Corinne James

Altimeter-derived surface circulation in the large–scale NE Pacific Gyres.: Part 1. seasonal variability

Abstract The seasonal variability of sea surface height (SSH) and currents are defined by analysis of altimeter data in the NE Pacific Ocean over the region from Central America to the Alaska Gyre. The results help to clarify questions about the timing of seasonal maxima in the boundary currents. As explained below, the long-term temporal mean of the SSH values must be removed at each spatial point to remove the temporally invariant (and large) signal caused by the marine geoid. We refer to the resulting SSH values, which contain all of the temporal variations, as the ‘residual’ SSH. Our main findings are: 1. The maximum surface velocities around the boundaries of the cyclonic Alaska Gyre (the Alaska Current and the Alaska Stream) occur in winter, at the same time that the equatorward California Current is weakest or reversed (forming the poleward Davidson Current); the maximum surface velocities in the California Current occur in summer. These seasonal maxima are coincident with the large-scale atmospheric wind forcing over each region. 2. Most of the seasonal variability occurs as strong residuals in alongshore surface currents around the boundaries of the NE Pacific basin, directly connecting the boundaries of the subpolar gyre, the subtropical gyre and the Equatorial Current System. 3. Seasonal variability in the surface velocities of the eastward North Pacific Current (West Wind Drift) is weak in comparison to seasonal changes in the surface currents along the boundaries. 4. There is an initial appearance next to the coast and offshore migration of seasonal highs and lows in SSH, alongshore velocity and eddy kinetic energy (EKE) in the Alaska Gyre, similar to the previously-described seasonal offshore migration in the California Current. 5. The seasonal development of high SSH and poleward current residuals next to the coast appear first off Central America and mainland Mexico in May–June, prior to their appearance in the southern part of the California Current in July–August and their eventual spread around the entire basin in November–December. Similarly, low SSH and equatorward transport residuals appear first off Central America and Mexico in January–February before spreading farther north in spring and summer. 6. The maximum values of EKE occur when each of the boundary currents are maximum.

Comparison of the seasonal and interannual variability of phytoplankton pigment concentrations in the Peru and California Current systems

Monthly composite images from the global coastal zone color scanner (CZCS) data set are used to provide an initial illustration and comparison of seasonal and interannual variability of phytoplankton pigment concentration along the western coasts of South and North America in the Peru Current system (PCS) and California Current system (CCS). The analysis utilizes the entire time series of available data (November 1978 to June 1986) to form a mean annual cycle and an index of interannual variability for a series of both latitudinal and cross-shelf regions within each current system. Within 100 km of the coast, the strongest seasonal cycles in the CCS are in two regions, one between 34 o and 45oN and the second between 24 o and 29oN, each with maximum concentrations (>3.0 mg m -3) in May-June. Weaker seasonal variability is present north of 45oN and in the Southern California Bight region (32oN). Within the PCS, in the same 100-km-wide coastal region, highest (>45oS) and lowest ( 1.5 mg m -3) during the austral spring, summer, and fall, matching that evident throughout the CCS. Between these regions, off northern and central Chile, the seasonal maximum occurs during July-August (austral winter), contrary to the influence of upwelling favorable winds. Within the CCS, the dominant feature of interannual variability in the 8-year time series is a strong negative concentration anomaly in 1983, an E1 Nifio year. The relative value of this negative anomaly is strongest off central California and is followed by an even stronger negative anomaly in 1984 off Baja California. In the PCS, strong negative anomalies during the 1982-1983 E1 Nifio period are evident only off the Peruvian coast and are evident there only in the regions 100 km or more from the coast. Although negative anomalies associated with the E1 Nifio were not present at higher latitudes (more than approximately 20oS) in the PCS, the extremely sparse sampling weakens our confidence in the results of the interannual analysis in this region. An upper estimate of the systematic winter bias remaining in the global CZCS data after reprocessing with the multiple scattering algorithm is given in the appendix.

The 1997-1998 oceanic El Nino signal along the southeast and northeast Pacific boundaries—an altimetric view

Changes in the sea surface heights (SSH) and geostrophic currents along the eastern boundaries of the Pacific (North, Central and South America) are examined during the 1997–1998 El Nino using altimeter data and proxy winds. These show that ‘symmetric’ SSH signals left the equator and propagated into both Hemispheres in two episodes, with primary periods of high equatorial SSH during May–July and October–December 1997. These are the ‘distant signals’ from the mid-latitude perspective. As the signals spread poleward in each Hemisphere, their loss of symmetry demonstrates the degree to which they were altered by topographic features, local winds, and/or local currents. The first four EOFs are calculated for 2-D SSH fields in 10° wide strips along the eastern margins (60°N–60°S) and extending out along the equator from the coast to 110°W. These account for approximately 40% of the overall variability and represent the main features of the seasonal cycles and El Nino interannual variability. Snapshots of the 2-D SSH fields depict the structure of the El Nino signal at different phases of its evolution.

Altimeter-derived surface circulation in the large-scale NE Pacific Gyres.: Part 2: 1997–1998 El Niño anomalies

Abstract Changes in the sea surface heights (SSH) and geostrophic transports in the NE Pacific are examined during the 1997–1998 El Nino using altimeter data, sea level pressure (SLP) fields, proxy winds and satellite sea surface temperature (SST). Most of the signal occurs along the boundaries of the basin from Panama to the Alaska Peninsula. Changes in the SSH and alongshore transports along the boundaries are caused both by propagation of signals from the south (stronger between the equator and the Gulf of California) and by local and basin-scale winds (stronger between the Pacific Northwest and the Alaska Peninsula). Two periods of high SSH occur at the equator, May–July 1997 and October 1997–January 1998. The first coastal SSH signal moved quickly polewards to approximately 24°N in early June, then stalled and moved farther north during transient events in July–September. Large-scale wind forcing combined with the equatorial signals during the second period of high equatorial SSH (Fall 1997) to move the high SSH and poleward transports quickly around the Alaska Gyre. A connection between the boundary currents and the interior North Pacific developed as part of the large-scale response to the basin-scale winds, after changes in the boundaries. Decreases in anomalies of SSH and poleward transports began in January 1998 south of 40°N and in February 1998 farther north.

Altimeter-derived variability of surface velocities in the California Current System: 1. Evaluation of TOPEX altimeter velocity resolution

In this paper, we evaluate the temporal and horizontal resolution of geostrophic surface velocities calculated from TOPEX satellite altimeter heights. Moored velocities (from vector-averaging current meters and an acoustic Doppler current profiler) at depths below the Ekman layer are used to estimate the temporal evolution and accuracy of altimeter geostrophic surface velocities at a point. Surface temperature gradients from satellite fields are used to determine the altimeters horizontal resolution of features in the velocity field. The results indicate that the altimeter resolves horizontal scales of 50–80 km in the along-track direction. The rms differences between the altimeter and current meters are 7–8 cm s−1, much of which comes from small-scale variability in the oceanic currents. We estimate the error in the altimeter velocities to have an rms magnitude of 3–5 cm s−1 or less. Uncertainties in the eddy momentum fluxes at crossovers are more difficult to evaluate and may be affected by aliasing of fluctuations with frequencies higher than the altimeters Nyquist frequency of 0.05 cycles d−1, as indicated by spectra from subsampled current meter data. The eddy statistics that are in best agreement are the velocity variances, eddy kinetic energy and the major axis of the variance ellipses. Spatial averaging of the current meter velocities produces greater agreement with all altimeter statistics and increases our confidence that the altimeters momentum fluxes and the orientation of its variance ellipses (the statistics differing the most with single moorings) represent well the statistics of spatially averaged currents (scales of 50–100 km) in the ocean. Besides evaluating altimeter performance, the study reveals several properties of the circulation in the California Current System: (1) velocity components are not isotropic but are polarized, strongly so at some locations, (2) there are instances of strong and persistent small-scale variability in the velocity, and (3) the energetic region of the California Current is isolated and surrounded by a region of lower energy starting 500–700 km offshore. This suggests that the source of the high eddy energy within 500 km of the coast is the seasonal jet that develops each spring and moves offshore to the central region of the California Current, rather than a deep-ocean eddy field approaching the coast from farther offshore.

Satellite-Derived Variability in Chlorophyll, Wind Stress, Sea Surface Height, and Temperature in the Northern California Current System

[1] Satellite-derived data provide the temporal means and seasonal and nonseasonal variability of four physical and biological parameters off Oregon and Washington (41°-48.5°N). Eight years of data (1998-2005) are available for surface chlorophyll concentrations, sea surface temperature (SST), and sea surface height, while six years of data (2000-2005) are available for surface wind stress. Strong cross-shelf and alongshore variability is apparent in the temporal mean and seasonal climatology of all four variables. Two latitudinal regions are identified and separated at 44°-46°N, where the coastal ocean experiences a change in the direction of the mean alongshore wind stress, is influenced by topographic features, and has differing exposure to the Columbia River Plume. All these factors may play a part in defining the distinct regimes in the northern and southern regions. Nonseasonal signals account for ∼60-75% of the dynamical variables. An empirical orthogonal function analysis shows stronger intra-annual variability for alongshore wind, coastal SST, and surface chlorophyll, with stronger interannual variability for surface height. Interannual variability can be caused by distant forcing from equatorial and basin-scale changes in circulation, or by more localized changes in regional winds, all of which can be found in the time series. Correlations are mostly as expected for upwelling systems on intra-annual timescales. Correlations of the interannual timescales are complicated by residual quasi-annual signals created by changes in the timing and strength of the seasonal cycles. Examination of the interannual time series, however, provides a convincing picture of the covariability of chlorophyll, surface temperature, and surface height, with some evidence of regional wind forcing.

Altimeter observations of the Peru-Chile countercurrent

Data from Geosat and TOPEX altimeters are used to infer the structure of the Peru-Chile Countercurrent, a jet that flows from at least as far north as 10°S (historical data suggests 7°S) to 35°–40°S, maintaining its position between approximately 100–300 km offshore. Although the annual mean current cannot be determined from altimeter observations, the nearly antisymmetric patterns in spring and fall, combined with historical observations, suggest that the countercurrent is poleward at most times and is maximum in spring and minimum in fall. Previous studies have linked the offshore countercurrent at 7°S to the Equatorial Undercurrent west of the Galapagos Islands, suggesting that the countercurrent is part of a continuous flow that extends from the western equatorial Pacific to the region off Chile between 35°–40°S.

The large-scale summer circulation of the California current

Satellite data from the Geosat altimeter and the Advanced Very High Resolution Radiometer (AVHRR) are used to show the large-scale structure of the surface circulation of the California Current System in summer. These data show the connection between an equatorward jet and temperature front off Oregon that lies within 100 km of the coast, similar to that first observed in the 1960s and 1970s, and a jet that meanders along the convoluted offshore edge of a temperature front off California, as repeatedly observed in the 1980s.

The salinity signature of the cross-shelf exchanges in the Southwestern Atlantic Ocean: Satellite observations

Satellite-derived sea surface salinity (SSS) data from Aquarius and SMOS are used to study the shelf-open ocean exchanges in the western South Atlantic near 35°S. Away from the tropics, these exchanges cause the largest SSS variability throughout the South Atlantic. The data reveal a well-defined seasonal pattern of SSS during the analyzed period and of the location of the export of low-salinity shelf waters. In spring and summer, low-salinity waters over the shelf expand offshore and are transferred to the open ocean primarily southeast of the river mouth (from 36°S to 37°30′S). In contrast, in fall and winter, low-salinity waters extend along a coastal plume and the export path to the open ocean distributes along the offshore edge of the plume. The strong seasonal SSS pattern is modulated by the seasonality of the along-shelf component of the wind stress over the shelf. However, the combined analysis of SSS, satellite-derived sea surface elevation and surface velocity data suggest that the precise location of the export of shelf waters depends on offshore circulation patterns, such as the location of the Brazil Malvinas Confluence and mesoscale eddies and meanders of the Brazil Current. The satellite data indicate that in summer, mixtures of low-salinity shelf waters are swiftly driven toward the ocean interior along the axis of the Brazil/Malvinas Confluence. In winter, episodic wind reversals force the low-salinity coastal plume offshore where they mix with tropical waters within the Brazil Current and create a warmer variety of low-salinity waters in the open ocean. Key Points Satellite salinity sensors capture low-salinity detrainment events from shelves SW Atlantic low-salinity detrainments cause highest basin-scale variability In summer low-salinity detrainments cause extended low-salinity anomalies

Altimeter‐derived seasonal circulation on the southwest Atlantic shelf: 27°–43°S

Abstract Altimeter sea surface height (SSH) fields are analyzed to define and discuss the seasonal circulation over the wide continental shelf in the SW Atlantic Ocean (27°–43°S) during 2001–2012. Seasonal variability is low south of the Rio de la Plata (RdlP), where winds and currents remain equatorward for most of the year. Winds and currents in the central and northern parts of our domain are also equatorward during autumn and winter but reverse to become poleward during spring and summer. Transports of shelf water to the deep ocean are strongest during summer offshore and to the southeast of the RdlP. Details of the flow are discussed using mean monthly seasonal cycles of winds, heights, and currents, along with analyses of Empirical Orthogonal Functions. Principle Estimator Patterns bring out the patterns of wind forcing and ocean response. The largest part of the seasonal variability in SSH signals is due to changes in the wind forcing (described above) and changes in the strong boundary currents that flow along the eastern boundary of the shelf. The rest of the variability contains a smaller component due to heating and expansion of the water column, concentrated in the southern part of the region next to the coast. Our results compare well to previous studies using in situ data and to results from realistic numerical models of the regional circulation.