Archive for November 7th, 2009

Winds and Waves on the Eastern North Atlantic

Saturday, November 7th, 2009

Drake is heading into the strongest northward currents along the western side of the clockwise rotating eddy centered near 51 W.  He is having no trouble maintaining the sampling line, running straight along 26.5 N.  In the image below, we have increased the scale of the current vectors by a factor of 3 - instead of the white vectors representing the equivalent drift over 1 day, the green vectors represent the equivalent drift over 3 days.  Even though the currents reported by Drake are the average currents over the upper 1200 m of the ocean, the directions are in excellent agreement with the geostrophic surface currents calculated from the sea surface height measured bythe satellite altimeters as shown on the map.  The current magnitude observed by Drake is a steady 4 cm/sec.


On the other side of the Atlantic, Scarlet  is reporting stronger currents than expected from the sea surface height maps.  Scarlet's currents are averaged over the top 150 m.  Today the peak reached 23 cm/sec.  Here we are showing the white arrows which are equivalent to 1 day of drift. The strong currents to the northeast are moving us along a more northerly path than expected.  To compensate, we'll point Scarlet to the southeast.  The angle between the current vector and Scarlet's own velocity relative to the water is about 60 degrees.  We'll keep that angle about 60 for the night and early morning.  If currents rotate more to the north, we'll increase the angle to around 90.  If they rotate towards the east, we'll decrease it.  We are trying to maintain the track between the thin white path line that spirals into the green circle and the region of high uncertainty indicated by the red circle to our north.  Distance to the Spanish EEZ is down to 127 km.


So why are the surface currents derived from the satellite altimetric measurements of sea surface height in better agreement with the deep currents reported by Drake and the shallow currents reported by Scarlet?  Are the differences just random errors in the altimeter data?  One thing to check is for unresolved eddies in the sea surface height field.  For those we look at satellite sea surface temperature maps.  But when you pop up the SST for Scarlet, you get patchy data.  Overly the clouds and you see why.  That straight band of clouds running east-west across the Atlantic is not a stable configuration for the atmospheric Jet Streams.  Straight jets like to develop waves that grow rapidly in amplitude.


If we check out the Jet Stream forecast, we see the sine wave that now runs across the full Atlantic basin is indeed associated with the Jet Stream. It llooks like those upper level winds are heading right at Spain.  Nice if you are an airplane heading that way.  Not so good if you are a ship.


Jet Stream forecasters also overlay the cloud images on their forecasts for you so you can see the relation directly, saving you the trouble of importing the image into the universal  Google Earth.


How do the lower level winds look in response?  Are these causing the observed differences between Scarlet's depth averaged currents and the satellite altimeter's horizontally averaged surface currents? We can check out the surface winds from ship reports at Oceanweather.  Strong surface winds from the northeast today.  Ekman theory says we should be piling up water between the Azores and Spain right now, so thats not it.


Strong surface winds also mean strong surface waves.  Again heading to Oceanweather, we see forecast waves off the Spanish coast are running over 5 meters (15 feet).


So lets check the wave buoys at Puertos del Estado.

The Cabo Silleiro buoy is closest. Waves heights are approaching 8 m.  Periods are long, as expected with that nice long fetch, but 8 m is still pretty big waves.  As winter approaches, these big waves become more and more common off Spain.  So we race the winter.


JPL Executive summary for 11/7/2009

Saturday, November 7th, 2009

Executive Summary of 11/07/2009

Northwesterly winds are observed today. Wind amplitude has been decreasing and will reach a minimum later Saturday. NAM forecasts have shown excellent agreement with the buoy measurements. NAM indicates a break increase of winds late Saturday and early Sunday in the northeast direction. Very weak (~ 5 knots) wind conditions are predicted into Sunday in the northeast direction. We continue to get excellent SST images for Thursday. The ensemble forecasted surface current averaged over Thursday shows a good agreement with the HF radar data, although with somewhat weaker amplitude. The outflow from the NY harbor is clearly reproduced by the ensemble model. New images are added comparing the glider profiles of temperature and salinity with the four models. In general, the models are doing well in simulating these vertical profiles. Major discrepancies are also evident with large temperature (over 1 degree) and salinity (over 1 psu) differences. EO-1 satellite overpass will be later today, and the data/images from the onboard Hyperion hyperspectral imager are expected to be available Monday or Tuesday.