East-West Rainfall Modal Dipole

• East-West Rainfall Modal Dipole is an asymmetric pattern in the Indian monsoon where NW India experiences enhanced rainfall while NE India shows a decline.
• EOF analysis of high-resolution IMD data identifies EOF 3 as capturing 5.01% variance and correlating strongly with the Webster–Yang zonal wind shear index.
• Physical attribution distinguishes robust Arabian Sea processes—such as SST rise, SLP decline, and moisture flux increase—from the weaker BoB response.

The East-West Rainfall Modal Dipole is an emergent, spatially asymmetric mode of variability within the Indian summer monsoon rainfall system characterized by an out-of-phase relationship in precipitation trends between northwest and northeast India. Recent analyses reveal that this pattern, now encapsulated by the third empirical orthogonal function (EOF 3), represents a striking deviation from the canonical, more uniformly distributed monsoonal rainfall paradigm. This modal dipole is physically grounded in observed changes in regional atmospheric dynamics and thermodynamics, most notably those linking the Arabian Sea to northwest India and the Bay of Bengal to northeast India, and is robustly correlated with the Webster-Yang zonal wind shear index (Satheesh et al., 27 Dec 2025).

1. Empirical Orthogonal Function Analysis and Modal Quantification

The quantification of the east-west modal dipole is rooted in EOF analysis performed on July–August rainfall anomalies, $R(t,x,y)$, constructed from the high-resolution (0.25° × 0.25°) India Meteorological Department (IMD) rainfall data. The analysis removes the 1980–2024 climatic mean, yielding anomalies across the spatial domain. The spatial covariance matrix,

$C_{ij} = \frac{1}{T}\sum_{t=1}^{T} R(t,i)\,R(t,j),$

is diagonalized to extract EOFs $\phi^{(k)}_i$ and related principal components $\mathrm{PC}_k(t)$, which reconstruct the anomaly field as

$$R(t,i) = \sum_{k=1}^N \mathrm{PC}_k(t)\,\phi^{(k)}_i.$$

EOF 3 ($\phi^3$) emerges as a robust mode explaining 5.01 % of the total July–August variance, distinguishing itself by its unique east-west dipole signature [(Satheesh et al., 27 Dec 2025), Fig. 2(e)].

2. Spatial-Temporal Characteristics of the Dipole Mode

The spatial structure of EOF 3 is characterized by positive rainfall anomalies over northwest India (Gujarat, Rajasthan) and a southeastward extension down the peninsular region, paired with negative anomalies spanning the Gangetic Plains, northeast India, and the southwest coastline. Temporally, the principal component $\mathrm{PC}_3(t)$ exhibits multi-decadal low-frequency variability punctuated by a statistically significant positive trend post-1980. This trend manifests as an increased frequency of positive dipole phases over the last two decades, indicating persistent and intensifying asymmetric rainfall distribution [(Satheesh et al., 27 Dec 2025), Fig. 2(f)].

3. Physical Attribution via the Webster–Yang Index

To establish the physical credibility of this dipole, the analysis employs regression of rainfall anomalies onto the Webster-Yang Index (WYI), a measure of monsoonal zonal wind shear defined as

$$\mathrm{WY}(t) = \overline{U_{850}(t) - U_{200}(t)}_{(0–20°N,\,40–110°E)},$$

where $U_{850}$ and $U_{200}$ are zonal wind components at 850 and 200 hPa, respectively. The regression slope $C_{ij} = \frac{1}{T}\sum_{t=1}^{T} R(t,i)\,R(t,j),$0 at each gridpoint,

$C_{ij} = \frac{1}{T}\sum_{t=1}^{T} R(t,i)\,R(t,j),$1

generates a spatial pattern mirroring EOF 3. The spatial correlation between $C_{ij} = \frac{1}{T}\sum_{t=1}^{T} R(t,i)\,R(t,j),$2 and $C_{ij} = \frac{1}{T}\sum_{t=1}^{T} R(t,i)\,R(t,j),$3 exceeds 0.8, with the time series correlation $C_{ij} = \frac{1}{T}\sum_{t=1}^{T} R(t,i)\,R(t,j),$4 (p < 0.05), demonstrating that dynamic changes in wind shear are intimately linked to the east-west dipole's rainfall structure [(Satheesh et al., 27 Dec 2025), Fig. 5a].

4. Dynamical and Thermodynamical Drivers

Regional dynamical analysis partitions the mechanisms into Arabain Sea (AS) and Bay of Bengal (BoB) domains:

Arabian Sea:

• A persistent negative trend in sea level pressure (SLP), $C_{ij} = \frac{1}{T}\sum_{t=1}^{T} R(t,i)\,R(t,j),$5 hPa/month (≈ –0.24 hPa decade⁻¹), is accompanied by a positive specific humidity trend, $C_{ij} = \frac{1}{T}\sum_{t=1}^{T} R(t,i)\,R(t,j),$6 g/kg month⁻¹ (≈ 0.24 g kg⁻¹ decade⁻¹).
• Moisture flux convergence increases ($C_{ij} = \frac{1}{T}\sum_{t=1}^{T} R(t,i)\,R(t,j),$7), indicating a strengthening of moist air inflow.
• The poleward shift and strengthening of the Somali Jet's northern branch are reflected in zonal wind trends ($C_{ij} = \frac{1}{T}\sum_{t=1}^{T} R(t,i)\,R(t,j),$8 up to $C_{ij} = \frac{1}{T}\sum_{t=1}^{T} R(t,i)\,R(t,j),$9 m s⁻¹ month⁻¹), quantified by a jet index $\phi^{(k)}_i$0 ascending by +0.1 m s⁻¹ decade⁻¹.

Bay of Bengal:

• Here, the SLP trend ($\phi^{(k)}_i$1 hPa/month) is weak and statistically insignificant.
• The specific humidity trend ($\phi^{(k)}_i$2 g/kg month⁻¹) is smaller, and the associated moisture convergence is negligible.

These results indicate that enhanced northwest rainfall is dynamically coupled to processes in the Arabian Sea, whereas the atmospheric state over the BoB offers limited support for increased northeast rainfall [(Satheesh et al., 27 Dec 2025), Figs. 8, 10].

5. Trends in Sea Surface Temperature and Moisture Availability

A comparative assessment of sea surface temperature (SST) shifts, using ERSSTv5, HadISST, and COBE‐SST2 for July–August, reveals that the Arabian Sea has warmed by approximately +0.4 °C between the 1980–2002 and 2003–2024 periods, whereas the BoB has warmed by only +0.2 °C. The PDF of these SST differences, statistically significant at the 95% confidence level, underpins differing regional moisture availability. The combination of greater SST rise, SLP depression, and increased specific humidity over the Arabian Sea underlies enhanced moisture transport toward northwest India [(Satheesh et al., 27 Dec 2025), Fig. 12].

6. Synthesis of Dynamical-Thermodynamical Interactions

All lines of observational and analytical evidence converge to establish the east-west rainfall modal dipole as an emergent, physically coherent pattern linked to dynamical and thermodynamical changes over the Indian sector. EOF 3 encapsulates the variability and recent intensification of the dipole. The Arabian Sea's rapid SST warming, SLP decline, and humidity increase, together with the poleward-shifting Somali Jet, focus moisture delivery onto northwest India, while the BoB's tepid dynamical response fails to augment northeast rains. The robust association with the WYI confirms a central dynamical role for altered zonal shear. This emergent dipole has important implications for regional hydroclimatology, monsoon predictability, and water resource management in India (Satheesh et al., 27 Dec 2025).