SATELLITE VIEWS OF THE ARCTIC OCEAN FRESHWATER BALANCE

摘要

The freshwater balance of the Arctic Ocean involves atmospheric moisture and its movement; precipitation onto and evaporation from the land and ocean surface; runoff from rivers into the ocean; the growth, movement and melt of sea ice; and the movement of freshwater within the ocean itself. Satellite observations have contributed to a surprisingly large portion of these topics, some quite directly and others rather indirectly. Satellites generally cannot see into sea ice or the ocean, so we are left to deduce what we can about the variables we wish to estimate from signals from the surface. On the other hand the atmosphere is seen throughout its depth by satellites, so we can even get some vertical resolution of where water resides in the atmosphere. In this chapter we treat only the Arctic Ocean and its surrounding seas, and not the ocean's drainage basin. For the most part, the satellite data record becomes quite valuable around 1979, and it keeps getting better with more sensors, better coverage, and better production of data sets applicable to the large-scale interests of this book. We begin our consideration with ice extent and ice concentration, the variables with the most accepted and longest satellite record. From them, we can examine seasonal and longer-term changes of the horizontal coverage of ice. Next we face the issue of wanting to know the mass of sea ice, its thickness as well as its coverage. This subject is less developed, but we review promising new capabilities and later in the chapter some results about thickness deduced from concentration observations. This same use of concentration has been extended to some estimates of the ocean freshwater storage and budget. All satellite sensors that provide imagery have the potential to give us ice motion if ice 'features" can somehow be recognized at sequential times and "tracked"; we review ice motion data next. In particular, the ice mass balance of the Arctic Ocean can be described by specifying all sources and sinks of ice, and the ice flux through passages, especially Fram Strait, has been well studied with satellite ice motion data. The atmospheric moisture balance is quite amenable to satellite studies; moisture transports and the net exchange with the surface have been estimated. Because satellites do not see exactly what we want, the thread of the assimilation of satellite data into ice-ocean models runs through this chapter. This technique has much more potential for the book's subject than has yet been realized. This is a book about geophysics, not remote sensing, so this chapter does not stress the latter. However, we have tried to provide a short review of the capability to make each of these measurements and some of the uncertainties and pitfalls. Somewhat arbitrarily, we have put these reviews as introductions to some sections. Some readers may find them unnecessary or a distraction and can surely skip them. Excellent treatments of these issues are given by Massom and Carsey. We conclude our review with a summation of quantitative freshwater balance estimates from satellites, and a brief recapitulation of which variables are already well used for this problem and which variables hold unrealized potential.