The Atlantic Meridional Overturning Circulation (AMOC) is a key component of global ocean circulation. It transports warm, salty surface water northward in the Atlantic and returns colder, deeper water southward. Its strength depends heavily on deep water formation (sinking) in the subpolar North Atlantic, particularly in the Labrador and Irminger Seas near Greenland.

Slowdown occurs when surface waters in these sinking regions become less dense, reducing their tendency to sink and drive the overturning.

Observations and proxies (e.g., sea surface temperature “fingerprint” with cold blob + coastal warming, mid-depth equatorial warming) indicate weakening over recent decades, though rates and exact timing vary across studies, with some pauses due to natural variability.

Climate models consistently project further weakening this century (often 18–43% or 3–6 Sv in high-emissions scenarios), but full collapse this century is considered unlikely by many (though debated, with some lower-complexity models or high-resolution work suggesting higher risk under sustained high warming or rapid melt).

The “cold blob” (also called the North Atlantic warming hole) is a region of unusually cool sea surface temperatures south of Greenland, in contrast to the overall warming of the global ocean. It is widely linked to a slowdown in the Atlantic Meridional Overturning Circulation (AMOC), which includes the Gulf Stream system. Freshwater from melting Greenland ice reduces water density and slows the northward flow of warm water, leading to localized cooling.

This feature has been observed for decades and is considered a fingerprint of AMOC weakening, though ocean-atmosphere interactions and other factors also play roles.

According to the new research:

• Researchers incorporated the cold blob into climate models.
• It alters the upper-level jet stream (likely the subtropical or tropical easterly jet) in ways that shift moisture transport.
• This pulls more monsoon rainfall northwestward over India (e.g., toward regions like Rajasthan or Punjab) while suppressing storm systems and rainfall in other parts of the country.

This matches observed trends over the past ~25 years: heavier rains in northwest India and relatively drier conditions elsewhere, contributing to changes in monsoon patterns, floods, and droughts.

The mechanism involves atmospheric teleconnections — distant ocean temperature anomalies influencing large-scale circulation patterns, including jet streams and moisture convergence over South Asia.

There is a recent scientific study (as of June 1, 2026) linking a persistent cold patch in the North Atlantic to shifts in the Indian summer monsoon.

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Missing Summer Westerly Jet Barotropic Governor Effect Explains Climate Models—Observation Discrepancies in the Indian Monsoon Trends

Since around 1999, the Indian summer monsoon has shown a clear northwest–southeast dipole in rainfall:

• Northwest India (e.g., parts of Rajasthan, Punjab): +24.6% increase in monsoon rainfall.
• Indo-Gangetic Plains (core monsoon region): –4.4% decrease, contributing to drought-like conditions in some areas.

This shift has real-world impacts on agriculture, water resources, and flooding patterns, but standard coupled climate models (e.g., CMIP6 historical simulations) fail to reproduce it.

The Proposed Mechanism: Summer Westerly Jet + Barotropic Governor Effect

The paper identifies a teleconnection from North Atlantic sea surface temperature (SST) changes — specifically the cold blob linked to AMOC slowdown — that propagates through atmospheric dynamics over Asia:

1. North Atlantic Cold Blob Influence: The cold anomaly (south of Greenland) alters large-scale atmospheric circulation, sending signals via wave trains or teleconnections to the upper-level Asian summer westerly jet (part of the subtropical or mid-latitude jet over Eurasia).
2. Barotropic Governor Mechanism:
   • In fluid dynamics and atmospheric science, a “barotropic governor” refers to how a strong, large-scale zonal (west-east) flow — like an intensified jet core — can suppress or “govern” the growth of smaller-scale baroclinic eddies and disturbances (e.g., weather systems, storms, or monsoon depressions). phys.org
   • Post-1999 observations show enhanced barotropic energy conversion: Momentum focuses and strengthens the jet core while weakening flanking regions (east-west sides). This creates a more “rigid” or focused jet.
   • Result: Altered moisture transport and suppression of storm formation in certain areas. It modulates the local Hadley cell (overturning circulation), pulling moisture northwestward over India while reducing it elsewhere.

This explains the rainfall dipole and also broader mid-latitude increases in storm activity in recent decades.

Barotropic energy conversion

Barotropic energy conversion refers to the transfer of kinetic energy between the large-scale mean atmospheric flow (e.g., a jet stream) and smaller-scale disturbances or eddies (e.g., weather systems, monsoon depressions, or transient waves). It is a key process in atmospheric dynamics, particularly in the mid-latitudes and upper troposphere, and does not involve temperature contrasts or vertical structure in the simplest sense (unlike baroclinic processes).

Barotropic energy conversion occurs when eddies extract kinetic energy from (or give it back to) the mean flow through horizontal shear or deformation in the basic (mean) winds. A common mathematical form for the barotropic kinetic energy conversion term (from the eddy kinetic energy budget) is something like:

$$CK = -\overline{u’v’} \frac{\partial \overline{u}}{\partial y} – \overline{u’^2} \frac{\partial \overline{u}}{\partial x} + \dots$$CK = -\overline{u'v'} \frac{\partial \overline{u}}{\partial y} - \overline{u'^2} \frac{\partial \overline{u}}{\partial x} + \dots (full expression has four terms involving Reynolds stresses and mean flow gradients)

Where overbars are time means, primes are deviations (eddies), u and v are zonal and meridional winds. Positive values often indicate energy transfer from the mean flow to eddies (eddies grow at the expense of the jet), while negative values mean the reverse (eddies dampen, strengthening the mean flow).

The Discrepancy- Why Models Miss This

Coupled climate models (where ocean and atmosphere interact) systematically underestimate or fail to reproduce the observed North Atlantic cold blob and associated SST patterns. As a result, they show reversed barotropic energy conversion patterns and miss the jet modulation.

When researchers run prescribed SST experiments (forcing atmospheric models with observed North Atlantic temperatures, including the cold blob), the models successfully reproduce the observed jet changes, energy conversion, and rainfall dipole

This highlights a key model bias in simulating AMOC-related ocean changes and their remote atmospheric teleconnections.

Implications

Ties directly to AMOC slowdown mechanisms (freshwater input, stratification, etc.): The cold blob acts as a fingerprint, linking North Atlantic circulation changes to South Asian monsoon trends. This connects two potential “tipping elements.”

Improves understanding of why traditional models struggle with recent monsoon trends.

Suggests better incorporation of observed SST forcings or improved ocean dynamics in models could enhance predictions.

The barotropic governor concept also helps explain global mid-latitude storm trends.

This is an important advance in explaining model-observation discrepancies through a specific dynamical pathway.

Published:  AGU Advances

DOI:  10.1029/2025av002173

Authors: Nimmakanti Mahendra, Nagaraju Chilukoti, Xiaoqing Liu, Jasti S. Chowdary, Lei Wang, Matthew Huber

Abstract

The South Asian summer monsoon has exhibited a pronounced Northwest India-Indo-Gangetic Plains rainfall dipole since 1999, with northwest India experiencing a 24.6% increase, while rainfall in the Indo-Gangetic Plain has decreased by 4.4%.

This dipole pattern is absent in historical climate model simulations, and the underlying physical mechanisms responsible for it are not yet fully understood.

Using synthesis of observational and assimilated data and climate model simulations, we demonstrate that this dipole is driven by North Atlantic Sea Surface Temperature (SST) changes, which are transmitted through a “barotropic governor mechanism” and modulate Asian jet stream dynamics.

Enhanced barotropic energy conversion after 1999 generated momentum focusing in the jet core while weakening east-west flanking regions.

This fundamentally altered the monsoon circulation by modulating the local Hadley cell.

We find that this barotropic governor activation coincides with the North Atlantic “cold blob” attributed to a slowdown in the Atlantic Meridional Overturning Circulation (AMOC).

Coupled climate simulations over the historical era systematically fail to reproduce North Atlantic SST changes; thus, this atmospheric dynamical mechanism instead showed reversed barotropic energy conversion patterns and failed to reproduce this key teleconnection mechanism.

Prescribed SST experiments forced by observed North Atlantic SST changes support the proposed mechanism, successfully reproducing both observed jet dynamics and rainfall trends.

The identification of a North Atlantic-Asian teleconnection pathway modulated by the barotropic governor effect directly links the behavior of the AMOC and monsoon tipping elements to each other.