North-hemisphere ice-shelf melts on summer and grows on winter. I would expect appreciable changes on sea-level between seasons, but sea-level looks equal on winter than on summer.

Why doesn't sea level show seasonality?

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    $\begingroup$ Put an ice-cube in a glass and let it melt. How differs the height of the water in the glass before and after it melts? Exactly, not at all. $\endgroup$ – Erik Jul 9 '19 at 14:54
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    $\begingroup$ @Erik the ice-cube is stored on land in the winter so sea level gets a tiny bit lower. $\endgroup$ – trond hansen Jul 9 '19 at 15:01
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    $\begingroup$ @Erik The volume would actually decrease slightly. Ice is less dense than water. $\endgroup$ – JAB Jul 10 '19 at 2:40
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    $\begingroup$ @JAB The volume of "water + ice" would decrease. The volume of "water" doesn't. The thing you're forgetting is that part of the ice is outside of the water, and it's this part where the extra volume of ice (compared to the volume of the water it froze from) is stored. $\endgroup$ – Luaan Jul 10 '19 at 6:00
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    $\begingroup$ @Universal_learner Ok, this is very important information that should be in the question. The Mediterranian basin is a very different water system from the oceans generally. Because it is so weakly coupled to the sea through the strait of Gibraltar this dampens effects like the tides, etc. See : Variations of the seasonal sea level cycle in southern Europe Also, due to the constriction of the strait the seasonal signals can be significantly phase shifted by up to months (ie : spring/fall extrema vs winter/summer). $\endgroup$ – J... Jul 10 '19 at 16:17

Sea level has a strong seasonal signal. The annual variability is less than the daily changes associated with tidal forcing in most locations, but still can be on the order of 5-10 cm (maximum values about 15 cm).

The causes of the seasonal fluctuations are mostly associated with seasonal changes in wind intensity and patterns, changes in temperature that relate to thermal expansion, and in salinity (haline contraction) and river discharge fluctuations. The annual sea level cycle is only partially related to ice melt and this effect tends to be quite local.

The largest sea level seasonal cycles are associated with areas in the vicinity of large rivers with strong seasonal cycles (e.g., Bay of Bengal). Also, there is a lot of spatial variability in the seasonal cycle with the northern hemisphere having a larger signal (likely caused by the stronger seasonality in wind patterns).

An example of the seasonal cycle can be seen in the monthly data from the Permanent Service for Mean Sea Level (PSMSL) for Woods Hole, MA (USA) (in m offset to avoid negative values in the PSMSL database). The monthly data shows strong seasonal variability and also a clear trend. As the data is monthly averaged, the tidal oscillations are filtered out. Most sea level rise graphs tend to use annual data and thus the seasonal information is not included.

Woods Hole water level PSML

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    $\begingroup$ So this is a frame challenge :) Trust the oceanographer! Thanks for the nice answer btw, accepted. I never thougth on seasonality due to rivers input. $\endgroup$ – user12525 Jul 10 '19 at 9:22
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    $\begingroup$ @Universal_learner It's not really a frame challenge - your question simply contained a false presupposition. $\endgroup$ – J... Jul 10 '19 at 14:41

In the Arctic there is mostly floating sea ice and the mass of this does not change the sea level.

When snow falls on the floating sea ice the weight of the snow is the same as the weight of the sea water it displaces so there will not be any change in sea level from this.

But the snow falling on land during winter is water temporary removed from the sea, so the sea level will drop a little during winter. See seasonal changes in sea level.

In the Antactic winter, the sea level drop will be less as most of the snow will fall on the floating ice and on a narrow band of land ice around Antarctica. This is mainly due to the circumpolar wind around Antarcica where less moist air will blow into the interior of Antarctica.

The tiny bit of snow falling in the central part of Antarctica is called diamond dust, this is tiny flat ice crystalls; they will sublimate before or soon after they reach the ground due to the dry air and low temperature.

  • $\begingroup$ Keep in mind that sea ice changes the salinity of water, which in turn changes its density; so freezing (and remelting) salt water does cause drop (or raise) in sea levels. It's a small effect compared to land ice, though. It shouldn't be forgotten that even land ice has some effect over very long times too - we're still in a rebound from the last ice age, and some places are slowly drifting upwards since the weight of the ice has been removed. $\endgroup$ – Luaan Jul 10 '19 at 6:04
  • $\begingroup$ North hemisphere ice-shelf melting does not change sea level as you notice. If not we'd see some metters of difference I guess. Perhaps I could have asked why does not change if Antarctica stores more ice/snow on winter. $\endgroup$ – user12525 Jul 10 '19 at 9:32

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