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Dynamics and Predictability of the ENSO teleconnection to the Tropical North Atlantic

Periodic Reporting for period 1 - DPETNA (Dynamics and Predictability of the ENSO teleconnection to the Tropical North Atlantic)

Reporting period: 2015-06-01 to 2017-05-31

DPETNA aims at exploring the El Niño-Southern Oscillation (ENSO) influence on the tropical North Atlantic (TNA) sea surface temperature (SST). The SST variability in the TNA region has been observed to affect weather and climate in surrounding areas, including European heat waves, tropical Atlantic hurricane activity, the West African monsoon, and rainfall over north-eastern Brazil. Fishery production in the subtropical Atlantic is, likewise, closely related to TNA SST anomalies. Hence, better understanding the predictability sources of the TNA SST variability is of major relevance, not only for the climate forecasting community but also for different socio-economic sectors.
The scientific objective of DPETNA is to advance understanding of the simulation and prediction of TNA SSTs at seasonal-to-interannual timescales. The main goals are to gain insight into the dynamical mechanisms at work, and to assess the ability of current seasonal forecast systems in representing the ENSO-TNA remote connection. Climate model deficiencies and biases, which lead to uncertainties in their forecasts, will also be addressed. The novelties of DPETNA are (i) the evaluation of a new teleconnection mechanism to explain the timing of the inter-tropical relationship, (ii) the assessment of the ENSO contribution to SST skill over the TNA region, and (iii) the use of the most complete set to date of operational seasonal forecast systems. The latter, in particular, ensures that the outcomes derived from this fellowship may contribute to meet users’ needs.
DPETNA presents and describes a fourth potential mechanism to explain the ENSO teleconnection to the tropical North Atlantic in boreal spring. The continuous ENSO-induced atmospheric forcing in the tropical Atlantic via the remote Gill-type response plus the climatological springtime increase in SST variance over the TNA region may conceivably be underlying the ‘one-season lagged’ ENSO-TNA teleconnection. This mechanism would be at work for long-lasting ENSO events because of the in-phase Gill-type response, while no significant TNA SST anomalies are expected in case of shorter-lived ones according to this mechanism and in line with previous studies.

The teleconnection mechanism proposed in DPETNA does not come into conflict with the three other mechanisms proposed until now, but helps to address some unclear issues concerning the ENSO-TNA relationship for which they do not provide a satisfactory insight. In particular, the remote Gill-type response in the tropical Atlantic can provide the atmospheric forcing needed at low latitudes of the TNA during the ENSO decaying phase to trigger the air-sea coupling there at the right timing. Once the subtropical wind anomalies vanish towards spring, the air-sea interaction and SST anomalies remain only in the deep TNA; any ENSO-related signal north of 10ºN/15ºN tends to damp. The remote Gill-type mechanism constitutes a regional atmospheric forcing rather than the expression of the wind-evaporation-SST feedback. The results derived from DPETNA on the transition from a subtropical forcing associated with the extratropical ENSO teleconnection (in winter/late-winter; via Rossby wavetrain or/and Walker-Hadley) into a tropical forcing associated with the remote Gill-type response (in early-spring/spring) are consistent with previous studies reporting a similar winter-to-spring evolution in observations, atmosphere-only and coupled simulations.

DPETNA has shown that the ENSO-TNA teleconnection not only contributes substantially to the variability of TNA SSTs in boreal spring, but also largely determine its predictability and prediction skill. In this regard, the results suggest that, beyond model biases in SST climatology over the TNA, model biases in ENSO variability along the seasonal cycle are critical for the establishment and prediction of the ENSO-TNA teleconnection. In particular, the correct simulation of the ENSO decaying phase appears to be fundamental to capture the right timing of the remote influence.
The mechanism proposed in DPETNA hints at some unresolved elements of the ENSO teleconnection to the tropical Atlantic: such as the twin minima in tropospheric air temperature, lower-level anticyclonic anomalies over the TNA, and the anomalous easterly winds in the equatorial Atlantic. The remote Gill-type response, and the associated Rossby wave source, may also provide a dynamical framework to understand other extratropical ENSO teleconnections: such as the secondary ENSO-induced Rossby wavetrain to Europe for mid/late-winter, or the ENSO influence on Europe in spring. Hence, the outputs of DPETNA may certainly lead to future, unexplored research lines and have a continuation in the climate science community. Likewise, DPETNA has offered a favourable scope for substantial progress of seasonal-to-interannual predictions in the tropical Atlantic sector, e.g. hurricane activity, and neighbouring areas such as West Africa and Southern Europe. Thus, the outcome of DPETNA may also have an impact on future climate services.
Schematic diagram of the different mechanisms proposed to explain the ENSO-TNA teleconnection.