Total Annual Precipitation: What do you see?

Fig. 1: Too much or too little precipitation can affect ecosystems. picture credit

Fig. 1: Too much or too little precipitation can affect ecosystems. picture credit

For today’s post, we are going to look at the total annual precipitation for the past ~115 years in the Northern Chesapeake and Southern Chesapeake. While I will provide a few insights, my hope is that you’ll find your own notable observations!

A while back, we investigated return periods of total annual precipitation for the HadEX2 data set. This time series showed that the four greatest precipitation events in that record have occurred since 1970.

I had made the figures for today as supporting material for our current climate work, and felt like today was the day to finally display them!

North and South Chesapeake

What do I mean when I say North Chesapeake region? Figure 2 is a rough picture of how our study area is broken down.

Fig. 2:

Fig. 2: Our study area. Yellow diamonds are the North Chesapeake stations, blue diamond are South Chesapeake, and green triangles are the CBNERRS stations. The HadEX2 is the approximate bounds of this image.

We aggregated 9 GHCN-Daily weather stations as North Chesapeake and 9 as South Chesapeake using the 38.3°N latitude line as a divider.

We did this for two reasons. First, we aggregated stations in order to assess regional signals to better understand the climate variability organisms have experienced. Early on in this project, we demonstrated that individual weather stations at the CBNERRS were significantly correlated to all GHCN-Daily stations, suggesting that temperature patterns were a regional and not local effect. Second, the National Climate Assessment divided Maryland into the Northeast and Virginia into the Southeast; thus our approach is somewhat comparable but allows us to investigate the Chesapeake region specifically.

North Chesapeake Total Annual Precipitation

To begin, let me explain what Fig. 3 is showing. The thin gray lines are the year-to-year total amount of precipitation. The thick black line is a LOESS fit, also known as a local regression, which smooths a scatterplot. The red dashed line is the 95th percentile and blue is the 5th percentile over the entire time series. Thus, lines above the red line are very wet years and below the blue line are very dry years.

Fig. 3:

Fig. 3: Total annual precipitation in the Northern Chesapeake.

My hope is that you look at this image and find your own notable take homes!

But I will state a few things that jump out to me!

1930 and 1965 were dry years! And it looks  like the years near 1965 were also fairly dry, suggesting a prolonged dry period. You can read a little bit more about notable droughts here.

The three wettest years, similar with the HadEX2 data set, were all within the last 35 years of this 115 year record: 1979, 1996, and 2003. It turns out that 1996 was a very interesting year in the Maryland region. A handful of tropical storms made the latter part of this year one of the wettest on record!

Southern Chesapeake Total Annual Precipitation

Okay, so how does the Southern Chesapeake compare to the Northern Chesapeake (Fig. 4)?

Again, I hope you look at these figures are interpret the data for yourself!

But, since I cannot help but chime in….

Fig. 4:

Fig. 4: Total annual precipitation in the Southern Chesapeake.

The Southern Chesapeake experienced many of the same excursions above the 95th percentile and below the 5th percentile. Specifically 1930 and 1965 were very dry years, suggesting that drought was a larger-scale regional event for these years that would have affected most of the Chesapeake near-shore region.

Similarly, 1979, 1996, and 2003 were very wet years, again suggesting these were bay-wide events!

But there are some differences, demonstrating why a separate North-South analysis is helpful here! First, Southern Chesapeake has experienced some more recent “extra” dry years, including 2007. Interestingly, there were no 95th percentile exceedances before 1958, unlike the North, and 2009 appears to have been wetter comparatively.

Concluding Thoughts


Fig. 5: So you can compare, here is Fig 3 and 4 side by side!

One of the most important parts of the scientific process is interpreting data. While I have given some of my observations for the Total Annual Precipitation in the North and South Chesapeake, hopefully you “see” interesting facts that may be applicable to your own studies. I displayed Fig. 3 and 4 side by side for direct comparison.

Which years standout to you? Maybe you remember one of these wet or dry years. What about the magnitude of each event or comparing what the percentile amounts between North and South are?

With data, the observations are virtually endless!


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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2 Responses to Total Annual Precipitation: What do you see?

  1. Fawn Palmer says:

    Do you think that the trend for the southern Chesapeake upwards is related to the larger thermal capacity of the down Bay section? Recently it was announced that on average the Bay has warmed which would influence the moisture and hence precipitation. Do you think that the u-shaped northern Chesapeake trend is related to proximity to the piedmont and Appalachians, iow, a terrestrial topographic influence?

    • Kari Pohl Kari Pohl says:

      Hi Fawn, you bring up some really interesting hypotheses for the differences both observed and projected for the larger Northeast and Southeast. So thank you because this will be helpful to literature search and incorporate into our discussion!

      Increases in total annual precipitation for both our North and South regions is in part likely attributed to warming and thus more moisture. The North and South have warmed slightly differently, so this could be interesting to investigate with our temperature indices (stay tuned!)

      Another factor is likely changes in the larger-scale climate patterns. I’ve heard a few theories that air mass changes in the Northeast could in part explain the larger increases in precipitation. We have an area-weighed dataset that includes weather stations in the Piedmont and it follows the same pattern (high years and low years) as our North and South; so our North and South incorporates the signal from the mountainous regions, thus changes in that region will be reflected in our data. We should look into the proportion of Piedmont vs coastal plain situated stations to see how that could play into this difference!

      Lastly, there is also a likely contribution from large coastal storms. Our data suggests that the North had an intensification of winter precipitation amounts….while I cannot say what caused that change, it could be attributed to either (or both) Northeast changes in coastal winter storms or cross-continental storms (such as a mid-latitude cyclones).

      Thank you for you insights, and thoughts on this?

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