Influence of convection on stratospheric water vapor in the North American Monsoon region

We quantify the connection between deep convective occurrence and summertime 100-hPa water vapor anomaly over the North America (NA) region and find substantial consistency of their inter-annual variations and that the water vapor mixing ratio over the NA region is up to ~1 ppmv higher when deep convection occurs. We use a Lagrangian trajectory model to demonstrate that the structure and the location of the NA anticyclone, as well as the tropical upper tropospheric temperature, 5 determine how much contribution the deep convection could make to moistening the lower stratosphere. The deep convection mainly occurs over the Central Plains region, and most of the convectively moistened air is then transported to the center of the NA anticyclone and the anticyclonic structure helps maintain high water vapor content there. Our hypothesis explains both the summer seasonal cycle and inter-annual variability of the convective moistening efficiency in the NA region, and can provide valuable insight on modeling stratospheric water vapor. 10

To make this correlation clearer, Figs. 1b-d show scatter plots of the 5-day water vapor anomaly and deep convective occurrence over NA in June, July, and August. We find that deep convection increases 100-hPa NA stratospheric water vapor by up to~1 ppmv. The slope of the linear fit in Figs. 1.b-d represents the moistening efficiency, which is defined as the amount of water vapor content added per unit of deep convective occurrence in the stated month added. This moistening efficiency is significantly lower during June than July and August, which we will be explained in the next section. 90 One must be careful not to confuse correlation with causality. We therefore use the back trajectory model to demonstrate the causal relationship implied in Fig. 1. As discussed in section 2.2, we divided the 100 hPa MLS observations into two groups depending on whether they encountered the deep convection during the 5-day back trajectory or not. Fig. 1a shows the convective influence, the fraction of MLS observations that encountered convection and Fig. 2 shows the probability density function (PDF) of water vapor mixing ratio during June, July, and August 2005-2016 in the two groups. 95 We see that the no-convection group has a similar PDF shape in June, July, and August, with peak values around 4 ppmv.
For the MLS measurements that encountered convection, the peak of the PDF is 5-6 ppmv during July and August, and 4-5 ppmv during June, 0.37 ppmv, 0.62 ppmv, and 0.69 ppmv higher than the no-convection group during June, July, and August, respectively.
Our work in this section establishes that deep convection is increasing water vapor over the NA region. However, three 100 questions remain to be answered: First, can deep convection explain the spatial distribution of the water vapor anomaly? Second, why is the convection more effective in July and August than in June? Third, why is there inter-annual variability in the effectiveness of moistening (for example, June 2010 vs. June 2011)? These are three key questions we answer in the following sections.
4 Differences between June, July, and August 105 From June to August every year, the water vapor mixing ratio over NA shows positive anomalies relative to the zonal mean . Deep convection also frequently occurs during boreal summer, especially over the central US (contours in Fig.3 a-c, see also Cooney et al. (2018)). However, there is a discrepancy between the spatial distribution of the water vapor anomaly and deep convective occurrence: The deep convection occurs mainly over the Central Plains region, centered around 40 • N.
Large positive water vapor anomalies are observed over a broader longitude range south of 40 • N latitude.

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Our back trajectory calculations show that regions with high convective influence ratios ( Fig. 3d-f) tend to be collocated with large positive water vapor anomalies (Fig. 3 a-c, also black dashed contours in Fig. 3d-f). Here, we define the convective influence ratio as the number of MLS observations in each grid box that encountered deep convection during the past 5 days, divided by the total number of MLS observations in that grid box. This collocation suggests that the pattern of enhanced water vapor seen by MLS can be explained by frequent convection. It is worth mentioning that previous studies have also suggested There is also a similarity between the distribution of the time spent over NA ( Fig. 3g-i) and the convective influence ratio, indicating that the monsoon circulation tends to hold air that has flowed over convection in the NA region. This provides an 125 explanation for the observations in Figs. 3a-c showing that convection tends to be located north of the 100-hPa water vapor maximum.
The monsoon dynamics are also an essential factor in the seasonal cycle of water vapor anomaly. Here, seasonal cycle refers to the increase in water vapor mixing ratio through the summer, from June to July to August. There are two reasons for this.
First, the North American monsoon anticyclone (NAMA) forms in June and enlarges and becomes stable during July and 130 August (Clapp et al., 2019). This leads to increases in the average NA residence time from 3.4 to 4.5 to 4.9 days from June to August. This increases the convective influence (the fraction of MLS observations that encountered convection), with values in June, July, and August of 0.038, 0.081, and 0.091, respectively. What is happening here is that, later in the summer, the convectively moistened air is more likely to be confined within the NA region instead of being transported downwind by the zonal mean flow. As a result, the deep convection can moisten the NA region more efficiently late in the summer compared to 135 early in the summer.
The second reason is also connected to the changing dynamics during June, July, and August. Parcels tend to travel to lower latitudes during June (Fig. 4a), which leads them to experience colder temperatures at 100 hPa (Fig. reffig4b). This means that convectively moistened air experiences subsequent dehydration more frequently in June than in later months (Randel et al., 2015).

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The PDFs of the minimum saturation water vapor mixing ratio, which limits the amount of water in the parcel, indicates that parcels in June tend to have lower values (Fig. 4c). If we choose a minimum saturation water vapor mixing ratio of 5 ppmv as a threshold of effective moistening (stratospheric water vapor mixing ratio commonly won't exceed this value), then 88.0%, 97.8%, and 97.5% of the observations that encountered deep convection are effectively moistened in June, July and August, respectively. We calculate the effective convective influence ratio by dividing the number of convectively moistened 145 observations that have a minimum saturation water vapor mixing ratio over 5 ppmv by the total number of observations. The effective convective influence ratio is 0.039, 0.082, and 0.092 during June, July, and August, respectively.  (Fig. 6a). This means that parcels influenced by convection in June 2011 on average experience colder temperatures (Fig.   6b). Finally, the tropics were slightly cooler during June 2011 compared to June 2010, which further contributed to lower water vapor in NA. The net result of these differences is that convectively influenced parcels retain more water vapor in June 2010 160 than in June 2011 (Fig. 6c) due to differing monsoon dynamics. Thus, monsoon dynamics variability plays a significant role in the generating inter-annual variability of stratospheric water vapor in the NA region.

Conclusions
In this study, we investigated the contribution of convection to the water vapor in the North American (NA) monsoon region, including the seasonal cycle and interannual variation of convective contributions during the Northern Hemisphere summer. 165 We have shown that the deep convection moistens the lower stratosphere, adding on up to~1 ppmv to the summertime NA water vapor at 100 hPa based on the observations from MLS.
We have also shown that it is not the amount of convection alone that determines the impact on water vapor -NA monsoon dynamics also play a role. We note that the location of deep convection is not collocated with the maximum water vapor in NA, and this is due to high water vapor content being transported downstream by the monsoon circulation. The maximum water 170 vapor content appears near the center of the NA anticyclone.
We also analyzed the seasonal cycle of convective influence. During June, the NA monsoon circulation is located further south than during July and August, so air influenced by convection during June experiences colder temperatures while traveling to the tropics. Subsequent dehydration reduces the net moistening from convection during June compared to those other months. Variations in the monsoon dynamics can also lead to interannual variations in convective moistening through a similar 175 mechanism. We compare June 2010 and June 2011 and show that a more northerly monsoon circulation in June 2010 leads to convectively influenced air encountering warmer temperatures, leading to higher water vapor than in June 2011.
Our use of GridRad data as a source of convection is a limitation in our analysis because it only covers the continental US.
Much of the monsoonal deep convection also occurs over the Gulf of Mexico (Clapp et al., 2019), out of range of the NEXRAD stations. Future studies including convective data with a larger spatial extent may find that the deep convection over the Gulf 180 of Mexico influences NA stratospheric water vapor, but we do not expect this will conflict with the main conclusions from our paper.  Minimum saturation water vapor mixing ratios and lowest latitude are the minimum values along the path after the parcels encounter deep convection in the back trajectory model and prior to being observed by MLS.