Silvia A. Venegas

Atmosphere–Ocean Coupled Variability in the South Atlantic

Abstract The climate variability of the South Atlantic region is determined from 40 yr (1953–92) of Comprehensive Ocean–Atmosphere Data Set monthly sea surface temperature (SST) and sea level pressure (SLP) data using the empirical orthogonal function (EOF) and the singular value decomposition (SVD) analysis methods. The EOF method is applied to each field separately, whereas the SVD method is applied to both fields simultaneously. The significance of the atmosphere–ocean interaction is revealed by a strong resemblance between individual (EOF) and coupled (SVD) modes of SST and SLP. The three leading modes of coupled variability on interannual and interdecadal timescales are discussed in some detail. The first coupled mode, which accounts for 63% of the total square covariance, represents a 14–16-yr period oscillation in the strength of the subtropical anticyclone, accompanied by fluctuations of a north–south dipole structure in the SST. The atmosphere–ocean coupling is strongest during the southern summe...

Decadal climate oscillations in the Arctic: A new feedback loop for atmosphere‐ice‐ocean interactions

A combined complex empirical orthogonal function analysis of 40 years of annual sea ice concentration (SIC) and winter sea level pressure (SLP) data reveals the existence of an approximately 10-year climate cycle in the Arctic and subarctic. The cycle is characterized by a clockwise propagating signal in the SIC anomalies and a standing oscillation in the SLP anomalies, the latter being linked to a fluctuation between the two phases of the North Atlantic Oscillation. To describe the formation and evolution of the SIC and SLP anomalies associated with the cycle, a simple feedback loop is proposed.

Is There a Dominant Timescale of Natural Climate Variability in the Arctic

Abstract A frequency-domain singular value decomposition performed jointly on century-long (1903–94) records of North Atlantic sector sea ice concentration and sea level pressure poleward of 40°N reveals that fluctuations on the interdecadal and quasi-decadal timescales account for a large fraction of the natural climate variability in the Arctic. Four dominant signals, with periods of about 6–7, 9–10, 16–20, and 30–50 yr, are isolated and analyzed. These signals account for about 60%–70% of the variance in their respective frequency bands. All of them appear in the monthly (year-round) data. However, the 9–10-yr oscillation especially stands out as a winter phenomenon. Ice variability in the Greenland, Barents, and Labrador Seas is then linked to coherent atmospheric variations and certain oceanic processes. The Greenland Sea ice variability is largely due to fluctuations in ice export through Fram Strait and to the local wind forcing during winter. It is proposed that variability in the Fram Strait ice ...

Evidence for interannual and interdecadal climate variability in the South Atlantic

A singular value decomposition (SVD) analysis is used to determine the coupled modes of variability of monthly sea surface temperature (SST) and sea level pressure (SLP) data from the South Atlantic region, for the period 1953–1992. We find that the three leading SVD modes respectively account for 63%, 20% and 6% of the total square covariance. The first mode represents an approximately 15-year period oscillation in the strength of the subtropical anticyclone, accompanied by fluctuations of a north-south dipole structure in the SST. It appears to be linked to the global-scale interdecadal (15-year) joint mode in SST and SLP recently studied by Mann and Park. The second mode is characterized by east-west displacements of the anticyclone center, in association with strong 6 to 7-year period fluctuations of SST off the coast of Africa. The third mode is characterized by north-south displacements of the anticyclone and 4-year period fluctuations in the SST in a broad band across the central South Atlantic. This mode is strongly correlated with ENSO.

The Antarctic Circumpolar Wave: A Combination of Two Signals?

Abstract A simplified view of the possible mechanisms behind the Antarctic Circumpolar Wave (ACW) interannual variability is provided by a frequency-domain decomposition of several observed atmospheric and oceanic variables. Two significant interannual signals with different temporal and spatial characteristics are identified in the Southern Ocean, and most of the variance of the ACW in the interannual band can be accounted for by a linear combination of them. The first signal has a period of oscillation of around 3.3 yr and a zonal wavenumber-3 structure across the Southern Ocean. It involves self-sustained fluctuations inherent in the Southern Ocean and driven by coupled air–sea interactions in which the atmosphere and the ocean mutually force one another. This signal is represented by an atmospheric standing oscillation with centers at fixed locations around Antarctica and a propagating oceanic pattern, in which the surface Antarctic Circumpolar Current plays an essential role. The second signal has a ...

An interdecadal climate cycle in the South Atlantic and its links to other ocean basins

A singular value decomposition analysis and a combined complex empirical orthogonal function analysis are performed on 80 years of monthly sea surface temperature (SST) and sea level pressure (SLP) data from the South Atlantic region. The analyses reveal the existence of interdecadal fluctuations in the coupled atmosphere-ocean system with a period of around 20 years. The SST anomalies are observed to propagate anticyclonically around the South Atlantic basin following the subtropical gyre circulation. At the same time, a westward propagation of SLP anomalies across the basin generates changes in the atmospheric circulation that appear to reinforce such SST anomalies through anomalous exchanges of heat. It is thus proposed that the dominant physical processes involved in this interdecadal cycle include the horizontal advection of heat by the ocean currents and changes in the atmosphere-ocean heat fluxes through local air-sea interactions. The global SST and SLP patterns that accompany the different phases of the South Atlantic cycle are also presented. They show similarities with other well-known interdecadal signals observed by several investigators in other ocean basins (e.g., the low-frequency part of the North Atlantic Oscillation). This suggests that the South Atlantic signal described in this study may be a regional aspect of global interdecadal variability.

Sea ice, atmosphere and upper ocean variability in the Weddell Sea, Antarctica

A frequency domain singular value decomposition is performed on 20 years (1979-1998) of monthly sea ice concentration, sea ice drift and sea level pressure data in the Weddell Sea, Antarctica. Interannual oscillations with periods of around 3-4 years are found to dominate the variability in this region. Anomalous atmospheric patterns periodically reach the Weddell Sea from the west and perturb the sea ice circulation and distribution in the Weddell Gyre through changes in the intensity and direction of the climatological winds. Sea ice accumulates in the southeastern Weddell Sea every 3-4 years owing to two atmospherically driven processes: (1) weak ice export to the north due to a weak northward branch of the gyre (driven by weak southerly winds) and (2) large ice import from the east due to a strong East Wind Drift (driven by strong easterly winds along the coast). The opposite situation gives rise to a depletion of sea ice in the same region half a cycle later. Sea ice anomalies are then advected north-northeastward before turning eastward in the gyre circulation. The eastward propagation of ice anomalies along the ice margin accounts for the passage of the Antarctic Circumpolar Wave through the Atlantic sector of the Southern Ocean. A low frequency signal is also detected in the Weddell Sea variations, albeit rather speculatively in this 20-year-long record. Sea ice variability on this timescale appears to be associated with a change in the shape and characteristics of the Weddell Gyre circulation around 1990. This mode of variability implicates feedbacks between the gyre and Weddell Deep Water temperature variations, whose impact is observed near the Maud Rise topographic feature.

South Atlantic Multidecadal Variability in the Climate System Model

Abstract Strong multidecadal variability is detected in a 300-yr integration of the NCAR Climate System Model in the South Atlantic region, through the application of two signal recognition techniques: the multitaper method and singular spectrum analysis. Significant oscillations of a 25–30-yr period are found in the sea surface temperature, sea level pressure, and barotropic transport fields. A similar-scale signal is also captured in about one century-long observational records. A composite analysis of several model variables is performed based on the extremes of the sea surface temperature oscillation. The proposed mechanism for this basin-scale multidecadal signal involves changes in the intensity of the westerlies, associated with variability in the southward extension of the subtropical anticyclone, which drives changes in the ocean mass transport. This results in variability in the intensity of the Malvinas western boundary current and in the position of the Brazil–Malvinas confluence zone. Anomalo...

Decadal‐to‐interdecadal fluctuations of arctic sea‐ice cover and the atmospheric circulation during 1954–1994

Abstract The relationship between the Arctic and subarctic sea‐ice concentration (SIC) anomalies, particularly those associated with the decadal‐scale Greenland and Labrador Seas “Ice and Salinity Anomalies (ISAs) “, and the overlying atmospheric circulation fluctuations is investigated using the singular value decomposition (SVD) and composite map analysis methods. The data analyzed are monthly SIC and sea level pressure (SLP) anomalies, which cover the northern hemisphere poleward of 45°N and extend over the 41‐year period 1954–1994. The SVD1 (first) mode of the coupled variability, which accounts for 57% of the square covariance, is for the most part an atmosphere‐to‐ice forcing mode characterized by the decadal timescale. The aforementioned ISA anomalies are clearly captured by this mode whose SIC anomalies are dominated by a strong dipole across Greenland. However, as part of the same mode, there is also a weaker SIC dipole in the northern North Pacific which has opposite‐signed anomalies in the Sea ...

Oscillatory and Propagating Modes of Temperature Variability at the 3–3.5- and 4–4.5-yr Time Scales in the Upper Southwest Pacific Ocean

This paper investigates oscillatory and propagating patterns of normalized surface and subsurface temperature anomalies (from the seasonal cycle) in the southwest Pacific Ocean using an extended empirical orthogonal function (EEOF) analysis. The temperature data (and errors) are from the Digital Atlas of Southwest Pacific upper Ocean Temperatures (DASPOT). These data are 3 monthly in time (January, April, July, and October), 2° 2° in space, an d5mi n thevertical to 450-m depths. The temperature anomalies in the EEOF analysis are normalized by the objective mapping temperature errors at each grid point. They are also Butterworth filtered in the 3–7-yr band to examine interannual variations in the temperature field. The oscillating and propagating patterns of the modes are examined across four vertical levels: the surface, and 100-, 250-, and 450-m depths. The dominant mode EEOF (70% of the total variance of the filtered data) oscillates in a 4–4.5-yr quasi-periodic manner that is consistent with El Nino–Southern Oscillation (ENSO). Anomalies peak first at the surface in the subtropics between New Caledonia and Fiji (centered around 17°S, 177°E), then 6 months later in the tropical far west centered around the Solomon Islands (5°S, 153°–157°E), with a maximum at the base of the mixed layer (100 m) and upper thermocline (250 m), and then eastward in the northeast of the southwest Pacific region (0°–10°S, 160°E–180°). Mode 2 (25% variance of the filtered data) has a periodicity of 3–3.5 yr, with centers of action in all four vertical levels. The mode-2 patterns are consistent with variations in the subtropical gyre circulation, including the East Australian Current and its separation, and are continuous with the Tasman Front. Two spatial dipoles are apparent: (i) one in sea surface temperature (SST) at about 5°S, straddling west–east either side of the Solomon Islands, consistent with the classic Pacific-wide ENSO SST anomaly mode, and (ii) a subsurface dipole pattern, with centers in the Solomon Islands region at 100- and 250-m depths, and the western Tasman Sea (27°–33°S, 157°–161°E) at 250- and 450-m depths, consistent with dynamic changes in the gyre intensity.