New Zealand atmospheric river (AR) lifecycles are analyzed to examine the synoptic conditions that produce extreme precipitation and regular flooding. An AR lifecycle tracking algorithm, novel to the region, is utilized to identify the genesis location of New Zealand ARs: the location where moisture fluxes enhance and become distinct synoptic features capable of producing impactful weather conditions. Genesis locations of ARs that later impact New Zealand cover a broad region extending from the Southern Indian Ocean (90°E) into the South Pacific (170°W) with the highest genesis frequency being in the Tasman Sea. The most impactful ARs, associated with heavy precipitation, tend to originate from distinct regions based on landfall location. Impactful North Island ARs tend to originate from subtropical regions to the northwest of New Zealand, while impactful South Island ARs are associated with genesis over southeast Australia. The synoptic conditions of impactful AR genesis are identified with North Island ARs typically associated with a cyclone in the central Tasman Sea along with a distant, persistent low pressure off the coast of West Antarctica. South Island AR genesis typically occurs in conjunction with moist conditions over Australia associated with a zonal synoptic-scale wavetrain. The Madden–Julian oscillation (MJO) is examined as a potential source of variability that modulates New Zealand AR lifecycles. It appears that the MJO modulates AR characteristics, especially during Phase 5, typically bringing more frequent, slow moving ARs with greater moisture fluxes to the North Island of New Zealand.

Attention is increasingly being turned towards an investigation of extreme hydrometeorological events within the context of land-atmosphere coupling in the wider hydrological cycle, particularly with respect to the identification of compound and seesaw events. To examine these events, accurate soil moisture data are essential. Here, soil moisture from three reanalysis products (ERA5-Land, BARRA and ERA5) are compared to station observations from 12 sites across New Zealand for an average timespan of 18 years. Soil moisture data from all three reanalyses were subsequently used to investigate land-atmosphere coupling with gridded (observational) precipitation and temperature. Finally, compound (co-occurrence of hot and dry) and seesaw (transitions from dry to wet) periods were identified and examined. No best performing reanalysis dataset for soil moisture is evident (min r = 0.78, max r = 0.80). All datasets successfully capture the seasonal and residual component of soil moisture, but not the observed soil moisture trends at each location. Strong coupling between soil moisture and temperature occurs across the predominately energy-limited regions of the lower North Island and entire South Island. Consequently, these regions reveal a high frequency of compound period occurrence and potential shifts in land states to a water limited phase during compound months. A series of seesaw events are also detected for the first time in New Zealand (terminating an average of 17% of droughts), with particularly high frequency of seesaw event occurrence detected in previously identified areas of atmospheric river (AR) activity, indicating the likely wider significance of ARs for drought termination.

Here, we analyze the inter-relationships between weather types (WTs) and atmospheric rivers (ARs) around Aotearoa New Zealand (ANZ), their respective properties, as well as their combined and separate influence on daily precipitation amounts and extremes. Results show that ARs are often associated with 3-4 WTs, but these WTs change depending on the regions where ARs landfall. The WTs most frequently associated with ARs generally correspond to those favoring anomalously strong westerly wind in the mid-latitudes, especially for southern regions of ANZ, or northwesterly anomalies favoring moisture export from the lower latitudes, especially for the northern regions. WTs and ARs show strong within-type and inter-event diversity. The synoptic patterns of the WTs significantly differ when they are associated with AR occurrences, with atmospheric centers of actions being shifted so that moisture fluxes towards ANZ are enhanced. Symmetrically, the location, angle, and persistence of ARs appear strongly driven by the synoptic configurations of the WTs. Although total moisture transport shows weaker WT-dependency, it appears strongly related to zonal wind speed to the south of ANZ, or the moisture content of the air mass to the north. Finally, WT influence on daily precipitation may completely change depending on their association, or lack thereof, with AR events. WTs traditionally considered as favorable to wet conditions may conceal daily precipitation extremes occurring during AR days, and anomalously dry days or near-climatological conditions during non-AR days.