OS22B-01 INVITED
Long-term Changes in the Climate of the California Current, With Biological Impacts
The CalCOFI dataset in the southern California Current reveals a significant surface-intensified warming and stratification (buoyancy frequency) change across the 1976-77 climate regime shifts. However, the average depth of the thermocline, defined as the maximum gradient of temperature, did not change significantly across the regime shift. But as the surface heating changed the strength of stratification, it also changed the slope of the nitrate-temperature relation for the mid-depth waters (roughly 30m to 200m). This may have affected the quality of upwelled water and the depth from which it is drawn. These historical changes can be useful in anticipating the potential impact of global warming on the oceanic circulation off the coast of California. For example, an eddy- permitting ocean model forced with wind stresses, heat fluxes and open boundary conditions obtained from a global climate model forced by increased greenhouse gases predicts increased upper-ocean temperatures, and increased stratification along the coast. The vertical structure of the thermal response is similar to recent studies of global warming trends.
OS22B-02 INVITED
Equatorial Forcing of Annual SSH Signals off Western South America
Results from previous modeling studies have shown that equatorial signals (ENSO) affect the interannual variability of the coastal ocean off western North America (the California Current). The specific case of the 1997- 98 El Niño is well documented with respect to the movement of the high sea surface height (SSH) signal from the equator to the California Current, using altimeter data. On the other hand, the annual cycle of SSH and circulation off western North America is thought to be controlled by the regional winds and heat fluxes at mid- latitudes. Off western South America (in the Humboldt Current), the connection between mid-latitude and equatorial coastal ocean is more direct than in the Northern Hemisphere. Both inter-annual and intra-seasonal signals at mid-latitudes (20°-30°S) have been traced to the equator. More recently, a semi- annual component of the seasonal cycle of the thermocline depth off northern Chile has been identified and hypothesized to originate at lower latitudes. In this study we use altimeter SSH and numerical models of the oceanic circulation off western South America to investigate the influence of equatorial dynamics on the annual cycle at mid-latitudes. A basin-scale numerical model of the circulation and SSH, forced by NCEP surface winds, is used to force regional models of ocean circulation off Peru and Chile, with boundary conditions that either include or exclude the basin-scale model's equatorial signals. Differences between the circulation off Peru and Chile under the two types of boundary conditions quantify the degree to which the seasonal cycles are controlled by distant forcing. In particular, a deep signal with a peak in austral winter appears to be driven by remote sensing, as is a shallow signal with a peak in austral summer. Altimeter and in situ data are used to verify the results.
OS22B-03 INVITED
Oceanographic Observations in the Mexican Pacific Ocean to Understand the Pelagic Ecosystem Response to the Climate Variability and Climate Change (1997-2006)
In the northeastern Pacific Ocean we are developing a long-term oceanographic program (IMECOCAL) to understand the pelagic ecosystem response to the climate variability and climate change. The IMECOCAL program began in October 1997, and we are expecting to continue until at least 2008 year, with the CONACYT (Mexican Council of Science and Technology) and CICESE supports. The IMECOCAL program is quarterly visiting an oceanographic area of the southern California Current region off Baja California, with approximately 80 hydrographic stations. Also, we are planning two continuous sampling sites, one in northern, and another in the south of the Baja California Peninsula. One of our main goals is understand the interannual variability of the physical-biological interactions in the pelagic ecosystem, with the study of major oceanic physical processes, together with temporal changes of temperature and salinity in the water column, and their relationships with plankton fluctuations. Also, we are searching the long term signature of the climate change over the ocean, with sediments analysis collected at San Lazaro Basin, one of the few anoxic basins of the eastern Pacific Ocean. The program will be modeled the effects of climate variability on the structure of the pelagic ecosystem, selecting some planktonic key species. Also, using hydrographic and remote sensed information (SST, Color, and SSH), global models will be feed with local estimated phytoplankton photosynthetic parameters, to realize statistical analyses in order to define spatial and temporal variability of plankton biomass and primary production in this area.
OS22B-04
Effects of Climate on the Zooplankton of the California Current
Almost six decades of sampling of the California Current system, carried out by the CalCOFI program (California Cooperative Fisheries Investigation) complemented by a decade of observations from the IMECOCAL program (Investigaciones Mexicanas de la Corriente de California), have revealed changing patterns in zooplankton abundances, species composition, and distributions over interannual through multidecadal time scales. Interannual changes associated with ENSO variability are manifested as strong but transitory perturbations in the mean annual cycle in seasonal abundances (and distributions) of particular species. An investigation of longer- term change, limited to the region off southern California, shows a persistent decline in zooplankton volumes (a proxy for overall biomass of macrozooplankton) between 1977 and 1998 that is considered to be a response to the well documented shift in basin-scale climate forcing that occurred in 1976-77. Further examination of this decline in zooplankton volumes indicates that it was due principally to the disappearance of several salp species after 1977. Other species and functional groups did not decline after the change in climate regime, while some species have followed persistent secular trends that appear to be associated more with the phenomenon of long-term global warming. Differences in the regional responses to climate change throughout the California Current system have also been observed recently in the spatial distribution of zooplankton biomass and changes in latitudinal ranges of certain species. For example, zooplankton biomass in the Baja California region show typical values for the 1997-98 El Niño that were followed by a decrease during the sharp transition to the cool La Niña conditions in 1999. This contrasts with the nearby region off southern California that was characterized by reduced biomass during the El Niño period and the subsequent recovery during the La Niña. Another regional contrast in zooplankton distribution observed recently was the significant presence of subarctic euphausiid species off Baja California during July 2005, while the krill collapsed in the region off Oregon in the same period. It is reasonable to suspect that regional contrasts in the zooplankton abundance and species distributions may increase as a response to latitudinal shifts in habitat character due to global warming.
OS22B-05
Effects of Climate Change on Sardine Productivity in the California Current System
The Pacific sardine (Sardinops sagax caeruleus) is one of several coastal pelagic, planktivorous species of fish that provide important trophic links within the ecosystems of the major eastern and western boundary currents. Significant and persistent change in sardine productivity has occurred in the California Current over interdecadal periods in response to reorganization of basin-wide, ocean-atmosphere circulation. Less extreme, but still significant changes in sardine productivity are associated with interannual to decadal-scale climate variability. A precipitous decline of the sardine population began in the mid-1940s with a shift in climate leading to cooling of the California Current system. While the decline, and ultimately the collapse of the population, was exacerbated by intensive fishing, the sardine also suffered a severe reduction in productivity with the southward contraction of favorable thermal habitat that led to restriction of the population to the waters off Southern California and Baja California. This southward displacement resulted in geographic separation of the population from the region off central and northern California that is characterized by significantly higher concentrations of zooplankton that supported the previous levels of success in spawning and larval development. The climate shift in 1976-77 led to the recovery of the population and extension of its range of distribution northwards into the waters off British Columbia. The relation of reproductive success of the sardine population to interannual and decadal climate change was examined for the period 1982-2005 using a suite of seasonal indices representing climate processes and habitat conditions (including zooplankton food levels) occurring through the different stages in the sardine life cycle. We used both stepwise regression and EOF analyses to determine the association between levels of recruitment success and seasonal indices of the Pacific Decadal Oscillation, Ekman pumping (measured at 35°N, 122.5°W), coastal upwelling (at 36°N, 122°W) and zooplankton biomass (represented by values from the center of sardine spawning). The seasonal indices of the PDO are positively correlated with reproductive success, while an inverse relationship between the PDO indices and coastal upwelling is consistent with reduced equatorward flow during coastal warming that favors sardine reproduction. Results also show an unambiguous inverse relationship between Ekman pumping and sardine recruitment success indicating the negative influence of increased offshore transport on the survival of eggs and larvae. There is a surprising lack of association between recruitment success and zooplankton biomass, interpreted to mean that food was not limiting for sardine reproduction during the period analyzed (in the warm regime after 1977). Based on the results of this study, we anticipate that global warming will favor the maintenance of the sardine population over its present range from the Gulf of California into the waters of British Columbia throughout the current century.
OS22B-06
Surface mixed layer temperature and salinity on seasonal and interannual timescales off Baja California
Seasonal and interannual variability of temperature and salinity in the mixed layer of the southern part of the
California Current are examined. The data presented were gathered during thirty one cruises from 1997 to 2005
over a grid based on the Investigaciones Mexicanas de la Corriente de California (IMECOCAL) station plan, which
is the same that the CalCOFI station plan, from lines 100-130, out to station 80. The sampling interval was 3
month. For each survey, temperature and salinity were integrated from the bottom of the mixed layer to the surface
in each station. Empirical Orthogonal Functions analysis identified the seasonal and interannual variability in the
data. During spring when both the California Current and the coastal upwelling system intensify, both
temperature and salinity reached its seasonal minimum. During fall when the net heat flux maximum occurs and
both the California Current and coastal upwelling weakens, temperature reached its seasonal maximum. The
maximum of salinity occurred in winter, when the countercurrents are enhanced. The mixed layer temperature
was anomalously cold from 1999 to 2003, the contrary happened during January and July 1998, July 2003, and
April and July 2004. The mixed layer salinity was anomalously low during January 1998 and from 2003 to 2005,
the contrary happened during January 1998 and January 2002. These results are in general in agreement with
the ENSO cycles, but it remains to be examined the roll of anomalous advection of cold and fresh water of
Subartic origin.
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OS22B-07
Seasonal and Interannual Variability off Baja California
Mean fields, seasonal cycles, and interannual variability are examined for fields of satellite-derived chlorophyll pigment concentrations (CHL), sea surface height (SSH), and sea surface temperature (SST) during 1997-2002. The nalyses help to identify three dynamic regions: an upwelling zone next to the coast, the Ensenada Front in the north, and regions of repeated meanders and/or eddy variability west and southwest of Point Eugenia. High values of CHL are found in the upwelling zone, diminishing offshore. The exception is the area north of 31N (the Ensenada Front), where higher CHL are found about 150 km offshore. South of 31N, the long-term mean dynamic topography decreases next to the coast, creating isopleths of height parallel to the coastline, consistent with southward geostrophic flow. North of 31N the mean flow is toward the east, consistent with the presence of the Ensenada Front. The mean SST reveals a more north-south gradient, reflecting latitudinal differences in surface heating due to solar radiation. Harmonic analyses and EOFs reveal the seasonal and interannual patterns, including the region of repeated eddy activity to the west and southwest of Point Eugenia. A maximum CHL occurs in spring in most of the inshore regions, reflecting the growth of phytoplankton in response to the seasonal maximum in upwelling-favorable winds. SST and SSH anomalies are negative in the coastal upwelling zone in spring, also consistent with a response to the seasonal maximum in upwelling. When the seasonal cycle is removed, the strongest signal in the EOF time series is the response to the strong 1997-1998 El Niño, with a weaker signal representing La Niña (1998-1999) conditions. El Niño conditions consist of low chlorophyll, high SSH, and high SST, with opposite conditions during La Niña.
OS22B-08
Biological Implications of Internal Waves and Internal Tidal Bores in the Southern Part of the California Current System
Internal waves and internal tidal bores have been shown to cause rapid changes in temperature associated with vertical and horizontal displacements of water masses in many settings. Their role in the cross-shore transport of matter and energy implies that internal motions have important biological ramifications, particularly in biogeographic transition zones, such as the Southern California Current. Benthic invertebrates and macroalgae depend greatly on the nearshore pelagic environment for the transport of larvae and nutrients. We present 2 examples of how internal motions may modulate benthic populations in this transition zone by enhancing the transport of larvae or the provision of nutrients from offshore sites to the coast. At two sites along the Baja California peninsula, high-frequency variability in thermal structure and horizontal flows were observed. Physical observations show episodes (lasting about 1 h) of rapid variations (every 1 to 5 min) in horizontal flows and temperature in the nearshore water column. We show that the timing and intensity of larval settlement, as well as the residence of cold nutrient - rich water on the shelf, is related with internal motions just offshore. Variability in the direction from which these perturbations propagate may determine small-scale spatial patterns in demography of benthic populations. We discuss the importance of these events in modulating benthic invertebrate and macroalgal populations and how climate change in this transition zone may have important biological consequences via changes in thermocline depth and its subsequent impact on the occurrence of internal motions.