Greenhouse warming intensifies north tropical Atlantic climate variability

Variability of North Tropical Atlantic (NTA) sea surface temperature (SST), characterized by a near-uniform warming at its positive phase, is a consequential mode of climate variability. Modulated by El Niño–Southern Oscillation (ENSO) and the North Atlantic Oscillation, NTA warm anomalies tend to induce La Niña events, droughts in Northeast Brazil, increased frequency of extreme hurricanes, and phytoplankton blooms in the Guinea Dome. Future changes of NTA variability could have profound socioeconomic impacts yet remain unknown. Here, we reveal a robust intensification of NTA variability under greenhouse warming. This intensification mainly arises from strengthening of ENSO-forced Pacific-North American pattern and tropospheric temperature anomalies, as a consequence of an eastward shift of ENSO-induced equatorial Pacific convection and of increased ENSO variability, which enhances ENSO influence by reinforcing the associated wind and moist convection anomalies. The intensification of NTA SST variability suggests increased occurrences of extreme NTA events, with far-reaching ramifications.

Spurious North Tropical Atlantic precursors to El Niño

The El Niño-Southern Oscillation (ENSO), the primary driver of year-to-year global climate variability, is known to influence the North Tropical Atlantic (NTA) sea surface temperature (SST), especially during boreal spring season. Focusing on statistical lead-lag relationships, previous studies have proposed that interannual NTA SST variability can also feed back on ENSO in a predictable manner. However, these studies did not properly account for ENSO’s autocorrelation and the fact that the SST in the Atlantic and Pacific, as well as their interaction are seasonally modulated. This can lead to misinterpretations of causality and the spurious identification of Atlantic precursors for ENSO. Revisiting this issue under consideration of seasonality, time-varying ENSO frequency, and greenhouse warming, we demonstrate that the cross-correlation characteristics between NTA SST and ENSO, are consistent with a one-way Pacific to Atlantic forcing, even though the interpretation of lead-lag relationships may suggest otherwise.

Interannual sea surface chlorophyll-a signature in the tropical Atlantic

<div><span>Surface chlorophyll-<em>a </em>concentration (CHL-<em>a</em>) remotely observed by satellite shows a marked seasonal and interannual variability in the Tropical Atlantic, with potential consequences on the marine trophic web. Seasonal and interannual CHL-<em>a </em>variability peaks in boreal summer and shows maxima in the equatorial Atlantic region at 10˚W, spreading from 0 to 30˚W. In this study, we analyze how the remotely-sensed surface CHL-<em>a </em>responds to the leading climate modes affecting the interannual equatorial Atlantic variability over the 1998-2018 period, namely the Atlantic Zonal Mode (AZM) and the North Tropical Atlantic Mode (NTA, also known as the Atlantic Meridional Mode). The AZM is characterized by anomalous warming (or cooling) along the eastern equatorial band. In contrast, the NTA is characterized by an interhemispheric pattern of the sea surface temperature (SST), with anomalous warm (cold) conditions in the north tropical Atlantic region and weak negative (positive) SST anomalies south of the equator. We show that both modes significantly drive the interannual Tropical Atlantic surface CHL-<em>a </em>variability, with different timings and contrasted modulation on the eastern and western portions of the cold tongue area. Our results also reveal that the NTA slightly dominates (40%) the summer tropical Atlantic interannual variability over the last two decades, most probably because of a positive phase of the Atlantic multidecadal oscillation. For each mode of variability, we analyze an event characterized by an extreme negative sea surface temperature (SST) anomaly in the Atlantic equatorial band. Both modes are associated with a positive CHL-<em>a </em>anomaly at the equator. In 2002, a negative phase of the NTA led to cold SST anomaly and high positive CHL-<em>a </em>in the western portion of the cold tongue, peaking in June-July and lasting until the end of the year. In contrast, in 2005, a negative phase of the AZM drove cool temperature and positive CHL-<em>a </em>in the eastern equatorial band, with a peak in May-June and almost no signature after August. Such contrasted year to year conditions can affect the marine ecosystem by changing temporal and spatial trophic niches for pelagic predators, thus inducing significant variations for ecosystem functioning and fisheries.</span></div>

Dominant role of North tropical Atlantic 2017 warm event on equatorial variability

<p>Multiple lines of new evidence suggest that the Atlantic Ocean plays an active role in the modulation of global climate. Special attention deserves tropical Atlantic extreme events that have increased from 2000s causing severe winter conditions in the Euro-Atlantic region and originating the most devastating hurricane seasons on record (Foltz and McPhaden 2006; Bucham et al. 2014; Lim et al. 2018; Klotzbach et al. 2018). In 2017, the north Tropical Atlantic (NTA) experienced a profound warming, resembling the Atlantic Meridional Mode (AMM) pattern, that originated a destructive hurricane season with catastrophic social and economic damages (Klotzbach et al. 2018). Previous studies focused their attention on the description of the precursors and predictability of the 2017 hurricane season. Nevertheless, the impact of the 2017 NTA warming on equatorial SST variability has not been explored so far. Recent findings put forward the key role of the AMM-associated cross-equatorial wind to trigger oceanic waves that impact on equatorial SSTs (Martín-Rey and Lazar 2019; Foltz and McPhaden 2010).</p><p>Thus, in the present study, we investigate the connection between NTA and equatorial variability during 2017, as well as the importance of an accurate ocean forcing to correctly simulate this event. For such purpose, a suite of three initialized climate predictions, performed with the climate model EC-Earth (version3.3), are analyzed. Two sets of predictions apply a wind stress correction over the Tropical Atlantic (35S-35N) using two distinct wind stress products: ERA-Interim (ERAI) reanalysis and a new ERAI-corrected (ERA*) wind product, which are compared to a control prediction with model-generated wind stress (MOD). ERA* has been developed based on means of a geolocated scatterometer-based correction applied to the ERA-interim reanalysis (Trindade et al. 2019). The high-quality of the scatterometer stress-equivalent winds (Portabella and Stoffelen 2009; De Kloe et al., 2017) allows ERA* to contain some of the physical processes missing or misrepresented (i.e., small-scale ocean processes, such as wind-current interaction) in ERAI.</p><p>Using more realistic surface wind stress (ERAI or ERA* with respect to MOD) considerably improves the simulation of eastern NTA and equatorial warming. The novel wind stress product (ERA*) respect its precursor ERAI, better represents the off-shore warm SSTs in the NTA and along eastern equatorial Atlantic and south African coast. It is worth mentioning that oceanic wave activity proves highly sensitivity when forced by realistic ERAI and ERA* wind stress products. In the wind-corrected experiments, an anomalous wind stress curl north of the equator during March-April excites a downwelling Rossby wave that propagates to the west and is boundary reflected in June-July, becoming an equatorial downwelling Kelvin wave (dKW). This dKW displaces eastward favouring the development of an equatorial warming in late-summer and fall. ERA* does not show significant changes in the RW generation, but in the amplitude of equatorial KW during summer season.</p><p>Our results highlight the importance of using improved wind stress products to achieve a correct simulation of ocean wave activity and in turn equatorial Atlantic SST variability. This information is of great value for improving current seasonal forecast systems.</p>

Spurious North Tropical Atlantic pre-cursors to ENSO

The El Niño-Southern Oscillation (ENSO), the primary driver of year-to-year global climate variability, is known to influence the North Tropical Atlantic (NTA) sea surface temperature (SST), especially during boreal spring season. Focusing on statistical lead-lag relationships, previous studies have proposed that interannual NTA SST variability can also feed back on ENSO in a predictable manner. However, these studies do not properly account for ENSO’s autocorrelation and the fact that the SST in the Atlantic and Pacific, as well as their atmospheric interaction are seasonally modulated. This can lead to misinterpretations of causality and the spurious identification of Atlantic precursors for ENSO. Revisiting this issue under consideration of seasonality, time-varying ENSO frequency, and greenhouse warming, we demonstrate that the cross-correlation characteristics between NTA SST and ENSO, are fully consistent with a one-way Pacific to Atlantic forcing, even though the interpretation of lead-lag relationships may suggest otherwise.

Understanding the 2017 warm event in North Tropical Atlantic

<p>The Atlantic Ocean has suffered tremendous warming during recent decades as a consequence of anthropogenic forcing, modulated by the natural low frequency variability. Special attention should be paid to the high temporal frequency of warm interannual events in the North Tropical Atlantic (NTA) since the early 2000s, resulting in the most intense hurricane seasons on record (Hallam et al., 2017; Lim et al., 2018; Murakami et al., 2018; Klotzbach et al., 2018; Camp et al., 2018). Moreover, NTA sea surface temperature anomalies during boreal spring have been suggested as a potential precursor to the Equatorial Mode (Foltz and McPhaden, 2010ab; Burmeister et al., 2016; Martín-Rey and Lazar, 2019; Martín-Rey et al., 2019).<strong> </strong></p><p>This study aims to investigate the development of the 2017 NTA spring-summer warming event, which was the strongest of the last decade, as well as the importance of an accurate ocean forcingin the simulation of this event. For such purpose, a set of four simulations using distinct ocean wind forcing products, namely from the EC-Earth model, ERA-Interim (ERAi) reanalysis and a new ERAi-corrected ocean wind product (ERAstar), have been performed and analysed.The latter consists of average geolocated scatterometer-based corrections applied to ERAi output (Trindade et al., 2019).In this sense, ERAstar includes some of the physical processes missing or misrepresented by ERA-i, and corrects for large-scale NWP parameterization and dynamical errors.</p><p>The air-sea processes underlying the onset and development of the warm 2017 NTA event and the wave activity present in the equatorial Atlantic will be explored to determine their possible connection with the equatorial sea surface temperature variability. Furthermore, the comparison between the different experiments allows us to validate the new surface wind dataset and evaluate the importance of accurate, high-resolution ocean forcing in the representation of tropical Atlantic variability.</p>

Analysis of the Transport of Aerosols over the North Tropical Atlantic Ocean Using Time Series of POLDER/PARASOL Satellite Data

The time series of total, fine and coarse POLAC/PARASOL aerosol optical depth (AOD) satellite products (2005–2013) processed by the POLAC algorithm are examined to investigate the transport of aerosols over the North Tropical Atlantic Ocean, a region that is characterized by significant dust aerosols events. First, the comparison of satellite observations with ground-based measurements acquired by AERONET ground-based measurements shows a satisfactory consistency for both total AOD and coarse mode AOD (i.e., correlation coefficients of 0.75 and bias ranging from −0.03 to 0.03), thus confirming the robustness and performance of POLAC/PARASOL data to investigate the spatio-temporal variability of the aerosols over the study area. Regarding fine mode aerosol, POLAC/PARASOL data present a lower performance with correlation coefficient ranging from 0.37 to 0.73. Second, the analysis of POLAC/PARASOL aerosol climatology reveals a high contribution of the coarse mode of aerosols ( AOD c between 0.1 and 0.4) at long distance from the African sources, confirming previous studies related to dust transport. The POLAC/PARASOL data were also compared with aerosol data obtained over the North Tropical Atlantic Ocean from MACC and MERRA-2 reanalyses. It is observed that the total AOD is underestimated in both reanalysis with a negative bias reaching −0.2. In summary, our results thus suggest that satellite POLAC/PARASOL observations of fine and coarse modes of aerosols could provide additional constraints useful to improve the quantification of the dust direct radiative forcing on a regional scale but also the biogeochemical processes such as nutrient supply to the surface waters.