Submitted to Journal of Climate Revised version

• Main Authors: Reindert J. Haarsma, Wilco Hazeleger
• Other Authors: The Pennsylvania State University CiteSeerX Archives
• Format: Text
• Language: English
• Published: 2006
• Subjects: North Atlantic
• Online Access: http://citeseerx.ist.psu.edu/viewdoc/summary?doi=10.1.1.161.4524; http://www.knmi.nl/publications/fulltexts/cold_tongue_mode_jclim_revised_2.pdf

The extra-tropical atmospheric response to the equatorial cold tongue mode in the Atlantic Ocean has been investigated with the coupled ocean-atmosphere model SPEEDO. Similar as in the observations the model simulates a lagged co-variability between the equatorial cold tongue mode during late boreal summer and the east Atlantic pattern a few months later in early winter. The equatorial cold tongue mode attains its maximum amplitude during late boreal summer. However, only a few months later, when the ITCZ has moved southward, it is able to induce a significant upper tropospheric divergence which is able to force a Rossby wave response. The lagged co-variability is therefore the result of the persistence of the cold tongue anomaly and a favorable tropical atmospheric circulation a few months later. The Rossby wave energy is trapped in the South Asian subtropical jet and propagates circumglobal before it reaches the North Atlantic. Due to the local increase of the Hadley circulation, forced by the cold tongue anomaly, the subtropical jet over the North Atlantic is enhanced. The resulting increase in the vertical shear of the zonal wind increases the baroclinicity over the North Atlantic. This causes the non-linear growth of the anomalies due to transient eddy-feedbacks to be largest over the North Atlantic, resulting in an enhanced response over that region. 2