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William L. Chapman
and
John E. Walsh

Feedbacks resulting from the retreat of sea ice and snow contribute to the polar amplification of the greenhouse warming projected by global climate models. A gridded sea-ice database, for which the record length is now approaching four decades for the Arctic and two decades for the Antarctic, is summarized here. The sea-ice fluctuations derived from the dataset are characterized by 1) temporal scales of several seasons to several years and 2) spatial scales of 30°–180° of longitude. The ice data are examined in conjunction with air temperature data for evidence of recent climate change in the polar regions. The arctic sea-ice variations over the past several decades are compatible with the corresponding air temperatures, which show a distinct warming that is strongest over northern land areas during the winter and spring. The temperature trends over the subarctic seas are smaller and even negative in the southern Greenland region. Statistically significant decreases of the summer extent of arctic ice are apparent in the sea-ice data, and new summer minima have been achieved three times in the past 15 years. There is no significant trend of ice extent in the Arctic during winter or in the Antarctic during any season. The seasonal and geographical changes of sea-ice coverage are consistent with the more recent greenhouse experiments performed with coupled atmosphere–ocean models.

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William L. Chapman
and
John E. Walsh

Abstract

Monthly surface air temperatures from land surface stations, automatic weather stations, and ship/buoy observations from the high-latitude Southern Hemisphere are synthesized into gridded analyses at a resolution appropriate for applications ranging from spatial trend analyses to climate change impact assessments. Correlation length scales are used to enhance information content while limiting the spatial extent of influence of the sparse data in the Antarctic region. The correlation length scales are generally largest in summer and over the Antarctic continent, while they are shortest over the winter sea ice. Gridded analyses of temperature anomalies, limited to regions within a correlation length scale of at least one observation, are constructed and validated against observed temperature anomalies in single-station-out experiments. Trends calculated for the 1958–2002 period suggest modest warming over much of the 60°–90°S domain. All seasons show warming, with winter trends being the largest at +0.172°C decade−1 while summer warming rates are only +0.045°C decade−1. The 45-yr temperature trend for the annual means is +0.082°C decade−1 corresponding to a +0.371°C temperature change over the 1958–2002 period of record. Trends computed using these analyses show considerable sensitivity to start and end dates, with trends calculated using start dates prior to 1965 showing overall warming, while those using start dates from 1966 to 1982 show net cooling over the region. Because of the large interannual variability of temperatures over the continental Antarctic, most of the continental trends are not statistically significant. However, the statistically significant warming over the Antarctic Peninsula is the strongest and most seasonally robust in the spatial patterns of temperature change.

Composite (11-model) global climate model (GCM) simulations for 1958–2002 with forcing from historic aerosol and greenhouse gas concentrations show warming patterns and magnitudes similar to the corresponding observed trends for the 45-yr period. GCM projections for the rest of the twenty-first century, however, discontinue the pattern of strongest warming over the Antarctic Peninsula, but instead show the strongest warming over the Antarctic continent.

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John E. Walsh
and
William L. Chapman

Abstract

Because much of the deep water of the world's oceans forms in the high-latitude North Atlantic, the potential climatic leverage of salinity and temperature anomalies in this region is large. Substantial variations of sea ice have accompanied North Atlantic salinity and temperature anomalies, especially the extreme and long-lived “Great Salinity Anomaly” of the late 1960s and early 1970s. Atmospheric pressure data are used hem to show that the local forcing of high-latitude North Atlantic Ocean fluctuations is augmented by antecedent atmospheric circulation anomalies over the central Arctic. These circulation anomalies are consistent with enhanced wind-forcing of thicker, older ice into the Transpolar Drift Stream and an enhanced export of sea ice (fresh water) from the Arctic into the Greenland Sea prior to major episodes of ice severity in the Greenland and Iceland seas. An index of the pressure difference between southern Greenland and the Arctic-Asian coast reached its highest value of the twentieth century during the middle-to-late 1960s, the approximate time of the earliest observation documentation of the Great Salinity Anomaly.

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John E. Walsh
and
William L. Chapman

Abstract

The circulation of the Arctic atmosphere undergoes large fluctuations about its monthly and annual means. The statistics of Arctic sea level pressure and temperature are evaluated in order to place Arctic atmospheric variability into the context of fluctuations elsewhere. The persistence of monthly sea level pressure anomalies in the Arctic is smaller than in the subtropics but greater than in middle latitudes. This persistence is strongest in winter. Air temperature anomalies are less persistent in the Arctic than in lower latitudes, except during the autumn freeze-up season. Monthly Arctic pressure anomalies show a relatively strong association with concurrent anomalies in the North Atlantic, especially during the winter half of the year. Associations with North Pacific anomalies are weak. During the past twenty years, the greatest warming has occurred over Alaska, the North Atlantic marginal ice zone, and north central Asia. Cooling has occurred over much of Europe, especially Scandinavia. The COADS sea surface temperature changes support the pattern of temperature change derived from land station data. The pattern of recent high-latitude temperature change is consistent with and at least partially attributable to corresponding changes in the sea level pressure (gradient wind) field.

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David M. Fratantoni
and
William E. Johns

Abstract

A deep-towed instrument package has been developed to study the velocity and tracer signature of abyssal overflows in the northeastern Caribbean. Primary package components include a conductivity-temperature-depth (CTD) instrument and an acoustic Doppler current profiler (ADCP), allowing for simultaneous measurement of density, watermass tracers, and absolute velocity. A description of package construction and operation is supplemented by examples from a set of 17 deployments during two oceanographic cruises in January 1991 and March 1992. A new method for determining the three-dimensional position of the instrument package is described, based on the ability of the ADCP to acquire reference velocities corresponding to its motion over the seafloor. Factors affecting ADCP data quality are discussed, particularly those stemming from the low-scatterer environment at abyssal depths and the impact of large vertical accelerations due to surface wave-induced ship heave.

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William E. Johns
and
Friedrich Schott

Abstract

Current meter observations were collected from a three-dimensional array moored in the Florida Straits between December 1983 and June 1984 as part of the Subtropical Atlantic Climate Studies (STACS) program. Approximately one-fourth of the total subinertial velocity and temperature variance contained in these records is associated with meandering of the Florida Current on time scales ranging from several days to a few weeks. There approach to be no strong correlation between the occurrence of meanders and variations in Florida Current volume transport or local wind forcing.

Utilizing frequency-domain empirical mode analysis we find the most coherent, energetic meandering signals within two limited frequency bands centered near periods of 12 days and 5 days. These meanders propagate downstream (northward) with phase speeds and wavelengths of approximately (28 km d−1, 340 km) and (36 km d−1, 170 km) respectively. Periodic waveforms composed from these modes indicate an asymmetric meander pattern with wave crests and troughs leading on the eastern side of the Florida Straits. These meanders appear to be giving up significant energy to the mean flow through up-gradient eddy momentum and buoyancy fluxes in the cyclonic shear zone of the Florida Current, with the net energy transfer being generally dominated by barotropic (eddy momentum flux) processes.

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Tiago Carrilho Biló
and
William E. Johns

Abstract

The mean North Atlantic Deep Water (NADW, 1000 < z < 5000 m) circulation and deep western boundary current (DWBC) variability offshore of Abaco, Bahamas, at 26.5°N are investigated from nearly two decades of velocity and hydrographic observations, and outputs from a 30-yr-long eddy-resolving global simulation. Observations at 26.5°N and Argo-derived geostrophic velocities show the presence of a mean Abaco Gyre spanning the NADW layer, consisting of a closed cyclonic circulation between approximately 24° and 30°N and 72° and 77°W. The southward-flowing portion of this gyre (the DWBC) is constrained to within ~150 km of the western boundary with a mean transport of ~30 Sv (1 Sv ≡ 106 m3 s−1). Offshore of the DWBC, the data show a consistent northward recirculation with net transports varying from 6.5 to 16 Sv. Current meter records spanning 2008–17 supported by the numerical simulation indicate that the DWBC transport variability is dominated by two distinct types of fluctuations: 1) periods of 250–280 days that occur regularly throughout the time series and 2) energetic oscillations with periods between 400 and 700 days that occur sporadically every 5–6 years and force the DWBC to meander far offshore for several months. The shorter-period variations are related to DWBC meandering caused by eddies propagating southward along the continental slope at 24°–30°N, while the longer-period oscillations appear to be related to large anticyclonic eddies that slowly propagate northwestward counter to the DWBC flow between ~20° and 26.5°N. Observational and theoretical evidence suggest that these two types of variability might be generated, respectively, by DWBC instability processes and Rossby waves reflecting from the western boundary.

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Hartmut Peters
and
William E. Johns
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Hartmut Peters
and
William E. Johns

Abstract

Turbulence in the Red Sea outflow plume in the western Gulf of Aden was observed with an upward-looking, five-beam, 600-kHz acoustic Doppler current profiler (ADCP). The “Bottom Lander” ADCP was deployed on the seafloor in two narrow, topographically confined outflow channels south of Bab el Mandeb for periods of 18–40 h at three locations at 376-, 496-, and 772-m depths. Two deployments were taken during the winter season of maximum outflow from the Red Sea and two in the summer season of minimum outflow. These short-term observations exhibit red velocity spectra with high-frequency fluctuations of typically a few centimeters per second RMS velocity during strong plume flow as well as strong subtidal variations. In one winter season event, the plume flow was reduced by a factor of 4 over an 18-h time span. In variance-preserving form, velocity spectra show a separation at frequencies of 0.3–3 cycles per hour between low-frequency and high-frequency signals. The latter show significant coherence between horizontal and vertical velocity components; hence they carried turbulent stress. Based on a comparison with velocity spectra from atmospheric mixed-layer observations, the authors argue that large variance at frequencies of the order of 1 cph was possibly associated with bottom-generated, upward-propagating internal waves. One coherent feature that matched such waves was observed directly. Higher frequencies correspond to turbulent motions of energy-carrying scales. The turbulent Reynolds stress at heights above the bottom between 4 and 30–40 m was computed for most of the ADCP observations. Near the bottom, the streamwise turbulent stress and the streamwise velocity followed a quadratic drag law with drag coefficients ranging from 0.002 to 0.008. There was also significant spanwise stress, hinting at the three-dimensional nature of the boundary layer flow. The time–height variations of the stress and its spectrum proved to be complex, one of its most striking features being angles of up to ∼40° between the direction of the stress and that of the low-frequency flow. The turbulent shear production and eddy viscosity were also examined. On the technical side, the paper discusses the role of the fifth, center-beam velocity measurements in correcting for instrument tilt along with the effect of beam spreading in the 30° Janus configuration of the “regular” four ADCP beams. Instrumental noise and detection limits for the stress are also established.

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Silvia Matt
and
William E. Johns

Abstract

The Red Sea outflow exhibits strong seasonal variability in outflow transport due to effects of monsoon winds and seasonal fluctuations in buoyancy forcing. As it descends the continental slope in the western Gulf of Aden, it entrains significantly less-dense near-surface water, which itself varies on seasonal time scales. High-resolution hydrographic and direct velocity data collected during the 2001 Red Sea Outflow Experiment (REDSOX) are used herein to characterize and quantify the pathways of the Red Sea Outflow Water (RSOW) and the associated entrainment of Gulf of Aden Water. The outflow transport exhibits a maximum in winter of about 0.29 Sv (Sv ≡ 106 m3 s−1) at the exit of the Bab-el-Mandeb and approximately doubles to 0.56 Sv as it descends into the Gulf of Aden and entrains ambient water. In summer, the outflow is much weaker, reaching about 0.06 Sv at the strait and about 0.18 Sv downstream. The outflow plume divides into three distinct branches in winter, consisting of descending branches along two bathymetrically confined channels (the “Northern” and “Southern” channels, respectively), and an adjusted intrusion layer at shallower depths in the water column. Estimates of transport of “pure” Red Sea Outflow Water through salt flux conservation show the general partitioning of the outflow between the individual plumes, where the Northern Channel (NC) accounts for 52% of Red Sea Outflow Water, the Southern Channel (SC) carries 31%, and the intrusion layer (IL) the remaining 17%. The results also indicate that the transport of Red Sea Outflow Water is subject to considerable synoptic temporal variability that is unresolved by the present study.

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