Minima at Mallacoota and Gabo Island

On a Weather Forum in which I post I wrote on 13 May:

 The BoM site is showing a minimum of 6.8C at 0640 - and may get a little lower. The surprising value is the minimum for Gabo Island at 11.2C at 0557. I know Gabo is surrounded by relatively warm sea but that seems a very large difference over such a short distance especially with wind at all sites coming from WNW.

One response to this was 

Quite similar thing happens here, I'm 2km from the coast and the min was 6.9°C morning (identical to yesterday's), yet the min at Bellambi (6km NNE and right on coast) was 12.0°C with a land breeze all night. The ocean does make a big difference to minimums.

So I thought I should investigate a bit further.  Using the excellent Climate Data Online facility at BoM I got a year's minimum records (taking 2019 as sufficient for my needs) for the two sites (Mallacoota # 084084, hereafter Airport, and Gabo Island #084016, hereafter Gabo) to see how they compared.

I omitted any days on which  either site didn't report a minimum giving me 330 records.  There were 35 null values for Gabo one of which was also blank for the Airport.  The correlation coefficient between the two series was strong at 0.935.  As would be expected from that a scatterplot of the two series shows a good significant trend.

Very few of the observations were identical.  The average difference (Mallacoota-Gabo) was -1.8^oC.  Disregarding sign the average difference is  2.0 ^oC.

The great majority of the differences are between +1^oC.  and -4^oC.   They are summarised in this chart, with the red-hot snowflake indicating the approximate position of the reading of 13 May.  So it was a relatively large negative difference!

It is still a little surprising that there are such differences between two sites within ~16.5 km.  The airport site is 200 m from the sea and 22 m elevation and I suspect the Gabo site is a similar distance and 15m elevation.

The wind direction at both sites was similar on the night between 12 and 13 May, mainly WNW with the speed a little higher (~20 kph) at Gabo than the Airport (11 -15 kph).  Perhaps the wind coming across a few kilometres of sea warmed it up?

The only other difference I can think of is that there some buildings on Gabo - but I think ~50 m from the screen.  At the airport the screen is in an open paddock (covered with mown heath in 2019) with no nearby buildings and the nearest substantial vegetation over 100 m away.