Tag Archives | waves

Riding the Waves of the Seasonal Roller-Coaster

Average monthly wave heights at NDBC Buoy 44025

It is often said in Pennsylvania that March comes in like a lion and goes out like a lamb. And while this March felt more like a ride on an Arctic roller-coaster that wouldn’t end, the good news is that, based on past years, we should soon be on the downward slope towards more calmer weather.

In the Mid-Atlantic, the winter months usually bring with them strong storms and high winds, like the nor’easter we saw earlier this month. In the ocean, strong winds lead to larger significant wave heights, as can be seen in the graph above that depicts the average monthly wave heights off the coast of New Jersey over the course of a year.

This graph was created using 8 years of significant wave height data from NDBC Buoy 44025, which is a little more than 40 miles from the New Jersey coast. Each line represents a particular percentile level, indicating the percentage of measurements that fall below the indicated level. The 50% percentile level is commonly called the median average value. For each month, half of the measured data over the course of the 8 years fell above the median value while the other half fell below.

This graph shows a distinct difference between the seasons. The median wave height in December, January and February is around 4.5 feet, while in the summer months of June, July and August, the average is closer to 3 feet. While the median value is higher in winter months than summer ones, the change is even larger for the 80th percentile line. For that, we can thank those large winter storm events that turn the ocean into one rough ride.

Thankfully, spring will soon give way to summer, and if the past averages hold true, the summer months of this roller-coaster should include calmer waters.

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A Rough and Significant Winter

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It has been a rough winter in New Jersey, especially on the coast. First, Post Tropical Storm Sandy struck Atlantic County on October 29th 2012, becoming the costliest natural disaster in New Jersey’s history. Over the next 5 months, several additional strong storms made their way across the state, bringing with them heavy winds, coastal and inland flooding and significant snowfalls.

Strong storms, many of which are called Nor’easters, are common occurrences in the Mid-Atlantic during winter months. Their strong winds also lead to high waves in the ocean. But this past winter was rather exceptional.

The above graph shows an analysis of wave heights measured by NDBC Station 44025. Each bar depicts the maximum wave height reached for each of the 12 largest events (each lasting 2-3 days) recorded over the past eight years (from January 2005 to today). Of the 12 events with the highest waves, 6 of them have been in the last 6 months. The largest recorded wave event was, of course, due to Sandy. The 12th largest occurred during the nor’easter that struck earlier this month.

It’s important to note that this does not (yet) represent a significant trend. Taking the top 20 events into account, only the same 6 events occurred this winter. Extreme events can often occur in spurts, triggered by prevailing climatic conditions, so the likelihood of many major events coinciding is not uncommon. In addition, an 8-year dataset is far too limited to make any assumptions about long-term climate changes.

However, it should be quite clear from this evidence that New Jersey residents are certainly ready for Spring, and more importantly, calmer weather.

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Significant Waves

Significant wave heights at Station 44025

Last week a major snowstorm travelled across the continental United states, becoming a strong nor’easter over the Mid-Atlantic. While snowfall amounts in New Jersey were far less than some had predicted, the wind and waves that battered the coast were still quite severe. Dunes in Mantoloking, NJ that were heavily damaged last fall by Hurricane Sandy were again breached, causing flooding and further hindering repairs.

Wave heights at NOAA Station 44025, just 43 miles off the coast of New Jersey, reached 18.4 feet on the night of March 6th. The blue line above shows the significant wave heights measured by the NOAA buoy over the course of the last week.

The red horizontal lines signify the percentage of hourly wave measurements recorded between 2005 and 2012 that were less than the indicated height. The top line, at 31.6 feet, represents the maximum wave height reached during the 8-year record, which occurred as Hurricane Sandy made landfall.

The maximum wave height during last week’s storm reached the 99.9th percentile. Only 1 hourly measurement in 1000 hours of measurements (the equivalent of 42 days) ever reach this level. After the peak, wave heights remained between the 90 and 99.9th percentile for 3 days, which indicates the significance of this storm.

Matlab tip: If you’re interested in calculating significant wave heights at various percentile levels at other stations or for other parameters, here’s some code to play with.

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% Load in the concatenated NDBC Datafile
fid=fopen('data/44025_8yr.txt','r');
data = textscan(fid,'%4f %2f %2f %2f %2f %f %f %f %f %f %f %f %f %f %f %f %f %f %*[^\n]','HeaderLines',2,'CommentStyle','#');
fclose(fid);
 
% Calculate time and remove bad datapoints
dtime = datenum(data{1},data{2},data{3},data{4},data{5},0);
wvht = data{9};
wvht(find(wvht==99)) = NaN;
 
% Calculate percentile levels and convert meters to feet
wvd = sort(wvht(find(~isnan(wvht))));
disp([.9 wvd(round(length(wvd)*.9))*3.28084]);
disp([.99 wvd(round(length(wvd)*.99))*3.28084]);
disp([.999 wvd(round(length(wvd)*.999))*3.28084]);
disp([1 wvd(round(length(wvd)*1))*3.28084]);
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