Warning: This writeup was done to help organize
my thoughts prior to the presentation.
The actual presentation may have been quite different!
Presented at the Salinity from Space Session of the Ocean Sciences Section. (20 minute slot).
I. Motivation for study
COARE Enhance Monitoring Array
III. Diurnal Cycle Analysis
Before getting started, I would like to acknowledge Roger Lukas for providing some of the 3m sea surface salinity (SSS) data that I'll be discussing here.
The outline of the presentation is as follows:
I'll begin with a brief discussion of the motivation for studying the diurnal cycle of SSS and rainfall.
I'll then discuss the data and results from the analysis and will end with a summary.
Cronin, M. F., and M. J. McPhaden (1998): Upper ocean salinity balance
in the western equatorial Pacific. J. Geophys. Res., 103,
Why are we doing this study:
1) From a technical perspective, undersampling the diurnal cycle can produce aliasing. Also, depending upon which part of the dc is undersampled, the fields can be either biased high or low. This is a particularly important issue for polar orbiting satellite measurements.
2) In a recent study which has just come out in the
November issue of JGR, Mike and I found that on time scales of a month
to the record length (1-2 years), variability in western equatorial Pacific
SSS was due primarily to zonal advection of the fresh pool, and only weakly
correlated with local rainfall. The correlation between local rainfall
and SSS increased though for shorter time scales.
Here we ask, on the diurnal cycle, can we relate SSS variability to rainfall variability?
3) One might ask Why diurnal? Why not 2day or 3day cycles? The diurnal cycle in heat flux is one of the strongest clocks in nature. Every morning, the sun rises, and every evening it sets. This clock has a profound effect on the ocean surface, surface heating, and convection. To understand its effect on the full hydrological cycle, we must also ask, how the diurnal cycle in rainfall effects the sea surface salinity, and the buoyancy of the ocean surface.
4) Finally, to fully understand this hydrological cycle, we must understand the spatial and temporal variabiltiy. How does the dc depend upon its location relative to the SPCZ, ITCZ, equator,...? How does the MJO affect the dc?
Hourly rainfall is measured with an Optical Rain Gauge (ORG), which computes rain rates from scintillations caused by raindrops falling through a near-infrared light. ORGs rainrates are high relative to radar and satellite rainfall rates, although the correlations with various satellite rainrates is as good as the satellite to satellite correlations. Factory calibration are the largest source of error and are typically 20-25%, and occasionally larger. Because of this, ORGs are not used at present in the TAO array.
SSS and SST were measured by SEACAT T&C sensors mounted either on the toroidal bridal at 1m or on the wire at 3m. After postprocessing, SSS errors are approximately 0.02 psu.
These 2 sites also measured shortwave radiation. Thus, using the COARE v2.5b bulk algorith to compute the latent and sensible heat fluxes, and a bulk algorithm to estimate the net longwave radiation, we have hourly timeseries of the net surface heat flux at these 2 sites.
Note that while all the sites are within the convection region of the western equatorial Pacific, there is spatial variability in the mean., with stronger convection off equator in the South Pacific Convergence Zone and the Intertropical Convergence Zone in the northern hemisphere. On the equator, convection has a maximum near the center of the array. Later, I'll be discussing this in relation to the spatial variability of the diurnal cycle.
Notice that the low frequency dynamic range is about 1-2 psu. These downward 1-2 psu fresh spikes are caused by rain. The fact that they are spikes, rather than steps, is because of other processes such as mixing. and advection. Although smaller, these spikes are still evident in the composite.
Consistent with other investigators' results, we see peak rainfall near 4 am.
SSS also has a clear diurnal cycle, although it is weak -- peak to peak amplitude fo rthe composite is about 0.01 psu at 1m and smaller at 3m. Surprisingly, SSS is saltiest during the period of highest rainfall. What's happening is that nighttime mixing is injecting that freshwater below the surface and bringing up saltier water to the surface.
Although the freshwater from rainfall tends to stabilize the ocean surface, looking at the buoyancy scale on the right axes, we see that the destabilizing thermal buoyancy flux is 2-3 times larger. Daytime rainfall, although substantially weaker than the nighttime peak, is sufficiently large to produce a 0.01 psu freshening.
Not surprisingly, rainfall is more than twice as high during the convective periods. In both periods, rainfall peaks at nighttime. However, during the suppressed periods, there is also a secondary maximum in the afternoon.
Although the 3m SSS anomaly is essentially flat during the suppressed periods, the 1m SSS is as large as during the convective period. This is consistent with the formation of very shallow rain puddles due to afternoon showers during a period of light winds.
While most sites have a primary peak rainfall at 4 AM local, at 0156E the primary peak is in the afternoon, and at 0157.5E no clear diurnal cycle is present, -- perhaps because this is a shorter record, or because of its location near the center of the convection max on the equator (I don't know why this would be).
The 3m SSS diurnal cycle also has interesting spatial variability. The amplitude tends to increase to the west, perhaps because mixing is reduced. At 0154e, the peak to peak amplitude is about 0.015 psu at 3m. 0 156E 1m SSS anomaly has an amplitude of roughly 0.02 psu.
Also, quite interestingly, the dc phasing changes zonally. In the east at 0165E, the anomalous low salinity peaks at about 9AM; at 0161E and 156E, it peaks at about 3PM; in west at 154E, it peaks in the evening at about 8PM. As discussed earlier, the phasing of the SSS diurnal cycle is due to the relative phasing of the rainfall and diurnal mixing, ultimately between the thermal and rainfall buoyancy fluxes.
This phase shift in the SSS diurnal cycle causes the composite diurnal cycle to be much weaker than at individual sites.
In the western equatorial Pacific, rainfall occurs preferentially during nighttime at 4am. On the equator, and during suppressed periods there is also an afternoon rainfall maximum. At 0,156E, this afternoon peak was larger than the nighttime peak.
SSS at 1m and 3m have weak diurnal cycles, with 0.005 - 0.02 psu peak to peak amplitudes.
The SSS diurnal cycle is controlled by daytime rain and nighttime mixing. The relative strengths and timing of the daytime rain and diurnal mixing, cause large spatial variability in the phasing of the SSS diurnal cycle.
SST and SSS are not coupled on the diurnal time scale.
Although SSS is affected by the SST diurnal cycle, on this time scale,
buoyancy associated with the SSS anomaly is too weak to affect the SST
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Meghan F. Cronin
Pacific Marine Environmental Laboratory
7600 Sand Point Way NE
Seattle, WA 98115 USA
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