Higher profile ocean current data

The Ocean Prediction Center website (http://www.opc.ncep.noaa.gov) is an excellent first stop for ocean voyagers looking for weather information tailored toward their pursuits, and I have written frequently about the website and the products available there. Those who visit the site regularly have noticed a recent change in the front page of the site, and I thought I would write about that this time.

Until recently, there were three tabs on the front page: the “Atlantic Marine” tab leads the user to weather analysis and forecast products for the North Atlantic Ocean Basin; the “Pacific Marine” tab leads the user to weather analysis and forecast products for the North Pacific Ocean Basin; and “Unified Analysis” leads the user to various different surface analysis maps extending from western Europe and northern Africa to eastern Asia, and from northern South America north to sub-Arctic latitudes. Now a fourth tab has been added, leading the user to “Ocean Products.”

Two types of products are presented on this tab, and both are derived from the Navy Coastal Ocean Model. The two types of products show sea surface temperatures, and current velocities. These products have actually been operational for several years, but have been a little more difficult to locate, thus the reason for giving them a higher profile on the website. Clearly, those who undertake open ocean voyages will find data about the major currents of the world useful. Given the current speeds in the Gulf Stream and other warm western boundary, currents can occasionally push past four knots, and many recreational vessels travel at speeds no more than twice that. Time spent in these currents can significantly impact the speed a vessel makes. It is important, though, to realize exactly what these products are providing.

For both the sea surface temperature and current velocity, forecast information is given at three-hour intervals from an initial time forward 72 hours. This is model generated data, and while the models have become quite sophisticated in capturing the physics of the ocean processes and the interaction between the top of the ocean and the bottom of the atmosphere, and therefore provide reasonably accurate output, they are not perfect.

There will be occasions, particularly when strong storms are moving through an area, or the ocean features are changing more quickly than normal, or that the data is sparse that the models will have difficulty and the forecasts may differ significantly with the actual conditions that end up prevailing. Despite these occasional difficulties, though, on average, the models will do a fairly good job at locating the currents. There is other ocean data available on the Ocean Prediction Center website as well. On the left hand side there is a link to “Ocean Products” which, if clicked, expands to three more links, just the first two of which I will talk about. The “Ocean Forecasts” link simply leads to the same page as the tab described in the previous paragraph. The “Ocean Analysis” link, when clicked opens a new link “SST” and when this link is clicked, it expands to two more links. The “Navy Gulf Stream” link will take the user directly to the U.S. Navy produced analysis of the Gulf Stream. Unlike the model forecast data present above, this product represents the analysis of a professional oceanographer based on several data sources. Keep in mind that the ships that the Navy is interested in are large vessels capable of much higher speeds than most recreational vessels, so this analysis is focused on the larger scale features of the Gulf Stream and often does not indicate the smaller, more subtle eddies that are important to smaller, slower vessels. Still, some useful information can be gleaned from the chart.

The other link under “SST” is “GOES Imagery” and this takes the user to several satellite-derived products for both the Atlantic and the Pacific. Infrared satellites are able to detect sea surface temperatures, but only when there are no clouds blocking the view. Thus, these products are composites based on hourly observations over a five-day period leading up to the current date. For most ocean areas, there will be at least some period of time in a five-day stretch when there are no clouds, but this is not always the case. The temperatures shown are an average of the available data through the five-day period. By clicking on the “Observation Age” and “Observation Numbers” links under each region, users can determine when the most recent observation for a given location was available, and also how many hours during the five-day period yielded useable data for any location.

The satellite data is real data, as opposed to the model data discussed earlier, but it is important to realize that this data (among other data) is used as input for the models. Therefore, if there are not very many observations (or none at all) for a given region, it stands to reason that the model forecasts moving forward are going to have a higher error rate, and the model output should be used with caution when this situation occurs.

More extended periods of cloudiness are likely during the winter season when storms are larger and more frequent. If you will be voyaging in or near any major ocean currents, it is a good idea to become familiar with these resources which can aid in planning your passage. Private oceanographic consultants can also offer more expertise in passage planning by applying the available data to specific situations and coming up with recommendations.

By Ocean Navigator