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1. Anomalous Tropical Instability Wave activity hindered the development of the 2016/2017 La Niña

   https://doi.org/10.1175/JCLI-D-20-0399.1  

   Xue, Aoyun ; Zhang, Wenjun ; Boucharel, Julien ; Jin, Fei-Fei ( April 2021 , Journal of Climate)

   null (Ed.)

   Abstract Although the 1997/98 and 2015/16 El Niño events are considered to be the strongest on record, their subsequent La Niña events exhibited contrasted evolutions. In this study, we demonstrate that the extremely strong period of Tropical Instability Waves (TIWs) at the beginning of boreal summer of 2016 played an important role in hindering the subsequent La Niña’s development by transporting extra off-equatorial heat into the Pacific cold tongue. By comparing the TIWs contribution based on an oceanic mixed-layer heat budget analysis for the 1998 and 2016 episodes, we establish that TIW-induced nonlinear dynamical heating (NDH) is a significant contributor to the El Niño-Southern Oscillation (ENSO) phase transition in 2016. TIW-induced NDH contributed to around 0.4°C per month warming during the early boreal summer (May-June) following the 2015/16 El Niño’s peak, which is found to be an essential inhibiting factor that prevented the subsequent La Niña’s growth. A time-mean eddy kinetic energy analysis reveals that anomalous TIWs during 2016 mainly gained their energy from the baroclinic instability conversion due to a strong SST warming in the northeastern off-equatorial Pacific that promoted an increased meridional SST gradient. This highlights the importance of accurately reproducing TIW activity in ENSO simulation and the benefit of off-equatorial SST anomalies in the eastern Pacific as an independent precursor for ENSO predictions. 

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2. Reviews and syntheses: Physical and biogeochemical processes associated with upwelling in the Indian Ocean

   https://doi.org/10.5194/bg-18-5967-2021  

   Vinayachandran, Puthenveettil Narayana ; Masumoto, Yukio ; Roberts, Michael J. ; Huggett, Jenny A. ; Halo, Issufo ; Chatterjee, Abhisek ; Amol, Prakash ; Gupta, Garuda V. ; Singh, Arvind ; Mukherjee, Arnab ; et al ( January 2021 , Biogeosciences)

   Abstract. The Indian Ocean presents two distinct climate regimes. The north Indian Ocean is dominated by the monsoons, whereas the seasonal reversal is less pronounced in the south. The prevailing wind pattern produces upwelling along different parts of the coast in both hemispheres during different times of the year. Additionally, dynamical processes and eddies either cause or enhance upwelling. This paper reviews the phenomena of upwelling along the coast of the Indian Ocean extending from the tip of South Africa to the southern tip of the west coast of Australia. Observed features, underlying mechanisms, and the impact of upwelling on the ecosystem are presented. In the Agulhas Current region, cyclonic eddies associated with Natal pulses drive slope upwelling and enhance chlorophyll concentrations along the continental margin. The Durban break-away eddy spun up by the Agulhas upwells cold nutrient-rich water. Additionally, topographically induced upwelling occurs along the inshore edges of the Agulhas Current. Wind-driven coastal upwelling occurs along the south coast of Africa and augments the dynamical upwelling in the Agulhas Current. Upwelling hotspots along the Mozambique coast are present in the northern and southern sectors of the channel and are ascribed to dynamical effects of ocean circulation in addition to wind forcing. Interaction of mesoscale eddies with the western boundary, dipole eddy pair interactions, and passage of cyclonic eddies cause upwelling. Upwelling along the southern coast of Madagascar is caused by the Ekman wind-driven mechanism and by eddy generation and is inhibited by the Southwest Madagascar Coastal Current. Seasonal upwelling along the East African coast is primarily driven by the northeast monsoon winds and enhanced by topographically induced shelf breaking and shear instability between the East African Coastal Current and the island chains. The Somali coast presents a strong case for the classical Ekman type of upwelling; such upwelling can be inhibited by the arrival of deeper thermocline signals generated in the offshore region by wind stress curl. Upwelling is nearly uniform along the coast of Arabia, caused by the alongshore component of the summer monsoon winds and modulated by the arrival of Rossby waves generated in the offshore region by cyclonic wind stress curl. Along the west coast of India, upwelling is driven by coastally trapped waves together with the alongshore component of the monsoon winds. Along the southern tip of India and Sri Lanka, the strong Ekman transport drives upwelling. Upwelling along the east coast of India is weak and occurs during summer, caused by alongshore winds. In addition, mesoscale eddies lead to upwelling, but the arrival of river water plumes inhibits upwelling along this coast. Southeasterly winds drive upwelling along the coast of Sumatra and Java during summer, with Kelvin wave propagation originating from the equatorial Indian Ocean affecting the magnitude and extent of the upwelling. Both El Niño–Southern Oscillation (ENSO) and Indian Ocean Dipole (IOD) events cause large variability in upwelling here. Along the west coast of Australia, which is characterized by the anomalous Leeuwin Current, southerly winds can cause sporadic upwelling, which is prominent along the southwest, central, and Gascoyne coasts during summer. Open-ocean upwelling in the southern tropical Indian Ocean and within the Sri Lanka Dome is driven primarily by the wind stress curl but is also impacted by Rossby wave propagations. Upwelling is a key driver enhancing biological productivity in all sectors of the coast, as indicated by enhanced sea surface chlorophyll concentrations. Additional knowledge at varying levels has been gained through in situ observations and model simulations. In the Mozambique Channel, upwelling simulates new production and circulation redistributes the production generated by upwelling and mesoscale eddies, leading to observations of higher ecosystem impacts along the edges of eddies. Similarly, along the southern Madagascar coast, biological connectivity is influenced by the transport of phytoplankton from upwelling zones. Along the coast of Kenya, both productivity rates and zooplankton biomass are higher during the upwelling season. Along the Somali coast, accumulation of upwelled nutrients in the northern part of the coast leads to spatial heterogeneity in productivity. In contrast, productivity is more uniform along the coasts of Yemen and Oman. Upwelling along the west coast of India has several biogeochemical implications, including oxygen depletion, denitrification, and high production of CH4 and dimethyl sulfide. Although weak, wind-driven upwelling leads to significant enhancement of phytoplankton in the northwest Bay of Bengal during the summer monsoon. Along the Sumatra and Java coasts, upwelling affects the phytoplankton composition and assemblages. Dissimilarities in copepod assemblages occur during the upwelling periods along the west coast of Australia. Phytoplankton abundance characterizes inshore edges of the slope during upwelling season, and upwelling eddies are associated with krill abundance. The review identifies the northern coast of the Arabian Sea and eastern coasts of the Bay of Bengal as the least observed sectors. Additionally, sustained long-term observations with high temporal and spatial resolutions along with high-resolution modelling efforts are recommended for a deeper understanding of upwelling, its variability, and its impact on the ecosystem. 

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3. ENSO sentinels in the Americas' humid tropics: We need combined hydrometric and isotopic monitoring for improved El Niño and La Niña impact prediction

   https://doi.org/10.1002/hyp.15027  

   Sánchez‐Murillo, R. ; Birkel, C. ; Boll, J. ; Esquivel‐Hernández, G. ; Rojas‐Jiménez, L. D. ; Castro‐Chacón, L. ; Durán‐Quesada, A. M. ; Voarintsoa, N. R. G. ; Dee, S. G. ; Winguth, A. M. E. ( December 2023 , Hydrological Processes)

   This Scientific Briefing presents results from a nearly 10‐year hydrometric and isotope monitoring network across north‐central Costa Rica, a region known as a headwater‐dependent system. This monitoring system has recorded different El Niño and La Niña events and the direct/indirect effects of several hurricane and tropical storm passages. Our results show that El Niño‐Southern Oscillation (ENSO) exerts a significant but predictable impact on rainfall amount anomalies, groundwater level and spring discharge, as evidenced by second‐order water isotope parameters (e.g., line conditioned‐excess or line‐conditioned (LC)‐excess). Sea surface temperature anomaly (El Niño Region 3) is correlated with a reduction in mean annual and cold front rainfall across the headwaters of north‐central Costa Rica. During El Niño conditions, rainfall is substantially reduced (up to 69.2%) during the critical cold fronts period, limiting groundwater recharge and promoting an early onset of minimum baseflow conditions (up to 5 months). In contrast, La Niña is associated with increased rainfall and groundwater recharge (up to 94.7% during active cold front periods). During La Niña, the long‐term mean spring discharge (39 Ls⁻¹) is exceeded 63–80% of the time, whereas, during El Niño, the exceedance time ranges between 26% and 44%. The regional hydroclimatic variability is also imprinted on the hydrogen and oxygen isotopic compositions of meteoric waters. Drier conditions favoured lower LC‐excess in rainfall (−17.3‰) and spring water (−6.5‰), whereas wetter conditions resulted in greater values (rainfall = +17.5‰; spring water = +10.7‰). The lower and higher LC‐excess values in rainfall corresponded to the very strong 2014–2016 El Niño and 2018 La Niña, respectively. During the recent triple‐dip 2021–23 La Niña, LC‐excess exhibited a significant and consistently increasing trend. These findings highlight the importance of combining hydrometric, synoptic and isotopic monitoring as ENSO sentinels to advance our current understanding of ENSO impacts on hydrological systems across the humid Tropics. Such information is critical to constraining the 21st century projections of future water stress across this fragile region.

    

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4. Asymmetric response of Amazon forest water and energy fluxes to wet and dry hydrological extremes reveals onset of a local drought‐induced tipping point

   https://doi.org/10.1111/gcb.16933  

   Restrepo‐Coupe, Natalia ; O’Donnell Christoffersen, Bradley ; Longo, Marcos ; Alves, Luciana F. ; Campos, Kleber Silva ; da Araujo, Alessandro C. ; de Oliveira, Jr, Raimundo C. ; Prohaska, Neill ; da Silva, Rodrigo ; Tapajos, Raphael ; et al ( September 2023 , Global Change Biology)

   Understanding the effects of intensification of Amazon basin hydrological cycling—manifest as increasingly frequent floods and droughts—on water and energy cycles of tropical forests is essential to meeting the challenge of predicting ecosystem responses to climate change, including forest “tipping points”. Here, we investigated the impacts of hydrological extremes on forest function using 12+ years of observations (between 2001–2020) of water and energy fluxes from eddy covariance, along with associated ecological dynamics from biometry, at the Tapajós National Forest. Measurements encompass the strong 2015–2016 El Niño drought and La Niña 2008–2009 wet events. We found that the forest responded strongly to El Niño‐Southern Oscillation (ENSO): Drought reduced water availability for evapotranspiration (ET) leading to large increases in sensible heat fluxes (H). PartitioningETby an approach that assumes transpiration (T) is proportional to photosynthesis, we found that water stress‐induced reductions in canopy conductance (G_s) droveTdeclines partly compensated by higher evaporation (E). By contrast, the abnormally wet La Niña period gave higherTand lowerE, with little change in seasonalET. Both El Niño‐Southern Oscillation (ENSO) events resulted in changes in forest structure, manifested as lower wet‐season leaf area index. However, only during El Niño 2015–2016, we observed a breakdown in the strong meteorological control of transpiration fluxes (via energy availability and atmospheric demand) because of slowing vegetation functions (via shutdown ofG_sand significant leaf shedding). Drought‐reducedTandG_s, higherHandE, amplified by feedbacks with higher temperatures and vapor pressure deficits, signaled that forest function had crossed a threshold, from which it recovered slowly, with delay, post‐drought. Identifying such tipping point onsets (beyond which future irreversible processes may occur) at local scale is crucial for predicting basin‐scale threshold‐crossing changes in forest energy and water cycling, leading to slow‐down in forest function, potentially resulting in Amazon forests shifting into alternate degraded states.

    

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5. Parameterizing the nonlinear feedback on ENSO from tropical instability waves (TIWs) by nonlinear eddy thermal diffusivity

   https://doi.org/10.1007/s00382-023-06744-4  

   Xue, Aoyun ; Jin, Fei-Fei ; Zhang, Wenjun ; Boucharel, Julien ; Kug, Jong-Seong ( March 2023 , Climate Dynamics)

   As the dominant form of mesoscale variability in the equatorial eastern Pacific, Tropical Instability Waves (TIWs) are known to interact with the El Niño and Southern Oscillation (ENSO) in complex ways. TIWs activity is modulated by the ENSO state and also provide significant feedback on ENSO via nonlinear dynamic heating (NDH), acting as a source of asymmetry between the El Niño and La Niña phases. In this work, we show that the interannual variability of TIWs-induced heat flux and NDH can be approximately expressed in terms of the mean meridional temperature gradient as TIWs tend to transport heat downgradient of the temperature anomalies along the Sea Surface Temperature (SST) front. The TIWs-induced NDH can be quantified as an asymmetric negative feedback on ENSO by a nonlinear thermal eddy diffusivity which depends on the background TIWs pattern and the ENSO-related linear and nonlinear processes. This proposed parameterization scheme can capture well the direct ENSO modulation on TIWs activity, the combination effect arising from the nonlinear interaction between ENSO and the cold tongue annual cycle, and associated ENSO nonlinearity. This parameterization scheme is effectively tested using four ocean reanalysis datasets with different horizontal resolutions that exhibit contrasted patterns of TIWs activity. This scheme may be useful for assessing the TIWs-induced feedback on ENSO in mechanistic ENSO models to better understand the dynamics of ENSO complexity. 

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