The intricacies of sea ice formation

A common misconception about global warming is that it means warming everywhere on the globe. This is an understandable, if too literal, interpretation of the phrase for a non-scientist and is something that is often played upon by less intellectually honest participants in the debate. This is one reason why "climate change" and "climate disruption" are perhaps better descriptors, even if warming is what the global average temerature is and will do.

Given the above, the fact that sea ice in the antarctic has increased slightly since careful mearsurements began (around 1%) is a frequent, if easily debunked, talking point.

But why is the sea ice increasing?

(click here for a larger image in a new window) -- source: National Snow and Ice Data Center

This trend is happening even while atmospheric and oceanic temperatures exhibit warming (recall this is not the same area as the continental Antarctic).

Via an interesting comment on the "Antarctic Seaice is growing" article, a couple of abstracts of papers on this subject have come to my attention which I would like to share.

According to a paper by Jinlun Zhang [large PDF] in the JOURNAL OF CLIMATE Volume 20, Issue 11, there are two competing effects of atmospheric warming and increasing downward longwave radiation (an enhanced greenhouse effect). On one had, the warming from above together with a generally warming ocean below cause the expected reductions in ice formation from above. But on the other hand, this causes a decrease in the upper ocean salinity and an increase in the stratification of ocean layers which then inhibits overturning leading to a slower changing in the heat flux from water to ice. The way it plays out, the rate of decrease in melting from the bottom is faster than the decrease in ice growth from above, resulting in a net increase. Don't forget that these processes are constantly ongoing, sea ice, ice sheets and glaciers are always melting in one way even as they are accumulating in another, thus the rate of growth and its sign are determined by the balance of these processes.

The second paper [PDF] by authors Powell DC, Markus T and Stossel A, in the Journal of Geophysical Research, describes the effects of snowfall on sea ice growth. Again there are complicated and competing effects of increased snowfall (another sign of warming, warmer air can hold more moisture for percipitation). On one side, a thicker snow layer acts as a insulator between ice formation and the very cold atmosphere that would create more of it. Basically, sheltered from the frigid air above, the much warmer ocean water (never below 0oC, right?) is free to melt away at the ice from below. On the other hand, deep and heavy snows contribute to the conversion of snow to ice and on thinner layers of sea ice can supress the ice below sea level, causing flooding that ultimately freezes on top, again contributing to ice formation. The precise balance of these effects depends on just how much snowfall there is.

Fascinating stuff, I think. Just goes to how how complex such a seemingly simple process can be.

Here are the abstracts quoted in full:

Increasing Antarctic sea ice under warming atmospheric and oceanic conditions [PDF]
Author(s): Zhang JL (Zhang, Jinlun)
Source: JOURNAL OF CLIMATE Volume: 20 Issue: 11 Pages: 2515-2529 Published: JUN 1 2007

Abstract: Estimates of sea ice extent based on satellite observations show an increasing Antarctic sea ice cover from 1979 to 2004 even though in situ observations show a prevailing warming trend in both the atmosphere and the ocean. This riddle is explored here using a global multicategory thickness and enthalpy distribution sea ice model coupled to an ocean model. Forced by the NCEP-NCAR reanalysis data, the model simulates an increase of 0.20 x 10(12) m(3) yr(-1) (1.0% yr(-1)) in total Antarctic sea ice volume and 0.084 x 10(12) m(2) yr(-1) (0.6% yr(-1)) in sea ice extent from 1979 to 2004 when the satellite observations show an increase of 0.027 x 10(12) m(2) yr(-1) (0.2% yr(-1)) in sea ice extent during the same period. The model shows that an increase in surface air temperature and downward longwave radiation results in an increase in the upper-ocean temperature and a decrease in sea ice growth, leading to a decrease in salt rejection from ice, in the upper-ocean salinity, and in the upper-ocean density. The reduced salt rejection and upper-ocean density and the enhanced thermohaline stratification tend to suppress convective overturning, leading to a decrease in the upward ocean heat transport and the ocean heat flux available to melt sea ice. The ice melting from ocean heat flux decreases faster than the ice growth does in the weakly stratified Southern Ocean, leading to an increase in the net ice production and hence an increase in ice mass. This mechanism is the main reason why the Antarctic sea ice has increased in spite of warming conditions both above and below during the period 1979-2004 and the extended period 1948-2004.

Effects of snow depth forcing on Southern Ocean sea ice simulations [PDF]
Author(s): Powell DC, Markus T, Stossel A
Source: JOURNAL OF GEOPHYSICAL RESEARCH-OCEANS Volume: 110 Issue: C6 Article Number: C06001 Published: JUN 1 2005

Abstract: [1] The aim of this study is to investigate the competing effects of a snow layer's insulation and snow-ice formation on thermodynamic sea ice thickness growth in response to changes in precipitation. Using optimal interpolation to assimilate Special Sensor Microwave/ Imager satellite-derived snow depths into a dynamic-thermodynamic sea ice model, we create a daily assimilated snow depth product for the years 1992 - 2003. The assimilated snow depths are used to adjust National Centers for Environmental Prediction/ National Center for Atmospheric Research reanalysis precipitation rates which subsequently force the model's snow depths and freshwater input. These adjusted precipitation rates are used to create a best estimate snow depth climatology. This climatology provides the basis for a series of sensitivity experiments. Precipitation rates are varied from 0.0 to a doubling of the present day precipitation. Initially, sea ice volume decreases with increasing precipitation rate multiplying factor (PRMF) because of the insulation effects of a deeper snow layer. The turning point at which the insulation effect becomes balanced by the snow to ice conversion effect ranges from PRMF = 0.50 to PRMF = 0.75, depending upon the snow thermal conductivity and density. This suggests that with present-day precipitation rates the snow effect on Southern Ocean sea ice is dominated by snow-ice formation rather than the snow's insulation.

Thanks again to streamtracker.


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You lost a close tag, and have runaway micro-font.

By Physicalist (not verified) on 06 Aug 2008 #permalink

"never below 0oC, right?"

Well, not much below it for salt water ;)

[Indeed! Perhpas I should have been more precise and said "never below the freezing point" : ) coby]