The Indian Summer monsoon is often described as the heartbeat of our subcontinent. It nourishes our fields, fills our rivers, and shapes the socioeconomic lives of the people in the subcontinent. Yet, it also remains one of the most unpredictable features of Earth’s climate. Scientists and farmers alike have long noticed that El Niño, the periodic warming of the Pacific Ocean, often disrupts the Indian Summer monsoon, causing it to weaken or arrive late. The El Niño matures in November- February in the Pacific Ocean, and it delays the following Indian Summer Monsoon onset. But what if a small patch of warm water in our own backyard — the southeastern Arabian Sea (near the Kerala coast)— quietly works to restore balance when El Niño throws the system off? This hidden warm pool, known as the Arabian Sea Mini Warm Pool (MWP), forms every year during April and May, just before the onset of the Indian Summer Monsoon. My colleagues and I have been fascinated by this feature for years. Our study published in the Journal of Climate uncovered how this warm pool acts as a silent stabilizer — a kind of “self-correcting” mechanism of nature — that helps the monsoon recover from the disruptions caused by the previous El Niño.
If you look at satellite maps of sea surface temperature in May, you’ll notice that the waters in the southeastern Arabian Sea near the Kerala coast are the warmest in the entire Arabian Sea in April- May, often exceeding 30°C. This warm patch, only a few hundred kilometers wide, might look small on the map, but it plays a significant role in preparing the atmosphere for the monsoon onset. These patches of warm water cause local convection that helps to pull the southwesterly monsoon winds towards Kerala.
However, this warm pool doesn’t behave the same way every year. Sometimes it is strong and extensive; other times it is weak or absent. For years, scientists have wondered why. My PhD research aimed to uncover what drives its formation and how it connects to larger climate patterns like El Niño and the Indian Summer Monsoon.
El Niño typically peaks between November and January in the Pacific Ocean, but the Indian Ocean doesn’t react immediately. Instead, it responds with a delay of about four to five months. By April– May, the Indian Ocean begins to warm through a chain of events known as the Indian Ocean Capacitor Effect. Much like an electrical capacitor that stores and releases energy later, the Indian Ocean stores the influence of El Niño and releases it months afterward. Our study shows that this delayed response plays a key role in shaping the MWP near the Kerala coast; its intensity and area are strongly controlled by this slow but powerful Indo-Pacific connection. During the months following an El Niño, anomalous easterly winds (associated with the Indian Ocean Capacitor Effect) blow across the equatorial Indian Ocean. These winds create a weak wind zone in the southeastern Arabian Sea, where the MWP forms. Because weaker winds mean less surface cooling, the ocean retains its heat, and the MWP becomes stronger and more extensive. In other words, the Pacific Ocean whispers to the Indian Ocean, and the Indian Ocean responds by building up this pool of high sea surface temperature in the southeastern Arabian Sea near the Kerala coast.
El Niño is well known for delaying the Indian monsoon in the following year. The anomalous easterly winds blowing across the equator slow down the southwesterly monsoon winds toward India, pushing the rains to arrive later than usual. To understand how the Arabian Sea’s MWP fits into this story, we used a regional coupled atmosphere-ocean numerical model configured at the Centre for Atmospheric Sciences, IIT Delhi. Our simulations revealed a fascinating twist: the same anomalous easterly winds (associated with the Indian Ocean Capacitor Effect) that intensified the MWP also delayed the monsoon onset in Kerala. As the warm pool grows stronger, it creates a low-pressure area that starts pulling winds toward the Kerala coast. This local circulation acts like a magnet, drawing moist air from the ocean and nudging the monsoon back on track. So, while El Niño tries to delay the monsoon, the MWP quietly works to reduce the damage.
It is as if nature has built a feedback loop — one oceanic imbalance compensated by another. Our numerical model’s simulations show that without this balancing act, the monsoon would arrive much later, increasing the risk of early-season droughts. A good example comes from 2010, when climate scientists expected a delay in the monsoon following the strong 2009-10 El Niño. Yet, the India Meteorological Department (IMD) recorded the monsoon onset over Kerala on May 31 — a day earlier than usual (June 1 is considered to be a normal monsoon onset date in Kerala by IMD). Our model experiments revealed that this timely onset was largely due to the MWP. Without this MWP, the monsoon would likely have arrived a week later in Kerala, around June 7. This highlights the need for closely monitoring the behavior of the MWP to reduce the uncertainty in predicting how El Niño affects the following year's monsoon onset.
By uncovering how this warm patch of water helps balance the effects of El Niño, we can improve how we predict the monsoon — making seasonal forecasts more reliable and giving farmers and policymakers a crucial edge in planning for the season ahead. This study also reminds us of a more profound truth: even small things matter in our planet’s climate system. Though just a few hundred kilometers in area, the Arabian Sea MWP influences winds, rains, and lives across the Indian subcontinent. It shows how the oceans and atmosphere move in rhythm — and how a small warm pool off the Kerala coast can quietly shape the fate of a billion people waiting for the monsoon.
The monsoon has always been India’s most unpredictable yet indispensable gamble. Through our research, we are beginning to see that its mysteries are not just written in the skies above, but also in the hidden warmth of the oceans below. As the climate continues to change, understanding these subtle connections between the Pacific and the Indian Ocean will be key to predicting how the monsoon’s rhythm might shift in the future — and how we can adapt to it.
Sankar Prasad Lahiri and Vimlesh Pant are with the Centre for Atmospheric Sciences, Indian Institute of Technology Delhi, New Delhi, India. Takeshi Izumo is with the Institut de Recherche pour le Développement, UMR241 SECOPOL Laboratory, Université de la Polynésie Française, Tahiti, French Polynesia. K R Prakash is with Applied Physics Laboratory, University of Washington, Seattle, USA.
Views expressed are the authors’ own and don’t necessarily reflect those of Down To Earth