Two Oceans, One Basin: New Index Reveals How the Pacific and Atlantic Drive Amazon’s Compound Extremes

The Amazon Basin has spent the past four decades lurching between two increasingly violent states: floods that submerge floodplain forests and droughts that turn the world’s largest rainforest into a tinderbox. Now, a study published in Climate Dynamics has, for the first time, fused precipitation, air temperature, and river levels into a single index that captures these extremes as they truly occur—not as isolated events, but as compound hydroclimatic states in which drought arrives with heat and floods arrive with cold. The result is a new diagnostic tool, the Amazon Basin Compound Hydroclimatic Extremes Index, or AB-CHEX, and with it a strikingly clear picture of who is really in charge of the basin’s climate: two different oceans, depending on the season.

The research team, led by G. Botetano and including J. C. Espinoza, K. Takahashi, and colleagues from institutions in Peru, France, and Brazil, analyzed the period from 1982 to 2024, using satellite-derived precipitation from CHIRPS, temperature from the ERA5-Land reanalysis, sea surface temperatures from HadISST, and more than forty years of daily water level measurements from the Negro River gauge at the port of Manaus. The Manaus gauge is no ordinary data point. Because the Negro River responds to rainfall across roughly three million square kilometers of upstream Amazonia, its levels integrate the hydrological memory of the entire northwestern basin, lagging behind precipitation by about three months as floodplains and saturated soils buffer and release water.

The technical core of the study rests on empirical orthogonal function analysis, a statistical technique that decomposes complex climate fields into their dominant spatial patterns and corresponding time series. When the researchers applied this method to seasonal precipitation, temperature, and terrestrial water storage anomalies measured by the GRACE and GRACE-FO satellite gravimetry missions, they found something remarkable: the leading modes of all these variables were tightly coupled. Precipitation variability correlated with water storage at around 0.78 to 0.79, river levels tracked precipitation at roughly 0.76 to 0.81, and temperature—the sign-inverted, since heat accompanies drought—correlated with storage at 0.76 to 0.88. In other words, the Amazon does not experience a rainfall anomaly here and a temperature anomaly there; it moves as a coherent compound system.

AB-CHEX distills this coupling into a single number for each season. Negative phases below the 15th percentile mark dry–hot compound events, combining rainfall deficits, anomalous warmth, and falling rivers. Positive phases above the 85th percentile mark wet–cold events, the signature of the great floods. The index validated beautifully against history: it reproduces the documented droughts of 1998, 2005, 2010, 2015–16, and 2023–24, and the floods of 2009, 2012, and 2021–22. It also captures basin-scale water storage variability, correlating with the GRACE-derived storage component at 0.93 in the wet season and 0.88 in the dry season—evidence that the index reflects genuine basin-wide hydroclimatic dynamics rather than the quirks of any single dataset.

Then comes the headline finding. When the researchers constructed a trend-preserving version of the index, AB-CHEXLT, the wet and dry seasons revealed opposite long-term trajectories. During the extended wet season from November to April, the index shows a significant drift toward wetter and colder compound conditions, consistent with an intensifying hydrological cycle and the record flooding that has plagued northern Amazonia—the cumulative duration of flood emergencies at Manaus in the first two decades of this century already exceeds that of the entire twentieth century by about 20 percent. During the dry season from June to September, the index shows a sustained shift toward increasingly dry–hot conditions, and here the decomposition is damning: the linear trend explains nearly a quarter of total dry-season variance, and within that trend, warming alone accounts for 86.9 percent of the long-term signal.

And then there is 2024. Even after removing long-term trends, the AB-CHEX value for 2024 reached its lowest magnitude across both seasons in the entire 43-year record, marking an event the authors describe as unprecedented. The 2023–24 drought combined delayed rainfall onset, record-breaking heat, collapsing river levels, widespread canopy browning, and sharp declines in gross primary productivity, with pronounced soil moisture deficits and elevated vapor pressure deficit covering more than half the Amazon region during the austral springs of 2023 and 2024. Previous severe dry–hot events clustered in recognizable patterns—the wet-season extremes of 1998 and 2016 during strong El Niño years, the dry-season extremes of 2005, 2010, and 2023—but 2024 broke the mold in both seasons simultaneously.

To find the culprits, the team turned to composite analysis of atmospheric circulation and sea surface temperatures during extreme index phases. During the wet season, the story is fundamentally Pacific. Dry–hot years show pronounced mid-level subsidence between 300 and 600 hectopascals across equatorial Amazonia—a weakening of the ascending branch of the regional Hadley circulation that suppresses deep convection—paired with an eastward shift of the Walker-like zonal circulation that enhances ascent over the eastern Pacific. Sea surface composites reveal the classic El Niño fingerprint: significant warming over the Niño-3.4 region. Quantitatively, ENSO-related Pacific sea surface temperature anomalies explain 61 percent of wet-season AB-CHEX variability, while the regional Hadley circulation index alone correlates at 0.85, accounting for 72 percent. El Niño weakens the overturning cells and starves the basin of moisture; La Niña strengthens them, driving the great wet–cold floods.

The dry season tells a different and, in a warming world, more troubling story. Here the primary control is not the Pacific but the Caribbean–Tropical North Atlantic region. Dry–hot years show intensified subsidence between 500 and 800 hectopascals, a northward-shifted and strengthened descending branch of the regional Hadley cell, and marked warming over the Caribbean–TNA. Warm sea surface anomalies there weaken the meridional pressure gradient, slacken the trade winds, and choke off the Atlantic moisture inflow that sustains dry-season convection. The Caribbean–TNA sea surface temperature index correlates with dry-season AB-CHEX at 0.79, explaining 62 percent of its variability—and crucially, this relationship holds regardless of the Pacific state, remaining significant under warm, neutral, and cold Niño-1+2 conditions. When the researchers removed the shared variance between the two ocean basins using partial correlations, the Pacific influence on dry-season compound extremes vanished entirely, while the Caribbean–TNA influence on wet-season extremes persisted only as a secondary modulation of ENSO-driven variability.

The spatial consequences of these two forcing regimes differ profoundly. During wet-season dry–hot events, precipitation deficits exceeding 25 percent and temperature anomalies above 1.2 degrees Celsius concentrate over northeastern Amazonia, where ENSO’s Walker and Hadley circulation teleconnections bite hardest. Dry-season dry–hot events, driven by the more spatially uniform Caribbean–TNA forcing, produce rainfall reductions exceeding 45 percent and anomalies near plus one degree Celsius across the Peruvian, central, and southern Amazon—wider, deeper, and more severe. Land–atmosphere feedbacks amplify the damage: antecedent soil moisture deficits persist through hydrological memory, suppressing evapotranspiration and shifting the surface energy balance toward sensible heating, which reinforces both the warmth and the rainfall deficit. In total, roughly 68 percent of the basin exhibits significant compound coupling with the index during the wet season, rising to 76 percent during the dry season.

The implications reach beyond atmospheric science. The sustained warming of the Caribbean–TNA since the mid-2000s, the authors note, appears to be intensifying dry–hot compound conditions, strengthening land–atmosphere feedbacks, and amplifying hydroclimatic instability—layered on top of deforestation-driven delays in wet-season onset across southern Amazonia and rising tree mortality across the basin. Because all of AB-CHEX’s inputs are publicly available, routinely updated datasets, the index can be computed operationally by hydrometeorological services and early-warning centers across Amazonian countries, with the caveat that the two-to-three-month hydrological lag limits real-time applications. What the index makes unmistakably clear is that the Amazon’s future hinges on two oceans and a warming atmosphere acting in concert—and that the dry season, where warming already explains most of the long-term drift toward compound drought and heat, is where the rainforest’s margin of safety is eroding fastest.

Subject of Research: Large-scale ocean–atmosphere drivers of compound drought–heat and flood–cold extremes in the Amazon Basin from 1982 to 2024

Article Title: Large-scale climate drivers of extreme compound events in the Amazon from interannual to long-term timescales (1982–2024)

Article References: Botetano, G., Espinoza, J. C., Takahashi, K., Gutierrez Villarreal, R. A., Marengo, J., & Wongchuig, S. (2026). Large-scale climate drivers of extreme compound events in the Amazon from interannual to long-term timescales (1982–2024). Climate Dynamics, 64(10), Article 427. https://doi.org/10.1007/s00382-026-08382-y

Image Credits: AI Generated

DOI: 10.1007/s00382-026-08382-y

Keywords: Amazon Basin, compound extremes, drought, ENSO, El Niño, Tropical North Atlantic, Caribbean sea surface temperature, Hadley circulation, Walker circulation, river levels, GRACE, hydroclimatic variability