Thursday, May 7, 2015

Whether the Weather

One of the things that can most impact our lives out here at sea is the weather. Early on in the trip, we were sailing through nearly perfect conditions—the seas were mostly calm, the sky was crystal clear, and many of us spent time on deck after dark simply to stargaze. James Allen, a student on board measuring chlorophyll concentrations and organic matter dynamics, grew up around meteorology and has a pet interest in astronomy. He taught a group of us how to navigate the southern sky by finding the two pointer stars alongside the Southern Cross. (Be careful to avoid the false cross, similar but without the Pointers!) I had never seen the southern sky before this trip, and it took me wholly by surprise. The interior of the Pacific also qualifies as a ‘Dark Sky’ region, where light pollution is low enough for serious stargazing. We could often see the Milky Way after letting our eyes adjust for a few minutes. Many of us spent time outside during our off-watch time, pointing out new constellations to each other.

After we crossed the equator, we entered the Inter-Tropical Convergence Zone, a low pressure area around the equator. This area is typically known to sailors as The Doldrums, as the prevailing wind directions in this area are vertical—warm sea surface temperatures cause hot air to rise and recirculate at altitude instead of near the sea surface where it is useful for sailing. This low pressure system is also famous for another reason—rain. As water in the warm, rising air condenses, storms ranging in size from squalls to hurricanes are found all the way through the ITCZ. The weather of the ITCZ depends somewhat on the strength of the easterly trade winds that blow over the equator, just to the south. Strong trade winds mean cloudier skies and more storms, and vice-versa. We have certainly noticed the clouds over the past few days, as we’ve broken away from the equatorial region and crossed 5 °N. It’s no longer really ripe for stargazing outside, although we have been able to see some great cloud systems on the horizon, as well as some wonderful rainbows.

Can you see it raining near the horizon? 
There were actually three rainbows, but only two would come out in my photograph.

In one of my highlights for the week, we’ve also seen the ITCZ pop up in our daily weather report generated by the Navy. As we are one of the only ships reporting weather observations in this region, we can have a significant impact on weather models and the marine forecast! This week, the position of the ITCZ ‘followed’ us along 151 °W from 6 °N to 9 °N, simply because were reporting cloudier skies. Below, the left plot shows the position of the ITCZ as forecast by the National Weather Service on 1 May, with 151 °N marked as a vertical black line. On the right is the forecast by for 3 May. To make it easier to compare the positions between the panels, I draw a horizontal black line from our starting position on 1 May across both plots. It’s a short jump, but those few degrees of latitude of movement were partly because of us! 

One of the other ways that the ITCZ has been impacting our science is through the brief but intense episodes of rain we have been seeing. The Brown employs two technicians that are specifically in charge of the scientific survey equipment on board—survey technicians. One of the instruments they monitor is our underway thermosalinograph (TSG), which tracks the temperature and the salinity of the surface waters as we drive through them. This is a screenshot of the TSG readout, showing an abrupt drop in salinity (red line) during a small rainsquall that crossed over us as we sat on station during a recent CTD cast. You can see a small change in the temperature of the surface water (blue line) too.

Temperature and salinity from the underway TSG as we sat on station 77 (6 °N, May 1; data courtesy of the Ron Brown). 

Actually, we can see the broader oceanographic influence of the ITCZ rains in our larger underway TSG record as well. In general, the Pacific Ocean is fresher than the Atlantic, because it’s so much bigger and collects so much more precipitation. Nowhere is this more evident than through the ITCZ, where the frequent rain far overshadows any evaporation that might occur during the warmer temperatures. As we’ve traversed north, we saw this aggregate effect starting at about 1.5 °N.

Salinity from the underway TSG on our cruise track (courtesy of the Ron Brown) between the Equator and about 14 degrees North.  The heavy rain in the ITCZ can be seen as the salinity gets fresher (lower) between 6 and 9 degrees North.

The underway TSG is actually a pretty powerful oceanographic tool for precisely this reason; the same sensor can be used to observe a single rainsquall while we are sitting still, and to see the overall impact of the large annual precipitation of the ITCZ as we move. Using it well allows us to see both the forest and each of the trees.


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