, on the other hand, have larger contributions to convection in the Western Atlantic Ocean. Both the westward- and eastward-propagating signals contribute approximately equally in the Central Atlantic basin. The impacts of intraseasonal signals have evident seasonality. The MJO is stronger during November-April than May-October in all regions. The 20-100-day Rossby waves are stronger during November-April than May-October in the African monsoon region, and are equally strong for the two seasons and dominate convection variability during May-October in the Western and Central Atlantic basins. Of particular interest is that the MJO originated from the Indo-Pacific Ocean, and the quasi-biweekly Kelvin wave generated by convection in the Amazon region and western Atlantic basin can enhance as they propagate through the tropical Atlantic Ocean, amplifying their impacts on the African monsoon. On the other hand, Rossby waves can be generated either in the eastern equatorial Atlantic or West African monsoon region. They can strengthen while they propagate westward through the tropical Atlantic, producing large effects on the Western Atlantic, Caribbean Sea and Central America regions. These results imply that air-sea interaction in the Atlantic Ocean, and possibly interaction with local convective signals can modify the strengths of the MJO, Kelvin and Rossby waves, which have important implication for the prediction in the countries that surround the tropical Atlantic Ocean.

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