One way of defining summer in NZ is calling it the three months December, January and February. By that definition we have just passed the half-way mark. What has the weather been like at your place so far this summer?  

In a recent blog post by Peter Kreft, he reflected on the 2009 winter. I thought I'd do a similar thing but looking at summer so far. In their climate summary, NIWA noted that Dec 2009 was "very sunny in the north", with below normal soil moisture in the Far North, Central North Island and Eastern Bay of Plenty.  

Here is the average pressure pattern for December 2009:  

December 2009 sea level pressure; contours in hPa (same unit as millibar). Images courtesy U.S National Oceanic & Atmospheric Administration Earth System Research Laboratory


And here is the long term average for December, based on over 50 years of December data:  

December average sea level pressure


Note the higher than usual pressure values over the North Island in Dec 2009, together with a stronger anticyclonic flow (there is high pressure inside the 1016 hPa isobar in the top chart).  

In his post Peter referred to the importance of the upper air in driving our weather. So let's also take a look at what the upper air was doing in December. The next chart shows the variation of upper level pressure heights, but you can think of it as showing the areas in the upper atmosphere where the flow was cyclonic and anticyclonic. For example, the yellow area over South Australia was anticyclonic, and the blue area east of the North Island was cyclonic. 

December 2009 upper flows: blue/purple = cyclonic, yellow/red = anticyclonic.

 The long term average in the upper air for December looks like this:  

December average upper flow, colours as previous chart

Not surprisingly, in December last year we saw stronger anticyclonic conditions than usual (shaded light green) across the north Tasman Sea towards the North Island. Also, interestingly, the usually very strong anticyclonic area south of Chatham Islands was much weaker last month. 

You may now want to ask how January is shaping up. Here is the average pressure for the first twelve days:  

1 to 12 January 2010 sea level pressure (contours in Pa rather than hPa, so 101200 Pa = 1012 hPa)

And the long term average for January is:  

January average sea level pressure

So the anticyclonic anomaly over the northern North Island has continued from December into the first part of January at least. The tendency so far this summer towards more anticyclonic conditions, both at the surface of the Earth and in the upper air, has driven the drier than usual weather in the north.