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Isolating the source of nontidal oceanographic noise in seafloor pressure data is critical for improving the use of these data for seafloor geodetic applications. Residuals between nearby bottom pressure records have typically been used to remove the nontidal components, as these are largely common-mode. To evaluate the similarities between pairs of observed bottom pressure records at a range of water depths, we calculate the standard deviations of the time series of residuals between data from all site pairs, installed in the Hikurangi subduction zone offshore New Zealand. We find that the magnitude of the standard deviation depends more on relative water depth than the distance between sites. This confirms the result of previous studies from Cascadia that nontidal components are more similar along isobaths even if the distance between sites is large. Furthermore, in order to reduce noises, the required depth difference between site pairs also varies with site depths. [Plain Language Summary] Coherent signals of ocean bottom pressure are observed along common water depths within an ocean bottom pressure array offshore New Zealand. We statistically evaluated the similarity of the seafloor pressure collected in 2014 offshore the North Island's east coast, where the Pacific Plate dives or “subducts” along the Hikurangi subduction zone beneath the North Island. This is important for removal of noise caused by oceanographic processes, which must be done to detect centimeter-level vertical movement of the seafloor crust during slow slip events using seafloor pressure records. We measured the similarity of pairs of seafloor pressure records at a range of water depths. Similar to previous studies offshore the Cascadia subduction zone, our results confirm that seafloor pressure records from similar depths (but at large horizontal distances from each other) can be used effectively to reduce oceanographic noise in sea floor pressure data to reveal the seafloor crustal deformation.

We constrain orientations of the horizontal stress field from borehole image data in a transect across the Hikurangi Subduction Margin. This region experiences NW‐SE convergence and is the site of recurrent slow slip events. The direction of the horizontal maximum stress is E‐W at an active splay thrust fault near the subduction margin trench. This trend changes to NNW‐SSE in a forearc trench slope basin on the offshore accretionary wedge, and to NE‐SW in the onshore forearc. Multiple, tectonic, and geological processes, either individually or in concert, may explain this variability. The observed offshore to onshore stress rotation may reflect a change from dominantly compressional tectonics at the deformation front, to a strike‐slip and/or extensional tectonic regime closer to the Taupo Volcanic Zone, further inland. In addition, the offshore stress may be affected by topography and/or stress rotation around subducting seamounts, and/or temporal stress changes during the slow slip cycle.

Following a sequence of three Slow Slip Events (SSEs) on the northern Hikurangi Margin, between June 2015 and August 2016, a Mw 7.1 earthquake struck ~30 km offshore of the East Cape region in the North Island of New Zealand on the 2nd September 2016 (NZ local time). The earthquake was also followed by a transient deformation event (SSE or afterslip) northeast of the North Island, closer to the earthquake source area. We use data from New Zealand's continuous Global Positioning System (cGPS) networks to invert for the SSE slip distribution and evolution on the Hikurangi subduction interface. Our slip inversion results show an increasing amplitude of the slow slip towards the Te Araroa earthquake foreshock and mainshock area, suggesting a possible triggering of the Mw 7.1 earthquake by the later stage of the slow slip sequence. We also show that the transient deformation following the Te Araroa earthquake ruptured a portion of the Hikurangi Trench northeast of the North Island, further north than any previously observed Hikurangi margin SSEs. Our slip inversion and the coulomb stress calculation suggest that this transient may have been induced as a response to the increase in the static coulomb stress change downdip of the rupture plane on the megathrust. These observations show the importance of considering the interaction between slow slip events, seismic and aseismic events, not only on the same megathrust interface, but also on faults within the surrounding crust.

Using time-varying ocean bottom pressure (OBP) from the Gravity Recovery and Climate Experiment (GRACE), a 9-year in situ OBP record at the North Pole, and wind reanalysis products, we perform a linear regression analysis to identify primary predictor time series that enable us to create a proxy representation of the Arctic time-varying OBP that explains the largest fraction of the observed Arctic OBP variability. After cross-validation, two predictors – North Pole OBP record, and wind-OBP coupling from maximum covariance analysis – explain 50% of the total variance of the Arctic OBP. This work provides a means for bridging existing short gaps in GRACE measurements, and potentially longer future gaps that may result if GRACE and its follow-on mission do not overlap. The technique may be applicable to bridge gaps in GRACE measurements in other oceanic regions.

Classical tidal theory predicts that the lunar gravitational semidiurnal tide (L2) should induce perturbations in relative humidity (RH). Adiabatic expansion in divergent flow in advance of the L2 pressure minimum cools the air and reduces its saturation vapor pressure, thereby increasing the rate of condensation in saturated air parcels and causing the relative humidity (RH) of unsaturated parcels to rise. Here we detect a statistically significant L2 signature in precipitation rate (P) in a 15 year, eight times daily, global precipitation data set acquired in the Tropical Rainfall Measuring Mission. Consistent with tidal theory and with the notion that L2 modulates P mainly by perturbing RH, P varies out of phase with pressure, and it increases at a rate of about 10% per 1% increase in RH. These measurements thus provide a measure of the sensitivity of P to planetary-scale changes in RH. Analysis of solar semidiurnal (S2) tidal statistics yields similar results.

We investigate the influence of elastic heterogeneity on geodetic inversions of slow-slip events by inverting for slip distributions of four events along the Hikurangi Margin, New Zealand. We generate Green's functions using a finite element code in conjunction with a New Zealand-wide seismic velocity model to assign elastic properties. We find that these heterogeneous models typically require ~20% less slip than homogeneous models in cases where the slip is deep or there is reasonable geodetic coverage above the slipping region. In cases where the slip is shallow (and mostly offshore) and there is little geodetic coverage directly above the slipping region, the heterogeneous models can predict significantly larger amounts of slip (42% in our study). These changes in the predicted amounts of slip have important implications for quantifying slip budgets accommodated by slow slip at subduction zones worldwide.

Using continuous GPS (cGPS) data from the Hikurangi subduction zone in New Zealand, we show for the first time that stress changes induced by a local earthquake can arrest an ongoing slow slip event (SSE). The cGPS data show that the slip rate in the northern portion of the 2013/2014 Kapiti SSE decreased abruptly following a nearby intraslab earthquake. We suggest that deceleration of the Kapiti SSE in early 2014 occurred due to a tenfold increase in the normal stress relative to shear stress in the SSE source, induced by the nearby Mw 6.3 earthquake, consistent with expectations of rate and state friction. Our observation of an abrupt halting/slowing of the SSE in response to stress changes imposed by a local earthquake has implications for the strength of fault zones hosting SSEs and supports the premise that static stress changes are an important ingredient in triggering (or delaying) fault slip.

The nonlinear response of a spherical shallow water model to an imposed heat source in the presence of realistic zonal mean zonal winds is investigated numerically. The solutions exhibit elongated, meridionally tilted ridges and troughs indicative of a poleward dispersion of wave activity. As the speed of the jets is increased, the equatorial Kelvin wave is unaffected but the global Rossby wave train coalesces to form a compact, amplified quadrupole structure that bears a striking resemblance to the observed upper level structure of the Madden-Julian oscillation. In the presence of strong subtropical westerly jets, the advection of planetary vorticity by the meridional flow and relative vorticity by the zonally averaged background flow conspire to create the distinctive quadrupole configuration of flanking Rossby waves.

The lunar gravitational semidiurnal atmospheric tide L2 is detected in the geopotential field in the ERA-Interim reanalysis by regressing the data for each of the four daily analysis times onto periodic lunar half-synodic cycle time series and differencing the resulting regression maps. A robust pattern dominated by zonal wave number 2 emerges, which agrees with tidal theory and with previous observational studies based on station data in terms of its amplitude, phase, and seasonality at the Earth's surface. Above the 100 hPa level, the tidal signal amplifies with height and it is downward propagating; below that level it is almost barotropic. The L2 signal is resolvable even in data for individual months of individual years. Monthly time series of tidal amplitude and phase exhibit a well-defined pattern of seasonality year after year. Since the ERA forecast model has no L2 gravitational forcing, the source of its L2 signal must be in the observations.

[1] We present previously undocumented deep (25–45 km depth), moderate-duration (2–3 months) slow slip events (SSEs) directly downdip of known shallow (

[1] A global contrast between oceanic and continental lightning very low frequency energy is observed using the World Wide Lightning Location Network (WWLLN). Strokes over the ocean are found to be stronger on average than those over land with a sharp boundary along a majority of coastlines. A linear regression method is developed to account for the spatial and temporal variation of WWLLN in order to perform a multiyear and global analysis of stroke energy distributions. The results are corroborated with data from the Lightning Imaging Sensor, the Optical Transient Detector, and the Earth Networks Total Lightning Network. These systematic comparisons lead to the conclusion that there exists a strong difference in the energetics between land and ocean thunderstorms that results in a higher fraction of more powerful strokes over the oceans.

The climatology of annual mean tropical rain rate is investigated based on merged Tropical Rainfall Measuring Mission (TRMM) 3B42 data. At 0.25° × 0.25° spatial resolution, and 3-hourly temporal resolution, half the rain is concentrated within only ∼1% of the area of the tropics at any given instant. When plotted as a function of logarithm of rain rate, the cumulative contribution of rate-ranked rain occurrences to the annual mean rainfall in each grid box is S shaped and its derivative, the contribution-weighted rain rate spectrum, is Gaussian shaped. The 50% intercept of the cumulative contribution R50 is almost equivalent to the contribution-weighted mean logarithmic rain rate RL¯ based on all significant rain occurrences. The spatial patterns of R50 and RL¯ are similar to those obtained by mapping the fraction of the annual accumulation explained by rain occurrences with rates above various specified thresholds. The geographical distribution of R50 confirms the existence of patterns noted in prior analyses based on TRMM precipitation radar data and reveals several previously unnoticed features.

The leading empirical orthogonal function of the wintertime sea-level pressure field is more strongly coupled to surface air temperature fluctuations over the Eurasian continent than the North Atlantic Oscillation (NAO). It resembles the NAO in many respects; but its primary center of action covers more of the Arctic, giving it a more zonally symmetric appearance. Coupled to strong fluctuations at the 50-hPa level on the intraseasonal, interannual, and interdecadal time scales, this "Arctic Oscillation" (AO)can be interpreted as the surface signature of modulations in the strength of the polar vortex aloft. It is proposed that the zonally asymmetric surface air temperature and mid-tropospheric circulation anomalies observed in association with the AO may be secondary baroclinic features induced by the land-sea contrasts. The same modal structure is mirrored in the pronounced trends in winter and springtime surface air temperature, sea-level pressure, and 50-hPa height over the past 30 years: parts of Eurasia have warmed by as much as several K, sea-level pressure over parts of the Arctic has fallen by 4 hPa, and the core of the lower stratospheric polar vortex has cooled by several K. These trends can be interpreted as the development of a systematic bias in one of the atmosphere's dominant, naturally occurring modes of variability.

[1] We examine the relationship between hourly extreme precipitation and daily mean temperature across the Contiguous United States (CONUS) during the period 1950–2009. Logarithmically-transformed hourly extreme precipitation generally increases approximately linearly with the daily mean temperature. At most (about 80%) of the stations, regression slopes between hourly extreme precipitation and daily mean temperature are higher than 7%, the approximate Clausius-Clapyron rate. Stations located in the western coastal region exhibit the lowest regression slopes, with median regression slopes less than 7%, while stations in the northern tier of the CONUS showed higher regression slopes, with median regression slope of 16%. More stations (75%) exhibit regression slopes higher than 7% in summer than in winter (65%). Stations in the southern U.S. have higher regression slopes in winter than in summer, whereas stations in the northern U.S. have higher slopes in summer. It seems physically implausible that the intensity of extreme rainfall events could be as sensitive to local temperature as reflected by the regression slopes. Hence, these large values at least partially may be a consequence of factors relating to temperature gradients as opposed to temperature in its own right.

[1] Strong summertime anticyclonic wind anomalies over the Arctic Ocean, with anomalous flow toward the Fram Strait, during summer months of 2007 contributed to the record-low the Arctic sea-ice extent observed in September of that year. Had the summer winds over the Arctic during the summers of 2010 and 2011 been the same as those in 2007, September sea ice extent would have reached new record lows in those years as well. By regulating the flow of ice toward and through the Fram Strait, variations in low-level winds over the Arctic have contributed to the month-to-month, year-to-year, and decade-to-decade variability of sea ice extent.

[1] The relationship between lightning and NO2 over Indonesia is examined on daily and intraseasonal time scales based on lightning observations from the World Wide Lightning Location Network (WWLLN) and tropospheric NO2 column densities from the Global Ozone Monitoring Experiment (GOME‐2) satellite mission. Composites of the daily NO2 observations regressed onto lightning frequency reveal a plume of enhanced NO2 following a day of enhanced lightning. Lightning and NO2 also vary coherently with the intraseasonal Madden‐Julian Oscillation (MJO) in a manner distinct from the cloudiness signature, with variations of up to ∼50% of the annual mean. Citation: Virts, K. S., J. A. Thornton, J. M. Wallace, M. L. Hutchins, R. H. Holzworth, and A. R. Jacobson (2011), Daily and intraseasonal relationships between lightning and NO2 over the Maritime Continent, Geophys. Res. Lett., 38, L19803, doi:10.1029/2011GL048578.

[1] Deep non-volcanic tremor (NVT) is usually associated with episodic slow-slip events. New Zealand is one notable exception where numerous slow slip events have been identified, yet NVT has remained undetected. Here we present the first known case of triggered NVT at New Zealand's Hikurangi subduction margin. Following the Mw8.8 Chilean earthquake of February 27, 2010, we identify coherent high-frequency tremor signals that are in phase with, and modulated by, the passing Rayleigh waves. This is consistent with the surface wave triggering potential for strike-normal incidence on a low-angle thrust fault. After constraining the tremor depth on the plate interface, we locate the tremor source within 20 km of the source area of episodic slow slip. The tremor location is also near the edge of a region with high seismic attenuation that marks the boundary between dehydrated subducted slab and inferred hydrated, underplated sediment. We speculate that reduced interface friction and high fluid pressures provided by fluid-rich underplated sediment facilitates the tremor generation.

[1] A sub-monthly mode of non-tidal variability of ocean bottom pressure (OBP) is observed in a 5-year record of deep-sea bottom pressure at the North Pole. OBP records from other regions in the Arctic show that the North Pole non-tidal mass fluctuation is part of a non-propagating basin-coherent variation that is well represented by the ice-ocean model PIOMAS, with a basin-averaged winter-only RMS of 3.3 cm. Wavelet analysis of the modeled OBP shows that the basin-averaged mass variations are non-stationary and only significant during the winter. The basin-averaged OBP is strongly related to the meridional wind component over the Nordic Seas. The ocean response is consistent with episodic wind forcing driving a northward geostrophic slope current. The mass transport anomaly associated with the mode is significant relative to the annual net mean flow.

[1] Application of the method of partial least squares (PLS) regression to geophysical data is illustrated with two cases: (1) finding sea level pressure patterns over the North Pacific associated with dynamically-induced winter-to-winter variations in snowpack in the Cascade mountains of western Washington state and (2) finding patterns of sea surface temperature over the tropical oceans that modulate Atlantic hurricane activity on a year-to-year basis. In both examples two robust patterns in the “predictor field” are identified that, in combination, account for over half the variance in the target time series.

[1] The unprecedented retreat of first-year ice during summer 2007 was enhanced by strong poleward drift over the western Arctic induced by anomalously high sea-level pressure (SLP) over the Beaufort Sea that persisted throughout much of the summer. Comparison of the tracks of drifting buoys with monthly mean SLP charts shows a substantial Ekman drift. By means of linear regression analysis it is shown that Ekman drift during summer has played an important role in regulating annual minimum Arctic sea-ice extent in prior years as well. In combination, the preconditioning by events in prior years, as represented by an index of May multi-year ice, and current atmospheric conditions, as represented by an index of July–August–September SLP anomalies over the Arctic basin account for ∼60% of the year-to-year variance of September sea-ice extent since 1979.

[1] Interrelationships between year-to-year variations in September Arctic sea ice extent and summer sea level pressure and surface air temperature at high northern latitudes are examined making use of microwave satellite imagery and atmospheric data for the period 1979–2006. Linear trends and year-to-year variability about the linear trend lines are considered separately: the latter gives a clearer indication of the physical linkages between fields. Years with low September sea ice extent tend to be characterized by anticyclonic circulation anomalies over the Arctic, with easterly wind anomalies over the marginal seas where the year-to-year variability of sea ice concentration is largest. It is hypothesized that the summer circulation anomalies cause sea ice extent principally by way of the Ekman drift in the marginal seas. The associated surface air temperature anomalies also tend to be largest over the marginal seas, with positive anomalies over the regions of reduced sea ice.

[1] We present the first published evidence of uplift along the eastern coast of New Britain, Papua New Guinea based upon 230Th dating of uplifted Holocene coral reefs. Although uplift rates of the nearby Huon Peninsula are well constrained (∼2–3 m/ka), little is known about vertical motion along the eastern margin of New Britain. Based upon relative sea-level curves for Papua New Guinea, present sample elevations, and estimates of paleowater depth of four in-situ Holocene corals we calculate an average uplift rate of 1.6 ± 0.4 m/ka for the southeast coast of New Britain. We suggest underplating of subducted sediment as a possible mechanism for uplift of the reefs.

[1] We present continuous GPS observations of a surface deformation episode in the Manawatu region of the North Island, New Zealand, lasting 18 months from January 2004 to June 2005. We invert vertical and horizontal displacements of GPS sites during the event for slow slip on the Hikurangi subduction thrust. We find that up to 350 mm of slip occurred on the thrust in the transition zone between strong interseismic coupling and aseismic creep. If the entire slip in the Manawatu slow slip event had occurred instantaneously, it would have resulted in an Mw 7.0 earthquake. Given the large amount of slip, we expect that the recurrence interval for similar slow slip events may be ten years or more.

In October 2002, a surface displacement episode of 20-30 mm magnitude was observed over a ∼10 day period on two continuous Global Positioning System (GPS) instruments near Gisborne, North Island, New Zealand. We interpret this to result from slow slip on the northern Hikurangi subduction interface. Using ten years of regional campaign GPS (1995-2004) and recent continuous GPS data, we estimate the recurrence interval for similar events to be 2-3 yrs. In November 2004, a similar slow slip event occurred within this recurrence period. The 2002 event can be modeled by ∼18 cm of slow slip near the down-dip end of the seismogenic zone on the subduction interface offshore of Gisborne. The campaign GPS data show that the 2002 slow slip event had little effect on regional strain patterns. Copyright 2005 by the American Geophysical Union.

[1] Three of the past six summers have exhibited record low sea-ice extent on the Arctic Ocean. These minima may have been dynamically induced by changes in the surface winds. Based on results of a simple model that keeps track of the age of ice as it moves about on the Arctic Ocean, we show that the areal coverage of thick multi-year ice decreased precipitously during 1989–1990 when the Arctic Oscillation was in an extreme “high index” state, and has remained low since that time. Under these conditions, younger, thinner ice anomalies recirculate back to the Alaskan coast more quickly, decreasing the time that new ice has to ridge and thicken before returning for another melt season. During the 2002 and 2003 summers this anomalously younger, thinner ice was advected into Alaskan coastal waters where extensive melting was observed, even though temperatures were locally colder than normal. The age of sea-ice explains more than half of the variance in summer sea-ice extent.

[1] The structure of the Northern Hemisphere annular mode (NAM) is shown to be significantly different during warm and cold winters of the ENSO cycle. During warm winters the Arctic center of action of the annular mode is more prominent and extends deeper into Siberia, its related surface air temperature signal is stronger, and the compensating outer ring in the sea-level pressure field is much broader, encompassing the entire tropics. During cold winters the NAM signature is much more pronounced over the Pacific sector: the sea-level pressure pattern exhibits a North Pacific center of action, positively correlated with the Atlantic center, and pronounced deflections of the storm track are also evident. It is suggested that the former features are associated with the equatorward displacement of the jet stream and the storm tracks during warm winters of the ENSO cycle and the latter is related to the the more pronounced break in the subtropical jet stream over the Pacific sector during cold winters.