<p>The shift appears to match up to a large change in the number of cyclones that peak as tropical storms in the early part of the TC season (here defined as March-July), shown by the blue (tropical storms) versus purple (hurricanes) in this graph (made using the <a href="https://www.nhc.noaa.gov/data/" rel="nofollow noreferrer">HURDAT2 dataset from NHC</a>):</p>

<p><a href="https://i.sstatic.net/IhuZv.png" rel="nofollow noreferrer"><img src="https://i.sstatic.net/IhuZv.png" alt="enter image description here"></a></p>

<p><strong>It appears that a large part of the increase in tropical storms lines up with a coincident decrease in tropical depressions (shown in green).</strong><br>
It's notable that tropical depressions only start appearing in the dataset starting in 1963.  I don't know it for a fact, but because the timing lines up <a href="http://noaasis.noaa.gov/NOAASIS/ml/40yearsa.html" rel="nofollow noreferrer">with the advent of satellite weather data</a> it suggests TDs may have been added once satellite offered a better picture to find and analyze weaker systems.<br>
Therefore, it would seem one possible theory might be that there was an initial predilection to hold less organized systems at tropical depression status, but as data has increased from hurricane hunters, satellite resolution upgrades, and additional/more rapid data reports, more of these systems were progressively graded as tropical storms.</p>

<p>Looking into the data at full resolution, it appears the trend may be greatest in weak and moderate tropical storms (focus on the lesser change in dark blue colors, and in most of the pink bands):</p>

<p><a href="https://i.sstatic.net/Dvj7x.png" rel="nofollow noreferrer"><img src="https://i.sstatic.net/Dvj7x.png" alt="enter image description here"></a></p>

<hr>

<p><strong>Does this pattern also match the rest of the year?</strong></p>

<p>I'd suggest yes and no.  Here's the corresponding graph for the late parts of the season:</p>

<p><a href="https://i.sstatic.net/lRgyS.png" rel="nofollow noreferrer"><img src="https://i.sstatic.net/lRgyS.png" alt="enter image description here"></a></p>

<p>So it looks like:</p>

<ul>
<li>Tropical storms are still on the upswing in the late season, just not nearly as much (the trend line for early season goes from 0.5 to 2.5, for late season it goes from 1 to 2).</li>
<li>Cyclones maxing as tropical depressions have also sharply dropped away in the late season, closely matching the early season pattern (indeed, late season TDs were less to start with, in parallel with the smaller TS increase).</li>
<li>Also note that the number of hurricanes themselves have increased in the late season by over 50% (while remaining fairly constant in the early season).</li>
</ul>

<p>You can look into the trends in each storm type in the following set of graphs:  </p>

<p><strong>TD/TS/HUR 50 years: <a href="https://i.sstatic.net/IhuZv.png" rel="nofollow noreferrer">early</a>, <a href="https://i.sstatic.net/4AXzz.png" rel="nofollow noreferrer">mid</a>, <a href="https://i.sstatic.net/lRgyS.png" rel="nofollow noreferrer">late</a>, <a href="https://i.sstatic.net/HUC8k.png" rel="nofollow noreferrer">total</a><br>
TS/HUR 100 years: <a href="https://i.sstatic.net/U6PW1.png" rel="nofollow noreferrer">early</a>, <a href="https://i.sstatic.net/tcqag.png" rel="nofollow noreferrer">mid</a>, <a href="https://i.sstatic.net/hdKNH.png" rel="nofollow noreferrer">late</a>, <a href="https://i.sstatic.net/0EvwL.png" rel="nofollow noreferrer">total</a><br>
By strength full history: <a href="https://i.sstatic.net/Dvj7x.png" rel="nofollow noreferrer">early</a>, <a href="https://i.sstatic.net/3N3oF.png" rel="nofollow noreferrer">mid</a>, <a href="https://i.sstatic.net/dgydn.png" rel="nofollow noreferrer">late</a>, <a href="https://i.sstatic.net/hlEOj.png" rel="nofollow noreferrer">total</a></strong><br>
<sub><sup>(Note these analyses don't include 19 unnamed TCs in 1967-69 where no intensity is given in HURDAT)</sup></sub></p>

<hr>

<p>Many certainly do propose climate change as a possible mechanism to shift the intensity and numbers of TCs.  Quite a few warming modeling studies have suggested there would be an increase in tropical storms.  It appears that <a href="https://journals.ametsoc.org/doi/full/10.1175/BAMS-D-11-00262.1" rel="nofollow noreferrer">this paper</a> (Kunkel et al 2009) has some interesting discussion into the topic in the hurricanes and typhoons section.  The conclusion of:</p>

<blockquote>
  <p>In summary, robust detection of trends in Atlantic and western North
  Pacific TC activity is significantly constrained by data heterogeneity
  and deficient quantification of internal variability. Attribution of
  past TC changes is further challenged by a lack of consensus on the
  physical linkages between climate forcing and TC activity. As a
  result, attribution of any observed trends in TC activity in these
  basins to anthropogenic forcing remains controversial.</p>
</blockquote>

<p>seems to offer a fair reflection of the overall situation as of now in tropical cyclones and possible connections to <a href="https://en.wikipedia.org/wiki/Global_warming" rel="nofollow noreferrer">climate change</a>.  There's certainly plenty of debate over how significantly climate change is influencing the shifts seen; some will draw more conclusions than others.</p>

<p>Regardless of whether it's due to changes in identification practices or other causes like human climate change, it's clear there are more named systems in the early season, and that would certainly often result in an earlier first name.</p>

<p>On the other hand, is it due particularly to <a href="http://www.usno.navy.mil/NOOC/nmfc-ph/RSS/jtwc/pubref/References/GUIDE/chap2/se302.htm" rel="nofollow noreferrer">sufficient ocean temperatures</a> existing earlier in the season over wider areas?  It would seem odd that such a change would mostly impact only the number of tropical storms, and not change the number of hurricanes.  But further analysis should certainly be done and the other <a href="http://wxmaps.org/pix/hurpot" rel="nofollow noreferrer">MPI factors</a>.</p>

<p>I do offer one final small postulate, based both upon these graphs and upon my own perceptions... perhaps at least a small part of the upswing in tropical storms might be due to an increase in the buzz at the start of each new year.  It seems there is more and more talk and "excitement" and interviews as each year get started.  And I wonder if that may prime forecasters to make earlier designations sometimes.</p>

CC BY-SA 4.0
2018-05-26T15:02:47.177Z
(Also note that these graphs only include the time that a system is within the given months.  So for example a hurricane like Georges in 1998 will show up in the late (Oct-Feb) set as only a 25 kt max TD as that was its peak within that period.  I took a quick look to see if the trends might be a fluke of storm timing changes around the cutoffs, but it doesn't appear likely.
JeopardyTempest
CC BY-SA 4.0
2018-05-26T15:05:28.317Z
@gansub good thinking, yeah, that makes sense... so maybe the looks should be at PV/extratropical mechanisms rather than SSTs, given SSTs are more often marginal in development within the bookends of the season anyways.
JeopardyTempest
CC BY-SA 4.0
2018-05-28T23:26:17.957Z
@Universal_learner I'd say it's always about of contributing factors in total, sometimes more of one, sometimes more of another.  As a whole sea surface temperatures are very important.  But in early season/late season storms, I'm suggesting (in the Atlantic at least) that vorticity often may prove core to the development... but gansub might offer more/better thoughts.
JeopardyTempest
CC BY-SA 4.0
2020-10-06T21:12:22.763Z
[Here's](https://i.sstatic.net/L3QxY.png) some more straightforward evidence that the trend in how NHC makes advisories is significantly changing overall... perhaps I'll rework this answer to better reflect the changes as a whole, as I admit my answer is a rather meandering wall of text :-(
JeopardyTempest
CC BY-SA 4.0
2020-10-06T22:27:01.373Z
[And another](https://i.sstatic.net/YkP12.png) that more echos this answer's direction.
JeopardyTempest
CC BY-SA 4.0
2020-10-07T20:03:33.923Z
@gansub it certainly makes sense that there should be such gradients, https://journals.ametsoc.org/mwr/article/141/6/1963/71760/A-Global-Climatology-of-Baroclinically-Influenced suggests quite a decent percentage of development in all basins includes extratropical interaction... but has there been a marked change in the seasonality of such gradients?  Or changes in conditions that support such PV gradients transitioning into TCs?  If you can answer those, it's a worthwhile answer to the question :-)
JeopardyTempest
CC BY-SA 4.0
2018-05-26T13:17:44.720Z
2018-05-27T05:36:51.823Z
2018-05-27T05:36:51.823Z
JeopardyTempest
<p><a href="https://twitter.com/BMcNoldy/status/999722611297972224" rel="noreferrer">This tweet</a> from Brian McNoldy includes a graph suggesting that tropical cyclones are typically getting named earlier in the year in the Atlantic:</p>

<p><a href="https://i.sstatic.net/7xgim.jpg" rel="noreferrer"><img src="https://i.sstatic.net/7xgim.jpg" alt="TC earliest name"></a></p>

<p>What are the causes for this?</p>

CC BY-SA 4.0
2018-05-26T09:59:28.553Z
@gansub Perhaps.  I'll add a separate analysis shortly, but it's taken longer when I realized a few more graphs were needed.
JeopardyTempest
CC BY-SA 4.0
2018-05-26T13:41:01.993Z
@gansub, I'll try to go back later and throw an ENSO phase plot together (got a link to any specific dataset you'd be looking for?) Universal_learner, doing EPAC may take a little time, though there should be a file for it, and got enough setup that it wouldn't take too long to graph that earliest date, so will try to answer that as well at some point.
JeopardyTempest
CC BY-SA 4.0
2018-05-27T00:07:30.577Z
@Universal_learner Sorry, indeed abbreviations like that would be hard to translate. EPAC = Eastern Pacific. We have thorough datasets easily available for the Atlantic and Eastern Pacific, but his graph was only about the Atlantic. I will make a separate graph for the Eastern Pacific region, and look into ENSO criteria, because yes I do agree that that will reveal more about the pattern and possible causes. I'd say my answer, while long, is more a lot of statistical comparisons than really in depth technical meteorology. But can understand it's tough. I will try to clean up language/acronyms.
JeopardyTempest
<tropical-cyclone><data-analysis><atlantic>
Why are tropical cyclones forming earlier in the North Atlantic basin?
JeopardyTempest
# Why are tropical cyclones forming earlier in the North Atlantic basin?

(Asked by: username_0 on 2018-05-26)

[This tweet](https://twitter.com/BMcNoldy/status/999722611297972224) from Brian McNoldy includes a graph suggesting that tropical cyclones are typically getting named earlier in the year in the Atlantic:

[![TC earliest name](https://i.sstatic.net/7xgim.jpg)](https://i.sstatic.net/7xgim.jpg)

What are the causes for this?

**Tags:** tropical-cyclone, data-analysis, atlantic

> **Comments:**
> **username_0**: @gansub Perhaps.  I'll add a separate analysis shortly, but it's taken longer when I realized a few more graphs were needed.  
> **username_0**: @gansub, I'll try to go back later and throw an ENSO phase plot together (got a link to any specific dataset you'd be looking for?) Universal_learner, doing EPAC may take a little time, though there should be a file for it, and got enough setup that it wouldn't take too long to graph that earliest date, so will try to answer that as well at some point.  
> **username_0**: @Universal_learner Sorry, indeed abbreviations like that would be hard to translate. EPAC = Eastern Pacific. We have thorough datasets easily available for the Atlantic and Eastern Pacific, but his graph was only about the Atlantic. I will make a separate graph for the Eastern Pacific region, and look into ENSO criteria, because yes I do agree that that will reveal more about the pattern and possible causes. I'd say my answer, while long, is more a lot of statistical comparisons than really in depth technical meteorology. But can understand it's tough. I will try to clean up language/acronyms.  

---

## 1 answers

### Answer by username_0 on 2018-05-26. Score: 4

The shift appears to match up to a large change in the number of cyclones that peak as tropical storms in the early part of the TC season (here defined as March-July), shown by the blue (tropical storms) versus purple (hurricanes) in this graph (made using the [HURDAT2 dataset from NHC](https://www.nhc.noaa.gov/data/)):

[![enter image description here](https://i.sstatic.net/IhuZv.png)](https://i.sstatic.net/IhuZv.png)

**It appears that a large part of the increase in tropical storms lines up with a coincident decrease in tropical depressions (shown in green).**  
It's notable that tropical depressions only start appearing in the dataset starting in 1963. I don't know it for a fact, but because the timing lines up [with the advent of satellite weather data](http://noaasis.noaa.gov/NOAASIS/ml/40yearsa.html) it suggests TDs may have been added once satellite offered a better picture to find and analyze weaker systems.  
Therefore, it would seem one possible theory might be that there was an initial predilection to hold less organized systems at tropical depression status, but as data has increased from hurricane hunters, satellite resolution upgrades, and additional/more rapid data reports, more of these systems were progressively graded as tropical storms.

Looking into the data at full resolution, it appears the trend may be greatest in weak and moderate tropical storms (focus on the lesser change in dark blue colors, and in most of the pink bands):

[![enter image description here](https://i.sstatic.net/Dvj7x.png)](https://i.sstatic.net/Dvj7x.png)

---

**Does this pattern also match the rest of the year?**

I'd suggest yes and no. Here's the corresponding graph for the late parts of the season:

[![enter image description here](https://i.sstatic.net/lRgyS.png)](https://i.sstatic.net/lRgyS.png)

So it looks like:

* Tropical storms are still on the upswing in the late season, just not nearly as much (the trend line for early season goes from 0.5 to 2.5, for late season it goes from 1 to 2).
* Cyclones maxing as tropical depressions have also sharply dropped away in the late season, closely matching the early season pattern (indeed, late season TDs were less to start with, in parallel with the smaller TS increase).
* Also note that the number of hurricanes themselves have increased in the late season by over 50% (while remaining fairly constant in the early season).

You can look into the trends in each storm type in the following set of graphs:

**TD/TS/HUR 50 years: [early](https://i.sstatic.net/IhuZv.png), [mid](https://i.sstatic.net/4AXzz.png), [late](https://i.sstatic.net/lRgyS.png), [total](https://i.sstatic.net/HUC8k.png)  
TS/HUR 100 years: [early](https://i.sstatic.net/U6PW1.png), [mid](https://i.sstatic.net/tcqag.png), [late](https://i.sstatic.net/hdKNH.png), [total](https://i.sstatic.net/0EvwL.png)  
By strength full history: [early](https://i.sstatic.net/Dvj7x.png), [mid](https://i.sstatic.net/3N3oF.png), [late](https://i.sstatic.net/dgydn.png), [total](https://i.sstatic.net/hlEOj.png)**  
(Note these analyses don't include 19 unnamed TCs in 1967-69 where no intensity is given in HURDAT)

---

Many certainly do propose climate change as a possible mechanism to shift the intensity and numbers of TCs. Quite a few warming modeling studies have suggested there would be an increase in tropical storms. It appears that [this paper](https://journals.ametsoc.org/doi/full/10.1175/BAMS-D-11-00262.1) (Kunkel et al 2009) has some interesting discussion into the topic in the hurricanes and typhoons section. The conclusion of:

> In summary, robust detection of trends in Atlantic and western North
> Pacific TC activity is significantly constrained by data heterogeneity
> and deficient quantification of internal variability. Attribution of
> past TC changes is further challenged by a lack of consensus on the
> physical linkages between climate forcing and TC activity. As a
> result, attribution of any observed trends in TC activity in these
> basins to anthropogenic forcing remains controversial.

seems to offer a fair reflection of the overall situation as of now in tropical cyclones and possible connections to [climate change](https://en.wikipedia.org/wiki/Global_warming). There's certainly plenty of debate over how significantly climate change is influencing the shifts seen; some will draw more conclusions than others.

Regardless of whether it's due to changes in identification practices or other causes like human climate change, it's clear there are more named systems in the early season, and that would certainly often result in an earlier first name.

On the other hand, is it due particularly to [sufficient ocean temperatures](http://www.usno.navy.mil/NOOC/nmfc-ph/RSS/jtwc/pubref/References/GUIDE/chap2/se302.htm) existing earlier in the season over wider areas? It would seem odd that such a change would mostly impact only the number of tropical storms, and not change the number of hurricanes. But further analysis should certainly be done and the other [MPI factors](http://wxmaps.org/pix/hurpot).

I do offer one final small postulate, based both upon these graphs and upon my own perceptions... perhaps at least a small part of the upswing in tropical storms might be due to an increase in the buzz at the start of each new year. It seems there is more and more talk and "excitement" and interviews as each year get started. And I wonder if that may prime forecasters to make earlier designations sometimes.

> **Comments:**
> **username_0**: (Also note that these graphs only include the time that a system is within the given months.  So for example a hurricane like Georges in 1998 will show up in the late (Oct-Feb) set as only a 25 kt max TD as that was its peak within that period.  I took a quick look to see if the trends might be a fluke of storm timing changes around the cutoffs, but it doesn't appear likely.  
> **username_0**: @gansub good thinking, yeah, that makes sense... so maybe the looks should be at PV/extratropical mechanisms rather than SSTs, given SSTs are more often marginal in development within the bookends of the season anyways.  
> **username_0**: @Universal_learner I'd say it's always about of contributing factors in total, sometimes more of one, sometimes more of another.  As a whole sea surface temperatures are very important.  But in early season/late season storms, I'm suggesting (in the Atlantic at least) that vorticity often may prove core to the development... but gansub might offer more/better thoughts.  
> **username_0**: [Here's](https://i.sstatic.net/L3QxY.png) some more straightforward evidence that the trend in how NHC makes advisories is significantly changing overall... perhaps I'll rework this answer to better reflect the changes as a whole, as I admit my answer is a rather meandering wall of text :-(  
> **username_0**: [And another](https://i.sstatic.net/YkP12.png) that more echos this answer's direction.  
> **username_0**: @gansub it certainly makes sense that there should be such gradients, https://journals.ametsoc.org/mwr/article/141/6/1963/71760/A-Global-Climatology-of-Baroclinically-Influenced suggests quite a decent percentage of development in all basins includes extratropical interaction... but has there been a marked change in the seasonality of such gradients?  Or changes in conditions that support such PV gradients transitioning into TCs?  If you can answer those, it's a worthwhile answer to the question :-)  

---

