Seelye Martin

Surface air temperature and its changes over the past 150 years

We review the surface air temperature record of the past 150 years, considering the homogeneity of the basic data and the standard errors of estimation of the average hemispheric and global estimates. We present global fields of surface temperature change over the two 20-year periods of greatest warming this century, 1925–1944 and 1978–1997. Over these periods, global temperatures rose by 0.37° and 0.32°C, respectively. The twentieth-century warming has been accompanied by a decrease in those areas of the world affected by exceptionally cool temperatures and to a lesser extent by increases in areas affected by exceptionally warm temperatures. In recent decades there have been much greater increases in night minimum temperatures than in day maximum temperatures, so that over 1950–1993 the diurnal temperature range has decreased by 0.08°C per decade. We discuss the recent divergence of surface and satellite temperature measurements of the lower troposphere and consider the last 150 years in the context of the last millennium. We then provide a globally complete absolute surface air temperature climatology on a 1° × 1° grid. This is primarily based on data for 1961–1990. Extensive interpolation had to be undertaken over both polar regions and in a few other regions where basic data are scarce, but we believe the climatology is the most consistent and reliable of absolute surface air temperature conditions over the world. The climatology indicates that the annual average surface temperature of the world is 14.0°C (14.6°C in the Northern Hemisphere (NH) and 13.4°C for the Southern Hemisphere). The annual cycle of global mean temperatures follows that of the land-dominated NH, with a maximum in July of 15.9°C and a minimum in January of 12.2°C.

Variations in Surface Air Temperature Observations in the Arctic, 1979-97

The statistics of surface air temperature observations obtained from buoys, manned drifting stations, and meteorological land stations in the Arctic during 1979‐97 are analyzed. Although the basic statistics agree with what has been published in various climatologies, the seasonal correlation length scales between the observations are shorter than the annual correlation length scales, especially during summer when the inhomogeneity between the ice-covered ocean and the land is most apparent. During autumn, winter, and spring, the monthly mean correlation length scales are approximately constant at about 1000 km; during summer, the length scales are much shorter, that is, as low as 300 km. These revised scales are particularly important in the optimal interpolation of data on surface air temperature (SAT) and are used in the analysis of an improved SAT dataset called International Arctic Buoy Programme/Polar Exchange at the Sea Surface (IABP/POLES). Compared to observations from land stations and the Russian North Pole drift stations, the IABP/POLES dataset has higher correlations and lower rms errors than previous SAT fields and provides better temperature estimates, especially during summer in the marginal ice zones. In addition, the revised correlation length scales allow data taken at interior land stations to be included in the optimal interpretation analysis without introducing land biases to grid points over the ocean. The new analysis provides 12-h fields of air temperatures on a 100-km rectangular grid for all land and ocean areas of the Arctic region for the years 1979‐97. The IABP/POLES dataset is then used to study spatial and temporal variations in SAT. This dataset shows that on average melt begins in the marginal seas by the first week of June and advances rapidly over the Arctic Ocean, reaching the pole by 19 June, 2 weeks later. Freeze begins at the pole on 16 August, and the freeze isotherm advances more slowly than the melt isotherm. Freeze returns to the marginal seas a month later than at the pole, on 21 September. Near the North Pole, the melt season length is about 58 days, while near the margin, the melt season is about 100 days. A trend of 118C (decade)21 is found during winter in the eastern Arctic Ocean, but a trend of 218C (decade)21 is found in the western Arctic Ocean. During spring, almost the entire Arctic shows significant warming trends. In the eastern Arctic Ocean this warming is as much as 28C (decade)21. The spring warming is associated with a trend toward a lengthening of the melt season in the eastern Arctic. The western Arctic, however, shows a slight shortening of the melt season. These changes in surface air temperature over the Arctic Ocean are related to the Arctic Oscillation, which accounts for more than half of the surface air temperature trends over Alaska, Eurasia, and the eastern Arctic Ocean but less than half in the western Arctic Ocean.

The contribution of Alaskan, Siberian, and Canadian coastal polynyas to the cold halocline layer of the Arctic Ocean

Numerous Arctic Ocean circulation and geochemical studies suggest that ice growth in polynyas over the Alaskan, Siberian, and Canadian continental shelves is a source of cold, saline water which contributes to the maintenance of the Arctic Ocean halocline. The purpose of this study is to estimate for the 1978–1987 winters the contributions of Arctic coastal polynyas to the cold halocline layer of the Arctic Ocean. The study uses a combination of satellite, oceanographic, and weather data to calculate the brine fluxes from the polynyas; then an oceanic box model is used to calculate their contributions to the cold halocline layer of the Arctic Ocean. This study complements and corrects a previous study of dense water production by coastal polynyas in the Barents, Kara, and Laptev Seas. Recurrent polynyas form on the Canadian and Alaskan coasts from Banks Island to the Bering Strait and on the Siberian coast from the Bering Strait to the New Siberian Islands. In the Bering Sea, polynyas form in Norton Sound, south and west of St. Lawrence Island, and in the Gulf of Anadyr. Two regions that account for almost 50% of the total dense water production are the Siberian coastal polynyas in the adjacent regions of the Gulf of Anadyr and Anadyr Strait and the Alaskan coastal polynyas which occur along the coast from Cape Lisburne to Point Barrow. For all of the western Arctic coastal regions examined, the mean annual total brine flux is 0.5±0.2 Sv. Combination of this flux with the contribution from the Barents, Kara, and Laptev Seas, which is recalculated from data in the earlier study, shows that over the entire Arctic, coastal polynyas generate about 0.7–1.2 Sv of dense water. This compares well with the theoretical estimates of 1–1.5 Sv. Because an unknown fraction of the Barents, Kara, and Laptev brine flux must go to the Eurasian Basin deep water, the coastal polynyas alone cannot renew the halocline layer. Other potential brine generation mechanisms include overall freezing on the shelves and the response of the ice to infrequent violent storms. For example, during February 1982 an intense storm generated a large region of low ice concentration in the eastern Chukchi Sea over Barrow Canyon. The refreezing of the region was followed by the flow of a dense plume down Barrow Canyon. Although the ocean dynamical response to this refreezing needs to be established, the possible response of a Barrow Canyon flow to this refreezing event suggests that the overall refreezing in response to infrequent violent storms may be a potential source of the additional brine needed to maintain the Arctic Ocean halocline.

The production of ice and dense shelf water in the Okhotsk Sea polynyas

This paper examines the ice and dense water production in the Okhotsk Sea coastal polynyas for the 1990–1995 winters. The dominant polynyas occur on the northwest and northern shelves and in Shelikhov Bay. We use an algorithm developed for the special sensor microwave/imager (SSM/I) to derive for each polynya the area and composition of thin ice and open water and a heat flux algorithm to derive the ice and brine production. Historic oceanographic observations show that the northwest shelf is the only North Pacific region where the σϑ = 26.8 potential density surface outcrops to the surface and is also that part of the Okhotsk shelf where the densest water is observed to occur. In support of these observations, we find that the northwest shelf polynya is the dominant ice and brine producer, contributing on average about 55% of the total production. Shelikhov Bay is the second largest producer with about 25% of the total; this region has been previously neglected by both oceanographic and remote sensing studies. Using a combination of two dense water production models, we find that the 6 year average dense water production lies between 0.2–0.4 Sv. The ice and brine production for the dominant northwest shelf vary interannually by a factor of 3, while the production from all the northern polynyas varies by factor of 2. The source of the variability for the northwest shelf comes from the fact that the southwest-to-northeast trend of the coastline and the mean winter geostrophic wind velocities are roughly parallel, which means that small variations in the wind direction yield large changes in the ice production.

A laboratory and theoretical study of the boundary layer adjacent to a vertical melting ice wall in salt water

In an experimental and theoretical study we model the convection generated in the polar oceans when a fresh-water ice wall melts in salt water of uniform far-field temperature T ∞ , and salinity S ∞ . Our laboratory results show that there are three different flow regimes which depend on T ∞ and S ∞ . First, when T ∞ and S ∞ lie between the maximum density curve and the freezing curve, the flow is only upward. Secondly, for the oceanic case 30 [les ] S ∞ [les ] 35‰ and T ∞ S ∞ but for T ∞ > 20°C, the flow reverses: at the top of the ice there is a laminar bidirectional flow above a downward turbulent flow. To model the turbulent upward flow theoretically, we numerically solve the governing equations in similarity form with a spatially varying eddy diffusivity that depends on the density difference between the ice-water interface and the far-field. The laboratory data then allows us to evaluate the dependence of eddy diffusivity on T ∞ and S ∞ . The results show that the magnitude of the eddy diffusivity is of the same order as the molecular viscosity and that both mass injection at the interface and opposed buoyancy forces must be included in a realistic flow model. Finally, we use an integral approach to predict the far-field conditions that yield the high-temperature flow reversal and obtain a result consistent with our observations.

Estimation of the thin ice thickness and heat flux for the Chukchi Sea Alaskan coast polynya from Special Sensor Microwave/Imager data, 1990-2001

[1] One of the largest Arctic polynyas occurs along the Alaskan coast of the Chukchi Sea between Cape Lisburne and Point Barrow. For this polynya, a new thin ice thickness algorithm is described that uses the ratio of the vertically and horizontally polarized Special Sensor Microwave/Imager (SSM/I) 37-GHz channels to retrieve the distribution of thicknesses and heat fluxes at a 25-km resolution. Comparison with clear-sky advanced very high resolution radiometer data shows that the SSM/I thicknesses and heat fluxes are valid for ice thicknesses less than 10–20 cm, and comparison with several synthetic aperture radar (SAR) images shows that the 10-cm ice SSM/I ice thickness contour approximately follows the SAR polynya edge. For the twelve winters of 1990–2001, the ice thicknesses and heat fluxes within the polynya are estimated from daily SSM/I data, then compared with field data and with estimates from other investigations. The results show the following: First, our calculated heat losses are consistent with 2 years of over-winter salinity and temperature field data. Second, comparison with other numerical and satellite estimates of the ice production shows that although our ice production per unit area is smaller, our polynya areas are larger, so that our ice production estimates are of the same order. Because our salinity forcing occurs over a larger area than in the other models, the oceanic response associated with our forcing will be modified. INDEX TERMS: 4540 Oceanography: Physical: Ice mechanics and air/sea/ice exchange processes; 3360 Meteorology and Atmospheric Dynamics: Remote sensing; 4504 Oceanography: Physical: Air/sea interactions (0312); 4572 Oceanography: Physical: Upper ocean processes; 4207 Oceanography: General: Arctic and Antarctic oceanography; KEYWORDS: Chukchi Sea, coastal polynya, remote sensing

Variability and trends in sea ice extent and ice production in the Ross Sea

the trend in ice production. The increase in brine rejection in the Ross Shelf Polynya associated with the estimated increase with the ice production, however, is not consistent with the reported Ross Sea salinity decrease. The locally generated sea ice enhancement of Ross Sea salinity may be offset by an increase of relatively low salinity of the water advected into the region from the Amundsen Sea, a consequence of increased precipitation and regional glacial ice melt.

A simple model study of the Arctic Ocean freshwater balance, 1979-1985

The freshwater budget of the Arctic Ocean from autumn 1979 to autumn 1985 is examined using a simple ice-ocean model. The ice model, described in detail by Thomas et al. [this issue], uses data from drifting buoys to determine the velocity field and data from passive microwave satellites to determine the concentration field. The resulting fluxes of momentum and salt are then used to drive the ocean model. The model “grid” consists of seven broad regional cells in which we compute average quantities such as salinity profiles. The domain extends down to 200 m depth. The results indicate that the interannual variability of mixed layer salinity (MLS) is greater in the western Arctic than in the eastern Arctic. The interannual variability of MLS in the Arctic Ocean as a whole is quite small in this simulation but is still as large or larger than the trend predicted by Manabe et al. [1991] due to increasing atmospheric CO2. The results of our freshwater analysis indicate that there was an increase of 45% above the mean in the freshwater export through Fram Strait during 1982. This increase occurred in both the sea ice and ocean components, although mostly in the former. A decrease in the freshwater outflow through the Canadian Archipelago (which generally constitutes about 34% of the total) occurred at about the same time. Finally, a simple experiment was run in which river and precipitation fluxes were increased to the level predicted by Manabe et al. [1991] by the end of a 100-year increased CO2 simulation. The resulting increase in oceanic freshwater flux through the Canadian Archipelago is about 60% more than that through the Fram Strait, which might have an influence on the preferred location for deep water formation in a warmer climate.

Dense water production on the northern Okhotsk shelves: Comparison of ship‐based spring‐summer observations for 1996 and 1997 with satellite observations

For the northern Okhotsk Sea polynyas, five Russian CTD surveys taken during 1995 to 1997 are used to examine the evolution of the polynya dense water. The surveys show that consistent with other investigations, the largest potential densities are 26.99 σθ, and the densest water occurs in Sakhalin and Shelikhov Bays. The surveys also suggest that the Shelikhov water drains directly into the deep Okhotsk, while on the northern shelves, gravity currents transport the dense water west to Sakhalin Bay. For comparison, determination of the polynya sizes and ice production from satellite passive microwave and meteorological observations shows that polynyas occur on the northwest shelf (NWS) between Ayan and Okhotsk City, on the northern shelf between Okhotsk City and Magadan, and in Shelikhov Bay. In contrast, the observations show that Sakhalin Bay is a region of land fast ice with no polynyas, so that the dense water observed here cannot form locally. For all polynyas the satellite observations show that the NWS contributes 60 to 70% of the total ice production, and due to the warmer 1997 air temperatures, the 1996 production is about 1.5 times the 1997 value. An estimate of the ice production from the surveys shows a similar regional distribution and enhancement of the 1996 production, with the satellite and ship estimates in agreement within their error bars. Finally, analysis of the dense water outflow shows that the upper Okhotsk Sea Mode Water has a renewal time of about 4 years; the lower part, about 14 years.

A laboratory study of frost flower growth on the surface of young sea ice

Frost flowers are fragile ice crystals containing salt which grow to a height of 10–30 mm on the surface of young sea ice. Such flowers are observed all over the Arctic. The importance of the flowers and their accompanying slush layer is that they provide a rapid way to change the surface albedo and increase the surface roughness of young sea ice. This paper describes a laboratory technique for growing frost flowers and the physical processes which accompany the growth. The study was carried out in a saltwater tank located in a cold room. To grow frost flowers, we alternately cool the surface of the growing sea ice with a fan, then supply it with water vapor from a vaporizer. For these conditions and a room temperature of −22°C, the frost flowers begin to grow when the ice thickness reaches 5–8 mm. The flowers form at random locations on the ice and grow vertically to a height of 10–15 mm while spreading laterally from their original sites. Beneath the flowers, the surface is initially dry; then as the flowers spread laterally, a high-salinity slush layer forms beneath them. This layer, which forms only under the flowers, grows to a thickness of 5 mm in 48 hours and has a characteristic lateral scale of 100–200 mm. The salinity of the slush layer is about 80 psu, compared with a frost flower salinity of 100 psu. Within 24 hours of their appearance, the flowers grow to cover 75–90% of the surface. A surface water budget for the flowers and slush layer shows that most of the water in the flowers and slush layer comes from the ice interior, not from the vaporizer. This implies that an external vapor source may be important in determining the initial growth of the flowers but not in their subsequent development.