Sea Breezes

Thou the sound of “light sea breezes” sounds rather nice to me, when cruising or staying at the beach cottage, it is a term that slightly concerns me with a 300nm race to be completed.

As we all know a sea breeze is in very common terms the breeze caused by the difference in temperature when the land and adjacent sea is heated. The land (in a theoretical world) will heat quicker than the sea. Once heated the land heats the air in contact with it, which will rise and be replaced by air which rushes in from offshore. The reverse will happen at night, the sea does not cool rapidly due to the specific heat capacity of water (think tomatoes in your toasted sandwich) as the land. Air then flows from the land to the sea.

Source: Adapted from graphic by National Oceanic Atmospheric Administration, Jet Stream.

The above description is very much sea breeze 101. In the real/non-physics world, the sea/land breeze circulations are affected by a number of other meteorological parameters including: 

• Soil moisture, increased soil moisture has been associated by decreased sea breeze frequency and strength.
• Sea surface temperature, lower sea surface temperature creates a larger temperature differential and therefore stronger sea breezes.
• Overall stability of the lower atmosphere. Very strong subsidence under high pressure can prevent air from rising, thus preventing the formation of cells. 
• Land topography, the greater the elevation, the cooler it is, so  less heating that occurs.

The Auckland/Hauraki Gulf sea breeze has been well described by various researchers , however it is a somewhat of a difficult  due to  local breezes from three different harbours interacting to form the final wind field.

Sea breezes in the Hauraki Gulf area are generally found in the summer and early autumn months with fine days and weak synoptic flows.  Their occurrence is correlated to the Southern Oscillation Index (SOI), an index used to quantify the El Niño/La Niña circulations of the Pacific Ocean. In periods of SOI close to zero sea breezes develop up to one day in five in October-December and March-May and once every three days in January-February. In periods of larger SOI the frequency is less due to the increase in synoptic wind, however sea breeze development has been observed one day in seven between October-May during such periods

In the absence of a geostrophic wind, immature sea breezes or ‘bay breezes’ tend to develop in the mornings reaching speeds of 5-8 knots by 11 am. These bay breezes are small areas of wind that move onshore in a variety of directions from ‘bays’ to the nearest land. The directions of the Auckland bay breezes are shown below.

Map showing elementary development of sea breeze in the absence of synoptic flow - source Mc Gill (1987)

In the early afternoon the bay breezes may mature in a true sea breeze with peaks wind speeds of between 15 to 26 knots. The direction of the wind of the mature breeze is dependent on the synoptic situation. If there is no geostrophic wind present at the time of sea breeze development, a convergence zone forms in the middle of the Auckland Isthmus and a small heat low develops in the south Auckland area as shown below.

Map showing mature of sea breeze in the absence of synoptic flow – source Mc Gill(1987)

If the prevailing wind is from the southwest the convergence zone is formed in a similar area to the no wind situation and then pushed east, and a large heat low forms over the central North Island thereby leading to large variations in wind speed and direction as the sea breeze develops as shown below.

Map showing mature sea breeze in south west flow – source Mc Gill (1987)

In north east flow conditions the sea breeze convergence zone is displaced towards the west as shown below.

Map showing mature sea breeze in north easterly flow – source Mc Gill (1987)

More tomorrow….

Mc Gill A.J (1987) “Sea breeze circulations about Auckland”:New Zealand Meteorological Service, Scientific Report 29, New Zealand Meteorological Service