Tag: positive IOD

The Indian Ocean Dipole (IOD), a pattern of sea- surface temperature differences across the tropical Indian Ocean, shows a statistically detectable teleconnection to early- winter (especially December) rainfall patterns and storm activity over the North Atlantic- European- Mediterranean (NAEM) region.

This link is described in a 2025 Geophysical Research Letters paper by M. Reale and colleagues (including A. Raganato, F. D’Andrea, M. Adnan Abid, A. Hochman, and others) and explained in a popular summary by lead author Marco Reale on phys.org (28 October 2026).

Key background

• The positive IOD phase features warmer- than- usual waters in the western tropical Indian Ocean relative to the east (the reverse for the negative phase). It typically peaks in boreal autumn.
• Tropical SST anomalies alter where air rises and where latent heat is released, disturbing large- scale atmospheric circulation. These effects can propagate far from the tropics as teleconnections, not by moisture literally traveling from the Indian Ocean to Europe, but by changing the atmospheric flow itself.

Main findings (based on 1979–2024 data)

Researchers examined atmospheric pressure, multiple rainfall metrics (totals, wet-day frequency, wet- spell length), and cyclone tracks, while statistically isolating the IOD signal from its linear relationship with El Niño- Southern Oscillation (ENSO).

• The clearest signal appears in December.
• Positive IOD conditions are linked to a north– south rainfall contrast: wetter conditions over parts of the northern Atlantic and drier conditions farther south, including the western-central Mediterranean. This involves changes in total precipitation, the frequency of wet days, and the duration of wet spells.
• Cyclone (storm) activity is reduced over the eastern Atlantic and western Mediterranean in December.
• The broader signal weakens later in winter (January- February). A positive NAO- like pressure pattern over the NAEM region emerges in December and shifts eastward through February.

Atmospheric mechanism

The changes in cyclone track density are primarily driven by variations in the Eady growth rate (a measure of baroclinic instability that favors extratropical storm development). These are tied to enhanced vertical wind shear associated with a strengthened meridional (north- south) temperature gradient over the NAEM region. In short, the IOD modulates the large-scale environment that supports (or suppresses) storm growth.

Implications and caveats

The relationship is statistical, it describes tendencies across many years, not a deterministic forecast for any single December. Other climate drivers and internal atmospheric variability still matter, and the study does not itself demonstrate improved forecast skill. However, it suggests that autumn IOD conditions could provide useful information for sub- seasonal to seasonal predictions of early- winter hydroclimate in the NAEM region, particularly when focusing on specific months, regions, and rainfall characteristics rather than a bulk winter average.

In essence, Europe’s early- winter weather is shaped not only by nearby Atlantic and Mediterranean systems but also by remote tropical Indian Ocean conditions from the preceding autumn.

Response of Early Winter Precipitation and Storm Activity in the North Atlantic–European–Mediterranean Region to Indian Ocean SST Variability

Abstract (summary)

The study examines the response of winter precipitation and storm activity in the North Atlantic- European- Mediterranean (NAEM) region to the Indian Ocean Dipole (IOD) over 1979–2024 (using ERA5 data).

• A positive NAO- like pattern appears over the NAEM in December and shifts eastward through February.
• The IOD modulates precipitation (total amounts, event frequency, and wet-spell duration).
• The teleconnection is primarily significant in December.
• Cyclone activity decreases in December over the East Atlantic and Western Mediterranean.
• These cyclone- track density changes are driven mainly by variations in the Eady Growth Rate, linked to enhanced vertical wind shear from a strengthened meridional temperature gradient.

The results highlight a significant remote impact of the autumn IOD on early-winter hydroclimate in the NAEM region and its potential value for improving sub- seasonal to seasonal predictions.⁠

Key Points

• Winter precipitation and storm activity show a mainly December long- term significant response to the Indian Ocean Dipole.
• Spatial changes in environmental baroclinicity drive the observed changes in storm activity in December.
• Latitudinal variations of baroclinicity are linked to stronger vertical wind shear driven by an enhanced meridional temperature gradient.

Methods (briefly)

• Data: ERA5 reanalysis (1979–2025), detrended anomalies.
• IOD index: Dipole Mode Index (DMI) for October, with the linear ENSO (NINO-3.4) influence removed to isolate the residual IOD signal (iod_RES).
• Storm activity: Objective cyclone tracking on 6-hourly MSLP fields; track density on a 2° grid.
• Baroclinicity: Eady Growth Rate (EGR) between 850 and 500 hPa.
• Analysis: Linear regression of atmospheric and precipitation fields onto the residual autumn IOD index. Precipitation extremes use Climdex indices (e.g., total wet-day precipitation, number of wet days, consecutive wet days, etc.).

Main results

• Strongest and most significant circulation response (positive NAO- like MSLP pattern) occurs in December. The pattern weakens and shifts eastward later in winter.
• Positive residual IOD is associated with a north- south precipitation dipole (wetter conditions farther north, drier in the western- central Mediterranean), affecting totals, frequency of wet days, and wet- spell length.
• Reduced cyclone track density over the eastern Atlantic and western Mediterranean in December, consistent with reduced Eady Growth Rate driven primarily by changes in vertical wind shear (linked to a stronger north- south temperature gradient).

Conclusions

The autumn IOD exerts a robust remote influence on early- winter (especially December) hydroclimate and storm activity over the NAEM region via changes in baroclinicity.

This supports the value of incorporating Indian Ocean variability into sub-seasonal- to- seasonal forecast systems for Europe and the Mediterranean. The authors note the need for further work on how climate change may alter the stationarity of this teleconnection.

The full paper (including figures of MSLP, precipitation indices, cyclone tracks, EGR, wind shear, etc., plus supporting information) is available open access via the Wiley link or institutional repositories (e.g., Oxford Research Archive, HAL).

Title: Response of Early Winter Precipitation and Storm Activity in the North Atlantic–European–Mediterranean Region to Indian Ocean SST Variability

Journal: Geophysical Research Letters, (Volume 52, Issue 20, e2025GL116732)

DOI: 10.1029/2025gl116732.

First published: 16 October 2025

Authors: M. Reale, A. Raganato, F. D’Andrea, M. Adnan Abid, A. Hochman, N. R. Chowdhury, S. Salon, F. Kucharski