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From boreal spring to summer Tropical Atlantic inter-annual variability

Periodic Reporting for period 1 - FESTIVAL (From boreal spring to summer Tropical Atlantic inter-annual variability)

Reporting period: 2019-01-16 to 2021-01-15

The tropical Atlantic variability is thought to be controlled by two air-sea couple patterns, the Meridional (MM) and Equatorial Mode (EM), peaking in boreal spring and summer respectively. Both modes exhibit pronounced environmental and socioeconomic impacts on the surrounding countries. Previous studies suggested possible interaction between these modes, but without reaching a consensus about the underlying mechanisms and the timing of the connection. Thus, determining the existence of MM-EM connection is essential to completely understand the tropical Atlantic variability and anticipate its climate impacts. Thus, FESTIVAL project aims to investigate the connection between the traditional MM and EM, understanding the underlying air-sea mechanisms and its multidecadal modulation. The main conclusions achieved in FESTIVAL project are:

1) Evolving modes, from winter to summer, emerge in the tropical Atlantic basin during the 20th century.

2) The evolving modes interact between them at inter-decadal time scales along the historical record, associated with large-scale ocean decadal patterns.

3) The North Tropical Atlantic SSTs act as a precursor of equatorial Atlantic variability up to 6 months in advance.

4) An oceanic mechanism based on the boundary-reflection of Rossby wave into an equatorial Kelvin wave is the main responsible to connect the MM and EM. However, its effectiveness in generating equatorial SST variability during summer months is modulated by the local wind forcing.

5) The evolving modes have a pronounced impact over the precipitation regime over Africa and South-America that changes from winter to summer season.
During the development of FESTIVAL project, the following results have been achieved:

1) Determination of the existence of evolving tropical Atlantic interannual modes along the 20th century and their multidecadal modulation.

Using a set of observational, reanalysis datasets and historical CMIP6 simulations, the existence of evolving (from winter to summer) tropical Atlantic modes has been demonstrated using a novel approach based on extended Maximum Covariance analysis (EMCA). Remarkably, the evolving modes illustrate different (same-phase and opposite-phase) connections between the traditional Meridional Mode (MM) and Equatorial Mode (EM) patterns and exhibit interdecadal fluctuations that seem to be caused by natural low-frequency patterns (i.e: Atlantic and Pacific Decadal Variability).

2) Characterization of the air-sea processes involved in the MM-EM connection.

The emergence of the positive (negative) phase of MM enhances (weakens) the surface pressure gradients, reinforcing (reducing) the North Equatorial Countercurrent during boreal spring (NECC). Noticeably, an anomalous wind curl north-of-equator triggers a downwelling (upwelling) Rossby wave that propagates westward and is boundary reflected into an equatorial Kelvin wave (KW). The displacement of the downwelling (upwelling) KW along the equatorial band reduces (increases) the zonal SSH gradient that weakens (reinforces) the Equatorial Undercurrent (EUC) during summer months. The RW-reflected mechanism is key to carry out the MM-EM connection, although its effectiveness to create the equatorial SSTs depends on favorable/unfavorable local wind forcing.

3) Assessment of the importance of realistic surface winds in the MM-EM connection.

The importance of realistic wind forcing in the MM-EM connection has been investigated for the 2017 event with a suite of forecast experiments with the EC-Earth3 model. Realistic wind stress forcing products from ERA-interim (ERAI) reanalysis and the novel ERA* product has been used and compared with the baseline model forecast. The use of realistic wind stress considerably improves the generation of TNA SSTs during boreal spring and equatorial anomalies in summer-fall. Outstandingly, correct surface winds appear a key element to trigger the RW-reflected mechanism that connects the MM and EM.

4) Evaluation of evolving modes impact on the precipitation regime over African and South American continent.

The evolving tropical Atlantic modes produces a substantial seasonally-varying impact on the rainfall anomalies over surrounding countries. During the first evolving mode, South American continent exhibits a meridional dipole of precipitation that is completely reserved from boreal winter to summer months. Over Africa, opposite rainfall anomalies are found in Angola and Namibian coast during boreal winter, while monopole rainfall signature is seen in South-Africa and East African countries from spring to summer. The second evolving mode causes a strong north-south rainfall gradient over South America in boreal spring that turns into a same-sign above-normal conditions during summer. Additionally, zonal dipoles of precipitation appear between the western-south African coast and East Africa in boreal winter-spring, contrasting with drought conditions over Gulf of Guinea in summer.

5) Evaluation of the north tropical Atlantic SST variability as precursor of equatorial SSTs.

The existence of evolving modes, and in turn, MM-EM connections puts forward the the predictive skill of boreal summer equatorial SST variability (up to 0.6) from the previous winter TNA SST anomalies. The precursor role of TNA SSTs is found for the entire 20th century, although the type of connection, same-sign and opposite-sign, alternates along the observational record.

The above-mentioned results have been presented in international meetings, scientific seminars and are recovered in 4 scientific publications (1 under review, 1 to be submitted and 2 in-preparation).
FESTIVAL has demonstrated the existence of evolving tropical Atlantic modes along the historical record.These evolving modes reflect different connections between the traditional Meridional (MM) and Equatorial Mode (EM). This result provides, for the first time, an integrated view of the boreal spring and summer interannual variability, modifying the prevailing thought of MM and EM as independent phenomena. Moreover, this MM-EM linkage suffers interdecadal variations that seem to be induced by natural low-frequency variability patterns. Outstandingly, the north tropical variability appears as a significant precursor (up to 6 months in advance) to generate the equatorial Atlantic variability for the entire 20th century. This information is quite valuable to improve the current seasonal forecast systems. Furthermore, FESTIVAL project has brought to light the key role of ocean waves to carry out the MM-EM connections. although its effectiveness is mediated by the favorable/unfavorable local wind forcing. Remarkably, using realistic wind stress products is essential to reproduce the RW-reflected mechanism and thus the MM-EM linkages. This information can help the climate modelers to achieve an accurate simulation of tropical Atlantic variability. The emergence of evolving modes cause varying rainfall patterns over African and South-American countries from winter to summer months. According to the strong linkage between rainfall and agriculture, these results can allow the local governments and insurance companies to adapt specific strategies to guarantee a sustained development.
Evolving tropical Atlantic modes
Impact of evolving modes on precipitation regime
Multidecadal Modulation of evolving modes