Swell waves hit southwestern coast hard as remote oceans turn stormier

A series of recent, long-period swell surge events, on the coast of Kerala and southwestern Tamil Nadu, indicates a trend of intense storms in the southernmost oceans of the world and their faraway impacts. Scientists call for better adherence to forecasts.

The unpredictable high waves pounded beaches and villages on the coast, damaging boats and houses, flooding low-lying areas in at least three events from late March till May this year.

South Indian fishers call these events “Kallakkadal” or stealthy sea, denoting waves without wind, usually observed before the monsoon starts. “You won’t know these swells would grow so big suddenly and crash hard on the shore. They have such an impact and scour away a lot of sand and destroy coastal structures,” said Aloysius Gomez, a science teacher and president of Radio Monsoon, an online weather service of Thiruvananthapuram fishers.

The recent “Kallakkadal” — now a scientific term— event had disastrous impacts as the swell waves lashed parts of Kanniyakumari,  Thiruvananthapuram, Kollam, Alappuzha and Thrissur districts. The March 31 event flooded 50 houses in north Kanniyakumari. People were evacuated. Two events followed — in early April and May 4-5. The last event killed five medical students and interns in Kanyakumari.

“The coasts get flooded in these events. However, it was the first time we saw waves becoming so strong. It’s very unusual,” said a fisherman in Puthukurichy village about 20 km northwest of Thiruvananthapuram city centre, referring to the March 31 event.

Boats parked on the shore were damaged in nearby villages. “I heard the news in the afternoon and reached a shore. By the time I reached there was water all around,” said a fisherman in Puthenthope, south of Puthukurichy. “Then we hired tractors and pulled all the vessels up the shore.”

In Poovar village of south Thiruvananthapuram, the sandy seashore up to the beach road, 500 m from the height tide line, were inundated. “The water receded by late evening, leaving a trail of dirt and debris,” said Alice Geetha, a local social worker.

Waves sent by distant storms

Scientists from the Indian National Centre for Ocean Information Services (INCOIS) attributed the events to storms in the South Atlantic, about 10,000 km away from Thiruvananthapuram. “There were intense storms there with 10-15 metre waves,” said T.N. Balakrishnan Nair, group director of ocean modelling, applied research and services at INCOIS while talking to Mongabay-India.

During the pre-monsoon season, intermittent high-period swell waves (15-17 seconds), with heights of 2 to 2.5 metres often lead to rough seas near shore and coastal flooding. These swell waves are different from the short, choppy, wind waves produced by local winds blowing along the water’s surface. Unlike the choppy waves, these swell waves are smooth and travel far from their point of origin, largely unaffected by the local wind. Interacting waves from different places at different directions at different speeds mark the ocean surface.

Long-period swell waves pack in more energy than short-period ones, and close to the shore they break and crash with tremendous energy, destroying coastal features. Generated by storms or sustained wind events in the Southern Ocean or South Indian Ocean, they take five or six days to reach and hit India’s southern parts.

An INCOIS 2016 study explained how swells from the Southern Ocean reach the Arabian Sea and cause high wave events. Earlier studies showed how “Kallakkadal” has remote origins.

The INCOIS study noted that long-period swells require intense winds over a longer duration and sustained over a large ocean area. Waves of around 18 seconds (time taken to cross a point) with significant energy require wind speeds of over 72 km per hour blown over 1000 km. Slow moving, often stationary, isolated systems called cut-off lows can prove idle these long-period waves, the study noted.

A more recent study showed that the forecasts of the timing of high swell events along the North Indian coasts can be erroneous by 12 hours if the Southern Ocean sea ice concentration is not included in the model. The sea ice concentrations fluctuate seasonally. INCOIS scientists said their people models and observation network is constantly updated.

Swell period, this time, was the longest recorded in the country at 25 seconds, INCOIS scientists told Mongabay India. “It is the long period swell, combined with high tide conditions, that caused coastal flooding in low-lying areas,” Balakrishnan said.

Southern Atlantic is becoming stormy this season

Scientists said the swell events could be possibly connected to changing storm trends in the world’s southernmost oceans. This year, the South Atlantic is becoming stormier than usual, for instance. University of Colorado scientists have warned, “A warmer-than-normal tropical Atlantic provides a more conducive dynamic and thermodynamic environment for hurricane formation and intensification.”

The swell surge event of early May was caused by a storm 10,000 km away from the Indian coast on April 26, 2024, in the southern Atlantic Ocean. It moved towards the Southern Indian Ocean. This caused the high energy swell propagation at 24 seconds intervals hitting the southern tip of India on May 4, INCOIS scientists stated.

The prevailing El Niño conditions are likely to move to neutral in the next few weeks and then to La Niña by the peak of the Atlantic hurricane season, typically increases Atlantic hurricane activity, Colorado scientists added.

Meanwhile, National Oceanic and Atmospheric Administration (NOAA)’s forecast predicts above-normal hurricane activity in the Atlantic this year. The season, June 1–November 30, has an 85% chance of being more active than usual, the U.S. scientific and regulatory agency that forecasts weather, has said.

The Southern Ocean is turning stormier, too.

Human interventions

While climate change is likely to worsen swell surge events, human intervention can also influence their impact. Southwestern India’s sandy coasts are increasingly replaced by hard structures such as seawalls, groynes, and breakwaters as studies show.

While breakwaters are built to aid the safe landing and launching of fishing craft, groynes capture the sediment drift and protect the beach. However, at river mouths and estuaries, these structures often worsen erosion. They have turned coastlines into granite seawalls that hamper the natural coastal dynamics, such as the beach-building process. Such natural processes are essential for sandy shores that cushion the impact of high waves.

“The southwestern coast is exceptionally dynamic due to its configuration – short continental shelf and rapid littoral (shore) currents,” noted D. Nandakumar, professor and former head department of geography at the University College, Thiruvananthapuram. He has studied the local coast for decades.

The crowded fishing village of Anjengo, the location of a 17th-century British fort, that usually gets flooded in wave attacks, is an example. The village lost its beaches after a fishing harbour came up south of it. Repeated attempts at building seawalls could only save part of the village, and every season it loses operations of its beach.

The human factors also include forecasting, dissemination and forecast usage. “Warnings were issued, but nobody took them seriously (at first),” Balakrishnan Nair said.

Balakrishnan Nair said the public memory is often short, and so is the “memory of the system.”

Scientists call for better models, observations, forecasts, dissemination to the last mile, and public awareness to ensure effective use of the forecasts.

Banner image: A series of recent long-period swell surge events on the coast of Kerala and southwestern Tamil Nadu has damaged properties and killed people. Image by Vincy Lopez.