Suzanne Dickinson

Ocean currents evident in satellite wind data

Satellite-mounted radar scatterometers designed to quantify surface winds over the ocean actually measure the relative motion between the air and the ocean surface. Estimates of the wind stress from conventional surface wind measurements are usually derived neglecting ocean currents. However, when the relative motion is used, the differences in the estimated stress can be as large as 50% near the equator and may even reverse sign during an El Nino. This assertion is supported by the strong relationship between the surface currents measured by the Tropical Atmosphere-Ocean (TAO) array in the Pacific Ocean and the differences between the winds estimated from scatterometer data and those measured by TAO anemometers. The fact that the scatterometer measures relative motion, and not wind alone, makes scatterometer-derived stress a more accurate representation of the boundary condition needed for both atmospheric and oceanic models than stress fields derived neglecting ocean currents.

The Signature of the Midlatitude Tropospheric Storm Tracks in the Surface Winds

Abstract Storm-track analysis is applied to the meridional winds at 10 m and 850 hPa for the winters of 1999–2006. The analysis is focused on the North Atlantic and North Pacific Ocean basins and the Southern Ocean spanning the region south of the Indian Ocean. The spatial patterns that emerge from the analysis of the 850-hPa winds are the typical free-tropospheric storm tracks. The spatial patterns that emerge from the analysis of the surface winds differ from the free-tropospheric storm tracks. The spatial differences between the surface and free-tropospheric storm tracks can be explained by the influence of the spatial variability in the instability of the atmospheric boundary layer. Strongly unstable boundary layers allow greater downward mixing of free-tropospheric momentum (momentum mixing), and this may be the cause of the stronger surface storm tracks in regions with greater instability in the time mean. Principal component analysis suggests that the basin-scale variability that is reflected in th...

NSCAT tropical wind stress maps: Implications for improving ocean modeling

Using wind vectors from the NASA scatterometer (NSCAT), daily maps of pseudostress have been constructed for the tropical Pacific Ocean and compared with pseudostress maps derived from the European Centre for Medium-Range Weather Forecasts (ECMWF) 10-m wind product. The map resolution for the NSCAT pseudostress maps was selected using both a statistical measure of the expected mapping errors and tests on realistic wind fields. The selected map resolution is 5 days and 2°, which minimizes residual effects from the NSCAT sampling pattern, while maximizing temporal and spatial resolution. Comparisons with the ECMWF maps showed significant differences in most regions, corresponding to mean wind speeds of 2–3 m s−1, particularly in the Intertropical Convergence Zone (ITCZ) and at 20°S and 20°N. A canonical correlation analysis between NSCAT and ECMWF fields showed a high degree of correlation of temporal variations and systematic differences in spatial structure. In the NSCAT fields the ITCZ is narrower, stronger, and is located 1–2° latitude farther south than in the ECMWF fields. The high degree of correlation between the two fields suggests that “hybrid” fields can be generated. The dynamical implications of the differences in wind forcing are illustrated using estimates of the Sverdrup stream function and the Ekman pumping. A simple reduced-gravity, linear vorticity model, forced by both the ECMWF and hybrid winds to examine predicted differences in ocean response, showed higher skill for the NSCAT winds.

Thermometry in high magnetic fields at low temperatures

We have investigated the behavior of carbon resistor and capacitive thermometers from 4.2 to below 0.1 K and to 19 T. We observe, in agreement with previous work at higher temperatures, a nonmonotonic magnetoresistance in a standard carbon resistor for thermometry below 1.0 K. We provide the reader with an expression for the magnetoresistance which may be used, with proper precalibration for a particular sensor, to determine the temperature from a reading of the resistance and the magnetic field. In addition, we discuss epoxy mixing chambers for dilution refrigerators and discuss results which indicate that certain materials can be magnetized and demagnetized at low temperatures. Finally, we report measurements on two types of capacitive temperature sensors which also exhibit magnetoeffects.

Seasonal ice loss in the Beaufort Sea: Toward synchrony and prediction

The seasonal evolution of sea ice loss in the Beaufort Sea during 1979–2012 is examined, focusing on differences between eastern and western sectors. Two stages in ice loss are identified: the Day of Opening (DOO) is defined as the spring decrease in ice concentration from its winter maximum below a value of 0.8 areal concentration; the Day of Retreat (DOR) is the summer decrease below 0.15 concentration. We consider three aspects of the subject, i.e., (i) the long-term mean, (ii) long-term linear trends, and (iii) interannual variability. We find that in the mean, DOO occurs earliest in the eastern Beaufort Sea (EBS) owing to easterly winds which act to thin the ice there, relative to the western Beaufort Sea (WBS) where ice has been generally thicker. There is no significant long-term trend in EBS DOO, although WBS DOO is in fact trending toward earlier dates. This means that spatial differences in DOO across the Beaufort Sea have been shrinking over the past 33 years, i.e., these dates are becoming more synchronous, a situation which may impact human and marine mammal activity in the area. Retreat dates are also becoming more synchronous, although with no statistical significance over the studied time period. Finally, we find that in any given year, an increase in monthly mean easterly winds of ∼1 m/s during spring is associated with earlier summer DOR of 6–15 days, offering predictive capability with 2–4 months lead time.

Comparisons between the TAO Buoy and NASA Scatterometer Wind Vectors

There was an opportunity to compare 10 months of collocated National Aeronautics and Space Administration scatterometer (NSCAT) wind vectors with those from the Tropical Atmosphere Ocean (TAO) buoy array, located in the tropical Pacific Ocean. Over 5500 data pairs, from nearly 70 buoys, were collocated in the calibration/ validation effort for NSCAT. These data showed that the wind speeds produced from the NSCAT-1 model function were low by about 7%‐8% compared with TAO buoy winds. The revised model function, NSCAT-2, produces wind speeds with a bias of about 1%. The scatterometer directions were within 208 (rms), meeting accuracy requirements, when compared to TAO data. The mean direction bias between the TAO and the NSCAT vectors (regardless of model function) is about 98 with the scatterometer winds to the right of the TAO winds, which may be due to swell. The statistics of the two datasets are discussed, using component biases in lieu of the speed bias, which is naturally skewed. Using ocean currents and buoy winds measured along the equator, it is shown that the scatterometer measures the wind relative to the moving ocean surface. In addition, a systematic effect of rain on the NSCAT wind retrievals is noted. In all analyses presented here, winds less than 3 m s 21 are removed, due to the difficulty in making accurate low wind measurements.

A Study of Near-Surface Winds in Marine Cyclones Using Multiple Satellite Sensors

Abstract Synoptic-scale surface wind fields are developed using data from two Special Sensor Microwave/Imager (SSM/1) passive radiometers and the European Remote Sensing Satellite-1 (ERS-1) scatterometer for the lifetime of two northern Pacific winter storms. The data are merged with surface wind output from a planetary boundary layer (PBL) model. The inputs to the PBL model are from the European Centre for Medium-Range Weather Forecasts (ECMWF) global analyses. A method for interpolating these analyses to the time of the satellite passage, while preserving fundamental storm characteristics, was developed. Composite wind fields are constructed each time one or more of the satellites views the storms. These composite wind fields are compared to the near-surface wind fields from ECMWF analyses within the storm domain and to buoy wind data on a point-by-point basis. Comparison with ECMWF analyses shows that the composite winds are slightly higher and contain more mesoscale variability. Comparison with buoy d...

Comparisons of Scatterometer and TAO Winds Reveal Time-Varying Surface Currents for the Tropical Pacific Ocean*

Abstract The differences between Tropical Atmosphere Ocean (TAO) anemometer and QuikSCAT scatterometer winds are analyzed over a period of 3 yr. Systematic differences are expected owing to ocean currents because the anemometer measures absolute air motion, whereas a radar measures the motion of the air relative to the ocean. Monthly averaged collocated wind differences (CWDs) are compared with available near-surface current data at 15-m depth from drifters, at 25-m depth from acoustic Doppler current profilers (ADCPs), and at 10-m depth from current meters and with geostrophic currents at the surface from the TOPEX/Poseidon radar altimeter. Because direct current observations are so sparse, comparisons are also made with climatological currents from these same sources. Zonal CWDs are in good agreement with the zonal current observations, particularly from 2°S to 2°N where there are strong currents and a robust seasonal cycle, with the altimeter-derived anomalous currents giving the best match. At higher ...

Evolution of summer Arctic sea ice albedo in CCSM4 simulations: Episodic summer snowfall and frozen summers

The albedo of Arctic sea ice is calculated from summertime output of twentieth century Community Climate System Model v.4 (CCSM4) simulations. This is compared with an empirical record based on the generalized observations of the summer albedo progression along with melt onset dates determined from remote sensing. Only the contributions to albedo from ice, snow, and ponds are analyzed; fractional ice area is not considered in this assessment. Key factors dictating summer albedo evolution are the timing and extent of ponding and accumulation of snow. The CCSM4 summer sea ice albedo decline was found, on average, to be less pronounced than either the empirical record or the CLARA-SAL satellite record. The modeled ice albedo does not go as low as the empirical record, nor does the low summer albedo last as long. In the model, certain summers were found to retain snow on sea ice, thus inhibiting ice surface melt and the formation or retention of melt ponds. These “frozen” summers were generally not the summers with the largest spring snow accumulation, but were instead summers that received at least trace snowfall in June or July. When these frozen summers are omitted from the comparison, the model and empirical records are in much better agreement. This suggests that the representation of summer Arctic snowfall events and/or their influence on the sea ice conditions are not well represented in CCSM4 integrations, providing a target for future model development work.

The phenology of Arctic Ocean surface warming

Abstract In this work, we explore the seasonal relationships (i.e., the phenology) between sea ice retreat, sea surface temperature (SST), and atmospheric heat fluxes in the Pacific Sector of the Arctic Ocean, using satellite and reanalysis data. We find that where ice retreats early in most years, maximum summertime SSTs are usually warmer, relative to areas with later retreat. For any particular year, we find that anomalously early ice retreat generally leads to anomalously warm SSTs. However, this relationship is weak in the Chukchi Sea, where ocean advection plays a large role. It is also weak where retreat in a particular year happens earlier than usual, but still relatively late in the season, primarily because atmospheric heat fluxes are weak at that time. This result helps to explain the very different ocean warming responses found in two recent years with extreme ice retreat, 2007 and 2012. We also find that the timing of ice retreat impacts the date of maximum SST, owing to a change in the ocean surface buoyancy and momentum forcing that occurs in early August that we term the Late Summer Transition (LST). After the LST, enhanced mixing of the upper ocean leads to cooling of the ocean surface even while atmospheric heat fluxes are still weakly downward. Our results indicate that in the near‐term, earlier ice retreat is likely to cause enhanced ocean surface warming in much of the Arctic Ocean, although not where ice retreat still occurs late in the season.