The ocean is creeping higher along California's coastline, and it's not just a few inches of difference.

Water levels off Southern California have been running up to two feet higher than they did at this time last year.

Those extra feet of ocean water are already helping push high tides into coastal neighborhoods, and it could make an even bigger difference when winter storms arrive.

El Niño is a major cause

El Niño can send enormous underwater waves of warm water, called Kelvin waves, thousands of miles across the Pacific Ocean.

(MORE: El Niño Kelvin wave turns Newport Beach streets into a lagoon)

These aren't the familiar waves that crash onto the beach. They're incredibly long, slow-moving bulges of warm water that travel eastward along the equator.

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When they reach the Americas, they turn north and travel along the coastline. 

They're already making their way up the West Coast, bringing warmer water and higher-than-normal sea levels with them.

A higher starting water level means waves can reach farther up beaches, push closer to seawalls and coastal infrastructure, and carry more energy into vulnerable shorelines.

It can also accelerate beach erosion and wear away dunes and bluffs that normally help protect coastal communities.

(MORE: The 'Pineapple Express' is a firehose. El Niño just turned it up)

Not a good sign ahead of winter

Winter can bring large storm waves and strong onshore winds, and if those conditions arrive while the ocean is already elevated by El Niño and unusually high tides, there's less room for the water to go before it spills into places that are normally dry.

(MORE: Next king tide could arrive Thanksgiving week)

That combination could mean more frequent coastal flooding, damage to waterfront properties and roads and waves reaching farther inland than residents might expect.

California's Coastal Commission is already urging coastal communities to prepare for flooding and erosion as El Niño strengthens.

And there's still a lot of winter ahead. The highest astronomical tides of the year are still approaching, meaning the ocean could get another boost just as winter storm season ramps up.