During 17 months in 1971-1973 Broenkow and Smethie (1978) prepared maps of near-surface temperature, salinity, dissolved oxygen and inorganic nutrients. These tracers were used to infer the direction of water movement and residence time inside the bay. An El Niño event in the winter of 1972-73 produced flood conditions in the Salinas, Pajaro and San Lorenzo Rivers resulting in low salinity (< 33 psu) in the northern part of the Bay (Figure 9). From the gauged discharge from these rivers a residence time of fresh water of 4 to 12 days was computed for January-March. Comparison of nearshore temperature with estimated insolation rates yielded similar residence time values. The median replacement time of surface waters in the bay is on the order of a week (Broenkow and Smethie state a range of 2 to 12 days). Such a replacement time requires a net alongshore current speed of about 5-15 cm/sec. Most AVHRR satellite images show a distinct warming of Bay waters and most often show that warmest waters are in the northern bight (Figure 3 and Figure 8).
Currents inside Monterey Bay have been measured by a variety of techniques. Recent and continuing work at the Navy Postgraduate School using Coastal Ocean Dynamics Applications Radar (CODAR; Paduan and Neal 1992, Paduan et al. 1995) has resolved tidal current ellipses in the south and central bay. Maximum semi-daily current speeds were about 20 cm/sec. The overall non-tidal flow appeared to be cyclonic with a net northward flow within the bay. A year-long (1976-1977) moored current meter record was obtained 2 km west of the Salinas River mouth in the Bay. Daily averaged currents at this location were northward about 2/3 of the year with maximum daily averaged speeds of about 15 cm/sec (Figure 10). Average northward non-tidal current speeds were about 5 cm/sec while southward speeds were 3 cm/sec. Current meter observations near Point Santa Cruz during the summer of 1976 were predominantly seaward, and between November and January the direction reversed and flow was eastward and into the Bay. Similar current meter records near the mouth of the Pajaro River in 1979-80 showed northward flow with monthly average speeds of about 15 cm/sec (Breaker and Broenkow 1994).
A number of drifter measurements of currents in Monterey Bay show generally consistent northward or cyclonic flow inside the bay. Moomey (1973) released 13 drogues across the center of Monterey Bay. Those drogue trajectories showed clearly cyclonic flow over the period of one day: inflow was observed over the southern half of the bay and outflow over the northern half. The drift was not corrected for wind drift and some of observed displacements may have been due to horizontal wind shear. Twelve day-long drifter measurements in 1976-77 were made off the Salinas River mouth as part of the predesign study for the Monterey Peninsula Water Quality Control Board regional sewer outfall. Those wind-corrected drogue trajectories showed tidal ellipses consistent with observed tidal current speeds. Seven of the twelve trajectories produced a net northward movement, two a net southward movement while the other three generally showed little net drift (Figure 11).
Surface wind drift inside Monterey Bay has been measured using drift cards (Blaskovich 1973, Griggs 1974). Both drift card studies show that surface waters (< 1 m) move essentially downwind at about 3% of the wind speed. Similar studies elsewhere generally agree. With regard to surface drift, it is important to note the large sea breeze-land breeze effect present in Monterey Bay (see Climate and Meteorology section). Because surface drift responds to wind forcing, the movement of surface material - be it dead sea birds, Velella "fleets"(see Sandy Beach and Pelagic Zone sections), spilled oil or sewage - are affected by this diurnal periodicity. At Moss Landing in summer, the sea breeze typically begins at 1000 local time, and peak wind speeds of 20 m/sec (10 knots) are attained by 1600 hours. During the evening, winds diminish to perhaps 5 m/sec and the direction may change from onshore to offshore. At Moss Landing the prevailing onshore wind direction is from 270 to 300°True. Paduan et al.'s (1995) recent CODAR surface current measurements show onshore flow of diurnal periodicity aligned with the direction of the Salinas Valley
The CODAR technique promises to shed new light on surface currents in the Bay, and to extend the measurements from the few isolated current meter records to nearly the full areal extent of the Bay. The first such long term mean surface current map for August through September 1994 (Paduan et al. 1995) showed cyclonic flow inside Monterey Bay: weakly eastward near Pt. Pinos, northward past the Salinas River mouth and strongly seaward past Santa Cruz. These CODAR results agree generally with mean flow inferred from earlier tracer distributions (Broenkow and Smethie 1978) and current meter records (Breaker and Broenkow 1994). The CODAR results go far beyond what was previously known about the surface currents in the Bay, and for the first time oceanographers are able to draw streamline maps depicting surface water movement through the Bay (Figure 12). Recently, real time CODAR and meteorological observations have been made available on the Internet through the Real-time Environmental Information Network and Analysis System (REINAS) project (see IX. Selected Physical Oceanography Resources).
The northern Bay is protected from the prevailing northwesterly winds while the south and central Bay are directly exposed. The northern bay is consistently warmer, as shown by shipboard observations (Broenkow and Smethie 1978, Rosenfeld et al. 1994) and satellite imagery (see for example, Figure 3 and Figure 8). The north Bay is nutrient depleted (about 7 µM/l in NO3 and 0.5 µM/l in PO4) compared with the mid- and south Bay, and Broenkow and Smethie (1978) showed that the lowered nutrient concentrations are consistent with primary productivity values of 1g C/m2/day. Graham et al. (1992) observed zooplankton distributions in the north Bay that they described to be "upwelling shadow." Here they observed a frontal area between cool offshore waters and warmer, nutrient-depleted north Bay waters.
None of the extant data on currents inside Monterey Bay show the presence of strong (approaching 50 cm/sec i.e. 1 knot) currents in surface waters. However, strong oscillating currents have been measured near the bottom in Monterey Submarine Canyon (Shepard et al. 1979) and by investigators at the Naval Postgraduate School (Gatje and Pizinger 1965, Dooley 1968, Njus 1968, Hollister 1975). Generally the deep currents were aligned with local direction of the canyon axis, at speeds up to 60 cm/sec. These near-bottom currents appear to have subtidal periods between six and nine hours, and the net flow is predominantly upcanyon. Shepard et al. (1979) attribute these currents to the propagation of internal waves up Monterey Submarine Canyon. The occurrence of strong currents in the canyon have been noted by remotely operated vehicle (ROV) from the Monterey Bay Aquarium Research Institute (C. Harrold, pers. comm.) from which highly turbid bottom waters and scoured bottom sediments indicated speeds of 100 cm/sec at a depth of 250 m. Eittreim et al. (1989) reported evidence for vigorous bottom currents of 25 to 60 cm/sec near 1000 m depth during an ALVIN dive. Breaker and Broenkow (1994) show a beam transmissometer profile in the canyon axis where the benthic nepthaloid layer extends from about 600 m to the bottom at 960 m, presumably a result of those strong flows. The presence of episodic flows in the canyon is further confirmed by Garfield et al. (1994) who described a 1.9 km displacement of a bottom-mounted acoustical transducer, along the canyon axis just outside Monterey Bay. This event was presumably caused by a turbidity flow associated with the October 1989 Loma Prieta earthquake ( see Geology section.
Section IV. El Niño