Flooding of U.S. coastlines due to sea level rise is a reality, plain and simple. This is referred to as high-tide flooding, and it’s distinguished from flooding caused by crashing waves or extreme rainfall. High-tide flooding in the U.S., which is measured by NOAA tide gauges that continuously record surrounding water levels (1), occurs nearly three times as often today as it did in 2000, and the frequency is accelerating along most Atlantic and Gulf Coast locations. Flooding that used to happen only during big storms now happens for a variety of reasons: a stiff onshore breeze, slowing of the Gulf Stream, a Hawaiian eddy or a full-moon tide. Also, as we discuss below, ENSO (El Niño/Southern Oscillation, the entire El Niño and La Niña system) has an important effect on coastal flooding.
High-tide flooding varies by region, but usually occurs when water levels reach at least 1 to 2 feet higher than average high tide (2). More frequent high-tide flooding is converting saltmarshes to mudflats, changing coastal ecosystems, infiltrating groundwater, degrading storm and wastewater systems, flooding streets, and slowing commutes and commerce alike. In short, our critical infrastructure and natural resources are in the tide’s way, and the tide is not backing down!
But you can prepare!
Due to the cumulative toll of high-tide flooding, the effort and cost of response are an increasing burden on communities. Until lasting comprehensive solutions are put into place to protect communities from rising seas, guidance for the next year will become ever more important (think annual emergency-response budgeting). Since 2015-16, when the last strong El Niño occurred, NOAA has been providing an annual and seasonal coastal flood outlook to give coastal communities guidance about what they are likely to experience in terms of coastal flooding in the coming year. (Check out footnote 3 for some additional background.)
How does ENSO factor into these coastal flood outlooks? El Niño in particular can really kick things up, because sea levels are already so close to the brim in many communities thanks to climate change. History shows that during El Niño, flooding occurs along many U.S. coastlines even more frequently (see figure below for a comparison of high-tide flooding with El Niño and without El Niño). In contrast, La Niña tends to decrease or have no effect on high-tide flooding frequencies in many regions. Thus, the El Niño that recently emerged is the leading cause of higher predictions above and beyond the trend imparted by sea level rise itself in the 2023/2024 NOAA Annual High-Tide Flooding Outlook. This is a change from the last several years when we were in La Niña.
Beyond the brim from coast-to-coast
So how does El Niño increase high-tide flooding on both U.S. coastlines? Briefly, El Niño effects tend to increase the number of high-tide flooding days during the Northern Hemisphere’s cool season (roughly October to April – the reason why the annual outlook follows a May-April meteorological year definition) by reinforcing the seasonal processes that cause high-tide flooding in the first place.
First off, El Niño causes a regional rise in sea levels and thus higher-than-normal spring (full or new moon) tides along the U.S. Pacific and Atlantic coastlines. Along the equator in the Pacific Ocean, El Niño weakens the typical east-to-west surface winds (i.e. a weakening of the Walker circulation), which triggers eastward propagating equatorial Kelvin waves, spreading warmer-than-average ocean temperatures into the eastern equatorial Pacific Ocean. Once these waves traverse the entire equatorial Pacific, they hit the South American coast, splitting north and south along the Pacific coastlines. These waves that travel along the coasts are called “coastal Kelvin waves” and these warmer waters elevate sea levels along the U.S. West Coast for several months at a time, often equal to a decade’s worth of sea level rise.
In addition to processes rooted in the ocean, El Niño can raise sea levels on both coasts by changing atmospheric pressure patterns in both the North Pacific and North Atlantic. These pressure patterns modify the winds, helping to pile up the water along the coasts even more (4).
Also, El Niño affects regional storm frequencies of both midlatitude storms (e.g., nor’easters) and hurricanes. Because of the large-scale atmospheric circulation anomalies caused by El Niño, the North Pacific subtropical west-to-east jet stream shifts farther south and is more elongated in the west-east direction across the southern U.S. This tends to bring more water vapor and storms than normal along the California and Mid-Atlantic coastlines. On the other hand, these conditions generally suppress tropical storm activity within the Atlantic basin (5), but in the end, the scale is still tilted toward higher overall storminess (and effects like beach erosion) along the U.S. East and West coasts during El Niño.
High-tide flooding varies by region, but usually occurs when water levels reach at least 1 to 2 feet higher than average high tide (2). More frequent high-tide flooding is converting saltmarshes to mudflats, changing coastal ecosystems, infiltrating groundwater, degrading storm and wastewater systems, flooding streets, and slowing commutes and commerce alike. In short, our critical infrastructure and natural resources are in the tide’s way, and the tide is not backing down!
But you can prepare!
Due to the cumulative toll of high-tide flooding, the effort and cost of response are an increasing burden on communities. Until lasting comprehensive solutions are put into place to protect communities from rising seas, guidance for the next year will become ever more important (think annual emergency-response budgeting). Since 2015-16, when the last strong El Niño occurred, NOAA has been providing an annual and seasonal coastal flood outlook to give coastal communities guidance about what they are likely to experience in terms of coastal flooding in the coming year. (Check out footnote 3 for some additional background.)
How does ENSO factor into these coastal flood outlooks? El Niño in particular can really kick things up, because sea levels are already so close to the brim in many communities thanks to climate change. History shows that during El Niño, flooding occurs along many U.S. coastlines even more frequently (see figure below for a comparison of high-tide flooding with El Niño and without El Niño). In contrast, La Niña tends to decrease or have no effect on high-tide flooding frequencies in many regions. Thus, the El Niño that recently emerged is the leading cause of higher predictions above and beyond the trend imparted by sea level rise itself in the 2023/2024 NOAA Annual High-Tide Flooding Outlook. This is a change from the last several years when we were in La Niña.
Beyond the brim from coast-to-coast
So how does El Niño increase high-tide flooding on both U.S. coastlines? Briefly, El Niño effects tend to increase the number of high-tide flooding days during the Northern Hemisphere’s cool season (roughly October to April – the reason why the annual outlook follows a May-April meteorological year definition) by reinforcing the seasonal processes that cause high-tide flooding in the first place.
First off, El Niño causes a regional rise in sea levels and thus higher-than-normal spring (full or new moon) tides along the U.S. Pacific and Atlantic coastlines. Along the equator in the Pacific Ocean, El Niño weakens the typical east-to-west surface winds (i.e. a weakening of the Walker circulation), which triggers eastward propagating equatorial Kelvin waves, spreading warmer-than-average ocean temperatures into the eastern equatorial Pacific Ocean. Once these waves traverse the entire equatorial Pacific, they hit the South American coast, splitting north and south along the Pacific coastlines. These waves that travel along the coasts are called “coastal Kelvin waves” and these warmer waters elevate sea levels along the U.S. West Coast for several months at a time, often equal to a decade’s worth of sea level rise.
In addition to processes rooted in the ocean, El Niño can raise sea levels on both coasts by changing atmospheric pressure patterns in both the North Pacific and North Atlantic. These pressure patterns modify the winds, helping to pile up the water along the coasts even more (4).
Also, El Niño affects regional storm frequencies of both midlatitude storms (e.g., nor’easters) and hurricanes. Because of the large-scale atmospheric circulation anomalies caused by El Niño, the North Pacific subtropical west-to-east jet stream shifts farther south and is more elongated in the west-east direction across the southern U.S. This tends to bring more water vapor and storms than normal along the California and Mid-Atlantic coastlines. On the other hand, these conditions generally suppress tropical storm activity within the Atlantic basin (5), but in the end, the scale is still tilted toward higher overall storminess (and effects like beach erosion) along the U.S. East and West coasts during El Niño.
