Frequency of warm water events

7767889108_0f7bc62cba_bWe have talked a bunch about submerged aquatic vegetation (SAV)! And with good reason.

If you like blue crabs, then you should like SAV since it provides a great shelter for juvenile blue crabs to avoid being eaten!

As we discussed last week, SAV can be critically harmed if water temperatures get too warm. Dieback events have been observed in 2002, 2005, and 2010 with warm water temperatures likely being a contributing factor.

Today we will look at an image that I have found particularly helpful and insightful for looking at how the frequency of warm water events has changed over the last ~100 years in Chesapeake Bay.

Plot of the Week

TX90p is an index which calculates the percentage of days when the warmest daily temperature was above the 90th percentile (That’s 34°C or 93.2°F for the summer).

TN90p is an index which calculates the percentage of days when the coolest daily temperature was above the 90th percentile (That’s 23°C or 73.4°F for the summer).

You can think of this as the amount of warmer-than-normal summer days and nights!

Figure 1:

Figure 1: The TX90p and TN90p indices for summer in the Chesapeake Bay near-shore region.

What this plot suggests is that when more than 25% of the summer had either the daily maximum or minimum temperature exceed the 90th percentile, SAV was likely thermally stressed. For example, those years with known SAV dieback events (2002, 2005, and 2010), all show up!

Lucky for us, that 90th percentile for the hottest daily temperature is just above the temperature where eelgrass gets thermally stressed, so in a way, TX90p is a proxy for eelgrass heat stress!

2005 was a particularly interesting year. TX90p was not particularly high, but TN90p was! In other words, the summer nights stayed warm, so the water temperatures never got a chance to cool down at night.

Like blue crabs? Then you should like SAV!

Like blue crabs? Then you should like SAV!

Why We Care

The shallow water ecosystems in Chesapeake Bay are experiencing more heat stress. Over the past century, the TX90p summer index has increased by 3.6 to 8.9%, which means that days with temperatures >34°C increased by an average of  5.5 days (that’s almost a whole week of extra hot days!).

And our future climate projections predict that summer TX90

p will continue to increase by 3.8 to 8.1%.

In other words, the potential for a heat-caused SAV dieback event has increased, and is likely to continue increasing over the next few decades!

More to come!

Kari Pohl

About Kari Pohl

I am a post-doctoral researcher at NOAA and the University of Maryland (Center for Environmental Science at Horn Point Laboratory). My work investigates how climate variability and extremes affect the diverse ecosystems in Chesapeake Bay. I received a Ph.D. in oceanography from the University of Rhode Island (2014) and received a B.S. in Environmental Science and a B.A. in Chemistry from Roger Williams University (2009). When I am not busy being a scientist, my hobbies include running, watching (and often yelling at) the Boston Bruins, and taking photos of my cat.
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