Overview - NPS



Naval Postgraduate School

OC3570 Operational Oceanography and Meteorology

Winter Cruise

19 January to January 31

A Comparison of Physical properties between Summer 2004 to Winter 2006

LT Boyd

Overview

On January 19 2006 the Naval Postgraduate School embarked on the Research Vessel(R/V) Point Sur and conducted research off the California coast. R/V Point Sur left Moss Landing and proceeded south to Fish Harbor. The topic of this paper is the comparison of summer 2004 data to Winter 2006.

[pic] Figure 1 MATLAB plot of the coastline and the ships track. Notice the end of the first leg and beginning of the second leg track is not included. The corners on the track are denoted by thick dark vertical lines on the figures at approximately 170nm and 560nm. Total distance from Moss Landing goes from 0 to 800nm.

California Current System

In order to compare summer 2004 versus winter 2006 it is important to understand the physical oceanography of the California Current System. This system is made up of the California current and the California Undercurrent (Davidson). The California current flows southward along the shore from the Washington-Oregon border to Southern California. This basic current is modified by seasonal variations in wind direction that give California's nearshore region its three more or less distinct "oceanic seasons."[i] The core of the California Current is approximately 350 km off the coast of California. Within the coastal regime, sea surface flow undergoes a seasonal reversal. During the late fall and winter the direction is primarily poleward while equatorward flow dominates during the spring and summer. The equatorward flow is coupled with the intensification of northwesterly winds that generally parallel the central California coastline. Wind intensity is proportional to the barometric pressure difference between the North Pacific High and the thermal low pressure centered in southern Nevada and California. This pressure gradient begins to form and strengthen in the spring. The sudden strengthening of the northwesterly winds, usually in March- May, may result in the "spring transition" in which upwelling commences. Surface waters are advected offshore, and equatorward geostrophic flow is established. During late fall, the North Pacific High weakens and migrates southward and the thermal low disappears. The surface flow reverses to poleward and can be regarded as the surface signature of the California Undercurrent, although some investigators refer to this poleward current as the Davidson Current (this is debatable).[ii] An important concept to keep in mind is that typically temperature and salinity contrast are normally large near the surface due to differences between the low temperature, high salinity of upwelled coastal waters and the warmer temperature, low salinity of Subarctic waters carried South by the CC jet.[iii]

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Figure 2 This sea surface temperature measurement shows warm sea surface temps moving toward the shore during the summer month of June 1993.[iv] This is included to show how the California current can bring warm temperatures equatorward. These “jets” can cause eddies along the coast and may be the cause for some of the salinity variations seen in figure 4. Also Notice the coastal upwelling temperature decreases.

Physical Properties Observed

During the course of the cruise CTD cast were made at various location along the track (figure 1). These conductivity-temperature-depth meter (CTD) measurements were taken at stations 1-65. With the exception of the coastal casts, most CTD sample were cast at approximately 1000m. Sampled properties were temperature, salinity, oxygen, fluorometer, par, transmissivity, potential density anomaly, and spice. The data was in row, column format and was entered, calculated and plotted on Surfer. The summer 2004 (August) data was subtracted from winter 2006 (January) data, to yield the difference plots. The charts have been cut off at the 500m level as most of the observed differences took place above this depth.

Temperature

The temperature difference is plotted in figure 3; notice that the surface is colder during the winter. The winter 2006 temperature is also warmer below approximately the 50 meter mark. The data also shows that during the summer months there is a shoaling of the thermocline close to shore. There are zero changes below the 125m depth. Also seen is some upwelling near the 750 nm mark. This shows definite seasonal variation, as summer surface temperatures are warmer in conjunction with longer days. This temperature difference may help explain the salinity difference observed in the next figure.

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Figure 3 Temperature difference in degrees Celsius.

Salinity difference

Between the two hundred and three hundred distances from Moss Landing there is a noticeable salinity increase down to 150 meters in the winter profile. This may be caused by an equatorward jet of the current system bringing fresher subarctic water into the region during the summer months. The reduction of salinity in the measured region during the winter months may be due to the poleward flow of the California undercurrent.

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Figure 3 Salinity difference in ppt.

Oxygen

There is very little difference in oxygen between summer 2004 and winter 2006. The variations below the surface just suggest that the water has been oxygenated, by some surface interaction.

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Figure 4 Oxygen levels in υm/kg

Flurometer, PAR and Transmissity

Notice elevated fluorometer (figure 6) readings near the shelf in winter months. This increase in chlorophyll fluorescence indicates the amount of phytoplankton in the area. Overall fluorometer readings are lower in the winter than in the summer. Due to summers higher sea surface temperature, longer days and coastal upwelling this is to be expected. Also there are relatively fertile waters further off the coast between the 160 and 560 nm mark.

The photosynthetically active radiation (PAR) readings (figure 7) follow the summer profiles as the higher PAR indicates a higher number of photons available for photosynthesis. Overall there are lower PAR readings during the winter months, with the exception of the coastal region.

Transmissivity (figure 8) which measures light (beam) transmission is used to determine the amount of particulate matter in the water. Since this light transmission is altered by absorption and scattering we will see lower levels of transmissivity when there is a high level of particulate matter (phytoplankton) in the water. Since there are definitely reduced levels of transmissivity during August 2004, this also confirms the increase in phytoplankton levels in figure 6. In figures 8 we see the transmissivity levels are higher in the winter in the areas were there was high fluorometer readings and PAR readings during the summer 2004.

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Figure 6 Fluorometer difference

PAR

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Figure 7 PAR difference

Tranmissivity

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Figure 8 Difference of Transmissivity in percent of transmission

Potential Density

A lower potential density anomaly corresponds to the higher temperature and lower salinity in figures 3 and 4. There is little variability below the 200m depth. There is an increase in the density anomaly around 750-800nm range. This is due to the slight upwelling that is found in figure 2, during the summer months.

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Figure 9

Spice

The winter months are less spicy, than the summer months on the surface and the spicier waters that reside just below the surface. This is expected and reflects the profiles in figure 3 and 4.

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Figure 10 spice difference

Conclusions/Results

The scope of this research was to provide more analysis and a comparison of seasonal variation of the complex California Current System. Generally the data reflects the expected seasonal variations that are found during these times of the year. Although the normal upwelling period ends in September, the data for summer 2004 shows less upwelling than was expected as I had expected that there would be stronger upwelling near the coast.

The increased fluorometer and PAR readings as well as the reduced transmissivity measurements are generally what I expected to see during the summer months.

This paper did not cover potential density or potential energy, but this may be interesting to look at in future studies. A look at geostrophic flow along the track might be of interest for future research.

Figure Appendix

The following figures are the summer and winter plots. These were used to get the previous mentioned difference plots (figures 1-10).

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Fluorometer 06

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