1. Storm Surge is the response of the ocean to changing atmospheric pressure and winds.
Inverted Barometer is the theoretical isostatic relationship given by:
For every hPa fall in pressure, sea level rises by 1 cm; and
For every hPa rise in pressure, sea level falls by 1 cm.
Total Residual is storm surge + tidal residual, or equivalently, the sea level signal (smoothed to remove high-frequency oscillations) less forecast tide.
2. Around New Zealand, storm surge is mainly driven by changing atmospheric pressure
and can be approximated by inverted barometer.
3. The reason wind has little effect on storm surge around New Zealand is
that our low pressure systems tend to move quite quickly across our region,
so the wind never stays in one direction for long enough for the ocean to respond.
This is in contrast to places like the North Sea or the Bay of Bengal, where deep
depressions can sit for days or weeks giving high wind from a constant direction that
draws water away from leeward shorelines and piles it up onto windward shorelines.
4. From time to time we get southerly storms
that generate wind setup. The Wahine Storm (Cyclone Giselle) in 1968 was one of these.
Even so, the storm surge rose to only 750 mm, not several m that can occur in the
the North Sea or the Bay of Bengal.
5. From time to time, we experience the effect of huge storms in the Southern Ocean.
These storms can draw water from our east coast region causing water levels to drop inexplicably by
up to 600 mm.
6. We also experience coastal trapped waves (CTWs).
When the pressure falls sharply and
sea level rises in response, the rotation of the Earth (Coriolis effect) can cause a
sort of rebound wave that propagates around both islands.
It's called a Kelvin Wave and it propagates with the land on its left
(in the northern hemisphere Kelvin Waves go in the opposite direction).
You can sometimes see a CTW in the storm surge record, just after an event when storm
surge has risen sharply to over 200 mm in height, then fallen away.
If there is a small trailing wave a day or so later, it is likely to be a CTW.
CTWs are largest at the coast and they decay away to zero out at the continental shelf break.