Despite Weakening, Tropical Cyclone Amphan Still Capable of Producing High-Impact Storm Surge

As Tropical Cyclone Amphan tracked towards the coast of India and Bangladesh in the Bay of Bengal on Tuesday, dry air and wind shear began weakening the storm. While this is good news that may serve to reduce some wind impacts, Amphan is still capable of producing a high-impact storm surge for three reasons.

Satellite image of Tropical Cyclone Amphan, provided by University of Wisconsin CIMSS.

Reason #1: Concave-shaped Coastline and Shallow Water Depth

The first reason is that the physical geography and coastal profile of the northern part of the Bay of Bengal is very efficient at generating high storm surges because of the concave shape of the coastline and the shallow water depth, or bathymetry.

The concave shape and shallow bathymetry, or water depth, in the northern Bay of Bengal, enhances storm surge levels. Graphic Link

This may seem counterintuitive, as most people expect locations with deeper offshore water to have a higher potential for coastal flooding. However, when tropical cyclones displace tremendous amounts of water into shallow areas with concave-shaped coastlines, the water gets trapped and has nowhere to go but “up”, as it floods the landscape.

The concave shape and relatively shallow water along the Georgia coast, including adjacent portions of Florida and South Carolina, make it more susceptible to coastal flooding. Graphic: Hal Needham

By contrast, areas with deep bathymetry generally experience smaller storm surges but higher waves. Storm surges and waves differ in that a storm surge is a dome of water that rises across an extended area, whereas a wave is thin pulse of water that pushes towards the coast. We see waves every time we go to the beach, but large storm surge events are much more rare.

The graphics below depict the differences in storm surge potential when tropical cyclones strike shallow deltas, like the northern Bay of Bengal, compared to locations with deeper water, like mid-oceanic volcanic islands.

Profile of coastal flooding near a shallow delta. Note the flat topography both onshore and offshore, the large storm surge that can travel inland for 30 miles, and the small waves. Graphic: Hal Needham
Profile of coastal flooding near mid-oceanic volcanic islands. Note the deep water, steep topography, small storm surge and large waves. Graphic: Hal Needham

The effects of coastline shape and water depth are so important for storm surge generation that weaker storms striking shallow deltas can produce higher storm surges than stronger storms hitting deep coastlines. For example, Hurricane Isaac (2012), generated a storm surge exceeding 12 feet (3.66 meters) in South Louisiana, even though the storm was stalled out as a category-1 hurricane with maximum sustained winds of 80 mph. By contrast, Hurricane Iniki (1992) struck the Hawaiian Islands as a category-4 hurricane with maximum sustained winds of 145 mph, but only generated a 6-foot (1.83-meter) storm surge (U.S. Department of Commerce 1993). The water along the Hawaiian coast is considerably deeper than Louisiana.

Hurricane Isaac (2012) generated a substantial storm surge, even though the storm was classified as a category-1 hurricane. Image Link

The shape and water depth of the Bay of Bengal are so efficient at generating storm surge that the area has broken many weather records. The highest credible storm surge event in history occurred in Bangladesh in 1876, as the storm tide reached 45 feet (13.7 meters) (Dube et al. 1997). The deadliest storm surge on record happened in this same region in 1970, when Cyclone Bhola killed approximately 300,000 people (Frank and Husain 1971).

While Tropical Cyclone Amphan is not forecast to inflict such horrific losses, we should always pay attention when tropical cyclones visit this part of the world.

Reason #2: Strong Pre-Landfall Winds and Large Wind Field

Amphan’s strong pre-landfall winds have already displaced tremendous amounts of water towards shore. This water starts piling up along the coastline long before the more intense winds arrive.

I found that storm surge heights correlate best with the wind speed 18 hours before landfall for basins with shallow water in a journal article in which I was the lead author in 2014. Although this study used wind and surge data from more than 100 tropical cyclones that struck the U.S. Gulf Coast, the physics for the Bay of Bengal has similarities and we should pay close attention to pre-landfall cyclonic winds in that basin as well.

Correlation between storm surge levels and maximum sustained wind speeds at three hour increments preceding landfall. Source: Earth Interactions.

The importance of pre-landfall winds is evident when considering that Hurricane Katrina (2005) generated a higher storm surge than Hurricane Camille (1969), although both storms made landfall along the Mississippi Coast in the United States, and Camille was classified as a category-5 hurricane at landfall, whereas Katrina was a category-3 storm. While Katrina’s large wind field certainly contributed to its large and extensive storm surge, the storm was considerably more powerful before landfall, generating maximum sustained winds of 152 knots (175 mph) 18 hours before landfall in Mississippi. These winds were within 11 knots (13) mph of Camille’s winds at the same time interval, and Camille’s wind field was geographically smaller, thereby enabling Katrina to generate a larger storm surge.

Satellite image of Hurricane Katrina on Sunday, August 28, 2005. This image shows the impressive size and profile of the storm the day before landfall. At this time, winds easily exceeded the category-5 threshold. Image: NOAA

The Joint Typhoon Warning Center (JTWC) tracking map for Tropical Cyclone Amphan estimates that maximum sustained winds reached 100 knots (115 mph) around May 19, 2020 at 1800 UTC, approximately 18 hours before landfall. This is equivalent to a category-3 hurricane, which would be classified as a major hurricane in the U.S.

Prior to that time the winds had exceeded 140 knots (160 mph), reaching the equivalent to category-5 in the U.S. Although the JTWC forecasts Amphan’s intensity to drop to 85 knots (97 mph) before landfall, the stronger pre-landfall winds, displacing massive amounts of water into a smallow sub-basin of water, will generate substantial storm surge.

Tracking map from the Joint Typhoon Warning Center provides estimates of storm intensity, position and the geographic size of various wind fields. This map shows a forecast for Amphan’s winds to weaken prior to landfall. Source: https://www.metoc.navy.mil/jtwc/jtwc.html

The JTWC map also shows the geographic area of various wind fields, revealing that Amphan has a broad wind field capable of pushing much water. For example, on Tuesday morning, the radius of Amphan’s 50-knot (57-mph) winds extended at least 120 nautical miles from the center of circulation. This broad wind field will enhance storm surge levels.

Reason #3: This Region has Experienced Many Changes

The third reason Tropical Cyclone Amphan could generate a high-impact storm surge along the coast of India and Bangladesh is that the region has observed many changes that exacerbate impacts of coastal flooding.

A surprising change is a loss of memory about the horrific cyclone history in this part of the world. When I built the first global storm surge database, I noticed that the period of hyperactivity in the Bay of Bengal between 1960 and 1970 was the most active on record anywhere in the world. Consider this: In this 11-year period, Bangladesh observed seven storm surge events exceeding 8.8 meters (28.9 feet), meaning they had seven storm surges higher than Hurricane Katrina in 11 years. Since then, the region has been relatively tranquil, except for isolated cyclones, like the 1999 cyclone that hit Orissa, India.

The table below shows the 10 highest storm surge events for the Northern Indian Ocean basin, taken from Needham et al. (2015). Original data are from Dube et al. (1997). Water levels are listed in meters.

RankHeight (m)YearMaximum Surge LocationCountry
113.70a1876Precise Location UnknownBangladesh
212.00b1737SunderbansIndia
212.00b1864Calcutta and SurroundingsIndia
49.60a1966Precise Location UnknownBangladesh
59.10a1960Precise Location UnknownBangladesh
59.10a1963Precise Location UnknownBangladesh
59.10a1970North of ChittagongBangladesh
88.80a1961Precise Location UnknownBangladesh
88.80a1961Precise Location UnknownBangladesh
88.80a1967Precise Location UnknownBangladesh
Water heights: a= storm tide; b = storm surge

Since the period of tropical cyclone hyperactivity ended in the 1970s, this region has experienced several changes that make it quite vulnerable to coastal flood impacts. Population growth places a tremendous number of people in harm’s way in Bangladesh, the most densely populated mega-country on the planet, where the density is 2,600 per square mile (Streatfield and Karar 2008). By contrast, the population density would only be 1,740 per square mile if everyone in the world was placed in the U.S.

Bangladesh is a low-lying, densely populated country vulnerable to coastal flooding. Image Link

Combining the dense population with rapidly rising sea levels, produces a dangerous combination of factors for flood impacts even when a powerful cyclone is not bearing down on the coastline. We can only hope that the many people along the coast of India and Bangladesh heed local warnings and do everything within their power to protect themselves from the severe coastal flooding that is likely from Tropical Cyclone Amphan.

The latest advisory from the India Meteorological Department forecasts storm surge levels to reach as high as 4-5 meters (13.2 – 16.5 feet) in the Parganas region and 3-4 meters (10-13.2 feet) in areas of West Bengal. Given the geographic size, pre-landfall wind speeds, coastal shape and shallow water of the region, those water levels seem probable.

[See “housekeeping note” below the references about how to contact me]

Non-Linked References

Dube, S.K., A.D. Rao, P.C. Sinha, T.S. Murty, N. Bahulayan, 1997: Storm surge in the Bay of Bengal and Arabian Sea: The problem and its prediction. Mausam, 48, 283-304.

Frank, N.L., and S.A. Husain, 1971: Deadliest tropical cyclone in history. Bulletin of the American Meteorological Society, 52, 438-&.

Streatfield, P.K., and Z.A. Karar, 2008: Population Challenges for Bangladesh in the Coming Decades. Journal of Health, Population and Nutrition, 26, 261-272.

U.S. Department of Commerce, 1993: Natural Disaster Survey Report, Hurricane Iniki, September 6-13, 1992. Report available online at: http://www.nws.noaa.gov/om/assessments/iniki/iniki1.pdf.

“Housekeeping Note”

Thank you for taking time to read my first blog post on my new site. I am in the process of launching a new blog site for 2020 Atlantic Hurricane Season. Within the next few weeks I hope to have the site better developed.

In the mean time, if you would like to reach out to me to discuss flood impacts from Tropical Cyclone Amphan, please send me an email to: hal@marineweatherandclimate.com. I am available for phone and video interviews.

You can also check out these websites for my partners and affiliated projects:

The U-Surge Project (www.u-surge.net)

U-Surge is an international storm surge data project, which provides the first data-driven storm surge analyses across seven global water basins.

CNC Catastrophe and National Claims (https://adjustingexpectations.com/)

CNC provides more than 30 years of expertise in the flood insurance and adjusting market. I work with CNC to innovate cutting-edge technologies that improve flood risk analysis and coastal resiliency.

Marine Weather and Climate (www.marineweatherandclimate.com)

Marine Weather and Climate has specialized in data-driven science communication to help improve resiliency for flood-prone coastal communities for more than a decade. The “Science Communication” tab provides extensive links to print and video media projects.

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