Geographic variation in oceanography: semi-diurnal, diurnal, and mixed tides

Oceanography

Oceanic Tides Differ Greatly Depending on Location

The tides of the ocean are unique to geographical location. Each individual location is affected differently by the pull of the moon and sun on the earth's bodies of water (National Oceanic and Atmospheric Administration 2010). Thus, location is a primary determinate of the types of tides, and how ocean water levels are affected during both high and low tides.

Semi-diurnal tides are most common. In the span of one day, the earth's rotation hits the point of high tide, where there is the greatest force pulling the water outward, then returns to low tide, and back again to the original point of high tide (National Oceanic and Atmospheric Administration 2010). According to research, "This results in two high tides and two low tides in a day," (Ocean Link 2010). These types of tides are most common on the Eastern Atlantic Seaboard, which explains how the Florida Keys and Connecticut tides are both semi-diurnal (Pidwirny 2008). They also occur closer to the equator, based on the incline of the moon's orbit around the earth (Ocean Link 2010). However, the data is recovered from a winter month, where the sun's inclination is at the highest point. Research states that "During these months the 'bulges' in the ocean are offset the most from the equator, and it is most likely to encounter only one tide cycle per day, or diurnal tides," (Ocean Link 2010).

Diurnal tides are much different. In these types of tides, there is only one high and one low tide per 24-hour period. They tend to occur in extremes of the northern and southern hemispheres (Ocean Link 2010). Diurnal tides occur typically in the northern Gulf of Mexico (Pidwirny 2008). Thus, Eagle Point Texas, the only point in the northern region of the Gulf of Mexico covered by the data is the only diurnal tide

Lastly, mixed tides show a combination of the elements of the two tide types. They are common on the West Coast of the United States (Pidwirny 2008). Hence, the Alaskan tide covered in this research is a mixed tide, showing elements of both diurnal and semi-diurnal tides. According to research, mixed tides show "successive high-water and low-water stands differ appreciably," (Pidwirny 2008). Therefore, these tides have the greatest disparity between high and low tide water levels.

The data correlates with the West coast of Alaska being dominated by mixed tides. There are two low and high tides per day, because Alaska returns to the same point of tide twice a day. However, it is clearly a mixed tide here. This data set shows the greatest disparity between the tides, with enormous differences between the high and low tide water level measurements. Around January 9th there is a greater disparity between the water levels of the tides. The high tide increases, but at the same time the low tide decreases in water level. There is a great decrease seen in the context of the low tide, the water levels drop dramatically, as seen in the graphs. On January 11 at 3:00 AM, high tide reaches its first peak at 31.248 feet at high tide. The corresponding low tide of this same day hits at 10 am at around 3.517 feet. There is a dramatic difference between the two tides occurring twice a day. On that same day, the second round high tide is at a height of 29.588 at 5 pm, a lesser degree than the first tide -- but the low tide is still incredibly different, at 5.09 feet at 11 pm. Then around the 16, the tides move towards the more neutral disparity that was seen earlier in the month. Only to once again increase in disparity later on in the month, this shows a pattern of an increase twice a month.

Key West

Key West in Florida also shows semi-diurnal tides, with less disparity seen in the Alaskan tide readings. Key West has its largest disparity in the second week of January, and then evens out for the rest of the month. The highest tide occurs on January 11 at 2 am 2.177 feet. The lowest actually comes the day before on January 10 at 8 am -0.67 feet.

Strangely, it resembles the Connecticut tide more than the Texan tide, which correlates with research stating that much of the Atlantic Seaboard sees semi-diurnal tides. The west coast of Florida shows more diurnal tides that would be more similar to Eagle Point Texas data.

Eagle Point

This is a clear indication of a diurnal tide. There is one high and one low tide per day, which is contrasting to the other three geographical points. There are much less dramatic differences between the height of high and low tide. High tide never gets above 1 foot, with low tide only maxing out at a little over -1 foot. Thus, there is very little difference, which is typical with diurnal tide patterns. Texas is dominated with only small changes, with some periods showing very little changes between high and low tide