The Bering Sea: A Puzzle of Wind Speed
The 12Z surface analysis over the North Pacific basin caught my eye yesterday morning (January 20). That's because there was a 1055 mb high centered just to the north of Wrangle Island over the Arctic Ocean (between the Chukchi Sea and East Siberian Sea) and two lows, a 973-mb cyclone over the Bering Sea and a 965-mb low centered over the northern Gulf of Alaska. Check out the cropped 12Z surface analysis below; here's a political map to get your bearings (I circled the general region of interest).
A portion of the 12Z surface analysis over the North Pacific basin on January 20, 2013. Alaska is on the right. Full analysis. Courtesy of the Ocean Prediction Center.
Needless to say, there was an impressive pressure gradient across the region, particularly over the Bering Sea. Before I show you the wind analyses over this region, check out the GFS model analysis of mean sea-level isobars on a North-Pole view (a different map projection than the OPC surface analysis above). I hope that you agree that the GFS had a fairly good handle of the pressure pattern over the region at 12Z.
I decided to look at the wind analyses at 100 meters, an altitude where friction is a bit less important compared to the earth's surface. At any rate, the 12Z GFS analysis of isotachs and streamlines at 100 meters (below; larger image) shows that easterly winds had speeds greater than 50 knots (but less than 60 knots) over the Bering Sea. This observation might be a bit surprising, given the impressive pressure gradient over the Bering Sea and the fact that there were stronger winds father south on the back side of the 965-mb low over the Gulf of Alaska, where the pressure gradient was not quite as large (revisit the 12Z GFS model analysis of MSL isobars).
The 12Z GFS model analysis of isotachs (color-filled in knots) and streamlines at 100 meters. Larger image. Courtesy of Penn State.
What's up with that? Let's assume that, at 100 meters, friction is not as important...so let's neglect it (as a general rule, the effects of friction typically vanish by 500 mb, where the standard height is 5500 meters). Our assumption of neglecting friction means that there are essentially two forces at work here: the pressure gradient and the Coriolis forces.
I'll write a blog about the Coriolis force (Coriolis effect) one day, but, for now, I'll just emphasize that it acts 90 degrees to the right of a moving air parcel in the Northern Hemisphere. Moreover, the magnitude of the Coriolis force depends on latitude (more precisely, the sine of latitude) and the speed at which an air parcel is moving (mathematical representation).
I'll bet you never thought you'd see math in a Weather Underground blog, but meteorology is a science, and mathematics is the language of science.
Okay, after the initial shock has worn off, check out this idealized set-up in the east-west direction (I'll keep this as simple as I can, but I'd also have to tackle the north-south direction in order to complete the analysis). The black curve represents a portion of the local isobar (or height contour). The pressure-gradient force is indicated by the blue vector pointing to your left (toward lower pressure...the red "L"), and its magnitude depends on density (the Greek letter rho) as well as the variation of pressure, p, in the east-west direction (along the x axis). The blue vector pointing to the right represents the Coriolis force. Along the x axis, its magnitude depends on the north-south component of the geostrophic wind (note the subscript, g), which is the idealized wind that results from the balance of the pressure-gradient and Coriolis forces).
Now let's apply what we've learned to yesterday's 12Z surface analysis over the Bering Sea and northern Gulf of Alaska. Why were wind speeds at 100 meters over the Bering Sea, where there was a dramatically tight packing of isobars (large pressure-gradient force) noticeably lower than the wind speeds in the wake of the low-pressure system over the northern Gulf of Alaska (where the pressure-gradient force was a bit weaker)?
Recall that the magnitude of the Coriolis force depends on the sine of the latitude, which increases with increasing latitude. Specifically, the sine of latitude is larger over the Bering Sea than it is over the northern Gulf of Alaska. Thus, even though the pressure gradient is larger over the Bering Sea, the larger value of the sine of latitude over the Bering Sea allows the magnitude of the Coriolis force to balance the pressure-gradient force at a somewhat lower wind speed. I'm not saying that the 50-knot wind over the Bering Sea (as shown on the GFS model analysis) is nothing to sneeze at, but it's perhaps not as fast as you might initially think after looking at the pressure gradient. Indeed, if you look closely at OPC's 12Z surface analysis, you can see wind speeds that belie the locally strong pressure gradient (I can make out two observations of 20 knots and 25 knots).
Here endeth the lesson.