February 1, 2021: Online Ocean Circulation and Climate Dynamics Colloquium

online 11 am
Webex meeting room: https://geomar.webex.com/geomar/j.php?MTID=m143bbdd58a12de29b32fc44306046cdf
Meeting number (access code): 174 648 4002
Meeting password: rfZZUv3Ju33

Traditionally, the interannual Tropical Atlantic variability (TAV) is thought to be governed by two independent air-sea coupled modes denoted as Meridional Mode (MM) and Equatorial Mode (EM), peaking in boreal spring and summer respectively. During past decades, several studies suggested a possible connection between the MM and EM, but without reaching a consensus about its frequency, type and associated mechanisms.
In the framework of the FESTIVAL project (MSCA-IF-H2020), we have demonstrated the existence of two evolving modes, from winter-to-summer, during the observational record. The first evolving mode displays an anomalous warming covering the entire tropical basin with maximum anomalies in north tropical Atlantic in boreal spring and equatorial band in summer months. The second evolving mode exhibits a persistent inter-hemispheric SST gradient from boreal spring to summer season. These patterns are referred to different connections, same-sign and opposite-sign, between the traditional MM and EM. Remarkably, the evolving modes appears to be differently inter-connected along the record, associated with changes in the Pacific and Atlantic Ocean background states.
The emergence of evolving modes from 2000 (i.e: 2005, 2009, 2010 and 2017), have been associated with severe winter conditions in the Euro-Atlantic sector or intense Atlantic hurricane seasons. In particular, we have investigated the mechanism connecting the north tropical Atlantic and equatorial variability during 2017 and the role played by the wind forcing through a suite of forecast simulations with the EC-Earth3.3 model. A free-running reference prediction (MOD) is compared with two additional forecasts in which ERA-interim (ERAI) and the novel corrected ERA-I product (ERA*) are prescribed over the tropical Atlantic region. MOD forecast is not able to correctly capture the observed NTA 2017 warm pattern just with initialized conditions. ERAI and ERA*, with respect to MOD, illustrate a significant improvement in 2017 SST simulation over eastern NTA (off-Mauritania) and along the Senegal coast from boreal spring to summer. The ocean activity proves a high sensitivity to realistic surface winds. Stronger north-equatorial wind curl is found in ERA* and ERAI forecasts, respect to MOD, from February to May. This excites a downwelling Rossby wave (dRW) that propagates westward as a second baroclinic mode (~ 0.49m/s). The dRW is boundary-reflected and displaces along the equatorial band during July-August, resembling the first baroclinic mode (~ 2.97m/s). Using the novel ERA*, compared with its predecessor ERAI, allows for enhancing the propagation of equatorial downwelling Kelvin wave during boreal summer.

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